[Federal Register Volume 66, Number 13 (Friday, January 19, 2001)]
[Rules and Regulations]
[Pages 5706-5910]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-996]


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DEPARTMENT OF LABOR

Mine Safety and Health Administration

30 CFR Part 57

RIN 1219-AB11


Diesel Particulate Matter Exposure of Underground Metal and 
Nonmetal Miners

AGENCY: Mine Safety and Health Administration (MSHA), Labor.

ACTION: Final rule.

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SUMMARY: This rule establishes new health standards for underground 
metal and nonmetal mines that use equipment powered by diesel engines.
    This rule is designed to reduce the risks to underground metal and 
nonmetal miners of serious health hazards that are associated with 
exposure to high concentrations of diesel particulate matter (dpm). DPM 
is a very small particle in diesel exhaust. Underground miners are 
exposed to far higher concentrations of this fine particulate than any 
other group of workers. The best available evidence indicates that such 
high exposures put these miners at excess risk of a variety of adverse 
health effects, including lung cancer.
    The final rule for underground metal and nonmetal mines would 
establish a concentration limit for dpm, and require mine operators to 
use engineering and work practice controls to reduce dpm to that limit. 
Underground metal and nonmetal mine operators would also be required to 
implement certain ``best practice'' work controls similar to those 
already required of underground coal mine operators under MSHA's 1996 
diesel equipment rule. These operators would also be required to train 
miners about the hazards of dpm exposure.
    By separate notice, MSHA has published a rule to reduce dpm 
exposures in underground coal mines.

DATES: The provisions of the final rule are effective March 20, 2001. 
However, Sec. 57.5060 (a) will not apply until July 19, 2002 and 
Sec. 57.5060 (b) will not apply until January 19, 2006.

FOR FURTHER INFORMATION CONTACT: David L. Meyer, Director, Office of 
Standards, Regulations, and Variances, MSHA, 4015 Wilson Boulevard, 
Arlington, VA 22203-1984. Mr. Meyer can be reached at [email protected] 
(Internet E-mail), 703-235-1910 (voice), or 703-235-5551 (fax). You may 
obtain copies of the final rule in alternative formats by calling this 
number. The alternative formats available are either a large print 
version of the final rule or the final rule in an electronic file on 
computer disk. The final rule also is available on the Internet at 
http://www.msha.gov/REGSINFO.HTM.

SUPPLEMENTARY INFORMATION:

I. Overview of the Final Rule

    This Part: (1) Summarizes the key provisions of the final rule; and 
(2) summarizes MSHA's responses to some of the fundamental questions 
raised during the rulemaking proceeding--the need for the rule, the 
ability of the agency to accurately measure diesel particulate matter 
(dpm) in underground metal and nonmetal mine environments, and the 
feasibility of the requirements for this sector of the mining industry.

(1) Summary of Key Provisions of the Final Rule

    The final rule applies only to underground areas of underground 
metal and nonmetal mines.
    The final rule requires operators: (A) To observe a concentration 
limit where miners normally work or travel by the application of 
engineering controls, with certain limited exceptions, compliance with 
which will be determined by MSHA sampling; (B) to observe a set of best 
practices to minimize dpm generation; (C) to limit engines newly 
introduced underground to those meeting basic emissions standards; (D) 
to provide annual training to miners on dpm hazards and controls; and 
(E) to conduct sampling as often as necessary to effectively evaluate 
dpm concentrations at the mine. A list of effective dates for the 
provisions of the rule follows this summary.
    (A) Observe a limit on the concentration of dpm in all areas of an 
underground metal or nonmetal mine where miners work or travel, with 
certain specific exceptions. The rule would limit dpm concentrations to 
which miners are exposed to about 200 micrograms per cubic meter of 
air--expressed as 200DPM g/m 3. However, 
the rule expresses the limit so as to reflect the measurement method 
MSHA will be using for compliance purposes to determine dpm 
concentrations. That method is specified in the rule itself. As 
discussed in detail in response to Question 2, the method analyzes a 
dust sample to determine the amount of total carbon present. Total 
carbon comprises 80-85% of the dpm emitted by diesel engines. 
Accordingly, using the lower boundary of 80%, a concentration limit of 
200DPM g/m 3 can be achieved by 
restricting total carbon to 160TC g/m 3. 
This is the way the standard is expressed:

    After January 19, 2006 any mine operator covered by this part 
shall limit the concentration of diesel particulate matter to which 
miners are exposed in underground areas of a mine by restricting the 
average eight-hour equivalent full shift airborne concentration of 
total carbon, where miners normally work or travel, to 160 
micrograms per cubic meter of air (160TC g/m 
3).

    All underground metal and nonmetal mines would be given a full five 
years to meet this limit, which is referred to in this preamble as the 
``final'' concentration limit. However, starting July 19, 2002, 
underground metal and nonmetal mines have to observe an ``interim'' dpm 
concentration limit--expressed as a restriction on the

[[Page 5707]]

concentration of total carbon of 400 micrograms per cubic meter 
(400TC g/m 3). The interim limit would 
bring the concentration of whole dpm in underground metal and nonmetal 
mines to which miners are exposed down to about 500 micrograms per 
cubic meter. No limit at all on the concentration of dpm is applicable 
for the first eighteen months following promulgation. Instead, this 
period would be used to provide compliance assistance to the metal and 
nonmetal mining community to ensure it understands how to measure and 
control diesel particulate matter concentrations in individual 
operations.
    In general, a mine operator has to use engineering or work practice 
controls to keep dpm concentrations below the applicable limit. The use 
of administrative controls (e.g., the rotation of miners) is explicitly 
barred. The use of personal protective equipment (e.g., respirators) is 
also explicitly barred except in two situations noted below. An 
operator can filter the emissions from diesel-powered equipment, 
install cleaner-burning engines, increase ventilation, improve fleet 
management, or use a variety of other readily available controls; the 
selection of controls is left to the operator's discretion.
    Special extension. The rule provides that if an operator of a metal 
or nonmetal mine can demonstrate that there is no combination of 
controls that can, due to technological constraints, be implemented by 
January 19, 2006, MSHA may approve an application for an additional 
extension of time to comply with the dpm concentration limit. Such a 
special extension is available only once, and is limited to 2 years. To 
obtain a special extension, an operator must provide information in the 
application adequate for MSHA to ensure that the operator will: (a) 
Maintain concentrations at the lowest limit which is technologically 
achievable; and (b) take appropriate actions to minimize miner exposure 
(e.g., provide suitable respiratory protection during the extension 
period).
    It is MSHA's intent that primary responsibility for analysis of the 
operator's application for a special extension will rest with MSHA's 
district managers. District managers are the most familiar with the 
conditions of mines in their districts, and have the best opportunity 
to consult with miners as well. At the same time, MSHA recognizes that 
district managers may need assistance with respect to the latest 
technologies and solutions being used in similar mines elsewhere in the 
country. Accordingly, the Agency intends to establish within its 
Technical Support directorate in Arlington, Va., a special panel to 
consult on these issues, to provide assistance to district managers, 
and to give final approval of any application for a special extension.
    Special rule for employees engaged in inspection, maintenance or 
repair activities. The final rule provides that with the advance 
approval of the Secretary, employees engaged in such activities may 
work in concentrations of dpm exceeding the applicable concentration 
limit. However, the Secretary may only approve such work under three 
circumstances: when the activities are to be conducted are in areas 
where miners work or travel infrequently or for brief periods of time; 
when the miners work exclusively inside enclosed and environmentally 
controlled cabs, booths and similar structures with filtered breathing 
air; or when the miners work in shafts, inclines, slopes, adits, 
tunnels and similar workings that are designated as return or exhaust 
air courses and that are used for access into the mine or egress from 
the mine. Moreover, to approve such an exception, the Secretary must 
determine that it is not feasible to reduce the concentration of dpm in 
these areas, and that adequate safeguards (including personal 
protective equipment) will be employed to minimize the dpm exposure of 
the miners involved.
    An operator plan providing such details must be submitted; it is 
MSHA's intent to review these in the same manner as applications for a 
special extension. Such plans can only be approved for one year, but 
may be resubmitted each year.
    Compliance determinations with concentration limit. Measurements to 
determine noncompliance with the dpm concentration limit will be made 
directly by MSHA, rather than having the Agency rely upon operator 
samples. Under the rule, a single Agency sample, using the sampling and 
analytical method prescribed by the rule, is explicitly deemed adequate 
to establish a violation.
    The rule requires that if an underground metal or nonmetal mine 
exceeds the applicable limit on the concentration of dpm, a diesel 
particulate matter control plan must be established and remain in 
effect for 3 years. The purpose of such plans is to ensure that the 
mine has instituted practices that will demonstrably control dpm levels 
thereafter. Reflecting current practices in this sector, the plan does 
not have to be preapproved by MSHA. The plan must include information 
about the diesel-powered equipment in the mine and applicable controls. 
The rule requires operator sampling to verify that the plan is 
effective in bringing dpm levels down below the applicable limit, using 
the same sampling and analytical methods as MSHA, with the records kept 
at the mine site with the plan to facilitate review. Failure of an 
operator to comply with the requirements of the dpm control plan or to 
conduct adequate verification sampling is a violation of the rule; MSHA 
is not be required to sample to establish such a violation.
    (B) Observe best practices. The rule requires that operators 
observe the following best practices to minimize the dpm generated by 
diesel-powered equipment in underground areas:
     Only low-sulfur (0.05% or less) diesel fuel may be used. 
The rule does not at this time require the use of ultra-low sulfur fuel 
by the mining community. MSHA is aware that the Environmental 
Protection Agency issued final regulations addressing emissions 
standards (December 2000) for new model year 2007 heavy-duty diesel 
engines and the low-sulfur fuel rule. The regulations require ultra-low 
sulfur fuel be phased in during 2006-2010.
     Only EPA-approved fuel additives may be used.
     Approved diesel engines have to be maintained in approved 
condition; the emission related components of non-approved engines have 
to be maintained in accordance with manufacturer specifications; and 
any installed emission devices have to be maintained in effective 
operating condition.
     Equipment operators are authorized and required to tag 
equipment with potential emissions-related problems, and tagged 
equipment has to be promptly referred for a maintenance check by 
persons qualified by virtue of training or experience to perform the 
maintenance.
    (C) Limit newly introduced engines to those meeting basic emission 
standards. The rule requires that, with the exception of diesel engines 
used in ambulances and fire-fighting equipment, any diesel engines 
added to the fleet of an underground metal or nonmetal mine after 
January 19, 2001 must either be an engine approved by MSHA under Part 7 
or Part 36, or an engine meeting certain EPA requirements on 
particulate matter specified in the rule. Since not all engines are 
MSHA approved, this ensures a wide variety of choice in meeting the 
engine requirements of this rule.
    (D) Provide annual training to miners on dpm hazards and controls. 
Mines using diesel-powered equipment must annually train miners exposed 
to dpm

[[Page 5708]]

in the hazards associated with that exposure, and in the controls being 
used by the operator to limit dpm concentrations. An operator may 
propose including this training in the Part 48 training plan.
    (E) Conduct sampling as often as necessary to effectively evaluate 
dpm concentrations at the mine. The purpose of this requirement is to 
assure that operators are familiar with current dpm concentrations so 
as to be able to protect miners. Since mine conditions vary, MSHA is 
not requiring a specific schedule for operator sampling, nor a specific 
sampling method. The Agency will evaluate compliance with this sampling 
obligation by reviewing evidence of operator compliance with the 
concentration limit, as well as information retained by operators about 
their sampling. Consistent with the statute, the rule requires that 
miners and their representatives have the right to observe any operator 
monitoring--including any sampling required to verify the effectiveness 
of a dpm control plan.
    Summary of Effective Dates. As of March 20, 2001, operators must 
comply with the requirement that new engines added to a mine's 
inventory be either MSHA approved or meet the listed EPA standards.
    As of March 20, 2001, underground metal and nonmetal mine operators 
must comply with the requirement to provide basic hazard training to 
miners who are exposed underground to dpm and the best practice 
requirements listed above under (B).
    As of July 19, 2002, underground metal and nonmetal mine operators 
must also comply with the interim dpm concentration limit of 400 
micrograms of total carbon per cubic meter of air.
    Finally, as of January 19, 2006, all underground metal and nonmetal 
mines have to comply with a final dpm concentration limit.
    MSHA intends to provide considerable technical assistance and 
guidance to the mining community before the various requirements go 
into effect, and be sure MSHA personnel are fully trained in the 
requirements of the rule. A number of actions have already been taken 
toward this end. The Agency held workshops on this topic in 1995 which 
provided the mining community an opportunity to share advice on how to 
control dpm concentrations. The Agency has published a ``toolbox'' of 
methods available to mining operators to achieve reductions in dpm 
concentration, often referred to during the rulemaking proceedings. 
MSHA also developed a computer spreadsheet template which allows an 
operator to model the application of alternative engineering controls 
to reduce dpm, which it has published in the literature and 
disseminated to the mining community. The Agency is committed to 
issuing a compliance guide for mine operators providing additional 
advice on implementing the rule.
    A note on surface mines. Surface areas of underground mines, and 
surface mines, are not covered by this rule. In certain situations the 
concentrations of dpm at surface mines may be a cause for concern: 
e.g., production areas where miners work in the open air in close 
proximity to loader-haulers and trucks powered by older, out-of-tune 
diesel engines, shops, or other confined spaces where diesel engines 
are running. The Agency believes, however, that these problems are 
currently limited and readily controlled through education and 
technical assistance. The Agency would like to emphasize, however, that 
surface miners are entitled to the same level of protection as other 
miners; and the Agency's risk assessment indicates that even short-term 
exposures to concentrations of dpm like those observed may result in 
serious health problems. Accordingly, in addition to providing 
education and technical assistance to surface mines, the Agency will 
also continue to evaluate the hazards of diesel particulate exposure at 
surface mines and will take any necessary action, including regulatory 
action if warranted, to help the mining community minimize any hazards.

(2) Summary of MSHA's Responses to Several Fundamental Questions About 
This Rule

    During the rulemaking proceeding, the mining community raised some 
fundamental questions about: (A) The need for the rule; (B) the ability 
of the agency to accurately measure diesel particulate matter (dpm) in 
underground metal and nonmetal mine environments; and (C) the 
feasibility of the requirements for this sector of the mining industry. 
MSHA gave serious considerations to these questions, has made some 
adjustments in the final rule and its economic assessment as a result 
thereof, and has provided detailed responses in this preamble. These 
responses are briefly summarized here.
    (A) The need for the rule. MSHA has to act in accordance with the 
requirements of the Mine Safety and Health Act. Section 101(a)(6)(A) of 
the Act specifies that any health standard must:

    * * * [A]dequately assure, on the basis of the best available 
evidence, that no miner will suffer material impairment of health or 
functional capacity even if such miner has regular exposure to the 
hazards dealt with by such standard for the period of his working 
life.

    The Mine Act also specifies that the Secretary of Labor 
(Secretary), in promulgating mandatory standards pertaining to toxic 
materials or harmful physical agents, base such standards upon:

    * * * [R]esearch, demonstrations, experiments, and such other 
information as may be appropriate. In addition to the attainment of 
the highest degree of health and safety protection for the miner, 
other considerations shall be the latest available scientific data 
in the field, the feasibility of the standards, and experience 
gained under this and other health and safety laws. Whenever 
practicable, the mandatory health or safety standard promulgated 
shall be expressed in terms of objective criteria and of the 
performance desired. [Section 101(a)(6)(A)].

    Thus, the Mine Act requires that the Secretary, in promulgating a 
standard, based on the best available evidence, attain the highest 
degree of health and safety protection for the miner with feasibility a 
consideration. (More information about what constitutes ``feasibility'' 
is discussed below in item C).
    In proposing this rule, MSHA sought comment on its risk assessment, 
which it published in full as part of the preamble to the proposed 
rule. In that risk assessment, the agency carefully laid out the 
evidence available to it, including shortcomings inherent in that 
evidence. Although not required to do so by law, MSHA had this risk 
assessment independently peer reviewed, and incorporated the reviewers 
recommendations. The reviewers stated that:

    * * * principles for identifying evidence and characterizing 
risk are thoughtfully set out. The scope of the document is 
carefully described, addressing potential concerns about the scope 
of coverage. Reference citations are adequate and up to date. The 
document is written in a balanced fashion, addressing uncertainties 
and asking for additional information and comments as appropriate. 
(Samet and Burke, Nov. 1997).

    Based on the information in that risk assessment, the agency made 
some tentative conclusions. First, its tentative conclusion that miners 
are exposed to far higher concentrations of dpm than anybody else. The 
agency noted that median concentrations of dpm had been observed in 
individual dieselized metal and nonmetal underground mines up to 180 
times as high as average environmental exposures in the most heavily 
polluted urban areas and up to 8 times as high as median exposures 
estimated for the most heavily exposed

[[Page 5709]]

workers in other occupational groups. Moreover, MSHA noted its 
tentative conclusion that exposure to high concentrations of dpm can 
result in a variety of serious health effects. These health effects 
include: (i) Sensory irritations and respiratory symptoms serious 
enough to distract or disable miners; (ii) premature death from 
cardiovascular, cardiopulmonary, or respiratory causes; and (iii) lung 
cancer. After a review of all the evidence, MSHA tentatively concluded 
that:
    (1) The best available evidence is that the health effects 
associated with exposure to dpm can materially impair miner health or 
functional capacity.
    (2) At levels of exposure currently observed in underground mining, 
many miners are presently at significant risk of incurring these 
material impairments over a working lifetime.
    (3) The reduction in dpm exposures that is expected to result from 
implementation of the rule proposed by the agency for underground metal 
and nonmetal mines would substantially reduce the significant risks 
currently faced by underground metal and nonmetal miners exposed to 
dpm.
    During the hearings and in written comments, some representatives 
of the mining industry raised a number of objections to parts of MSHA's 
proposed risk assessment, thus questioning the scientific basis for 
this rulemaking. It has been asserted that MSHA's observations of dpm 
concentrations in underground metal and nonmetal mines do not 
accurately represent exposures in the industry. It has been asserted 
that if dpm concentrations are not this high in general, or only on an 
intermittent basis, then the agency is incorrect in determining that 
the conditions in these mines put miners at significant risk of 
material impairment of their health. Moreover it has been asserted that 
there is insufficient evidence to establish a causal connection between 
dpm exposure and significant adverse health effects, that the agency 
has no hard evidence that reducing exposures to a particular level will 
in fact reduce the risks, and that it has no rational basis for 
selecting the concentration limit it did. In addition, it has been 
asserted that the risks of dpm exposure at any level are not well 
enough established to provide the basis for regulation at this time, 
and that action should be postponed pending the completion of various 
studies now underway that might shed more light on these risks.
    MSHA has carefully evaluated all of these comments, and the 
evidence submitted in support of these positions. The agency's risk 
assessment has been modified as a result.
    Exposures of underground metal and nonmetal miners. MSHA has 
clarified the charts of exposure measurements in Part III of this 
preamble to ensure that they fully reflect all studies in the record.
    MSHA has not and does not claim that the actual exposure 
measurements in the record are a random or fully representative sample 
of the industry. What they do show is that exposures far higher than 
those which have been observed in other industries can and do occur in 
an underground mining environment.
    Moreover, MSHA also placed into the record of the proposed rule 
several studies it had recently conducted in which dpm concentrations 
for several underground metal and nonmetal mines were estimated based 
upon the actual equipment and dpm controls currently available in those 
mines. Those simulations were performed using a software tool known as 
the Estimator (described in detail in an appendix to Part V of the 
preamble of the proposed rule, and since published in the literature 
(Haney and Saseen, April 2000). These studies of specific mines 
demonstrated that the type of equipment found in such mines, even after 
the application of current ventilation and controls, can be expected to 
produce localized high concentrations of dpm. The agency acknowledged 
that these simulations were conducted in mines that were not typical 
for the industry (they were chosen because the agency thought dpm 
concentrations might be particularly difficult to control in these 
mines, which turned out not to be the case); nevertheless, they 
indicate what is likely to be the case in at least some sections of 
many underground metal and nonmetal mines. To the extent that an 
individual mine has no covered mining areas with concentrations higher 
than those observed in other industries, it will not be impacted by the 
concentration limit established through this rulemaking. That is 
because the rule does not eliminate exposures, or even to reduce them 
to a safe level, but only to reduce them to the levels observed in 
other industries.
    The nature of risks associated with dpm exposure. Although there 
were some commenters who suggested that symptoms reported by miners 
working around diesel equipment might be due to the gases present 
rather than dpm, there was nothing in the comments that changed MSHA's 
conclusions about the health problems associated with dpm exposure.
    There are a number of studies quantifying significant adverse 
health effects--as measured by lost work days, hospitalization and 
increased mortality rates--suffered by the general public when exposed 
to concentrations of fine particulate matter like dpm far lower than 
concentrations to which some miners are exposed. The evidence from 
these fine particulate studies was the basis for recent rulemaking by 
the Environmental Protection Agency \1\ to further restrict the 
exposure of the general public to fine particulates, and the evidence 
was given very widespread and close scrutiny before that action was 
made final. Of particular interest to the mining community is that 
these fine particulate studies indicate that smokers and those who have 
pre-existing pulmonary problems are particularly at risk. Many 
individual miners in fact have such pulmonary problems and are 
especially susceptible to the adverse health effects of inhaling fine 
particles.
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    \1\ The basis for the PM2.5 NAAQS was a large body of 
scientific data indicating that particles in this size range are 
responsible for the most serious health effects associated with 
particulate matter. The evidence was thoroughly reviewed by a number 
of scientific panels through an extended process. The proposed rule 
resulted in considerable public attention, and hearings by Congress, 
in which the scientific evidence was further discussed. Moreover, 
challenges to the EPA's determination that this size category 
warranted rulemaking were rejected by a three-judge panel of the DC 
Circuit Court. (ATA v. EPA, 175 F.3d 1027, D.C. Circuit 1999).
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    Although no epidemiological study is flawless, numerous 
epidemiological studies have shown that long term exposure to diesel 
exhaust in a variety of occupational circumstances is associated with 
an increased risk of lung cancer. With only rare exceptions, involving 
relatively few workers and/or observation periods too short to reliably 
detect excess cancer risk, the human studies have consistently shown a 
greater risk of lung cancer among workers exposed to dpm than among 
comparable unexposed workers. When results from the human studies are 
combined, the risk is estimated to be 30-40 percent greater among 
exposed workers, if all other factors (such as smoking habits) are held 
constant. The consistency of the human study results, supported by 
experimental data establishing the plausibility of a causal connection, 
provides strong evidence that chronic dpm exposure at high levels 
significantly increases the risk of lung cancer in humans.
    Moreover, all of the occupational studies indicating an increased 
frequency of lung cancer among workers exposed to dpm involved exposure 
levels estimated, on average, to be far below levels observed in 
underground mines. Except for miners, the workers

[[Page 5710]]

included in these studies were exposed to average dpm levels below the 
limit established by this rule.
    As noted in Part III, MSHA views extrapolations from animal 
experiments as subordinate to results obtained from human studies. 
However, it is noteworthy that dpm exposure levels recorded in some 
underground mines have been of the same order of magnitude that 
produced tumors in rats.
    Based on the scientific data available in 1988, the National 
Institute for Occupational Safety and Health (NIOSH) identified dpm as 
a probable or potential human carcinogen and recommended that it be 
controlled. Other organizations have made similar recommendations. Most 
recently, the National Toxicology Program listed dpm as ``reasonably 
anticipated to be a human carcinogen'' in the Ninth Edition (Year 2000) 
of the National Report on Carcinogens.
    The relationship between exposures and risks. Commenters noted 
MSHA's caution about trying to define a quantitative relationship 
between dpm exposure and particular health outcomes. They roundly 
attacked the agency's benefit analysis and a NIOSH paper reviewing 
quantification efforts as implying that such a relationship could be 
established in a valid way.
    As MSHA acknowledged in the preamble to the proposed rule, the 
scientific community has not yet widely accepted any exposure-response 
relationship between the amount of dpm exposure and the likelihood of 
adverse health outcomes (63FR 58167). There are, however, two lung 
cancer studies in the record that show increasing risk of lung cancer 
with increasing levels of dpm exposure. Quantitative results from these 
studies, both conducted specifically on underground miners, can be used 
to estimate the reduction in lung cancer risk expected when dpm 
exposure is reduced in accordance with this rule. Depending on the 
study and method of statistical analysis used, these estimates range 
from 68 to 620 lung cancer deaths prevented, over an initial 65-year 
period, per 1000 affected miners with lifetime (45-year) exposure to 
dpm.
    NIOSH and the National Cancer Institute (NCI) are collaborating on 
a cancer mortality study designed to provide additional information in 
this regard. The study is projected to take about seven years.
    Notwithstanding this situation, MSHA believes the Agency is 
required under its statute to take action now to protect miners' 
health. As noted by the Supreme Court in an important case on risk 
involving the Occupational Safety and Health Administration, the need 
to evaluate risk does not mean an agency is placed into a 
``mathematical straightjacket.'' Industrial Union Department, AFL-CIO 
v. American Petroleum Institute, 448 U.S. 607, 100 S.Ct. 2844 (1980). 
The Court noted that when regulating on the edge of scientific 
knowledge, absolute scientific certainty may not be possible, and:

so long as they are supported by a body of reputable scientific 
thought, the Agency is free to use conservative assumptions in 
interpreting the data * * * risking error on the side of 
overprotection rather than underprotection. (Id. at 656).

This advice has special significance for the mining community, because 
a singular historical factor behind the enactment of the current Mine 
Act was the slowness of the mining community in coming to grips with 
the harmful effects of other respirable dust (coal dust).
    It is worth noting that while the cohort selected for the NIOSH/NCI 
study consists of underground miners (specifically, underground metal 
and nonmetal miners), this choice is in no way linked to MSHA's 
regulatory framework or to miners in particular. This cohort was 
selected for the study because it provides the best population for 
scientists to study. For example, one part of the study would compare 
the health experiences of miners who have worked underground in mines 
with long histories of diesel use with the health experiences of 
similar miners who work in surface areas where exposure is 
significantly lower. Since the general health of these two groups is 
very similar, this will help researchers to quantify the impacts of 
diesel exposure. No other population is likely to be as easy to study 
for this purpose. But as with any such epidemiological study, the 
insights gained are not limited to the specific population used in the 
study. Rather, the study will provide information about the 
relationship between exposure and health effects that will be useful in 
assessing the risks to any group of workers in a dieselized industry.
    Because of the lack of a generally accepted dose-response 
relationship, some commenters questioned the agency's rationale in 
picking a particular concentration limit: 160TC g/
m3 or around 200DPM g/m3. 
Capping dpm concentrations at this level will eliminate the worst 
mining exposures, and bring miner exposures down to a level 
commensurate with those reported for other groups of workers who use 
diesel-powered equipment. The proposed rule would not bring 
concentrations down as far as the proposed ACGIH TLVR of 
150DPM g/m3. Nor does MSHA's risk 
assessment suggest that the proposed rule would completely eliminate 
the significant risks to miners of dpm exposure.
    In setting the concentration limit at this particular value, the 
Agency is acting in accord with its statutory obligation to attain the 
highest degree of safety and health protection for miners that is 
feasible. The Agency's risk assessment supports reduction of dpm to the 
lowest level possible. But feasibility considerations dictated 
proposing a concentration limit that does not completely eliminate the 
significant risks that dpm exposure poses to miners.
    The Agency specifically explored the implications of requiring 
mines in this sector to comply with a lower concentration limit than 
that being adopted. The results, discussed in Part V of this preamble, 
indicate that although the matter is not free from question, it still 
may not be feasible at this time for the underground metal and nonmetal 
mining industry as a whole to comply with a significantly lower limit 
than that being adopted. The Agency notes that since this rulemaking 
was initiated, the efficiency of hot gas filters has improved 
significantly, the dpm emissions from new engines continue to decline 
under EPA requirements, and the availability of ultra-low sulfur fuel 
should make controls even more efficient than at present.
    The agency also explored the idea of bridging the gap between risk 
and feasibility by establishing an ``action level''. In the case of 
MSHA's noise rule, for example, MSHA adopted a ``permissible exposure 
level'' of a time-weighted 8-hour average (TWA8) of 90 dBA 
(decibels, A-weighted), and an ``action level'' of half that amount--a 
TWA8 of 85 dBA. In that case, MSHA determined that miners 
are at significant risk of material harm at a TWA8 of 85 
dBA, but technological and feasibility considerations preclude the 
industry as a whole, at this time, below a TWA8 of 90 dBA. 
Accordingly, to limit miner exposure to noise at or above a 
TWA8 of 85 dBA, MSHA requires that mine operators must take 
certain actions that are feasible (e.g., provide hearing protectors).
    MSHA considered the establishment of a similar ``action level'' for 
dpm--probably at half the proposed concentration limit, or 
80TC g/m3. Under such an approach, mine 
operators whose dpm concentrations are above the ``action level'' would 
be required to implement a series of ``best practices''--e.g., limits 
on fuel types,

[[Page 5711]]

idling, and engine maintenance. Only one commenter supported the 
creation of an Action Level for dpm. However, this commenter suggested 
that such an Action Level be adopted in lieu of a rule incorporating a 
concentration limit requiring mandatory compliance. The agency 
determined it is feasible for the entire underground mining community 
to implement these best practices to minimize the risks of dpm exposure 
without the need for a trigger at an Action Level.
    Some of the comments suggesting that the agency had no rational 
basis for setting the exposure limit at 160TC g/
m3 seem to suggest that the statute itself does not provide 
the Agency with adequate guidance in this regard. The Agency recognizes 
that the Supreme Court has scheduled argument on a case that raises the 
question of how specific a regulatory statute must be with respect to 
how an agency must make standards determinations in order to be deemed 
a constitutional delegation of authority from the Congress. A decision 
is not expected until 2001. However, unless and until determined 
otherwise, MSHA presumes the Mine Act does pass constitutional muster 
in this regard, consistent with the existing case law concerning the 
very similar Occupational Safety and Health Act.
    (B) The ability of the agency to accurately measure diesel 
particulate matter (dpm) in underground metal and nonmetal mine 
environments. As MSHA noted in the preamble to the proposed rule, there 
are a number of methods which can measure dpm concentrations with 
reasonable accuracy when it is at high concentrations and when the 
purpose is exposure assessment. Measurements for the purpose of 
compliance determinations must be more accurate, especially if they are 
to measure compliance with a dpm concentration of 200DPM 
g/m3 or lower. Accordingly, MSHA noted that it 
needed to address a number of questions as to whether such any existing 
method could produce accurate, reliable and reproducible results in the 
full variety of underground mines, and whether the infrastructure 
(samplers and laboratories) existed to support such determinations. 
(See 63 FR 58127 et seq.).
    MSHA concluded that there was no method suitable for such 
compliance measurements in underground coal mines, due to the inability 
of the available methods to distinguish between dpm and coal dust. 
Accordingly, the agency developed a rule for the coal mining sector 
that does not depend upon ambient dpm measurements.
    By contrast, the agency tentatively concluded that by using a 
sampler developed by the Bureau of Mines, and an analytical method 
developed by the National Institute for Occupational Safety and Health 
(NIOSH) to detect the total amount of carbon in a sample, MSHA could 
accurately measure dpm levels at the required concentrations in 
underground metal and nonmetal mines. While not requiring operators to 
use this method for their own sampling, MSHA did commit itself through 
provisions of the proposed rule to use this approach (or a method 
subsequently determined by NIOSH to provide equal or improved accuracy) 
for its own sampling. Moreover the agency proposed that MSHA sampling 
be the sole basis upon which determinations would be made of compliance 
by metal and nonmetal mine operators with applicable compliance limits, 
and that a single sample would be adequate for such purposes. 
Specifically, proposed Sec. 57.5061 provided as follows:

Sec. 57.5061  Compliance Determinations

    (a) A single sample collected and analyzed by the Secretary in 
accordance with the procedure set forth in paragraph (b) of this 
section shall be an adequate basis for a determination of 
noncompliance with an applicable limit on the concentration of 
diesel particulate matter pursuant to Sec. 57.5060.
    (b) The Secretary will collect and analyze samples of diesel 
particulate matter by using the method described in NIOSH Analytical 
Method 5040 and determining the amount of total carbon, or by using 
any method subsequently determined by NIOSH to provide equal or 
improved accuracy in mines subject to this part.

    This part of MSHA's proposed rule received considerable comment. 
Some commenters challenged the accuracy, precision and sensitivity of 
NIOSH Analytical Method 5040. Some challenged whether the amount of 
total carbon determined by the method is a reliable way to determine 
the amount of dpm. Others questioned whether the sampler developed by 
the Bureau of Mines would provide an accurate sample to be analyzed, 
and whether such samplers and analytical procedures would be 
commercially available. Commenters also questioned the use of a single 
sample as the basis for a compliance determination, and the use of area 
sampling in compliance determinations. These comments are addressed 
elsewhere in this preamble (section 3 of Part II, and in connection 
with section 5061 in Part IV).
    Here, MSHA summarizes its views on the most common assertion made 
by commenters: that the sampling and analytical methods the agency 
proposed to use are not able to distinguish between dpm and various 
other substances in the atmosphere of underground metal and nonmetal 
mines--carbonates and carbonaceous minerals, graphitic materials, oil 
mists and organic vapors, and cigarette smoke.
    Interferences: what MSHA said in preamble to proposed rule. In the 
preamble to the proposed rule, MSHA recognized that there might be some 
interferences from other common organic carbon sources in underground 
metal and nonmetal mines: specifically, oil mists and cigarette smoke. 
The agency noted it had no data on oil mists, but had not encountered 
the problem in its own sampling. With respect to cigarette smoke, the 
agency noted that: ``Cigarette smoke is under the control of operators, 
during sampling times in particular, and hence should not be a 
consideration.'' (63FR 58129)
    The agency also discussed the potential advantages and 
disadvantages of using a special device on the sampler--a submicron 
impactor--to eliminate certain other possible interferences (See Figure 
I-1). The submicron impactor stops particles larger than a micron from 
being collected by the sampler, while allowing the smaller dpm to be 
collected. Thus, an advantage of using the impactor would be to ensure 
that the sampler was not inadvertently collecting materials other than 
dpm. However MSHA pointed out that while samples in underground metal 
and nonmetal mines could be taken with a submicrometer impactor, this 
could lead to underestimating the total amount of dpm present (63FR 
58129). This is because the fraction of dpm particles greater than 1 
micron in size in the environment of noncoal mines can be as great as 
20% (Vuk, Jones, and Johnson, 1976).

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    Interferences: comments and MSHA efforts to verify. Many commenters 
asserted that no matter how it is performed in underground metal and 
nonmetal mines, the sampling and analysis proposed by MSHA to determine 
the amount of diesel particulate present would suffer from one or more 
of the aforementioned interferences. A number asserted that their own 
measurements using this approach provided clear evidence of such 
interferences. Although MSHA repeatedly asked for actual data and 
information about the procedures used to verify these assertions, very 
little was provided. Nevertheless, rather than conclude that these 
assertions were baseless, MSHA decided to attempt to verify these 
assertions itself. Accordingly, appropriate field and laboratory 
measurements were conducted toward this end, the results written up in 
appropriate fashion, and added to the record of this rulemaking. The 
agency has taken those results into account in ascertaining what weight 
to give to the assertions made by commenters and how to deal with those 
assertions supported by its measurements.
    As described in detail in section 3 of Part II, MSHA's 
verifications demonstrate that the submicron impactor can eliminate any 
interferences from carbonates, carbonaceous minerals, and graphitic 
ores. Accordingly, although use of the impactor will result in an 
undercount of dpm, the final rule provides that MSHA will always use 
the submicron impactor in compliance sampling.
    MSHA's verifications also demonstrated that oil mists as well as 
cigarette smoke, can in fact, under certain circumstances, create 
interferences even with the use of the impactor. MSHA presumes the same 
would happen with organic vapors. The verifications demonstrated that 
the problems occur in the immediate vicinity of the interferent (e.g., 
close to a drill or smoker). However, the verifications also 
demonstrated that the interference dissipates when the sampling device 
is located a certain distance away from the interferent.
    Accordingly, as detailed in the discussion of section 5061 in Part 
IV of this preamble, MSHA's sampling strategy for dpm will take these 
problems into account. For example, if a miner works in an enclosed cab 
all day and smokes, MSHA will not place a sampler in that cab or on 
that miner. If a miner works part of a day drilling, MSHA will not 
place a sampler on that miner. But MSHA can, for example, take an area 
sample in an area of a mine where drilling is being performed without 
concern about interferences from oil mists if it locates the sampler 
far enough away from the drill. MSHA's compliance manual will provide 
specific instructions to inspectors on how to avoid interferences.
    The organic interferences (diesel mist, smoking) could be avoided 
by only analyzing a sample for elemental carbon, pursuant to the NIOSH 
method. As it indicated in the preamble to the proposed rule, however, 
MSHA does not at this time know the ratio between the amount of 
elemental carbon and the amount of dpm. Accordingly, rather than deal 
with the uncertainties in all samples which this approach would 
present, MSHA is going to use a method (i.e., sampling and analyzing 
for both organic carbon and elemental carbon) that, if properly 
applied, provides accurate results.
    (C) The feasibility of the requirements for this sector of the 
mining industry. The Mine Act generally requires MSHA to set the 
standard that is most protective of miner health while still being 
technologically and economically feasible. In addition, consistent with 
the Regulatory Flexibility Act, the agency pays particular attention to 
the impact of any standard on small mining operations.
    (1) Technological feasibility of the rule. It has been clear since 
the beginning of this rulemaking that if technological feasibility was 
an issue, it would be in the context of requiring all underground metal 
and nonmetal mines to meet a particular limit. While the Mine Act does 
not require that each mine be able to meet a standard for it to be 
considered technologically feasible--only that the standard be feasible 
for the industry as a whole--the extent to which various mines might 
have a problem complying is the evidence upon which this conclusion 
must be based.
    Accordingly, MSHA evaluated the technological feasibility of the 
concentration limit in the underground

[[Page 5713]]

metal and nonmetal sector by evaluating whether it was possible, using 
a combination of existing control approaches, to reach the 
concentration limit even in situations in which the Agency's engineers 
determined that compliance might be the most difficult. In this regard, 
the Agency examined how emissions generated by the actual equipment in 
four different underground mining operations could be controlled. The 
mines were very diverse--an underground limestone mine, an underground 
(and underwater) salt mine, and an underground gold mine. Yet in each 
case, the analysis revealed that there are available combinations of 
controls that can bring dpm concentrations down to well below the final 
limit--even when the controls that needed to be purchased were not as 
extensive as those which the Agency is assuming will be needed in 
determining the costs of the final rule. (The results of these analyses 
are discussed in Part V of the preamble, together with the methodology 
used in modeling the results--just as they were discussed in the 
preamble accompanying the proposed rule.) As a result of these studies, 
the Agency has concluded that there are engineering and work practice 
controls available to bring dpm concentrations in all underground metal 
and nonmetal mines down to the required levels.
    The best actions for an individual operator to take to come into 
compliance with the interim and final concentration limits will depend 
upon an analysis of the unique conditions at the mine. The final rule 
provides 18 months after it is promulgated for MSHA to provide 
technical assistance to individual mine operators. It also gives all 
mine operators in this sector an additional three and a half years to 
bring dpm concentrations down to the proposed final concentration 
limit--using an interim concentration limit during this time which the 
Agency is confident every mine in this sector can timely meet. And the 
rule provides an opportunity for a special extension for an additional 
two years for mines that have unique technological problems meeting the 
final concentration limit.
    As noted during 1995 workshops co-sponsored by MSHA on methods for 
controlling diesel particulate, many underground metal and nonmetal 
mine operators have already successfully determined how to reduce 
diesel particulate concentrations in their mines. MSHA has disseminated 
the ideas discussed at these workshops to the entire mining community 
in a publication, ``Practical Ways to Control Exposure to Diesel 
Exhaust in Mining--a Toolbox''. The control methods are divided into 
eight categories: use of low emission engines; use of low sulfur fuel; 
use of aftertreatment devices; use of ventilation; use of enclosed 
cabs; diesel engine maintenance; work practices and training; fleet 
management; and respiratory protective equipment. Moreover, MSHA 
designed a model in the form of a computer spreadsheet that can be used 
to simulate the effects of various controls on dpm concentrations. 
(This model is discussed in Part V of the preamble.) This makes it 
possible for individual underground mine operators to evaluate the 
impact on diesel particulate levels of various combinations of control 
methods, prior to making any investments, so each can select the most 
feasible approach for his or her mine.
    (2) Economic Feasability of the Rule. The underground metal and 
nonmetal industry uses a lot of diesel-powered equipment, and it is 
widely distributed. Accordingly, MSHA recognizes that the costs of 
bringing mines into compliance with this rule will be widely felt in 
this sector (although, unlike underground coal mines, this sector did 
not have to comply with MSHA's 1996 diesel equipment rule).
    In summary, the costs per year to the underground metal and 
nonmetal industry are about $25.1 million. The cost for an average 
underground metal and nonmetal mine is expected to be about $128,000 
annually.
    The Agency's initial cost estimates of $19.2 million a year were 
challenged during the rulemaking proceeding. As a result, the Agency 
reconsidered the costs.
    In its initial estimate of the costs for the industry to comply 
with the concentration limit, MSHA assumed that a variety of 
engineering controls, such as low emission engines, ceramic filters, 
oxidation catalytic converters, and cabs would be needed on diesel 
powered equipment. Most of the engineering controls would be needed on 
diesel equipment used for production, while a small amount of diesel 
equipment that is used for support purposes would need engineering 
controls. In addition to these controls, MSHA assumed that some 
underground metal and nonmetal mines would need to make ventilation 
changes in order to meet the proposed concentration limits.
    Specifically, in the PREA, MSHA assumed that: (1) the interim 
standard would be met by replacing engines, installing oxidation 
catalytic converters, and improving ventilation; and (2) the final 
standard would be met by adding cabs and filters. Comments on the PREA 
and data collected by the Agency since publication of the proposed rule 
indicate that engine replacement is more expensive than originally 
thought and filters are more effective relative to engine replacement. 
The revised compliance strategy, upon which MSHA bases its revised 
estimates of compliance costs, reverses the two most widely used 
measures. MSHA now anticipates that: (1) the interim standard will be 
met with filters, cabs, and ventilation; and (2) the final standard 
will be met with more filters, ventilation, and such turnover in 
equipment and engines as will have occurred in the baseline. This new 
approach uses the same toolbox and optimization strategy that was used 
in the PREA. Since relative costs are different, however, the tools 
used and cost estimated are different.
    (3) Impact on small mines. As required by the Regulatory 
Flexibility Act, MSHA has performed a review of the effects of the 
proposed rule on ``small entities''.
    The Small Business Administration generally considers a small 
mining entity to be one with less than 500 employees. MSHA has 
traditionally defined a small mine to be one with less than 20 miners, 
and has focused special attention on the problems experienced by such 
mines in implementing safety and health rules. Accordingly, MSHA has 
separately analyzed the impact of the rule on three categories of 
mines: large mines (more than 500 employees), middle size mines (20-500 
employees), and small mines (those with less than 20 miners).
    As required by law, MSHA has also developed a preliminary and final 
regulatory flexibility analysis. The Agency published its preliminary 
Regulatory Flexibility Analysis with its proposed rule and specifically 
requested comments thereon; the agency's final Regulatory Flexibility 
Analysis is included in the Agency's REA. In addition to a succinct 
statement of the objectives of the rule and other information required 
by the Regulatory Flexibility Act, the analysis reviews alternatives 
considered by the Agency with an eye toward the nature of small 
business entities.
    In promulgating standards, MSHA is required to protect the health 
and safety of all the Nation's miners and may not include provisions 
that provide less protection for miners in small mines than for those 
in larger mines. But MSHA does consider the impact of its standards on 
even the smallest mines when it evaluates the feasibility of various 
alternatives. For example, a major reason why MSHA concluded it

[[Page 5714]]

needed to stagger the effective dates of some of the requirements in 
the rule is to ensure that it would be feasible for the smallest mines 
to have adequate time to come into compliance.
    MSHA recognizes that smaller mines may need particular assistance 
from the agency in coming into compliance with this standard. Before 
the dpm concentration goes into effect in 18 months, the Agency plans 
to provide extensive compliance assistance to the mining community. The 
metal and nonmetal community will also have an additional three and a 
half years to comply with the final concentration limit, which in many 
cases means these mines may have a full five years of technical 
assistance before any engineering controls are required. MSHA intends 
to focus its efforts on smaller operators in particular--training them 
in measuring dpm concentrations, and providing technical assistance on 
available controls. The Agency will also issue a compliance guide, and 
continue its current efforts to disseminate educational materials and 
software.
    (4) Benefits of the final rule Benefits of the rule include 
reductions in lung cancer. In the long run, as the mining population 
turns over, MSHA estimates that a minimum of 8.5 lung cancer deaths 
will be avoided per year.\2\
---------------------------------------------------------------------------

    \2\ This lower bound figure could significantly underestimate 
the magnitude of the health benefits. For example the estimate based 
on the mean value of all the studies examined is 49 lung cancer 
deaths avoided per year.
---------------------------------------------------------------------------

    Benefits of the rule will also include reductions in the risk of 
death from cardiovascular, cardiopulmonary, or respiratory causes and 
in sensory irritation and respiratory symptoms. MSHA does not believe 
that the available data can support reliable or precise quantitative 
estimates of these benefits. Nevertheless, the expected reductions in 
the risk of death from cardiovascular, cardiopulmonary, or respiratory 
causes appear to be significant, and the expected reductions in sensory 
irritation and respiratory symptoms appear to be rather large.

II. General Information

    This part provides the context for this preamble. The nine topics 
covered are:
    (1) The role of diesel-powered equipment in underground metal and 
nonmetal mining in the United States;
    (2) The composition of diesel exhaust and diesel particulate matter 
(dpm);
    (3) The sampling and analytical techniques for measuring ambient 
dpm in underground metal and nonmetal mines;
    (4) Limiting the public's exposure to diesel and other final 
particulates-- ambient air quality standards;
    (5) The effects of existing standards--MSHA standards on diesel 
exhaust gases (CO, CO2, NO, NO2, and 
SO2), and EPA diesel engine emission standards--on the 
concentration of dpm in underground metal and nonmetal mines;
    (6) Methods for controlling dpm concentrations in underground metal 
and nonmetal mines;
    (7) MSHA's approach to diesel safety and health in underground coal 
mines and its effect on dpm;
    (8) Information on how certain states are restricting occupational 
exposure to dpm; and
    (9) A history of this rulemaking.
    Material on these subjects which was available to MSHA at the time 
of the proposed rulemaking was included in Part II of the preamble that 
accompanied the proposed rule. (63 FR 58123 et seq). Portions of that 
material relevant to underground metal and nonmetal mines is reiterated 
here (although somewhat reorganized), and the material is amended and 
supplemented where appropriate as a result of comments and additional 
information added to the record since the proposal was published.

(1) The Role of Diesel-Powered Equipment in Underground Metal and 
Nonmetal Mining in the United States

    Diesel engines, first developed about a century ago, now power a 
full range of mining equipment in underground metal and nonmetal mines, 
and are used extensively in this sector. This sector's reliance upon 
diesel engines to power equipment in underground metal and nonmetal 
mines appears likely to continue for some time.
    Historical Overview of Diesel Power Use in Mining. As discussed in 
the notice of proposed rulemaking, the diesel engine was developed in 
1892 by the German engineer Rudolph Diesel. It was originally intended 
to burn coal dust with high thermodynamic efficiency. Later, the diesel 
engine was modified to burn middle distillate petroleum (diesel fuel). 
In diesel engines, liquid fuel droplets are injected into a prechamber 
or directly into the cylinder of the engine. Due to compression of air 
in the cylinder the temperature rises high enough in the cylinder to 
ignite the fuel.
    The first diesel engines were not suited for many tasks because 
they were too large and heavy (weighing 450 lbs. per horsepower). It 
was not until the 1920's that the diesel engine became an efficient 
lightweight power unit. Since diesel engines were built ruggedly and 
had few operational failures, they were used in the military, railway, 
farm, construction, trucking, and busing industries. The U.S. mining 
industry was slow, however, to begin using these engines. Thus, when in 
1935 the former U.S. Bureau of Mines published a comprehensive overview 
on metal mine ventilation (McElroy, 1935), it did not even mention 
ventilation requirements for diesel-powered equipment. By contrast, the 
European mining community began using these engines in significant 
numbers, and various reports on the subject were published during the 
1930's. According to a 1936 summary of these reports (Rice, 1936), the 
diesel engine had been introduced into German mines by 1927. By 1936, 
diesel engines were used extensively in coal mines in Germany, France, 
Belgium and Great Britain. Diesel engines were also used in potash, 
iron and other mines in Europe. Their primary use was in locomotives 
for hauling material.
    It was not until 1939 that the first diesel engine was used in the 
United States mining industry, when a diesel haulage truck was used in 
a limestone mine in Pennsylvania, and not until 1946 was a diesel 
engine used in a coal mine. Today, however, diesel engines are used to 
power a wide variety of equipment in all sectors of U.S. mining. 
Production equipment includes vehicles such as haultrucks and shuttle 
cars, front-end loaders, hydraulic shovels, load-haul-dump units, face 
drills, and explosives trucks. Diesel engines are also used in support 
equipment including generators and air compressors, ambulances, fire 
trucks, crane trucks, ditch diggers, forklifts, graders, locomotives, 
lube units, personnel carriers, hydraulic power units, longwall 
component carriers, scalers, bull dozers, pumps (fixed, mobile and 
portable), roof drills, elevating work platforms, tractors, utility 
trucks, water spray units and welders.
    Current Patterns of Diesel Power Use in Underground Metal and 
Nonmetal Mining. Table II-1 provides information on the current 
utilization of diesel equipment in underground metal and nonmetal 
mines.

[[Page 5715]]



                      Table II-1.--Diesel Equipment in Underground Metal and Nonmetal Mines
----------------------------------------------------------------------------------------------------------------
                                                             Number of
                       Mine size                         underground mines   Number of mines   Number of Engines
                                                                 A            with diesels B           B
----------------------------------------------------------------------------------------------------------------
Small C................................................                134                 77                584
Large..................................................                130                119              3,414
All....................................................                264                196             3,998
----------------------------------------------------------------------------------------------------------------
(A) Number of underground mines is based on those reporting operations for FY1999 (preliminary data).
(B) Number of mines using diesels are based on January 1998 count, by MSHA inspectors, of underground metal and
  nonmetal mines that used diesel powered equipment, and the number of engines (the latter rounded to the
  nearest 25) was determined in the same count with reference to equipment normally in use.
(C) A ``small'' mine is one with less than 20 miners.

    As noted in Table II-1, a majority of underground metal and 
nonmetal mines use diesel-powered equipment.
    Diesel engines in metal and nonmetal underground mines, and in 
surface coal mines, range up to 750 HP or greater, although equipment 
size, and thus the size of the engine, can be limited by production 
requirements, the dimensions of mine openings, and other factors. By 
contrast, in underground coal mines, the average engine size is less 
than 150 HP. The reason for this disparity is the nature of the 
equipment powered by diesel engines. In underground metal and nonmetal 
mines, and surface mines, diesel engines are widely used in all types 
of equipment--both the equipment used under the heavy stresses of 
production and the equipment used for support. In underground metal and 
nonmetal mines, of the approximate 4,000 pieces of diesel equipment 
normally in use, about 1,800 units are used for loading and hauling. By 
contrast, the great majority of the diesel usage in underground coal 
mines is in support equipment.
    This fact is significant for dpm control in underground metal and 
nonmetal mines. As the horsepower size of the engine increases, the 
mass of dpm emissions produced per hour increases. (A smaller engine 
may produce the same or higher levels of particulate emissions per 
volume of exhaust as a large engine, but the mass of particulate matter 
increases with the engine size). Accordingly, as engine size increases, 
control of emissions may require additional efforts.
    Another factor relevant to control of dpm emissions in this sector 
is that fewer than 15 underground metal and nonmetal mines are required 
to use Part 36 permissible equipment because of the possibility of the 
presence of explosive mixtures of methane and air. The surface 
temperature of diesel powered equipment in underground metal and 
nonmetal mines classified as gassy must be controlled to less than 
400 deg.F. Such mines must use equipment approved as permissible under 
Part 36 if the equipment is utilized in areas where permissible 
equipment is required. These gassy metal and nonmetal mines have been 
using the same permissible engines and power packages as those approved 
for underground coal mines. (MSHA has not certified a diesel engine 
exclusively for a Part 36 permissible machine for the metal and 
nonmetal sector since 1985 and has certified only one permissible power 
package; however, that engine model has been retired and is no longer 
available as a new purchase to the industry). As a result, engine size 
(and thus dpm production of each engine) is more limited in these 
mines, and, as explained in section 6 of this part, the exhaust from 
these engines is cool enough to add a paper type of filtration device 
directly to the equipment.
    By contrast, since in nongassy underground metal and nonmetal mines 
mine operators can use conventional construction equipment in their 
production sections without the need for modifications to the machines, 
they tend to do so. Two examples are haulage vehicles and front-end 
loaders. As a result, these mines can and do use engines with larger 
horsepower and hot exhaust. As explained in section 6 of this part, the 
exhaust from such engines must be cooled by a wet or dry device before 
a paper filter can be used, or high temperature filters (e.g., 
ceramics) must be used.
    At this time, diesel power faces little competition from other 
power sources in underground metal and nonmetal mines. As can be seen 
from the chart, there are some small metal and nonmetal mines (less 
than 20 employees) which do not use diesel-powered equipment; most of 
these used compressed air for drilling and battery-powered rail 
equipment for haulage.
    It is unclear at this time, how quickly new ways to generate energy 
to run mobile vehicles will be available for use in a wide range of 
underground metal and nonmetal mining activities. New hybrid electric 
automobiles are being introduced this year by two manufacturers (Honda 
and Toyota); such vehicles combine traditional internal combustion 
power sources (in this case gasoline) with electric storage and 
generating devices that can take over during part of the operating 
period. By reducing the time the vehicle is directly powered by 
combustion, such vehicles reduce emissions. Further developments in 
electric storage devices (batteries), and chemical systems that 
generate electricity (fuel cells) are being encouraged by government-
private sector partnerships. For further information on recent 
developments, see the Department of Energy alternative fuels web site 
at http://www.afdc.doe.gov/altfuels.html, and ``The Future of Fuel 
Cells'' in the July 1999 issue of Scientific American. Until such new 
technologies mature, are available for use in large equipment, and are 
reviewed for safe use underground, however, MSHA assumes that the 
underground metal and nonmetal mining community's significant reliance 
upon the use of diesel-power will continue.

(2) The Composition of Diesel Exhaust and Diesel Particulate Matter 
(DPM)

    The emissions from diesel engines are actually a complex mixture of 
compounds, containing gaseous and particulate fractions. The specific 
composition of the diesel exhaust in a mine will vary with the type of 
engines being used and how they are used. Factors such as type of fuel, 
load cycle, engine maintenance, tuning, and exhaust treatment will 
affect the composition of both the gaseous and particulate fractions of 
the exhaust. This complexity is compounded by the multitude of 
environmental settings in which diesel-powered equipment is operated. 
Nevertheless, there are a few basic facts about diesel emissions that 
are of general applicability.
    The gaseous constituents of diesel exhaust include oxides of 
carbon, nitrogen and sulfur, alkanes and alkenes (e.g., butadiene), 
aldehydes (e.g., formaldehyde), monocyclic aromatics (e.g., benzene, 
toluene), and polycyclic aromatic hydrocarbons (e.g.,

[[Page 5716]]

 phenanthrene, fluoranthene). The oxides of nitrogen ( NOX) 
are worth particular mention because in the atmosphere they can 
precipitate into particulate matter. Thus, controlling the emissions of 
NOX is one way that engine manufacturers can control 
particulate production indirectly. (See section 5 of this part).
    The particulate components of the diesel exhaust gas include the 
so-called diesel soot and solid aerosols such as ash particulates, 
metallic abrasion particles, sulfates and silicates. The vast majority 
of these particulates are in the invisible sub-micron range of 100nm.
    The main particulate fraction of diesel exhaust is made up of very 
small individual particles. These particles have a solid core mainly 
consisting of elemental carbon. They also have a very surface-rich 
morphology. This surface absorbs many other toxic substances, that are 
transported with the particulates, and can penetrate deep into the 
lungs. There can be up to 1,800 different organic compounds adsorbed 
onto the elemental carbon core. A portion of this hydrocarbon material 
is the result of incomplete combustion of fuel; however, the majority 
is derived from the engine lube oil. In addition, the diesel particles 
contain a fraction of non-organic adsorbed materials. Figure II-1 
illustrates the composition of dpm.
    Diesel particles released to the atmosphere can be in the form of 
individual particles or chain aggregates (Vuk, Jones, and Johnson, 
1976). In underground coal mines, more than 90% of these particles and 
chain aggregates are submicrometer in size (i.e., less than 1 
micrometer (1 micron) in diameter). Dust generated by mining and 
crushing of material--e.g., silica dust, coal dust, rock dust--is 
generally not submicrometer in size. Figure II-2 shows a typical size 
distribution of the particles found in the environment of a mine that 
uses equipment powered by diesel engines (Cantrell and Rubow, 1992). 
The vertical axis represents relative concentration, and the horizontal 
axis the particle diameter. As can be seen, the distribution is 
bimodal, with dpm generally being well less than 1 m in size 
and dust generated by the mining process being well greater than 1 
m.

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    As shown on Figure II-3 (Majewski, W. Addy, Diesel Progress June, 
1998) diesel particulates have a bimodal size distribution which 
includes small nuclei mode particles and larger accumulation mode 
particles. As further shown, most of diesel particle mass is contained 
in the accumulation mode but most of the particle number can be found 
in the nuclei mode.
    The particles in the nuclei mode, also known as nanoparticles, are 
being investigated as to their health hazard relevance. The interest in 
these particles has been sparked by the finding that newer ``low 
polluting engines emit higher numbers of small particles than the old 
technology engines. Although the exact composition of diesel 
nanoparticles is not known, it was found that they may be composed of 
condensates (hydrocarbons, water, sulfuric acid). The amount of these 
condensates and the number of nanoparticles depends very significantly 
on the particulate sampling conditions, such as dilution ratios, which 
were applied during the measurement.
    Both the maximum particle concentration and the position of the 
nuclei and accumulation mode peaks, however, depend on which 
representation is chosen. In mass distributions, the majority of the 
particulates (i.e., the particulate mass) is found in the accumulation 
mode. The nuclei mode, depending on the engine technology and particle 
sampling technique, may be as low as a few percent, sometimes even less 
than 1%. A different picture is presented when the number distribution 
representation is used. Generally, the number of particles in the 
nuclei mode contributes to more than 50% of the total particle count. 
However, sometimes the nuclei mode particles represent as much as 99% 
of the total particulate number. The topic of nanoparticles is 
discussed further in section 5 of this Part.

(3) The Sampling and Analytical Techniques for Measuring Ambient dpm in 
Underground Metal and Nonmetal Mines

    As MSHA noted in the preamble to the proposed rule, there are a 
number of methods which can measure dpm concentrations with reasonable 
accuracy when it is at high concentrations and when the purpose is 
exposure assessment. Measurements for the purpose of compliance 
determinations must be more accurate, especially if they are to measure 
compliance with a dpm concentration as low as 200 g/m\3\ or 
lower. Accordingly, MSHA noted that it needed to address a number of 
questions as to whether any existing method could produce accurate, 
reliable and reproducible results in the full variety of underground 
mines, and whether the samplers and laboratories existed to support 
such determinations. (See 63 FR 58127 et.seq).
    MSHA concluded that there was no method suitable for such 
compliance measurements in underground coal mines, due to the inability 
of the available methods to distinguish between dpm and coal dust. 
Accordingly, the agency developed a rule for the coal mining sector 
that does not depend upon ambient dpm measurements.
    By contrast, the agency concluded that by using a sampler developed 
by the former Bureau of Mines, and an analytical method developed by 
the National Institute for Occupational Safety and Health (NIOSH), MSHA 
could accurately measure dpm levels at the required concentrations in 
underground metal and nonmetal mines. While not requiring operators to 
use this method for their own sampling, MSHA did commit itself to use 
this approach (or a method subsequently determined by NIOSH to provide 
equal or improved accuracy) for its own sampling. Moreover the agency 
proposed that MSHA sampling be the sole basis for determining 
compliance by metal and nonmetal mine operators with applicable 
compliance limits, and that a single sample would be adequate for such 
purposes. Specifically, proposed Sec. 57.5061 would have provided:
    Section 57.5061 Compliance determinations.
    (a) A single sample collected and analyzed by the Secretary in 
accordance

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with the procedure set forth in paragraph (b) of this section shall be 
an adequate basis for a determination of noncompliance with an 
applicable limit on the concentration of diesel particulate matter 
pursuant to Sec. 57.5060.
    (b) The Secretary will collect and analyze samples of diesel 
particulate matter by using the method described in NIOSH Analytical 
Method 5040 and determining the amount of total carbon, or by using any 
method subsequently determined by NIOSH to provide equal or improved 
accuracy in mines subject to this part.
    This part of MSHA's proposed rule received considerable comment. 
Some commenters challenged the accuracy, precision and sensitivity of 
NIOSH Analytical Method 5040. Some challenged whether the amount of 
total carbon determined by the method is a reliable way to determine 
the amount of dpm. Others questioned whether the sampler developed by 
the former Bureau of Mines would provide an accurate sample to be 
analyzed. Many commenters asserted that the analytical method would not 
be able to distinguish between dpm and various other substances in the 
atmosphere of underground metal and nonmetal mines--carbonates and 
carbonaceous minerals, graphitic materials, oil mists and organic 
vapors, and cigarette smoke. (It should be noted that commenters also 
questioned the use of a single sample as the basis for a compliance 
determination, and the use of area sampling in compliance 
determinations; these comments are reviewed and responded to in Part IV 
of this preamble in connection with the discussion of Sec. 57.5061.)
    The agency has carefully reviewed the information and data 
submitted by commenters. Where necessary to verify the validity of 
comments, MSHA collected additional information which it has placed in 
the record, and which in turn were the subject of an additional round 
of comments.
    Background. As discussed in section 2 of this part, diesel 
particulate consists of a core of elemental carbon (EC), adsorbed 
organic carbon (OC) compounds, sulfates, vapor phase hydrocarbons and 
traces of other compounds. The method developed by NIOSH provides for 
the collection of a sample on a quartz fiber filter. As originally 
conceived, the filter is mounted in an open face filter holder that 
allows for the sample to be uniformly deposited on the filter surface. 
After sampling, a section of the filter is analyzed using a thermal-
optical technique (Birch and Cary, 1996). This technique allows the EC 
and OC species to be separately identified and quantified. Adding the 
EC and OC species together provides a measure of the total carbon 
concentration in the environment.
    Studies have shown that the sum of the carbon (C) components (EC + 
OC) associated with dpm accounts for 80-85% of the total dpm 
concentration when low sulfur fuel is used (Birch and Cary, 1996). 
Therefore, in the preamble to the proposed rule, MSHA asserted that 
since the TC:DPM relationship is consistent, it provides a method for 
determining the amount of dpm. MSHA noted that the method can detect as 
little as 1 g/m3 of TC. Moreover, NIOSH has 
investigated the method and found it to meet NIOSH's accuracy criterion 
(NIOSH, 1995)--i.e., that measurements come within 25 percent of the 
true TC concentration at least 95 percent of the time.
    In the preamble to the proposed rule, MSHA recognized that there 
might be some interferences from other common organic carbon sources in 
underground metal and nonmetal mines: specifically, oil mists and 
cigarette smoke. The agency noted it had no data on oil mists, but had 
not encountered the problem in its own sampling. With respect to 
cigarette smoke, the agency noted that: ``Cigarette smoke is under the 
control of operators, during sampling times in particular, and hence 
should not be a consideration.'' (63 FR 58129).
    The agency also discussed the potential advantages and 
disadvantages of using a special device on the sampler to eliminate 
certain other possible interferences. NIOSH had recommended the use of 
a submicron impactor when taking samples in coal mines to filter out 
particles more than one micron in size. See Figure III-3. The idea is 
to ensure that a sample taken in a coal mine does not include 
significant amounts of coal dust, since the analytical method would 
capture the organic carbon in the coal dust just like the carbon in 
dpm. Coal dust is generally larger than one micron, while dpm is 
generally smaller than one micron. However, MSHA pointed out that while 
samples in underground metal and nonmetal mines could be taken with a 
submicrometer impactor, this could lead to underestimating the total 
amount of dpm present. This is because the fraction of dpm particles 
greater than 1 micron in size in the environment of noncoal mines can 
be as great as 20%.

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    MSHA also noted that while NIOSH Method 5040 requires no 
specialized equipment for collecting a dpm sample, the sample would 
most probably require analysis by a commercial laboratory. The agency 
noted it did not foresee the availability of qualified testing 
facilities as a problem. The agency likewise discussed the availability 
of the sampling device, and noted steps that were underway to develop a 
disposable sampler. (63 FR 58130)
    Sample Collection Methods. Some commenters raised questions about 
how dpm samples should be taken: using open face sampling, respirable 
sampling and submicron sampling. All three are discussed in NIOSH 
Analytical Method 5040. Because diesel particulate matter is primarily 
submicron in size any of the three sampling methods could be used.
    The choice of sample collection method considers the cost and 
potential interferences that the method can contribute. Regardless of 
the sampling method, the sampling media (filter) must be one that does 
not interfere with the analysis. For this reason a pre-fired quartz 
fiber filter has been chosen. The quartz fiber filter is capable of 
withstanding the temperatures from the analytical procedure. The filter 
is pre-fired to remove residual carbon, attached to the filter during 
manufacturing.
    Total Dust Sampling. Total dust sampling is the least expensive 
method to collect an airborne dust sample. It is commonly used to 
collect a sample that is representative of all the dust in the 
environment; i.e., the particles are not preclassified during the 
collection process. Total dust sampling can be performed using a filter 
cassette that allows the whole face of the filter to be exposed during 
collection of the sample (open face) or using a filter cassette with a 
small inlet opening (referred to as a closed face filter cassette). The 
latter method is used by MSHA for compliance sampling for total dust in 
the metal and nonmetal sector. Because the sample collected is 
representative of all the particulate matter in the environment, there 
is the potential for interference from mineral contaminants when 
sampling for diesel particulate matter. While in many cases the 
analytical results can be corrected for these interferences, in some 
instances the interferences may be so large that they can not be 
quantified with the analytical procedure, thus preventing the 
analytical result to be corrected for the interference.
    Additionally, MSHA has noted that in some cases when using the 
total dust sampler with the small inlet hole, distribution of the 
collected sample on the filter is not uniform. The distribution of 
sample is concentrated in the center of the filter. This can result in 
the effect of an interference being magnified. As a result, MSHA 
considers that total dust sampling is not an appropriate sampling 
method for the mining industry to use when sampling diesel particulate 
matter.
    Respirable Dust Sample Collection. Respirable dust sampling is 
commonly used when a size selective criteria for dust is required. The 
mining industry is familiar with size selective sampling for the 
collection of coal mine dust samples in coal mines and for collecting 
respirable silica samples in metal and nonmetal mines. For respirable 
dust sampling MSHA uses a 10 millimeter, Dorr Oliver nylon cyclone as a 
particle classifier to separate the respirable fraction of the aerosol 
from the total aerosol sampled. The use of this particle classifier 
would be suitable when sampling diesel particulate, provided 
significant amounts of interfering minerals are not present. This is 
because 90 percent of the diesel particulate is typically less than 1 
micrometer in size. Particles less than 1 micrometer in size pass 
through the cyclone and are deposited on the filter. While in many 
cases, these interferences could be removed during the analytical 
procedures, the analytical procedures alone can not be assured to 
remove the interferences when large amounts of mineral dust are 
present.
    Additionally, MSHA has observed that in some sampling equipment the 
cyclone outlet hole has been reduced when interfacing it with the 
filter capsule. MSHA has further observed that where this has occurred, 
the distribution of sample on the collection filter may not be uniform. 
In this circumstance the sample is also concentrated in the center of 
the filter which can result in the effect of a mineral interference 
being magnified. As a result, MSHA considers that respirable dust 
sampling is not a universally applicable sampling method for the mining 
industry to use for sampling diesel particulate matter.
    Submicron Dust Sample Collection. Since only a small fraction of a 
mineral dust aerosol is less than 1 micrometer in size, a submicrometer 
impactor (Cantrell and Rubow, 1992) was developed to permit the 
sampling of diesel particulate without sampling potential mineral 
interferences. The submicrometer impactor was initially developed to 
remove the interference from coal mine dust when sampling diesel 
particulate in coal mines. It was designed to remove the carbon coal 
particles, that are greater than 0.8 micrometer in size, when sampling 
for diesel particulate matter at a pump flowrate of 2.0 liters per 
minute. As a result the submicrometer impactor cleans potentially 
interfering mineral dust from the sample.
    As noted in the preamble to the proposed rule, use of this method 
to measure dpm does result in the exclusion of that portion of dpm that 
is not submicron in size, and this can be significant. On the other 
hand, this method avoids problems associated with the other methods 
described above. Moreover, as discussed in more detail below under the 
topic of ``interferences'', the submicron impactor can eliminate 
certain substances that in metal and nonmetal mines would otherwise 
make it difficult for the analytical method to be used for compliance 
purposes.
    Accuracy of Analytical Method, NIOSH Method 5040. Commenters 
challenged the accuracy, precision and sensitivity of the analytical 
method (NIOSH Method 5040) used for the diesel particulate analysis. 
MSHA has carefully reviewed these concerns, and has concluded that 
provided a submicron impactor is used with the sampling device in 
underground metal and nonmetal mines, NIOSH Method 5040 does provide 
the accuracy, precision and sensitivity necessary to use in compliance 
sampling for dpm in such mines.
    As noted above, NIOSH Method 5040 is an analytical method that is 
used to determine elemental and organic carbon content from an airborne 
sample. It is more versatile than other carbon analytical methods in 
that it differentiates the carbon into its organic and elemental carbon 
components. The method accomplishes this through a thermal optical 
process. An airborne sample is collected on a quartz fiber filter. A 
portion of the filter, (approximately 2 square centimeters in area) is 
placed into an oven. The temperature of the oven is increased in 
increments. At certain oven temperature and atmospheric conditions 
(helium, helium-oxygen), carbon on the filter is oxidized into carbon 
dioxide. The carbon dioxide gas is then passed over a catalyst and 
reduced to methane. The methane concentration is measured and carbon 
content is determined. Separation of different types of organic carbon 
is accomplished through temperature and atmospheric control. The 
instrument is programmed to increase temperature in steps over time. 
This step by step increase in temperature allows for differentiation 
between various types of organic carbon.

[[Page 5722]]

    A laser is used to differentiate the organic carbon from the 
elemental carbon. The laser penetrates the filter and when the laser 
transmittance reaches its initial value this determines when elemental 
carbon begins to evolve. The computer software supplied with the 
instrumentation indicates this separation by a vertical line. The 
separation point can be adjusted by the analyst. As a result, there may 
be small differences in the determination of organic and elemental 
carbon between analysts, but the total carbon (sum of elemental and 
organic carbon) does not change. The software also allows the analyst 
to identify and quantify the different types of organic carbon using 
identifiable individual peaks. This permits the mathematical 
subtraction of a particular carbon peak. This feature is particularly 
useful in removing contributions from carbonates or other carbonaceous 
minerals. In other total carbon methods, samples have to be acidified 
to remove carbonate interference. A thermogram is produced with each 
analysis that shows the temperature ramps, oven atmospheric conditions 
and the amount of carbon evolved during each step.
    A range of five separate sucrose standards between 10-100 
g/cm\2\ carbon are initially analyzed to check the linearity 
of the internal calibration determined using a constant methane 
concentration. This constant methane concentration is injected at the 
end of each analysis. To monitor this methane constant, sucrose 
standards are analyzed several times during a run to determine that 
this constant does not deviate by more than 5-10%.
    The method has the sensitivity to analyze environmental samples 
containing 1 to 10 g/m\3\ of elemental carbon. The method will 
be used in mining applications to determination total carbon 
contamination where the diesel particulate concentration will be 
limited to 400 g/m\3\TC and 160 g/
m\3\TC. NIOSH has reported that the lower limit of detection 
for the method is 0.1 g/cm\2\ elemental carbon for an oven 
pre-fired filter portion and 0.5 g/cm\2\ organic carbon for an 
oven pre-fired filter portion. For a full shift sample, this detection 
limit represents approximately 1 and 5 g/m\3\ of elemental and 
organic carbon, respectively. Additionally, NIOSH has conducted a round 
robin program to assess interlaboratory variability of the method. This 
study indicated a relative standard deviation for total carbon, of less 
than 15 percent.
    A typical diesel particulate thermogram is shown in Figure II-4. 
The thermogram generally contains five or six carbon peaks, one for 
each temperature ramp on the analyzer. The first four peaks (occurring 
during a helium atmosphere ranging from a temperature of 210C to 870C) 
are associated with organic carbon determination and the fifth and/or 
sixth peak (occurring during a helium/oxygen atmosphere ranging in 
temperature from 610C to 890C) is the elemental carbon determination.
    The fourth peak (temperature ~750C) is also where carbonate and 
other carbonaceous minerals are evolved in the analysis. For a diesel 
particulate sample without interferences present, this fourth peak is 
usually minimal as it is attributed to heavy distillant organics not 
normally associated with diesel operations in underground mining 
applications. If this peak is due to carbonate, the carbonate 
interference can be verified by analyzing a second portion of the 
sample after acidification as described in the NIOSH 5040 method. If 
the fourth peak is caused by some other carbonaceous mineral, the 
acidification process may not completely remove the interference and 
may, on occasion cause a positive bias to elemental carbon.
    As explained below in the discussion of interferences, these 
analytical interferences from carbonaceous materials can be corrected 
by using the submicron impactor preceded by a cyclone (respirable 
classifier) to collect diesel particulate matter samples, since nearly 
all the particles of these minerals are greater than 1 micrometer in 
size. Accordingly, MSHA has determined it should utilize a submicron 
impactor in taking any samples in underground metal and nonmetal mines, 
and has included this requirement in the rule. Specifically, 57.5061(b) 
now provides:
    (b) The Secretary will collect samples of diesel particulate matter 
by using a respirable dust sampler equipped with a submicrometer 
impactor and analyze the samples for the amount of total carbon using 
the method described in NIOSH Analytical Method 5040, except that the 
Secretary may also use any methods of collection and analysis 
subsequently determined by NIOSH to provide equal or improved accuracy 
for the measurement of diesel particulate matter in mines subject to 
this part.

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    In keeping with established metal and nonmetal sampling protocol, 
the samplers will be operated at a flow rate of 1.7 LPM. At a flow rate 
of 1.7 LPM, the cut point for the impactor is 0.9 micrometers.
    Any organic carbon detected at the fourth peak will be subtracted 
from the organic carbon portion of the sample analysis using the 
software supplied with the analytical program. The only samples that 
MSHA anticipates that will be acidified are those collected in trona 
mines. These samples contain a bicarbonate which evolves in several of 
the organic peaks but can be removed by acidification. Use of the 
submicron impactor will also insure a uniform distribution of diesel 
particulate and mineral dust on the filter.
    Some Commenters indicated that a uniform deposit of mineral dust 
was sometimes not obtained with certain respirable dust sampler 
configurations. For some commodities such as salt and potash, where 
carbonate may not be an interference, it is probably not necessary to 
sample with the submicron impactor. However, in order to be consistent, 
MSHA will sample all commodities using a respirable dust sampler 
equipped with a submicrom impactor, and has so noted in the rule.
    Proper use of sample blanks. Each set of samples collected to 
measure the diesel particulate concentration of a mine environment, 
must be accompanied by a field blank (a filter cassette that is treated 
and handled in the same manner as filters used to collect the samples) 
when submitted for analysis. The amount of total carbon determined from 
the analysis of the blank sample must be applied to (subtracted from) 
the carbon analysis of each individual sample. The field blank 
correction is applied to account for non-sampled carbon that attaches 
to the filter media. The blank correction is applied to the organic 
fraction as, typically, no elemental carbon is found on the blank 
filters.
    Failure to adjust for the blanks can lead to incorrect results, as 
was the case with samples collected by some commenters. While field 
blanks were submitted and analyzed with their samples, the field blank 
analytical results were not used to correct the individual samples for 
nonsampled carbon content. Typically the carbon content on the reviewed 
field blanks ranged from 2 to 3 g/square centimeter of filter 
area. For a one-hour sample, not using a blank correction of this 
magnitude, could result in an overestimate of 250 g/m\3\ of 
dpm (3 x 8.55 x 1000/(1.7 * 60)=250). For an eight-hour sample, not 
using a blank correction, could result in an overestimate of 30 
g/m\3\ of dpm (3 x 8.55 x 1000/(1.7* 480)=30).

Variability of Sample Blanks

    In response to the July 1, 2000, reopening of the record, one 
commenter submitted summary data from a study that examined diesel 
exposures in seven underground facilities where trona, salt, limestone, 
and potash were mined. The purpose of this study was to determine the 
precision and accuracy of the NIOSH 5040 method in these environments. 
According to the commenter, the study data ``provide strong evidence 
that the NIOSH 5040 Method * * * is not feasible as a measure of DPM 
exposure.'' The commenter's conclusion was based on five 
``difficulties'' that, according to the commenter, were documented when 
sampling for DPM using organic carbon or total carbon as a surrogate. 
These difficulties were:
    (1) High and variable blank values from filters;
    (2) High variability from duplicate punches from the same sampling 
filter;
    (3) Consistently positive interference when open-faced monitors 
were sampled side-by-side with cyclones;
    (4) Poor correlation of organic carbon to total carbon levels; and
    (5) Interference from limestone that could not be adequately 
corrected with acid-washing.
    As discussed elsewhere in this preamble, difficulties #3 and #5 
will be resolved by the use of a submicrometer impactor sampler. 
Difficulty #4, the lack of a strong correlation between organic carbon 
and total carbon, has long been recognized by MSHA. That is one of the 
reasons MSHA chose total carbon (TC=EC+OC) as the best surrogate to use 
for assessing DPM levels in underground metal and nonmetal mines. MSHA 
has never proposed using organic carbon as a surrogate measure of DPM.
    The summary data that the commenter submitted do not appear to 
demonstrate the first two items of ``difficulties'' with respect to TC 
measurements. Because MSHA has not experienced the difficulties of (1) 
high and variable blank values and (2) high variability between 
duplicate punches from the same sampling filter, MSHA also performed 
its own analysis of the data submitted by the commenter. MSHA's 
examination of the data included:
     Estimating the mean, within-mine standard deviation, and 
relative standard deviation (RSD) for blank TC values, based on the 
``Summary of Blank Sample Results'' submitted; and
     Estimating the variability (expressed as RSD) associated 
with the TC analysis of duplicate punches from the same filter, based 
on individual sample data submitted earlier by the same commenter for 
five of the mines.
    Based on the summary data, the overall average mean TC content per 
blank filter, weighted by the number of blank samples in each mine, was 
16.9 g TC. This represents the average value that would be 
subtracted from the TC measurement from an exposed sample before making 
a noncompliance determination. At a TC concentration of 160 g/
m3 (the final limit established by this rule), the TC 
accumulated on a filter after an 8-hour sampling period would be 
approximately 130 g. Therefore, these data show that the mean 
TC value for a blank is less than 13 percent of TC accumulated at the 
concentration limit, and an even lower percentage of total TC 
accumulated at concentrations exceeding the limit. MSHA considers this 
to be acceptable for samples used to make noncompliance determinations. 
Based on the same summary data presented for TC measurements on blank 
samples, the weighted average of within-mine standard deviations is 6.4 
g. Compared to TC values greater than or equal to 130 
g, this corresponds to an RSD no greater than 6.4/130 = 4.9 
percent. MSHA also regards this degree of variability in blank TC 
values to be acceptable for purposes of noncompliance determination.
    To estimate the measurement variability associated with analytical 
errors in the TC measurements, MSHA examined the individual TC results 
from duplicate punches on the same filter. These data were submitted 
earlier by the same commenter for five mines. As shown, by the 
commenter's summary table, data obtained from the first mine were 
invalid, leaving data from four mines (2-5) for MSHA's data analysis. 
Data were provided on a total of 73 filters obtained from these four 
mines, yielding 73 pairs of duplicate TC measurements, using the 
initial and first repeated measurement provided for both elemental and 
organic carbon. MSHA calculated the mean percent difference within 
these 73 pairs of TC measurements (relative to the average for each 
pair) to be 8.2 percent (95-percent confidence interval = 5.6 to 10.9 
percent). Based on the same data, MSHA calculated an estimated RSD = 
10.0 percent for the analytical error in a single determination of 
TC.\1\ Contrary

[[Page 5725]]

to the commenter's conclusion, this result supports MSHA's position 
that TC measurements do not normally exhibit excessive analytical 
errors.
---------------------------------------------------------------------------

    \1\ This estimate was obtained by first calculating the standard 
deviation of the differences between the natural logarithms of the 
TC measurements within each pair. Since each of these differences 
contains two TC determinations, and two corresponding analytical 
errors, this standard deviation was divided by the square root of 2. 
Using standard propagation of error formulas, the result provides a 
reasonably good estimate of the RSD over the range of TC values 
reported. MSHA used the same technique to estimate the RSD for the 
25 pairs of TC samples analyzed at different laboratories, as 
described below.
---------------------------------------------------------------------------

    This estimate of the RSD = 10.0 percent for TC measurements is also 
consistent with the replicated area sample results submitted by the 
commenter for the seven mines. In this part of the study, designed to 
evaluate measurement precision, 69 sets of simultaneous samples were 
collected at the seven mines. Each set, or ``basket,'' of samples 
normally consisted of five simultaneous samples taken at essentially 
the same location. Since the standard deviation of the TC measurements 
within each basket was based on a maximum of five samples, the standard 
deviation calculated within baskets is statistically unstable and does 
not provide a statistically reliable basis for estimating the RSD 
within individual baskets. However, as shown in the summary table 
submitted by the commenter, the mean RSD across all 69 baskets was 10.6 
percent. This RSD, which includes the effects of normal analytical 
variability, variability in the volume of air pumped, and variability 
in the physical characteristics of individual sampler units, is not 
unusually high, in the context of standard industrial hygiene practice.
    MSHA also examined data submitted by another commenter to estimate 
the total variability associated with TC sample analysis by different 
laboratories. Based on 25 pairs of simultaneous TC samples (using a 
cyclone) analyzed by different laboratories, this analysis showed a 
total RSD of approximately 20.6 percent. If the most extreme of three 
statistical outliers in these data is excluded, the result based on 24 
pairs is an estimated RSD of 11.7 percent. Like the first commenter's 
estimate of RSD = 10.6 percent, based on simultaneous samples analyzed 
at the same laboratory, these RSD's include not only normal analytical 
variability in a TC determination, but also variability in the volume 
of air pumped and variability in the physical characteristics of 
individual sampler units. The higher estimates, however, also cover 
uncertainty in a TC measurement attributable to differences between 
laboratories.
    Based on these analyses, MSHA has concluded that the data submitted 
to the record by commenters support the Agency's position that NIOSH 
Method 5040 is a feasible method for measuring DPM concentrations in 
underground M/NM mines.
    Availability of analysis and samplers. One of the concerns 
expressed by commenters was the limited number of commercial 
laboratories available to analyze diesel particulate samples, and the 
availability of required samplers. While MSHA will be doing all 
compliance sampling itself, and running the analyses in its AIHA 
accredited laboratory in Pittsburgh, pursuant to Sec. 57.5071 of the 
rule, operators in underground metal and nonmetal mines will be 
required to do environmental monitoring; and although they will not be 
required to use the same methods as MSHA to determine dpm 
concentrations, MSHA presumes that many will wish to do so. Moreover, 
there are certain situations (e.g., verification that a dpm control 
plan is working) where the rule requires operators to use this method 
(Sec. 57.5062(c)).
    Currently there are four commercial labs that have the capability 
to analyze for dpm using the NIOSH 5040 Method. These labs are: Sunset 
Laboratory, Forest Grove, Oregon and Chapel Hill, North Carolina; Data 
Chem, Salt Lake City, Utah; and Clayton Group Services, Detroit, MI. 
All of these labs, as well as including the NIOSH Laboratories in 
Cincinnati and Pittsburgh and the MSHA laboratory in Pittsburgh 
participate in a round robin analytical test to verify the accuracy and 
precision of the analytical method being used by each. As MSHA 
indicated in the preamble to its proposed rule, it believes that once 
there is a commercial demand for these tests, additional laboratories 
will offer such services.
    The cost of the analysis from the commercial labs is approximately 
$30 to $50 for a single punch analysis and a report. This is about the 
same amount as a respirable silica analysis. The labs charge another 
$75 to acidify and analyze a second punch from the same filter and to 
prepare an analytical report. The labs report both organic and 
elemental carbon. By using the submicron impactor, operators can 
significantly reduce the number of situations where acidification is 
required, and thus reduce the cost of sample analysis.
    The availability of samplers has been the subject of many 
comments--not so much because of concern about availability once the 
rule is in effect, but because of assertions that they are not 
available now. In particular, it has been alleged by some commenters 
that they have been unable to conduct their own ``independent 
evaluation'' of the NIOSH method because the agency has kept from them 
the samplers needed to properly conduct such testing. Some commenters 
even accused the agency of deliberately withholding the needed 
samplers.
    As indicated in MSHA's toolbox and the preamble to the proposed 
rule, the former Bureau of Mines (BOM) submitted information on the 
development of a prototype dichotomous impactor sampling device that 
separates and collects the submicron respirable particulate from the 
respirable dust sampled. Information on this sampling device has been 
available to the industry since 1992. A picture of the sampler is shown 
above as Figure II-3. The impactor plate is made out of brass and the 
nozzles are drilled. The former BOM made available to all interested 
parties detailed design drawings that permitted construction of the 
dichotomous impactor sampler by any local machine shop. NIOSH and MSHA 
had hundreds of these sampling devices made for use in their programs 
to measure dpm concentrations. Anyone could have had impactor samplers 
built by a local machine shop at a cost ranging from $50 to $100.
    In 1998, MSHA provided NIOSH with research funds for the 
development of a disposable sampling device that would have the same 
sampling characteristics as the BOM sampler, and including an impactor 
with the same sampling characteristics as the metal one. NIOSH awarded 
SKC the contract for the development of the disposable sampler. MSHA 
estimates the cost of the disposable sampler will be less than $50. The 
sampler is designed to interface with the standard 10 millimeter Dorr 
Oliver cyclone particle classifier and to fit in a standard MSHA 
respirable dust breast plate assembly. The quartz fiber filter used for 
the collection of diesel particulate in accordance with NIOSH Method 
5040 has been encapsulated in an aluminum foil to make handling during 
the analytical procedure easier. To reduce manufacturing expense (and 
therefore, sampler cost), the nozzle plate in the SKC sampler is made 
of plastic instead of brass. In order to ensure that the nozzles in the 
impaction plate would hold their tolerances during manufacturing, the 
plastic nozzle plate for the SKC sampler is fitted with synthetic 
sapphire nozzles. This nozzle plate and nozzle assembly have the same 
performance as the BOM-designed sampler.

[[Page 5726]]

    As of the time MSHA conducted its verification sampling for 
interferences, SKC had developed several prototypes of the disposable 
unit. However, testing of the devices by NIOSH indicated that a minor 
design modification was needed to better secure the impaction plate and 
nozzle plate to the sampler housing for a production unit. In its 
verification sampling, MSHA used both BOM designed and SKC prototype 
samplers. Prior to its verification tests, MSHA replaced the brass 
nozzle plates in the BOM design impactors with plastic nozzle-plates 
fitted with sapphire nozzles, as used in the SKC prototype sampler. 
However, because there was no change in nozzle geometry, this change in 
the BOM impactors did not affect their performance. During MSHA's 
verifications testing, no problems were experienced with dislodgement 
of the impaction plates or nozzle plates. The impactors used by MSHA in 
its verification sampling were not defective in any way, as suggested 
by several Commenters.
    Under the Mine Act, MSHA has no obligation to make devices 
available to the mining community to conduct its own test sampling or 
to verify MSHA's results, nor does the mining industry have any 
explicit authority under the Mine Act to ``independently evaluate'' 
MSHA's results. The responsibility for determining the accuracy of the 
device and method for sampling rests with the agency, not the mining 
community. Accordingly, although some commenters requested that MSHA 
remove its interference studies from the record, the agency declines to 
do so. These studies are discussed in more detail below; additional 
questions raised about the sampling devices used in the studies, and 
the procedures for that sampling, are discussed in that context.
    Some commenters initially asserted that their inability to conduct 
their own testing would prevent them from making comments of MSHA's 
verification studies. Based on the detailed comments subsequently 
provided, this initial concern appears to have been overstated.
    It appears from some of the comments on MSHA's studies that members 
of the mining community may have understood MSHA to say that use of an 
impactor sampler would remove all interferences. MSHA can find no such 
statement. As noted in more detail below, use of the impactor will 
remove most of the interferences (albeit at the cost of eliminating 
some dpm as well).
    Choice of Total Carbon as Measurement of Diesel Particulate Matter. 
MSHA asserted that the amount of total carbon (determined by the 
sampling and analytical methods discussed above) would provided the 
agency with an accurate representation of the amount of dpm present in 
an underground metal and nonmetal mine atmosphere at the concentration 
levels which will have to be maintained under the new standard. Some 
commenters questioned MSHA's statements concerning the consistency of 
the ratio between total carbon and diesel particulate, and the amount 
of that ratio. Other commenters suggested that elemental carbon may be 
a better indicator of diesel particulate because it is not subject to 
the interference that could effect a total carbon measurement.
    Under the approach incorporated into the final rule, the 
concentration of organic and elemental carbon (in g per square 
centimeter) are separately determined from the sample analysis and 
added together to determine the amount of total carbon. The 
interference from carbonate or mineral dust quantified by the fourth 
organic carbon peak is subtracted from the organic carbon results. The 
field blank correction is then subtracted from the organic analysis 
(the blank does not typically contain elemental carbon). Concentrations 
(time weighted average) of carbon are calculated from the following 
formula:
[GRAPHIC] [TIFF OMITTED] TR19JA01.099

Where:
    C=The Organic Carbon (OC) or Elemental Carbon (EC) concentration, 
in g/m\3\, measured in the thermal/optical carbon analyzer 
(corrected for carbonate and field blank).
    A=The surface area of the filter media used. The surface areas of 
the filters are as follows: quartz fiber filter without aluminum cover 
is 8.55 cm\2\; quartz fiber filter with aluminum cover is 8.04 cm\2\.

    The 80 percent factor MSHA used to establish the total carbon level 
equivalents of the 500 g/m\3\ and 200 g/m\3\ dpm 
concentration limits being set by the rule was based on information 
obtained from laboratory measurements conducted on diesel engines 
(Birch and Cary, 1996). Since the publishing of the proposed rule, this 
value has been confirmed by measurements collected in underground mines 
in Canada (Watts, 1999)
    MSHA agrees that the total carbon measurement is more subject to 
interferences than the elemental carbon measurement. However, because 
the ratio of elemental carbon to total carbon in underground mines is 
dependent on the duty cycle at which the diesel engine is operated 
(found to vary between 0.2 and 0.7), MSHA believes that total carbon is 
the best indicator of diesel particulate for underground mines. 
Additionally, MSHA has observed that some controls, such as filtration 
systems on cabs can alter the ratio of elemental to total carbon. The 
ratio can be different inside and outside a cab on a piece of diesel 
equipment. MSHA notes that NIOSH has asserted that the ratio of 
elemental carbon to dpm is consistent enough to provide the basis for a 
standard based on elemental carbon (``* * * the literature and the MSHA 
laboratory tests support the assertion that DPM, on average, is 
approximately 60 to 80% elemental carbon, firmly establishing EC as a 
valid surrogate for DPM''). However, while an average value for 
elemental carbon percent may be a useful measure for research purposes, 
data submitted by commenters show that elemental carbon can range from 
8 percent to 81 percent of total carbon.
    MSHA does not believe elemental carbon is a valid surrogate for dpm 
in the context of a compliance determination that, like all other metal 
and nonmetal health standards, can be based on a single sample. By 
contrast, as noted above, studies have shown that there is a consistent 
ratio between total carbon and dpm (from 80 to 85%). Moreover, although 
the ratio of the elemental carbon to organic carbon components obtained 
using the NIOSH Method 5040 may vary, total carbon determinations 
obtained with this method are very consistent, and agree with other 
carbon methods (Birch, 1999). Accordingly, while total carbon sampling 
does necessitate sampling protocols to avoid interferences, of the sort 
discussed below, MSHA has concluded that it would not be suitable at 
this time to use elemental carbon as a surrogate for dpm.
    Potential Sample Interferences/Contributions. As noted in the 
introduction to this section, many commenters asserted that the 
analytical method would not be able to distinguish between dpm and 
various other substances in the atmosphere of underground metal and 
nonmetal mines--carbonates and carbonaceous minerals, graphitic 
materials, oil mists and organic vapors, and cigarette smoke. The 
agency carefully reviewed the information submitted by commenters, both 
during the hearings and in writing, and found that it was in general 
insufficient to establish that such interferences would be a problem. 
Limitations in the data submitted by the

[[Page 5727]]

commenters included, for example, failure to utilize blanks, failure to 
blank correct sample results, open face and respirable samples that 
were collected in the presence of high levels of carbonate 
interference, the amount of carbonate interference was not quantified, 
dpm was not uniformly deposited on filters and sample punches were 
taken where the deposit was heaviest, failure to adjust sample results 
due to short sampling times, failure to consider the impact of 
interferences such as carbonate, oil mist, and cigarette smoke on dpm 
exposure.
    Rather than dismiss these assertions, however, the agency decided 
to conduct some investigations to verify the validity of the comments. 
As a result of these tests, the agency has determined that certain 
interferences can exist, within certain parameters; and was also able 
to demonstrate how these interferences can be minimized or avoided. The 
material which follows reviews the information MSHA has on this topic, 
including representative comments MSHA received on these verification 
studies. Part IV of this preamble reviews in some detail the 
adjustments MSHA has made to the proposed rule, and the practices MSHA 
will follow in compliance sampling, to avoid these interferences.
    General discussion of interference studies. As noted above, MSHA 
conducted the verifications to determine if the alleged interferences 
were in fact measurable in underground mining environments. At the same 
time, the studies gave MSHA an opportunity to identify sampling 
techniques that would minimize or eliminate the interferences, evaluate 
analytical techniques to minimize or eliminate the interferences from 
the samples, and develop a sampling and analytical strategy to assure 
reliable dpm measurements in underground mines.
    A total of six studies were conducted. One field study was 
conducted at Homestake Mine, a gold mine in Lead, South Dakota, three 
field studies were conducted at gold mines near Carlin, Nevada. These 
included Newmont, South Area Carlin Mine and Barrick Goldstrike. One 
study was conducted in the NIOSH Research Laboratory's experimental 
mine in Pittsburgh, Pennsylvania and one study conducted in a 
laboratory dust chamber at the NIOSH Pittsburgh Research Laboratory. 
For example the studies conducted at Carlin and Homestake were to 
evaluate interference from oil mist and the studies conducted at 
Homestake, Newmont and Barrick were to assess interference from 
carbonaceous dust. These locations were carefully selected in light of 
the assertions about interferences which had been made by commenters.
    Despite the care that went into designing where to conduct the 
verification samples, there were a number of comments asserting the 
samples were not representative. For example, it was asserted that MSHA 
did not sample a representative particle size distribution and sampled 
the wrong material (i.e., ores with the highest carbon content). On the 
contrary the samples that MSHA collected were representative of the 
respirable and submicron fractions of the dust in the environment as 
well as the total dust in the environment. Therefore, MSHA believes 
that the particle size distribution of the samples collected were 
representative. Also, MSHA obtained a bulk sample of the various ores 
tested. While the samples collected at the crushers were low carbon 
content (0-10.3%), the carbon content (30.3%) of the ore collected at 
the underground mining area sampled at Carlin was similar to the high 
carbon content (31.4%) ores obtained at Barrick. The sampling therefore 
included a cross section of the ores in question.
    Some commenters objected to the fact that no personal samples were 
collected in these studies. Packages of samplers were placed in areas 
that were close to the breathing zone of the workers. Upwind and 
downwind samples were used to determine the extent of the interference. 
The regulation recognizes the validity of area samples. As a result 
these samples provided valid information on interferences that are 
likely to be encountered during sampling by MSHA inspectors.
    More generally, commenters asserted that MSHA lacked enough studies 
for statistical analysis. MSHA notes again that the studies were 
conducted to verify specific industry assertions, and were properly 
designed to try and verify those assertions. However, the same studies 
which confirmed that such interferences could be measured in certain 
conditions were also able to determine that these interferences could 
not be measured, or were not significant in scope, if some of the 
conditions were changed. Part IV of this preamble discusses what 
actions the agency plans to take as a result of its current information 
on this matter.
    Some commenters asserted that MSHA made certain incorrect technical 
assumptions in its verification sampling: about the sampling method 
used to conclude that overall dust levels would meet MSHA's standards; 
about the concentration of EC in submicrometer dust; and about the 
variability of carbonaceous ores. With respect to the first point, the 
final sampling strategy adopted by MSHA for dpm allows for either 
personal or area sampling using a submicrometer sampler preceded by a 
respirable cyclone. Because of the sampling and analytic procedures, 
the only potential mineral interferent would be the graphitic 
contribution (elemental carbon). The carbonate and carbonaceous 
contribution would be eliminated or reduced by the use of the impactor 
sampler and using the software integration procedure described in 
Method 5040.
    With respect to the second point, the concentration of EC in the 
submicrometer dust, for personal and most area samples, the allowable 
silica exposure would limit the amount of submicrometer mineral dust 
sampled. This has been demonstrated for samples collected in coal mines 
where the coal dust contains high levels of elemental carbon, but the 
interference for EC from submicrometer samples has been less that 4 
g/m3.
    With respect to the last point which addresses the geology of the 
ore, MSHA acknowledges that there would be variation in the carbon 
content of the ore. However, it would be unlikely that the carbon 
content would exceed that of coal mine dust where the elemental carbon 
interference has been found to be negligible.
    The sampling was performed with the BOM designed or SKC prototype 
samplers as described in the prior section. All samplers used the more 
precise sapphire nozzles. Samples were collected using standard 
procedures developed by MSHA for assessing particulate concentrations 
in mine environments. Samples were analyzed for total carbon using 
NIOSH Method 5040. The analyses was performed by MSHA at the Pittsburgh 
Safety and Health Technology Center's Dust Division laboratory. For 
some samples a second analysis was performed using an acidification 
procedure.
    Commenters alleged a number of technical problems with how the 
sampling was performed. Some asserted that defective devices were used 
for the sampling, or that MSHA did not properly calibrate its 
equipment. MSHA did not experience any problems with the samplers, and 
did calibrate its equipment according to standard procedures. Some 
pointed out that MSHA conducted the verifications with samplers 
different from those required by the rule. MSHA presumes this comment 
reflects the fact that the proposed rule did not require an

[[Page 5728]]

impactor to be used; this is, however, the case with the final rule.
    Some commenters noted that MSHA voided some sample results and 
that, lacking further explanation, it might be assumed the agency 
simply eliminated those samples which gave results that did not agree 
with the conclusions it sought. The only samples that were voided were 
chamber samples. Some voided samples were higher than, and some void 
samples were lower than, the sample used. These were duplicate samples 
collected for short time periods. Samples were voided because they were 
inconsistent with other samples in the set of six samples collected. 
These inconsistencies as-well-as variability between other duplicate 
samples were attributed to short sample times. Voided sample results 
are shown for Homestake (1 of 12 impactors). No impactor samples were 
voided at Barrick nor at the Newmont crusher. In the Jackleg drill 
tests conducted at Carlin Mine, there were 2 of 6 impactor samples 
voided.
    Others asserted that MSHA failed to validate the design of the box 
which held the sampling equipment. In fact, all of the issues mentioned 
relative to the sampling box (i.e., pressure build up, leakage of 
chamber, impaction of particles, pump calibration) had been carefully 
examined by MSHA prior to the tests and found not to be a problem. 
Also, this sample chamber has been used extensively in other field 
tests where duplicate samples or a variety of samplers have been used 
and has worked extremely well.
    One commenter stated that these studies confirm that measurement 
interference cannot be eliminated by blank correction and longer sample 
times, and that the proposed single sample enforcement policy would not 
be representative of typical mine conditions. MSHA disagrees with this 
conclusion from the verification tests. The MSHA tests demonstrated 
that blank correction does eliminate a source of interference. The 
residual organic carbon indicated in several of the samples collected 
at crushers were attributed to short sample time and normal variation 
in the range of blank values. The verification tests did not address 
sample time. However, when converting the mass collected to a 
concentration, the mass is divided by the sample time. Dividing by a 
longer time will always reduce an interference caused by a positive 
bias.
    Other commenters alleged that there were problems with the MSHA 
personnel performing the studies. Some asserted these personnel failed 
to listen to suggestions made by representatives of mine companies who 
accompanied MSHA in their facilities during in-mine testing, 
suggestions which they assert would have corrected asserted problems in 
the testing procedure. Others simply assert that the MSHA personnel 
were biased, manipulated the data, and tried to conform the study 
results to those they wanted to find. It was also asserted that any 
potential for bias should have been removed through independent peer 
review of the results, or performance or confirmation of the studies by 
independent personnel or laboratories.
    The tests were designed and conducted by personnel from MSHA's 
Pittsburgh Safety and Heath Technology's Dust Division. This laboratory 
at this facility is AIHA accreditated, and its personnel are among the 
foremost experts in particulate sampling analysis in the mining 
industry. They are widely published and are accustomed to performing 
work that must survive legal and scientific scrutiny. Moreover, the 
personnel designing and performing these studies have more experience 
than anybody else with dust sampling in general, and with this 
particular measurement application. While the agency welcomes scrutiny 
of its work, and repetition by others, it also recognizes that such 
efforts take time. In this case, the agency elected to conduct tests to 
address specific concerns, given its obligation to respond to the risks 
to miners reviewed in Part III of this preamble. It did so using a 
sound study design and expert personnel, and has made the detailed 
results of its studies a matter of public record.
    In this regard, a number of commenters made reference to a study 
currently being conducted by NIOSH of possible interferences with the 
5040 method. Some of these commenters provided MSHA with a copy of what 
is apparently the final protocol for the study, asserted that it would 
provide better information than the verification studies conducted by 
MSHA, and urged the agency to wait for completion of this study.
    MSHA welcomes the NIOSH study, and will carefully consider its 
results--and the results of any other studies of this matter--in 
refining the compliance practices outlined in part IV of this preamble. 
But given the agency's obligation to respond to the risks to miners 
reviewed in Part III of this preamble, and the recommendations of NIOSH 
to take action in light of that risk, it would be inappropriate to 
await the results of another study.
    Carbonates and Carbonaceous Minerals. As noted in the discussion of 
the analytical method (NIOSH Method 5040), carbonates have been known 
to cause an interference when determining the total carbon content of a 
diesel particulate sample. Carbonates are generally in two forms--
carbonates such as limestone and dolomite and bicarbonate which is 
associated with trona (soda ash). As further noted, the amount of 
carbonate and bicarbonate collected on a sample can be significantly 
reduced or eliminated through the use of a submicrometer impactor. If 
the total carbon analysis of a sample indicates that a carbonate 
interference exists after the use of a submicrometer impactor, any 
remaining interfering effect may be removed or diminished using the 
acidification process described in NIOSH Method 5040.
    Carbonate interference can also be removed during the analytical 
process by mathematically subtracting the organic carbon quantified by 
the fourth peak in the thermogram. Because bicarbonate is evolved over 
several temperature ranges, subtraction of only one peak does not 
remove all of the interference from bicarbonate. As a result, the 
sample needs to be acidified to remove all of the bicarbonate 
interference.
    Commenters correctly pointed out that other carbonaceous minerals 
are not removed by the acidification process and in fact in some cases, 
the acidification process may cause a positive bias to the elemental 
carbon measurement. However, MSHA has verified that through the use of 
the submicrometer impactor, which reduces the mineral dust collected, 
combined with the subtraction of organic carbon quantified by the 
fourth organic carbon peak, this source of interference can be 
eliminated (PS&HTC-DD-505, PS&HTC-DD-509, PS&HTC-DD-510 and PS&HTC-DD-
00-523).
    MSHA has verified the use of a submicron impactor to remove 
carbonate interference through field and laboratory measurements. In 
the field measurements, simultaneous respirable and submicron dust 
samples were collected near crushing operations where there was no 
diesel equipment operating. In the laboratory measurements, a aerosol 
containing carbonate dust was introduced into a dust chamber and 
simultaneous submicron, respirable and total dust samples were 
collected. For both the field and laboratory measurements, the samples 
were analyzed for carbon using NIOSH Method 5040. Results of analysis 
of these samples showed that for respirable dust samples, acidification 
of the sample removed the carbonate.

[[Page 5729]]

Carbonate was evolved in the fourth peak of the organic portion of the 
analysis. The carbon evolved by the analysis was approximately 10 
percent of the carbonate collected on the gravimetric sample, roughly 
equating to 12 percent carbon contained in calcium carbonate tested 
(limestone). Sampling with the submicron impactor removed the carbonate 
and carbonaceous component from the sample. A commenter noted that in 
the dust chamber tests, organic carbon was reported, even though the 
carbonate was removed by sampling, acidification or software 
integration. This organic carbon was attributed to oil vapors leaking 
from the compressor that delivered the dust to the chamber. This oil 
leak was reported to MSHA after the tests were completed.
    Sample results further indicated that the total carbon mass 
determined for the respirable diesel particulate samples was 
approximately 95 percent of the diesel particulate mass determined 
gravimetrically and the total carbon mass determined from the impactor 
diesel particulate samples was approximately 82 percent of the 
respirable value. Use of the impactor reduced the amounts of carbonate 
collected on the sample by 90 percent.
    The difference between the respirable total carbon determinations 
and the gravimetric diesel particulate can be attributed to sulfates or 
other noncarbonaceous minerals in the diesel particulate. The 
difference between the submicron total carbon and the respirable total 
carbon determinations is attributed to the removal of diesel 
particulate particles that are greater than 0.9 micrometers in size. 
The difference between the carbonate measured by NIOSH Analytical 
Method 5040 and the gravimetric carbonate is attributed to impurities 
in the material. The expected ratio of evolved carbon from the 
carbonate to carbonate (C/CaCo3) would be 0.12 (12/(40 + 12 + 48)).
    Graphitic Minerals. Commenters reported that several ores, 
primarily associated with gold mines, contain graphitic carbon, and 
that this carbon shows up as elemental carbon in an airborne dust 
sample. MSHA has collected samples of this ore and has found that in 
fact this is true (PS&HTC-DD-505, PS&HTC-DD-509, PS&HTC-DD-510). MSHA 
has verified the use of a submicron impactor to remove graphitic carbon 
interference through field measurements.
    In the field measurements, simultaneous respirable and submicron 
dust samples were collected near crushing operations where there was no 
diesel equipment operating. For both the field and laboratory 
measurements, the samples were analyzed for carbon using NIOSH Method 
5040. Results of analysis of these samples showed that for respirable 
dust samples, several g/m3 of elemental carbon 
could be present in the sample.
    However, MSHA has found this interference is very small, and can be 
reduced still further through the use of the submicron impactor on the 
sampler. The highest elemental carbon content of the ores was less than 
5 percent. These ores also contain at least 20 percent respirable 
silica, as determined from samples collected near crushers where diesel 
particulate was not present. Based on a 20 percent respirable silica 
content in the dust in the environment, the allowable respirable dust 
exposure would be limited to 0.45 mg/m3. Based on a 5 
percent elemental carbon content in the sample, this sample could 
contain 23 g/m3 of elemental carbon. Typically 10 
percent of mineral dust is less than one micron. By using the submicron 
impactor, the interference from graphitic carbon in the ore would be 
less than 3 g/m3. Samples collected by MSHA, near 
crushing operations, using submicron impactors, did not contain 
elemental carbon.
    Accordingly, MSHA plans to sample for diesel particulate matter 
using submicron impactors to reduce the potential interference from 
carbonates, carbonaceous minerals and graphitic ores. As noted 
previously, this requirement is being specifically added to the 
regulation.
    Oil Mist and Organic Vapors. Commenters indicated that diesel 
particulate sample interference can occur from sampling around drilling 
operations and from organic solvents.
    To verify the existence and extent of any such interference, MSHA 
collected samples at stoper drilling, jack leg drilling and face 
drilling operations. The stoper drill and jack leg drill were 
pneumatic. The face drill was electrohydraulic. Interference from drill 
oil mist was observed for both the stoper drill and jack leg drill 
operations (PS&HTC-DD-505, PS&HTC-DD-511). Respirable and submicron 
samples were collected in the stope, the intake air to the stope and 
the exhaust air from the stope. Interference from drill oil mist was 
not found in submicron samples collected on the electrohydraulic face 
drill (PS&HTC-DD-505). The oil mist interference for the stoper drill 
was confined to the drill location due to the use of a high viscosity 
lube grease. The amount of interference in the stope on a submicron 
sample for the stoper drill was 4.5 g/m\3\ per hour of 
drilling. The interference from the oil mist on the jack leg operation 
extended throughout the mining stope area, but it did not extent into 
the main ventilation heading. The amount of interference in the stope 
on a submicron sample for the jack leg drill was 9 to 11 g/
m\3\ per hour of drilling. MSHA believes that similar interferences 
could occur when miners are working near organic solvents.
    Accordingly, this is an interference that can be addressed by not 
sampling too close to the source of the interference. As discussed in 
more detail in Part IV of this preamble, when MSHA collects compliance 
samples on drilling operations that produce an oil mist, or where 
organic solvents are used, personal samples will not be collected. 
Instead, an area sample will be collected, upwind of the driller or 
organic solvent source.
    A commenter suggested that the lack of organic carbon reduction 
from outside to inside the cab at Homestake Mine indicated additional 
sources of organic carbon that have not been identified. MSHA believes 
that the reduction in elemental but not organic carbon from outside to 
inside the cab at Homestake Mine was attributed to size distribution. 
The organic carbon is small enough to pass through a filter. The 
organic carbon in the cab could not have been generated from a source 
inside the cab or attributed to residual cigarette smoke as the air 
exchange rate for the cab was one air change per minute. The cab 
operator did not smoke.
    Cigarette Smoke. Cigarette smoke is a form of organic carbon. 
Commentors indicated that cigarette smoke can interfere with a diesel 
particulate measurement when total carbon is used as the indicator of 
dpm. Industry Commenters collected samples in a surface ``smoke room'' 
where the airflow and number of cigarettes were not monitored.
    To verify the existence and the extent of any such interference, 
MSHA took samples in an underground mine where controlled smoking took 
place. Two series of cigarette tests were conducted. A test site was 
chosen in the NIOSH, PRL, Experimental Mine. The site consisted of 
approximately 75 feet of straight entry. The entry was approximately 
18.5 feet wide and 6.2 feet high (115 square feet area). In the first 
test, the airflow rate through the test area was 6,000 cfm and 4 
cigarettes were smoked over a 120 minute period. In the second test, 
the airflow was 3,000 cfm and 28 cigarettes were smoked over a 210 
minute period. A control filter was used to adjust for organic carbon 
present on the filter media. MSHA collected samples on the smokers, 
twenty-five feet upwind of the smokers,

[[Page 5730]]

twenty-five feet downwind of the smokers and fifty feet downwind of the 
smokers. Results of the underground test did verify that smoking could 
be an interference on a dpm measurement.
    Analysis of the thermogram from the smoking test showed that 
cigarette smoke showed up only in the organic portion of the analysis. 
In this test with the cigarette smoke, a fifth organic peak was 
observed. This peak contributed approximately 0.5 g/m\2\ to 
the analysis. This would be equivalent to an 8 hour full shift 
concentration of 5 g/m\3\. The thermogram otherwise is not 
distinguishable from the organic portion of a thermogram for a diesel 
particulate sample. Analysis of the thermogram indicated that 30 
percent of the organic carbon appeared in the first organic peak, 15 
percent appeared in the second organic peak, 10 percent appeared in the 
third organic peak, 25 percent of the cigarette smoke appeared in the 
fourth organic peak, and 20 percent of the cigarette smoke appeared in 
the fifth organic peak. While the amount of carbon identified by the 
fourth organic peak can be quantified and mathematically subtracted 
from the amount of total carbon measured, the remaining three peaks, 
representing 83 percent of the total carbon associated with smoking, 
would be an interferrant to the diesel particulate matter measurement.
    However, the effect of cigarette smoke was even more localized to 
the smoker than the oil mist was to the stoper or jack leg drill 
operator. Twenty five feet upwind of the smoker, no carbon attributed 
to cigarette smoke was detected. For the smoker, each cigarette smoked 
would add 5 to 10 g/m\3\ to the exposure, depending on the 
airflow. Smoking 10 cigarettes would add 50 to 100 g/m\3\ to a 
worker's exposure. At both twenty five feet and fifty feet downwind of 
the smoker, after mixing with the ventilating air, the contribution of 
carbon attributed to smoking was reduced to 0.3 g/m\3\ for 
each cigarette smoked. Sampling twenty-five to fifty feet down wind of 
a worker smoking 10 cigarettes per day would add no more than 3 
g/m\3\ to the worker's exposure (PS&HTC-DD-518). The air 
velocities in this test (30 to 60 feet per minute) were relatively low 
compared to typical mine air velocities. The interference would be even 
less at the higher air velocities normally found in mines.
    Accordingly, as discussed in more detail in Part IV of this 
preamble, when MSHA collects compliance samples, miners will be 
requested not to smoke. If a miner does want to smoke while being 
sampled, and is not prohibited from doing so by the mine operator, the 
inspector will collect an area sample a minimum of twenty-five feet 
upwind or downwind of the smoker. Smokers working inside cabs will not 
be sampled.
    Summary of Conclusions from Verification Studies. In summary, MSHA 
was able to draw the following conclusions from these studies:
     As specified in NIOSH Method 5040, it is essential to use 
a blank to correct organic carbon measurements.
     Contamination (interference) from carbonate and 
carbonaceous minerals is evolved in the fourth organic peak of the 
thermogram.
     Interference from graphitic minerals may appear in the 
elemental carbon portion of the analysis.
     Interference from cigarette smoke and oil mist from 
pneumatic drills appears in several peaks of the organic analysis.
     Use of the submicron impactor removes the mineral 
interference from carbonate, carbonaceous minerals and graphitic 
minerals.
     Acidification is required to remove the interference from 
bicarbonate which maybe evolved in several of the organic peaks.
     Subtraction of the fourth organic peak by software 
integration can be used to correct for interference from carbonaceous 
minerals.
     Interference from cigarette smoke and oil mist from 
pneumatic drills is localized. It can be avoided by sampling upwind or 
downwind of the interfering source.
     Total carbon from cigarettes smoke and oil mist are small 
compared to emissions from a diesel engine.
     Sampling can be conducted down wind of the interfering 
source after the contaminated air current has been diluted with another 
air current.
    The magnitude of interferences measured during the verifications 
were small compared to the levels of total carbon measured in 
underground mines (as reported in Part III of this preamble). The 
discussion of section 5061 in Part IV of this preamble provides further 
information on how MSHA will take this information about interferences 
into account in compliance sampling; in addition, MSHA will provide 
specific guidance to inspectors as to how to avoid interferences when 
taking compliance samples.

(4) Limiting the Public's Exposure to Diesel and Other Fine 
Particulates--Ambient Air Quality Standards.

    Pursuant to the Clean Air Act, the Federal Environmental Protection 
Agency (EPA) is responsible for setting air pollution standards to 
protect the public from toxic air contaminants. These include standards 
to limit exposure to particulate matter. The pressures to comply with 
these limits have an impact upon the mining industry, which limits 
various types of particulate matter into the environment during mining 
operations, and a special impact on the coal mining industry whose 
product is used extensively in particulate emission generating power 
facilities. But those standards hold interest for the mining community 
in other ways as well, for underlying some of them is a large body of 
evidence on the harmful effects of airborne particulate matter on human 
health. Increasingly, that evidence has pointed toward the risks of the 
smallest particulates--including the particles generated by diesel 
engines.
    This section provides an overview of EPA's rulemaking efforts to 
limit the ambient air concentration of particulate matter, including 
its recent particular focus on diesel and other fine particulates. 
Additional and up-to-date information about the most current rulemaking 
in this regard is available on EPA's Web site, http://www.epa.gov/ttn/oarpg/naaqsfin/.
    EPA is also engaged in other work of interest to the mining 
community. Together with some state environmental agencies, EPA has 
actually established limits on the amount of particulate matter that 
can be emitted by diesel engines. This topic is discussed in the next 
section of this Part (section 5). Environmental regulations also 
establish the maximum sulfur content permitted in diesel fuel, and such 
sulfur content can be an important factor in dpm generation. This topic 
is discussed in section 6 of this Part. In addition, EPA and some state 
environmental agencies have also been exploring whether diesel 
particulate matter is a carcinogen or a toxic material at the 
concentrations in which it appears in the ambient atmosphere. 
Discussion of these studies can be found in Part III of this preamble.
    Background. Air quality standards involve a two-step process: 
standard setting by EPA, and implementation by each State.
    Under the law, EPA is specifically responsible for reviewing the 
scientific literature concerning air pollutants, and establishing and 
revising National Ambient Air Quality Standards (NAAQS) to minimize the 
risks to health and the environment associated with such pollutants. 
This review is to be conducted every five years. Feasibility of 
compliance by pollution sources is not supposed to be a factor in 
establishing NAAQS. Rather, EPA is required to set the level that 
provides

[[Page 5731]]

``an adequate margin of safety'' in protecting the health of the 
public.
    Implementation of each national standard is the responsibility of 
the states. Each must develop a state implementation plan that ensures 
air quality in the state consistent with the ambient air quality 
standard. Thus, each state has a great deal of flexibility in targeting 
particular modes of emission (e.g., mobile or stationary, specific 
industry or all, public sources of emissions vs. private-sector 
sources), and in what requirements to impose on polluters. However, EPA 
must approve the state plans pursuant to criteria it establishes, and 
then take pollution measurements to determine whether all counties 
within the state are meeting each ambient air quality standard. An area 
not meeting an NAAQS is known as a ``nonattainment area''.
    TSP. Particulate matter originates from all types of stationary, 
mobile and natural sources, and can also be created from the 
transformation of a variety of gaseous emissions from such sources. In 
the context of a global atmosphere, all these particles are mixed 
together, and both people and the environment are exposed to a 
``particulate soup'' the chemical and physical properties of which vary 
greatly with time, region, meteorology, and source category.
    The first ambient air quality standards dealing with particulate 
matter did not distinguish among these particles. Rather, the EPA 
established a single NAAQS for ``total suspended particulates'', known 
as ``TSP.'' Under this approach, the states could come into compliance 
with the ambient air requirement by controlling any type or size of 
TSP. As long as the total TSP was under the NAAQS--which was 
established based on the science available in the 1970s--the state met 
the requirement.
    PM10. When the EPA completed a new review of the 
scientific evidence in the mid-eighties, its conclusions led it to 
revise the particulate NAAQS to focus more narrowly on those 
particulates less than 10 microns in diameter, or PM10. The 
standard issued in 1987 contained two components: an annual average 
limit of 50 g/m\3\, and a 24-hour limit of 150 g/
m\3\. This new standard required the states to reevaluate their 
situations and, if they had areas that exceeded the new PM10 
limit, to refocus their compliance plans on reducing those particulates 
smaller than 10 microns in size. Sources of PM10 include 
power plants, iron and steel production, chemical and wood products 
manufacturing, wind-blown and roadway fugitive dust, secondary aerosols 
and many natural sources.
    Some state implementation plans required surface mines to take 
actions to help the state meet the PM10 standard. In 
particular, some surface mines in Western states were required to 
control the coarser particles--e.g., by spraying water on roadways to 
limit dust. The mining industry has objected to such controls, arguing 
that the coarser particles do not adversely impact health, and has 
sought to have them excluded from the EPA ambient air standards.
    PM2.5. The next scientific review was completed in 1996, 
following suit by the American Lung Association and others. A proposed 
rule was published in November of 1996, and, after public hearings and 
review by the Office Management and Budget, a final rule was 
promulgated on July 18, 1997. (62 FR 38651).
    The new rule further modifies the standard for particulate matter. 
Under the new rule, the existing national ambient air quality standard 
for PM10 remains basically the same--an annual average limit 
of 50 g/m3 (with some adjustment as to how this is 
measured for compliance purposes), and a 24-hour ceiling of 150 
g/m3. In addition, however, a new NAAQS has now 
been established for ``fine particulate matter'' that is less than 2.5 
microns in size. The PM2.5 annual limit is set at 15 
g/m3, with a 24-hour ceiling of 65 g/
m3.
    The basis for the PM2.5 NAAQS is a large body of 
scientific data suggesting that particles in this size range are the 
ones responsible for the most serious health effects associated with 
particulate matter. The evidence was thoroughly reviewed by a number of 
scientific panels through an extended process. The proposed rule 
resulted in considerable press attention, and hearings by Congress, in 
which this scientific evidence was further discussed. Moreover, 
challenges to EPA's determination that this size category warranted 
rulemaking were rejected by a three judge panel of the DC Circuit 
Court. (American Trucking Association vs. EPA, 275 F.3d 1027).
    Second, the majority of the panel agreed with challenges to the 
EPA's determination to keep the existing requirements on PM10 as a 
surrogate for the coarser particulates in this category (those 
particulates between 2.5 and 10 microns in diameter); instead, the 
panel ordered EPA to develop a new standard for this size category. 
(Op.Cit., *23.)
    Implications for the Mining Community. As noted earlier in this 
part, diesel particulate matter is mostly less than 1.0 micron in size. 
It is, therefore, a fine particulate; indeed, in some regions of the 
country, diesel particulate generated by highway and off-road vehicles 
constitutes a significant portion of the ambient fine particulate (June 
16, 1997, PM-2.5 Composition and Sources, Office of Air Quality 
Planning and Standards, EPA). Moreover, as noted in Part III of this 
preamble, some of the scientific studies of health risk from fine 
particulates used to support the EPA rulemaking were conducted in areas 
where the major fine particulate was from diesel emissions. 
Accordingly, MSHA has concluded that it must consider the body of 
evidence of human health risk from environmental exposure to fine 
particulates in assessing the risk of harm to miners of occupational 
exposure to diesel particulate. Comments on the appropriateness of the 
conclusion by MSHA, and whether MSHA should be working on a fine 
particulate standard rater than just one focused on diesel particulate 
are reviewed in Part III.

(5) The Effects of Existing Standards--MSHA Standards on Diesel Exhaust 
Gases (CO, CO2, NO, NO2, and SO2), and 
EPA Diesel Engine Emission Standards--on the Concentration of dpm in 
Underground Metal and Nonmetal Mines

    With the exception of diesel engines used in certain 
classifications of gassy mines, MSHA does not require that the 
emissions from diesel engines used in underground metal and nonmetal 
mines, as measured at the tailpipe, meet certain minimum standards of 
cleanliness. (Some states may require engines used in underground metal 
and nonmetal mines to be MSHA Approved.) This is in contrast to 
underground coal mines, where only engines which meet certain standards 
with respect to gaseous emissions are ``approved'' for use in 
underground coal mines. Indeed, as discussed in section 7 of this part, 
the whole underground coal mine fleet must now consist of approved 
engines, and the engines must be maintained in approved condition. 
While such restrictions do not directly control dpm emissions of 
underground coal equipment, they do have some indirect impact on them.
    MSHA does have some requirements for underground metal and nonmetal 
mines that limit the exposure of miners to certain gases emitted by 
diesel engines. Accordingly, those requirements are discussed here.
    Engine emissions of dpm in underground metal and nonmetal mines are 
gradually being impacted by Federal environmental regulations, 
supplemented in some cases by State restrictions. Over time, these 
regulations have required, and are continuing to

[[Page 5732]]

require, that new diesel engines meet tighter and tighter standards on 
dpm emissions. As these cleaner engines replace or supplement older 
engines in underground metal and nonmetal mines, they can significantly 
reduce the amount of dpm emitted by the underground fleet. Much of this 
section reviews developments in this area. Although this subject was 
discussed in the preamble of the proposed dpm rule (63 FR 58130 et 
seq.), the review here updates the relevant information.
    MSHA Limitations on Diesel Gases. MSHA limits on the exposure of 
miners to certain gases in underground mines are listed in Table II-2, 
for both coal mines and metal/nonmetal mines, together with information 
about the recommendations in this regard of other organizations. As 
indicated in the table, MSHA requires mine operators to comply with gas 
specific threshold limit values (TLVs) recommended by the 
American Conference of Governmental Industrial Hygienists (ACGIH) in 
1972 (for coal mines) and in 1973 (for metal and nonmetal mines).

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[[Page 5734]]

    To change an exposure limit at this point in time requires a 
regulatory action; the rule does not provide for their automatic 
updating. In 1989, MSHA proposed changing some of these gas limits in 
the context of a proposed rule on air quality standards. (54 FR 35760). 
Following opportunity for comment and hearings, a portion of that 
proposed rule, concerning control of drill dust and abrasive blasting, 
has been promulgated, but the other components are still under review.
    One commenter expressed concern that MSHA would attempt to regulate 
dpm together with diesel exhaust gases based on their additive or 
combined effects. As discussed in greater detail in Part IV of this 
preamble, MSHA does not, at this time, have sufficient information upon 
which to enforcement limits for dpm and diesel exhaust gases on the 
basis of their additive or combined effects, if any.
    Authority for Environmental Engine Emission Standards. The Clean 
Air Act authorizes the Federal Environmental Protection Agency (EPA) to 
establish nationwide standards for mobile vehicles, including those 
powered by diesel engines (often referred to in environmental 
regulations as ``compression ignition'' or ``CI'' engines). These 
standards are designed to reduce the amount of certain harmful 
atmospheric pollutants emanating from mobile sources: the mass of 
particulate matter, nitrogen oxides (which as previously noted, can 
result in the generation of particulates in the atmosphere), 
hydrocarbons and carbon monoxide.
    California has its own engine emission standards. New engines 
destined for use in California must meet standards under the law of 
that State. The standards are issued and administered by the California 
Air Resources Board (CARB). In many cases, the California standards are 
the same as the national standards; as noted herein, the EPA and CARB 
have worked on certain agreements with the industry toward that end. In 
other situations, the California standards may be more stringent.
    Regulatory responsibility for implementation of the Clean Air Act 
is vested in the Office of Transportation and Air Quality (formerly the 
Office of Mobile Sources), part of the Office of Air and Radiation of 
the EPA. Some of the discussion which follows was derived from 
materials which can be accessed from the agency's home page on the 
World Wide Web at (http://www.epa.gov/omswww/omshome.htm). Information 
about the California standards may be found at the CARB home page at 
(http://www.arb.ca.gov/homepage.htm).
    Diesel engines are generally divided into three broad categories 
for purposes of engine emissions standards, in accordance with the 
primary use for which the type of engine is designed: (1) light duty 
vehicles and light duty trucks (i.e., those engines designed primarily 
to power passenger transport or transportation of property); (2) heavy 
duty highway engines (i.e., those designed primarily to power over-the-
road truck hauling); and (3) nonroad vehicles (i.e., those engines 
designed primarily to power small equipment, construction equipment, 
locomotives and other non-highway uses).
    The exact emission standards which a new diesel engine must meet 
varies with engine category and the date of manufacture. Through a 
series of regulatory actions, EPA has developed a detailed 
implementation schedule for each of the three engine categories noted. 
The schedule generally forces technology while taking into account 
certain technological realities.
    Detailed information about each of the three engine categories is 
provided below; a summary table of particulate matter emission limits 
is included at the end of the discussion.
    EPA Emission Standards for Light-Duty Vehicles and Light Duty 
Trucks.\2\
---------------------------------------------------------------------------

    \2\ The discussion focuses on the particulate matter 
requirements for light duty trucks, although the current pm 
requirement for light duty vehicles is the same. The EPA regulations 
for these categories apply to the unit, rather than just to the 
engine itself; for heavy-duty highway engines and nonroad engines, 
the regulations attach to the engines.
---------------------------------------------------------------------------

    Current light-duty vehicles generally comply with the Tier 1 and 
National LEV emission standards. Particulate matter emission limits are 
found in 40 CFR Part 86. In 1999, EPA issued new Tier 2 standards that 
will be applicable to light-duty cars and trucks beginning in 2004. 
With respect to pm, the new rules phase in tighter emissions limits to 
parts of production runs for various subcategories of these engines 
over several years; by 2008, all light duty trucks must limit pm 
emissions to a maximum of 0.02 g/mi. (40 CFR 86.1811-04(c)). Engine 
manufacturers may, of course, produce complying engines before the 
various dates required.
    EPA Emissions Standards for Heavy-Duty Highway Engines. In 1988, a 
standard limiting particulate matter emitted from the heavy duty 
highway diesel engines went into effect, limiting dpm emissions to 0.6 
g/bhp-hr. The Clean Air Act Amendments of 1990 and associated 
regulations provided for phasing in even tighter controls on 
NOX and particulate matter through 1998. Thus, engines had 
to meet ever tighter standards for NOX in model years 1990, 
1991 and 1998; and tighter standards for PM in 1991 (0.25 g/bhp-hr) and 
1994 (0.10 g/bhp-hr). The latter remains the standard for PM from these 
engines for current production runs (40 CFR 86.094-11(a)(1)(iv)(B)). 
Since any heavy duty highway engine manufactured since 1994 must meet 
this standard, there is a supply of engines available today which meet 
this standard. These engines are used in mining in the commercial type 
pickup trucks.
    New standards for this category of engines are gradually being put 
into place. On October 21, 1997, EPA issued a new rule for certain 
gaseous emissions from heavy duty highway engines that will take effect 
for engine model years starting in 2004 (62 FR 54693). The rule 
establishes a combined requirement for NOX and Non-methane 
Hydrocarbon (NMHC). The combined standard is set at 2.5 g/bhp-hr, which 
includes a cap of 0.5 g/bhp-hr for NMHC. EPA promulgated a rulemaking 
on December 22, 2000 (65 FR 80776) to adopt the next phase of new 
standards for these engines. EPA is taking an integrated approach to: 
(a) Reduce the content of sulfur in diesel fuel; and thereafter, (b) 
require heavy-duty highway engines to meet tighter emission standards, 
including standards for PM. The purpose of the diesel fuel component of 
the rulemaking is to make it technologically feasible for engine 
manufacturers and emissions control device makers to produce engines in 
which dpm emissions are limited to desired levels in this and other 
engine categories. The EPA's rule will reduce pm emissions from new 
heavy-duty engines to 0.01 g/bhp-hr, a reduction from the current 0.1 
g/bhp-hr. MSHA assumes it will be some time before there is a 
significant supply of engines that can meet this standard, and the fuel 
supply to make that possible.
    EPA Emissions Standards for Nonroad Engines. Nonroad engines are 
those designed primarily to power small portable equipment such as 
compressors and generators, large construction equipment such as haul 
trucks, loaders and graders, locomotives and other miscellaneous 
equipment with non-highway uses. Engines of this type are the ones used 
most frequently in the underground coal mines to power equipment.
    Nonroad diesel engines were not subjected to emission controls as 
early as other diesel engines. The 1990 Clean Air Act Amendments 
specifically directed EPA to study the contribution of nonroad engines 
to air pollution, and

[[Page 5735]]

regulate them if warranted (Section 213 of the Clean Air Act). In 1991, 
EPA released a study that documented higher than expected emission 
levels across a broad spectrum of nonroad engines and equipment (EPA 
Fact Sheet, EPA420-F-96-009, 1996). In response, EPA initiated several 
regulatory programs. One of these set Tier 1 emission standards for 
larger land-based nonroad engines (other than for rail use). Limits 
were established for engine emissions of hydrocarbons, carbon monoxide, 
NOX, and dpm. The limits were phased in with model years 
from 1996 to 2000. With respect to particulate matter, the rules 
required that starting in model year 1996, nonroad engines from 175 to 
750 hp meet a limit on pm emissions of 0.4 g/bhp-hr, and that starting 
in model year 2000, nonroad engines over 750 hp meet the same limit.
    Particulate matter standards for locomotive engines were set 
subsequently (63 FR 18978, April, 1998). The standards are different 
for line-haul duty-cycle engine and switch duty-cycle engines. For 
model years from 2000-2004, the standards limit pm emissions to 0.45 g/
bhp-hr and 0.54 g/bhp-hr respectively for those engines; after model 
year 2005, the limits drop to 0.20 g/bhp-hr and 0.24 g/bhp-hr 
respectively.
    In October 1998, EPA established additional standards for nonroad 
engines (63 FR 56968). Among these are gaseous and particulate matter 
limits for the first time (Tier 1 limits) for nonroad engines under 50 
hp. Tier 2 emissions standards for engines between 50 and 175 hp 
include pm standards for the first time. Moreover, they establish Tier 
2 particulate matter limits for all other land-based nonroad engines 
(other than locomotives which already had Tier 2 standards). Some of 
the non-particulate emissions limits set by the 1998 rule are subject 
to a technology review in 2001 to ensure that the levels required to be 
met are feasible; EPA has indicated that in the context of that review, 
it intends to consider further limits for particulate matter, including 
transient emission measurement procedures. Because of the phase-in of 
these Tier 2 pm standards, and the fact that some manufacturers will 
produce engines meeting the standard before the requirements go into 
effect, there are or soon will be some Tier 2 pm engines in some sizes 
available, but it is likely to be a few years before a full size range 
of Tier 2 pm nonroad engines is available.
    Table II-3, EPA NonRoad Engine PM Requirements, provides a full 
list of the EPA required particulate matter limitations on nonroad 
diesel engines. For example, a nonroad engine of 175 hp produced in 
2001 must meet a standard of 0.4 g/hp-hr; a similar engine produced in 
2003 or thereafter must meet a standard of 0.15 g/hp-hr.

             Table II-3.--EPA Nonroad Engine PM Requirements
------------------------------------------------------------------------
                                                 Year first  PM limit (g/
             kW range                  Tier      applicable     kW-hr)
------------------------------------------------------------------------
kW8..............................            1         2000         1.00
                                             2         2005         0.80
8kW19.................            1         2000         0.80
19kW37................            1         1999         0.80
                                             2         2004         0.60
37kW75................            1         1998  ...........
                                             2         2004         0.40
75kW130...............            1         1997  ...........
                                             2         2003         0.30
130kW225..............            1         1996         0.54
                                             2         2003         0.20
225kW450..............            1         1996         0.54
                                             2         2001         0.20
450kW560..............            1         1996         0.54
                                             2         2002         0.20
kW>560...........................            1         2000         0.54
                                             2         2006         0.20
------------------------------------------------------------------------

    The Impact of EPA Engine Emission Standards on the Underground 
Metal and Nonmetal Mining Fleet. In the mining industry, engines and 
equipment are often purchased in used condition. Thus, many of the 
diesel engines in an underground mine's fleet may only meet older 
environmental emission standards, or no environmental standards at all.
    By requiring that underground coal mine engines be approved, MSHA 
regulations have led to a less polluting fleet in that sector than 
would otherwise be the case. Many highly polluting engines have been 
barred or phased out as a result. As noted in Part IV of this preamble, 
such a requirement for the underground metal and nonmetal sector is 
being added by this rulemaking; however, it will be some time before 
its effects are felt. Moreover, although the environmental tailpipe 
requirements will bring about gradual reduction in the overall 
contribution of diesel pollution to the atmosphere, the beneficial 
effects on mining atmospheres may require a long timeframe absent 
actions that accelerate the turnover of mining fleets to engines that 
emit less dpm.
    The Question of Nanoparticles. Comments received from several 
commenters on the proposed rule for diesel particulate matter exposure 
of underground coal miners raised questions relative to 
``nanoparticles'; i.e., particles found in the exhaust of diesel 
engines that are characterized by diameters less than 50 nanometers 
(nm). As the topic may be of interest to this sector as well, MSHA's 
discussion on the topic is being repeated in this preamble for 
informational purposes.
    One commenter was concerned about recent indications that 
nanoparticles may pose more of a health risk than the larger particles 
that are emitted from a diesel engine. This commenter submitted 
information demonstrating that nanoparticles emitted from the engine 
could be effectively removed from the exhaust using aftertreatment 
devices such as ceramic traps. Another commenter was concerned that 
MSHA's proposed rule for underground coal mines is based on removing 
95% of the particulate by mass. His concern was focused on the fact 
that this reduction in mass was attributed to those particles

[[Page 5736]]

greater than 0.1m but less than 1m and did not 
address the recent scientific hypothesis that it may be the very small 
nanopaticles that are responsible for adverse health effects. Based on 
the recent specific information on the potential health effects 
resulting from exposure to nanoparticles, this commenter did not 
believe that the risk to cancer would be reduced if exposure levels to 
nanoparticles increased. He indicated that studies suggest that the 
increase in nanoparticles will exceed 6 times their current levels.
    Current environmental emission standards established by EPA and 
CARB, and the particulate index calculated by MSHA, focus on the total 
mass of diesel particulate matter emitted by an engine--for example, 
the number of grams per some unit of measure (i.e., grams/brake-
horsepower). Thus, the technology being developed by the engine 
industry to meet the standards accordingly focuses on reducing the mass 
of dpm being emitted from the engine.
    There is some evidence, however, that some aspects of this new 
technology, particularly fuel injection, is resulting in an increase in 
the number of nanoparticles being emitted from the engine.

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[[Page 5738]]

    The formation of particulates starts with particle nucleation 
followed by subsequent agglomeration of the nuclei particles into an 
accumulation mode. Thus, as illustrated in Figure II-3, the majority of 
the mass of dpm is found in the accumulation mode, where the particles 
are generally between 0.1 and 1 micron in diameter. However, when 
considering the number of particles emitted from the engine, more than 
half and sometimes almost all of the particles (by number) are in the 
nuclei mode.
    Various studies have demonstrated that the size of the particles 
emitted from the new low emission diesel engines, has shifted toward 
the generation of nuclei mode particles. One study compared a 
comparable 1991 engine to its 1988 counterpart. The total PM mass in 
the newer engine was reduced by about 80%; but the new engine generated 
thousands of times more particles than the older engine (3000 times as 
much at 75 percent load and about 14,000 times as much at 25 percent 
load). One hypothesis offered for this phenomenon is that the cleaner 
engines produce less soot particles on which particulates can condense 
and accumulate, and hence they remain in nuclei mode. The accumulation 
particles act as a ``sponge'' for the condensation and/or adsorption of 
volatile materials. In the absence of that sponge, gas species which 
are to become liquid or solid will nucleate to form large numbers of 
small particles (diesel.net technology guide). Mayer, while pointing 
out that nanoparticle production was a problem with older engines as 
well, concurs that the technology being used to clean up pollution in 
newer engines is not having any positive impact on nanoparticle 
production. While there is scientific evidence that the newer engines, 
designed to reduce the mass of pollutants emitted from the diesel 
engine, emit more particles in the nuclei mode, quantifying the 
magnitude of these particles has been difficult because as dpm is 
released into the atmosphere the diesel particulate undergoes very 
complex changes. In addition, current testing procedures can produce 
spurious increases in the number of nanoparticles that would not 
necessarily occur under more realistic atmospheric conditions.
    Experimental work conducted at WVU (Bukarski) indicate that 
nanoparticles are not generated during the combustion process, but 
rather during various physical and chemical processes which the exhaust 
undergoes in after treatment systems.
    While current medical research findings indicate that small 
particulates, particularly those below 2m in size, may be more 
harmful to humans than the larger ones, much more medical research and 
diesel emission studies are needed to fully characterize diesel 
nanoparticles emissions and their impact on human health. If 
nanoparticles are found to have an adverse health impact by virtue of 
size and number, it could require significant adjustments in 
environmental engine emission regulation and technology. It could also 
have implications for the type of controls utilized, with some 
asserting that aftertreatment filters are the only effective way to 
limit the emission of nanoparticles and others asserting that 
aftertreatment filters may under certain circumstances limit the number 
of nanoparticles.
    Research on nanoparticles and their health effects is currently a 
topic of investigation. (Bagley et al., 1996, EPA Grant). Based on the 
comments received and a review of the literature currently available on 
the nanoparticle issue, MSHA believes that, at this time, promulgation 
of the final rules for underground coal and metal and nonmetal mines is 
necessary to protect miners. The nanoparticle issues discussed above 
will not be resolved for some time because of the extensive research 
required to address the questions raised.

(6) Methods for controlling dpm concentrations in underground metal and 
nonmetal mines

    As discussed in the last section, the introduction of new engines 
underground will certainly play a significant role in reducing the 
concentration of dpm in underground metal/nonmetal mines. There are, 
however, many other approaches to reducing dpm concentrations and 
occupational exposures to dpm in underground metal/nonmetal mines. 
Among these are: aftertreatment devices to eliminate particulates 
emitted by an engine; altering fuel composition to minimize engine 
particulate emission; maintenance practices and diagnostic systems to 
ensure that fuel, engine and aftertreatment technologies work as 
intended to minimize emissions; enhancing ventilation to reduce 
particulate concentrations in a work area; enclosing workers in cabs or 
other filtered areas to protect them from exposure; and work and fleet 
practices that reduce miner exposures to emissions.
    As noted in section 9 of this Part, information about these 
approaches was solicited from the mining community in a series of 
workshops in 1995, and highlights were published by MSHA as an appendix 
to the proposed rule on dpm ``Practical Ways to Control Exposure to 
Diesel Exhaust in Mining--a Toolbox.'' During the hearings and in 
written comments on this rulemaking, mention was made of all these 
control methods.
    This section provides updated information on two methods for 
controlling dpm emissions: aftertreatment devices and diesel fuel 
content. There was considerable comment on aftertreatment devices 
because MSHA's proposed rule would require high-efficiency particulate 
filters be installed on a certain percentage of the fleet in order to 
meet both the interim and final dpm concentration; and the current and 
potential efficiency of such devices remains an important issue in 
determining the technological and economic feasibility of the final 
rule. Moreover, some commenters strongly favored the use of oxidation 
catalytic converters, a type of aftertreatment device used to reduce 
gaseous emission but which can also impact dpm levels. Accordingly, 
information about such devices is reviewed here. With respect to diesel 
fuel composition, a recent rulemaking initiative by EPA, and actions 
taken by other countries in this regard, are discussed here because of 
the implications of such developments for the mining community.
    Emissions aftertreatment devices. One of the most discussed 
approaches to controlling dpm emissions involves the use of devices 
placed on the end of the tailpipe to physically trap diesel particulate 
emissions and thus limit their discharge into the mine atmosphere. 
These aftertreatment devices are often referred to as ``particle 
traps'' or ``soot traps'', but the term filter is often used. The two 
primary categories of particulate traps are those composed of ceramic 
materials (and thus capable of handling uncooled exhaust), and those 
composed of paper materials (which require the exhaust to first be 
cooled). Typically, the latter are designed for conventional 
permissible equipment mainly used in coal mining which have water 
scrubbers installed which cool the exhaust. However, another 
alternative that is now utilized in coal is the ``dry system 
technology'' which cools the diesel exhaust with a heat exchanger and 
then uses a paper filter. The dry system was first developed for oil 
shale mining applications where permissibility was required. However, 
when development of the oil shale industry faltered, manufacturers 
looked to coal mining for

[[Page 5739]]

application of the dry system technology. However, dry systems could be 
used as an alternative to the wet scrubbers for the relatively small 
number of permissible machines used in the metal/nonmetal industry. In 
addition, ``oxidation catalytic converters,'' devices used to limit the 
emission of diesel gases, and ``water scrubbers'', devices used to cool 
the exhaust gases, are discussed here as well, because they also can 
have a significant effect on limiting particle emission.
    Water Scrubbers. Water scrubbers are devices added to the exhaust 
system of certain diesel equipment. Water scrubbers are essentially 
metal boxes containing water through which the diesel exhaust gas is 
passed. The exhaust gas is cooled, generally to below 170 degrees F. A 
small fraction of the unburned hydrocarbons are condensed and remain in 
the water along with a portion of the dpm. Tests conducted by the 
former Bureau of Mines and others indicate that no more than 20 to 30 
percent of the dpm is removed. This information was presented in the 
Toolbox publication. The water scrubber does not remove any of the 
carbon monoxide, the oxides of nitrogen, or any other gaseous emission 
that remains a gas at room temperature so their effectiveness as 
aftertreatment devices is questionable.
    The water scrubber does serve as an effective spark and flame 
arrester and as a means to cool the exhaust gas when permissibility is 
required. Consequently, it is used in the majority of the permissible 
diesel equipment in mining as part of the safety components needed to 
gain MSHA approval.
    The water scrubber has several operating characteristics which keep 
it from being a candidate for use as an aftertreatment device on 
nonpermissible equipment. The space required on the vehicle to store 
sufficient water for an 8 hour shift is not available on some 
equipment. Furthermore, the exhaust contains a great deal of water 
vapor which condenses under some mining conditions creating a fog which 
can adversely effect visibility. Also, operation of the equipment on 
slopes can cause the water level in the scrubber to change resulting in 
water being blown out the exhaust pipe. Control devices are sometimes 
placed within the scrubber to maintain the appropriate water level. 
Because these devices are in contact with the water through which the 
exhaust gas has passed, they need frequent maintenance to insure that 
they are operating properly and have not been corroded by the acidic 
water created by the exhaust gas. The water scrubber must be flushed 
frequently to remove the acidic water and the dpm and other exhaust 
residue which forms a sludge that adversely effects the operation of 
the unit. These problems, coupled with the relatively low dpm removal 
efficiency, have prevented widespread use of water scrubbers as a dpm 
control device on nonpermissible equipment.
    Oxidation Catalytic Converters. Oxidation catalytic converters 
(OCCs) were among the first devices added to diesel engines in mines to 
reduce the concentration of harmful gaseous emissions discharged into 
the mine environment. OCCs began to be used in underground mines in the 
1960's to control carbon monoxide, hydrocarbons and odor. That use has 
been widespread. It has been estimated that more than 10,000 OCCs have 
been put into the mining industry over the years.
    Several of the harmful emissions in diesel exhaust are produced as 
a result of incomplete combustion of the diesel fuel in the combustion 
chamber of the engine. These include carbon monoxide and unburned 
hydrocarbons including harmful aldehydes. Catalytic converters, when 
operating properly, remove significant percentages of the carbon 
monoxide and unburned hydrocarbons. Higher operating temperatures, 
achieved by hotter exhaust gas, improve the conversion efficiency.
    Oxidation catalytic converters operate by, in effect, continuing 
the combustion process outside the combustion chamber. This is 
accomplished by utilizing the oxygen in the exhaust gas to oxidize the 
contaminants. A very small amount of material with catalytic 
properties, usually platinum or some combination of the noble metals, 
is deposited on the surfaces of the catalytic converter over which the 
exhaust gas passes. This catalyst allows the chemical oxidation 
reaction to occur at a lower temperature than would normally be 
required.
    For the catalytic converter to work effectively, the exhaust gas 
temperature must be above 370 degrees Fahrenheit for carbon monoxide 
and 500 degrees Fahrenheit for hydrocarbons. Most converters are 
installed as close to the exhaust manifold as possible to minimize the 
heat loss from the exhaust gas through the walls of the exhaust pipe. 
Insulating the segment of the exhaust pipe between the exhaust manifold 
and the catalytic converter extends the portion of the vehicle duty 
cycle in which the converter works effectively.
    The earliest catalytic converters for mining use consisted of 
alumina pellets coated with the catalytic material and enclosed in a 
container. The exhaust gas flowed through the pellet bed and the 
exhaust gas came into contact with the catalyst. Designs have evolved, 
and the most common design is a metallic substrate, formed to resemble 
a honeycomb, housed in a metal shell. The catalyst is deposited on the 
surfaces of the honeycomb. The exhaust gas flows through the honeycomb 
and comes into contact with the catalyst.
    Soon after catalytic converters were introduced, it became apparent 
that there was a problem brought about by the sulfur found in diesel 
fuels in use at that time. Most diesel fuels in the United States 
contained anywhere from 0.25 to 0.50 percent sulfur or more on a mass 
basis. In the combustion chamber, this sulfur was converted to 
SO2, SO3, or SO4 in various 
concentrations, depending on the engine operating conditions. In 
general, most of the sulfur was converted to gaseous SO2. 
When exhaust containing the gaseous sulfur dioxide passed through the 
catalytic converter, a large proportion of the SO2 was 
converted to solid sulphates which are in fact, diesel particulate. 
Sulfates can ``poison'' the catalyst, severely reducing its life.
    Recently, as described elsewhere in this preamble, the EPA required 
that diesel fuel used for over the road trucks contain no more than 500 
ppm sulfur. This action made low sulfur fuel available throughout the 
United States. MSHA, in its recently promulgated regulations for the 
use of diesel powered equipment in underground coal mines requires that 
this low sulfur fuel be used. MSHA is now extending this requirement 
for low sulfur fuel (500ppm) to underground metal/nonmetal mines in 
this final rule. When the low sulfur fuel is burned in an engine and 
passed through a converter with a moderately active catalyst, only 
small amounts of SO2 and additional sulfate based 
particulate are created. However, when a very active catalyst is used, 
to lower the operating temperature of the converter or to enhance the 
CO removal efficiency, even the low sulfur fuel has sufficient sulfur 
present to create an SO2 and sulfate based particulate 
problem. Consequently, as discussed later in this section, the EPA has 
notified the public of its intentions to promulgate regulations that 
would limit the sulfur content of future diesel fuel to 15 ppm for on-
highway use in 2006.
    The particulate reduction capabilities of some OCCs are significant 
in gravimetric terms. In 1995, the EPA implemented standards requiring 
older buses in urban areas to reduce the dpm emissions from rebuilt bus 
engines. (40

[[Page 5740]]

CFR 85.1403). Aftertreatment manufacturers developed catalytic 
converter systems capable of reducing dpm by 25%. Such systems are 
available for larger diesel engines common in the underground metal and 
nonmetal sector. However, as has been pointed out by Mayer, the portion 
of particulate mass that seems to be impacted by OCCs is the soluble 
component, and this is a smaller percentage of particulate mass in 
utility vehicle engines than in automotive engines. Moreover, some 
measurements indicate that more than 40% of NO is converted to more 
toxic NO2, and that particulate mass actually increases 
using an OCC at full load due to the formation of sulfates. In 
summation, Mayer concluded that the OCCs do not reduce the combustion 
particulates, produce sulfate particulates, have unfavorable gaseous 
phase reactions increasing toxicity, and that the positive effects are 
irrelevant for construction site diesel engines. Indeed, he indicates 
the negative effects outweigh the benefits. (Mayer, 1998. The Phase 1 
interim data report of the Diesel Emission Control-Sulfur Effects 
(DECSE) Program (a joint government-industry program to explore lower 
sulfur content that is discussed in more detail later in this section) 
similarly indicates that using OCCs under certain operating conditions 
can increase dpm emissions due to an increase in the sulfate fraction 
(DECSE Program Summary, Dec. 1999). Another commenter also notes that 
oxidation catalytic activity can increase sulfates and submicron 
particles under certain operating conditions.
    Other commenters during the rulemaking strongly supported the use 
of OCCs as an interim measure to reduce particulate and other diesel 
emission to address transitory employee effects that were mentioned in 
the proposed preamble. MSHA views the use of OCCs as one tool that mine 
operators can use to reduce the dpm emissions from certain vehicles 
alone or in combination of other aftertreatment controls to meet the 
interim and final dpm standards. The overall reduction in dpm emissions 
achieved with the exclusive use of an OCC is low compared to the 
reductions required to meet the standards. MSHA is aware of the 
negative effects produced by OCCs. However, with the use of low sulfur 
fuel and a catalyst that is formulated for low sulfate production, this 
problem can be resolved. Mine operators must work with aftertreatment 
manufacturers to come up with the best plan for their fleet for dpm 
control.
    Hot gas filters. Throughout this preamble, MSHA is referring to the 
particulate traps (filters) that can be used in the undiluted hot 
exhaust stream from the diesel engine as hot gas filters. Hot gas 
filters refer to the current commercially available particulate 
filters, such as ceramic cell, woven fiber filters, sintered metal 
filters, etc.
    Following publication of EPA rules in 1985 limiting diesel 
particulate emissions from heavy duty diesel engines, aftertreatment 
devices capable of significant reductions in particulate levels began 
to be developed for commercial applications.
    The wall flow type ceramic honeycomb diesel particulate filter 
system was initially the most promising approach. These consisted of a 
ceramic substrate encased in a shock and vibration absorbing material 
and covered with a protective metal shell. The ceramic substrate is 
arranged in the shape of a honeycomb with the openings parallel to the 
centerline. The ends of the openings of the honeycomb cells are plugged 
alternately. When the exhaust gas flows through the particulate trap, 
it is forced by the plugged end to flow through the ceramic wall to the 
adjacent passage and then out into the mine atmosphere. The ceramic 
material is engineered with pores in the ceramic material sufficiently 
large to allow the gas to pass through without adding excessive back 
pressure on the engine, but small enough to trap the particulate on the 
wall of the ceramic material. Consequently, these units are called wall 
flow traps.
    Work with ceramic filters in the last few years has led to the 
development of the ceramic fiber wound filter cartridge (SAE, SP-1073, 
1995). The ceramic fiber has been reported by the manufacturer to have 
dpm reduction efficiencies up to 80 percent. This system has been used 
on vehicles to comply with German requirements that all diesel engines 
used in confined areas be filtered. Other manufacturers have made the 
wall flow type ceramic honeycomb dpm filter system commercially 
available to meet the German standard.
    The development of these devices has proceeded in response to 
international and national efforts to regulate dpm emissions. However, 
due to the extensive work performed by the engine manufacturers on new 
technological designs of the diesel engine's combustion system, and the 
use of low sulfur fuel, particulate traps turned out to be unnecessary 
to comply with the EPA standards of the time for vehicle engines.
    These devices proved to be very effective at removing particulate 
achieving particulate removal efficiencies of greater than 90 percent.
    It was quickly recognized that this technology, while not 
immediately required for most vehicles, might be particularly useful in 
mining applications. The former Bureau of Mines investigated the use of 
catalyzed diesel particulate filters in underground mines in the United 
States (BOM, RI-9478, 1993). The investigation demonstrated that 
filters could work, but that there were problems associated with their 
use on individual unit installations, and the Bureau made 
recommendations for installation of ceramic filters on mining vehicles.
    Canadian mines also began to experiment with ceramic traps in the 
1980's with similar results (BOM, IC 9324, 1992). Work in Canada today 
continues under the auspices of the Diesel Emission Evaluation Program 
(DEEP), established by the Canadian Centre for Mineral and Energy 
Technology in 1996 (DEEP Plenary Proceedings, November 1996). The goals 
of DEEP are to: (1) Evaluate aerosol sampling and analytical methods 
for dpm; and (2) evaluate the in-mine performance and costs of various 
diesel exhaust control strategies.
    Perhaps because experience is still limited, the general perception 
within the mining industry of the state of this technology in recent 
years is that it remains limited in certain respects; as expressed by 
one commenter at one of the MSHA workshops in 1995, ``while ceramic 
filters give good results early in their life cycle, they have a 
relatively short life, are very expensive and unreliable.''
    One commenter reported unsuccessful experiments with ceramic 
filters in 1991 due to their inability to regenerate at low 
temperatures, lack of reliability, high cost of purchase and 
installation, and short life.
    In response to the proposed rule, MSHA received a variety of 
information and claims about the current efficiency of such 
technologies. Commenters stated that in terms of technical feasibility 
to meet the standards, the appropriate aftertreament controls are not 
readily available on the market for the types and sizes of equipment 
used in underground mines. Another commenter stated that MSHA has not 
identified a technology capable of meeting the proposed standards at 
their mine and they were not aware of any technology currently 
available or on the horizon that would be capable of attaining the 
standards. Yet another commenter stated that both ceramic and paper 
filters are not technically feasible at their mine because of the high 
operating temperatures needed to regenerate filters or the difficulties

[[Page 5741]]

presented by periodic removal of the filters for regeneration. Periodic 
removal of fragile ceramic filters subjects them to chipping and 
cracking and requires a large inventory of surplus filters. Commenter 
also stated that paper filters require exhaust gas cooling so that the 
paper filter does not burn. Commenter stated that they have been 
working with a manufacturer on installing one of these on a piece of 
equipment, but it is experimental and this installation was the first 
time a paper filter would be used on equipment of this size and type.
    In response to the paper filter comment, dry system technology as 
described above was first tested on a large haul truck used in oil 
shale mining and then later applied to coal mining equipment. Paper 
filter systems have also been successfully installed on coal mining 
equipment that is identical to LHD machines used in metal/nonmetal 
mines. Therefore this technology has been applied to engine of the type 
and size used in metal/nonmetal mines. Commenters have stated that 
filters are not feasible at this time from the above comments. However, 
MSHA believes that the technology needed to reduce dpm emissions to 
both the interim and final standards is feasible. Much work has 
occurred in the development of aftertreatment controls, especially OCCs 
and hot gas filters. Aftertreatment control manufacturers have been 
improving both OCCs and ceramic type filters to provide better 
performance and reliability. New materials are currently available 
commercially and new filter systems are being developed especially in 
light of the recent requirements in Europe and the new proposals from 
the EPA. Consequently, MSHA does not agree with the commenter 
concerning chipping of the traps when removed. As stated, manufacturers 
have designed systems to either be removed easily or even regenerated 
on the vehicle by simply plugging the unit in without removing the 
filter.
    Two groups in particular have been doing some research comparing 
the efficiency of recent ceramic models: West Virginia University, as 
part of that State's efforts to develop rules on the use of diesel-
powered equipment underground; and VERT (Verminderung der Emissionen 
von Realmaschinen in Tunnelbau), a consortium of several European 
agencies conducting such research in connection with major planned 
tunneling projects in Austria, Switzerland and Germany to protect 
occupational health and subsequent legislation in each of the three 
countries restricting diesel emissions in tunneling.
    The State of West Virginia legislature enacted the West Virginia 
Diesel Act, thereby creating the West Virginia Diesel Commission and 
setting forth an administrative vehicle to allow and regulate the use 
of diesel equipment in underground coal mines in West Virginia. West 
Virginia University was appropriated funds to test diesel exhaust 
controls, as well as an array of diesel particulate filters. The 
University was asked to provide technical support and data necessary 
for the Commission to make decisions on standards for emission 
controls. Even though the studies were intended for the Commission's 
work for underground coal, the control technologies tested are relevant 
to metal/nonmetal mines.
    The University reported data on four different engines and an 
assortment of configurations of available control devices, both hot gas 
filters and the DST system, a system which first cools the 
exhaust and then runs it through a paper filter. The range of 
collection efficiencies reported for the ceramic filters and oxidation 
catalysts combined fell between 65% and 78%. The highest collection 
efficiency obtained using the ISO 8 mode test cycle (test cycle 
described in rule) was 81% on the DST system (intended for 
coal use). The University did report problems with this system that 
would account for the lower than expected efficiency for a paper filter 
type system.
    VERT's studies of particulate traps are detailed in two articles 
published in 1999 which have been widely disseminated to the diesel 
community here through www.DieselNet.com. The March article focuses on 
the efficiency of the traps; the April article compares the efficiency 
of other approaches (OCCs, fuel reformulation, engine modifications to 
reduce ultra-fine particulates) with that of the traps. Here we focus 
only on the information about particulate traps.
    The authors of the March article report that 29 particulate trap 
systems were tested using various ceramic, metal and fiber filter media 
and several regeneration systems. The authors of the March article 
summarize their conclusions as follows:

    The results of the 4-year investigations of construction site 
engines on test rigs and in the field are clear: particulate trap 
technology is the only acceptable choice among all available 
measures. Traps proved to be an extremely efficient method to 
curtail the finest particles. Several systems demonstrated a 
filtration rate of more than 99% for ultra-fine particulates. 
Specific development may further improve the filtration rate.
    A two-year field test, with subsequent trap inspection, 
confirmed the results pertaining to filtration characteristics of 
ultra-fine particles. No curtailment of the ultra-fine particles is 
obtained with any of the following: reformulated fuel, new 
lubricants, oxidation catalytic converters, and optimization of the 
engine combustion.
    Particulate traps represent the best available technology (BAT). 
Traps must therefore be employed to curtail the particulate 
emissions that the law demands are minimized. This technology was 
implemented in occupational health programs in Germany, Switzerland 
and Austria.

    On the bench tests, it appears that the traps reduce the overall 
particulate matter by between 70 and 80%, with better results for solid 
ultrafine particulates; under hot gas conditions, it appears the non-
solid components of particulate matter cannot be dependably retained by 
these traps. Consistent with this finding, it was found that polycyclic 
aromatic hydrocarbons (PAHs) decreased proportionately to the 
gravimetric decrease of carbon mass. The tests also explored the impact 
of additives on trap efficiency, and the impact of back pressure.
    The field tests confirmed that the traps were easy to mount and 
retained their reliability over time, although regeneration was 
required when low exhaust temperatures failed to do this automatically. 
Electronic monitoring of back pressure was recommended. In general, the 
tests confirmed that a whole series of trap systems have a high 
filtration rate and stable long time properties and are capable of 
performing under difficult construction site conditions. Again, the 
field tests indicated a very high reduction (97-99%) of particulates by 
count, but a lower rate of reduction in terms of mass.
    Subsequently, VERT has evaluated additional commercially available 
filter systems. The filtration efficiency, expressed on a gravimetric 
basis is shown in the column headed ``PMAG--without additive''. The 
filtration efficiencies determined by VERT for these 6 filter systems 
range from 80.7% to 94.5%. The average efficiency of these filters is 
87%. MSHA will be updating the list of VERT's evaluated systems as they 
become available.
    VERT has also published information on the extent of dpm filter 
usage in Europe as evidence that the filter technology has attained 
wide spread acceptance. This information is included in the record of 
the coal dpm rulemaking where it has particular significance; it is 
noted here for informational purposes. The information isn't critical 
in this case because operators have a choice of controls. MSHA didn't 
explicitly add the latest VERT data to the Metal/

[[Page 5742]]

Nonmetal record during the latest reopening of the record. MSHA 
believes this information is relevant to metal/nonmetal mining because 
the tunneling equipment on which these filters are installed is similar 
to metal/nonmetal equipment. VERT stated that over 4,500 filter systems 
have been deployed in England, Scandinavia, and Germany. Deutz 
Corporation has deployed 400 systems (Deutz's design) with full flow 
burners for regeneration of filters installed on engines between 50-
600kw. The company Oberland-Mangold has approximately 1,000 systems in 
the field which have accumulated an average of 8,400 operating hours in 
forklift trucks, 10,600 operating hours in construction site engines, 
and 19,200 operating hours in stationary equipment. The company Unikat 
has introduced in Switzerland over 250 traps since 1989 and 3,000 
worldwide with some operating more than 20,000 hours. German industry 
annually installs approximately 1,500 traps in forklifts.

BILLING CODE 4510-43-P

[[Page 5743]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.057


BILLING CODE 4510-43-C

[[Page 5744]]

    Some commenters asserted that the VERT work was for relatively 
small engines and not for large engines, i.e., 600-700 hp, and hence 
could not be relied upon to demonstrate the availability of filters of 
such high efficiencies for the larger equipment used in some 
underground mines. MSHA believes this comment is misplaced. The 
efficiency of a filter is attributable to the design of the filter and 
not the size of the engine. VERT is documenting filter efficiencies of 
commercially available filters. It is customary in the industry, 
however, for the filter manufacturer to size the filter to fit the size 
of the engine. The mine operator must work with the filter manufacturer 
to verify that the filter needed will work for the intended machine. 
MSHA believes that this is no different for other types of options 
installed on machines for underground mining use.
    More information about the results of the VERT tests on specific 
filters, and how MSHA intends to use this information to aid the mining 
industry to comply with the requirements of the standards are discussed 
in Part IV of this preamble.
    The accumulated dpm must be removed from all particulate traps 
periodically. This is usually done by burning off the accumulated 
particulate in a controlled manner, called regeneration. If the diesel 
equipment on which the trap is installed has a duty cycle which creates 
an exhaust gas temperature greater than about 650 degrees Fahrenheit 
for more than 25 percent of the operating time, the unit will be self 
cleaning. That is, the hot exhaust gas will burn off the particulate as 
it accumulates. Unfortunately, only hard working equipment, such as 
load-haul-dump and haulage equipment usually satisfies the exhaust gas 
temperature and duration requirements.
    Techniques are available to lower the temperature required to 
initiate the regeneration. One technique under development is to use a 
fuel additive. A comparatively small amount of a chemical is added to 
the diesel fuel and burns along with the fuel in the combustion 
chamber. The additive is reported to lower the required regeneration 
temperature significantly. The additive combustion products are 
retained as a residue in the particulate trap. The trap must be removed 
from the equipment periodically to flush the residue. Another technique 
used to lower the regeneration temperature is to apply a catalyst to 
the surfaces of the trap material. The action of the catalyst has a 
similar effect as the fuel additive. The catalyst also lowers the 
concentration of some gaseous emissions in the same manner as the 
oxidation catalytic converter described earlier.
    A very active catalyst applied to the particulate trap surfaces and 
a very active catalyst in a catalytic converter installed upstream of 
the trap can create a situation in which the trap performs less 
efficiently than expected. Burning low sulfur diesel fuel, containing 
less than 500 ppm sulfur, will result in the creation of significant 
quantities of sulfates in the exhaust gas. These sulfates will still be 
in the gaseous state when they reach the ceramic trap and will pass 
through the trap. These sulfates will condense later forming diesel 
particulate. Special care must be taken in the selection of the 
catalyst formulation to ensure that sulfate formation is avoided. This 
problem is not present on systems which are designed with a catalytic 
converter upstream of a water scrubber. The gaseous phase sulfates will 
condense when contacting the water in the scrubber and will not be 
discharged into the mine atmosphere. Thus far, no permissible diesel 
packages have been approved which incorporate a catalytic converter 
upstream of the water scrubber.
    One research project conducted by the former Bureau of Mines which 
attempted this arrangement was unsuccessful. The means selected to 
maintain a surface temperature less than the 300 degrees Fahrenheit 
required for permissibility purposes caused the exhaust gas to be 
cooled to the point that the catalytic converter did not reach the 
necessary operating temperature. It would appear that a means to 
isolate the catalytic converter from the exhaust gas water jacket is 
necessary for the arrangement to function as intended.
    If the machine on which the particulate trap is installed does not 
work hard enough to regenerate the trap with the hot exhaust gas and 
the option to use a fuel additive or catalyzed trap is not appropriate, 
the trap can still be regenerated while installed on the machine. 
Systems are available whereby air is heated by an externally applied 
heat source and caused to flow through the particle trap with the 
engine stopped. The heat can be supplied by an electrical resistance 
element installed in front of the trap. The heat can also be supplied 
by a burner installed into the exhaust pipe in front of the trap fueled 
by an auxiliary fuel line. The fuel is ignited creating large 
quantities of hot gas. With both systems, an air line is also connected 
to the exhaust pipe to create a flow of hot gases through the 
particulate trap. Both systems utilize operator panels to control the 
regeneration process.
    Some equipment owners may choose to remove the particle trap from 
the machine to perform the regeneration. Particle traps are available 
with quick release devices that allow maintenance personnel to readily 
remove the unit from the machine. The trap is then placed on a 
specially designed device that creates a controlled flow of heated air 
that is passed through the filter burning off the accumulated 
particulate.
    The selection of the most appropriate means to regenerate the trap 
is dependent on the equipment type, the equipment duty cycle, and the 
equipment utilization practices at the mine.
    A program under the Canadian DEEP project is field testing dpm 
filter systems in a New Brunswick Mine. The project is testing four 
filter systems on trucks and scoops. The initial feedback from Canada 
is very favorable concerning the performance of filters. Operators are 
very positive and are requesting the vehicles equipped with the filters 
because of the noticeable improvement in air quality and an absence of 
smoke even under transient load conditions. One system being tested 
utilizes an electrical heating element installed in the filter system 
to provide the heated air for regeneration of the filter. This heating 
element requires that the filter be connected to an external electrical 
source at the end of the shift. Initial results have been successful.
    Paper filters. In 1990, the former Bureau of Mines conducted a 
project to develop a means to reduce the amount of dpm emitted from 
permissible diesel powered equipment using technologies that were 
available commercially and that could be applied to existing equipment. 
The project was conducted with the cooperation of an equipment 
manufacturer, a mine operator, and MSHA. In light of the fact that all 
permissible diesel powered equipment in coal and metal/nonmetal, at 
that time, utilized water scrubbers to meet the MSHA approval 
requirements, the physical characteristics of the exhaust from that 
type of equipment were the basis for the selection of candidate 
technologies. The technology selected for development was the pleated 
media filter or paper filter as it came to be called. The filter 
selected was an intake air cleaner normally used for over the road 
trucks. That filter was acceptable for use with permissible diesel 
equipment because the temperature of the exhaust gas from the water 
scrubber was less than 170 degrees F which was

[[Page 5745]]

well below the ignition point of the filter material.
    Recognizing that under some operating modes water would be 
discharged along with the exhaust, a water trap was installed in the 
exhaust stream before it passed through the filter. After MSHA 
conducted a thorough permissibility evaluation of the modified system, 
this filter was installed on a permissible diesel coal haulage vehicle 
and a series of in mine trials conducted. It was determined, by in mine 
ambient gravimetric sampling, that the particulate filter reduced dpm 
emissions by 95 percent compared to that same machine without the 
filter. The testing determined that the filters would last between one 
and two shifts, depending on how hard the equipment worked. (BOM, IC 
9324).
    Following the successful completion of the former Bureau of Mines 
mine trial, several equipment manufacturers applied for and received 
MSHA approval to offer the paper filter kits as options on a number of 
permissible diesel machines. These filter kits were installed on other 
machines at the mine where the original tests were conducted, and 
later, on machines at other mines. MSHA is not aware of any paper 
filters installed on permissible equipment in m/nm to date.
    Despite the initial reports on the high efficiency of paper 
filters, during the coal public hearings and in the coal comments on 
this rulemaking a number of commenters at the coal public hearings 
questioned whether in practice paper filters could achieve efficiencies 
on the order of 95% when used on existing permissible equipment. In 
order to determine whether it could verify those concerns, MSHA 
contracted with the Southwest Research Institute to verify the ability 
of such a filter to reduce the dpm generated by a typical engine used 
in permissible equipment. The results of this verification effort 
confirmed that paper filters has a dpm removal efficiency greater than 
95%. The information about MSHA's verification effort with respect to 
paper filters is discussed in detail in connection with the companion 
rule for the coal sector, where it has particular significance.
    Dry systems technology. As mentioned earlier, the most recently 
developed means of achieving permissibility with diesel powered 
equipment in the United States is the dry exhaust conditioning system 
or dry system. This system combines several of the concepts described 
above as well as new, innovative approaches. The system also solves 
some of the problems encountered with older technologies.
    The dry system in its most basic form consists of a heat exchanger 
to cool the exhaust gas, a mechanical flame arrestor to prevent the 
discharge of any flame from within the engine into the mine atmosphere, 
and a spark arrestor to prevent sparks for being discharged. The 
surfaces of all of these components and the piping connecting them are 
maintained below the 300 degrees F required by MSHA approval 
requirements. A filter, of the type normally used as an intake air 
filter element, is installed in the exhaust system as the spark 
arrestor. In terms of this dpm regulation, the most significant feature 
of the system is the use of this air filter element as a particulate 
filter. The filter media has an allowable operating temperature rating 
greater than the 300 degree F exhaust gas temperature allowed by MSHA 
approval regulations. These filters are reported to last up to sixteen 
hours, depending on how hard the machine operates.
    The dry system can operate on any grade without the problems 
encountered by water scrubbers. Furthermore, there is no problem with 
fog created by operation of the water scrubber. Dry systems have been 
installed and are operating successfully in coal mines on diesel 
haulage equipment, longwall component carriers, longwall component 
extraction equipment, and in nonpermissible form, on locomotives.
    Although the systems were originally designed for permissible 
equipment applications, they can also be used directly on 
nonpermissible equipment (whose emissions are not already cooled), or 
to replace water scrubbers used to cool most permissible equipment with 
a system that includes additional aftertreatment.
    Reformulated fuels. It has long been known that sulfur content can 
have a significant effect on dpm emissions. In its diesel equipment 
rule for underground coal mines, MSHA requires that any fuel used in 
underground coal mines have less than 0.05% (500 ppm) sulfur. EPA 
regulations requiring that such low-sulfur fuel (less than 500 ppm) be 
used in highway engines, in order to limit air pollution, have in 
practice ensured that this type of diesel fuel is available to mine 
operators, and they currently use this type of fuel for all engines.
    EPA has proposed a rule which would require further reductions in 
the sulfur content of highway diesel fuel. Such an action was taken for 
gasoline fuel on December 21, 1999.
    On May 13, 1999 (64 FR 26142) EPA published an Advance Notice of 
Proposed Rulemaking (ANPRM) relative to changes for diesel fuel. In 
explaining why it was initiating this action, EPA noted that diesel 
engines ``contribute greatly'' to a number of serious air pollution 
problems, and that diesel emissions account for a large portion of the 
country's particulate matter and nitrogen oxides a key precursor to 
ozone. EPA noted that while these emissions come mostly from heavy-duty 
truck and nonroad engines, they expected the contribution to dpm 
emissions of light-duty equipment to grow due to manufacturers' plans 
to greatly increase the sale of light duty trucks. These vehicles are 
now subject to Tier 2 emission standards whether powered by gasoline or 
diesel fuel, and such standards may be difficult to meet without 
advanced catalyst technologies that in turn would seem to require 
sulfur reductions in the fuel.
    Moreover, planned Tier 3 standards for nonroad vehicles would 
require similar action (64 FR 26143). The EPA noted that the European 
Union has adopted new specifications for diesel fuel that would limit 
it to 50 ppm by 2005, (an interim limit of 350 ppm by this year), that 
the entire diesel fuel supply in the United Kingdom should soon be at 
50 ppm, and that Japan and other nations were working toward the same 
goal (64 FR 26148). In the ANPRM, the EPA specifically noted that while 
continuously regenerating ceramic filters have shown considerable 
promise for limiting dpm emissions even at fairly low exhaust 
temperatures, the systems are fairly intolerant of fuel sulfur. 
Accordingly, the agency hopes to gather information on whether or not 
low sulfur fuel is needed for effective PM control (64 FR 26150). EPA's 
proposed rule was published in June 2000, (65 FR 35430) and proposed a 
sulfur limit of 15 ppm for on-highway use in 2006-2009.
    A joint government-industry partnership is also investigating the 
relationship between varying levels of sulfur content and emissions 
reduction performance on various control technologies, including 
particulate filters and oxidation catalytic convertors. This program is 
supported by the Department of Energy's Office of Heavy Vehicles 
Technologies, two national laboratories, the Engine Manufacturers 
Association, and the Manufacturers of Emission Controls Association. It 
is known as the Diesel Emission Control-Sulfur Effects (DECSE) Program; 
more information is available from its web site, http://www.ott.doe.gov/decse.
    MSHA expects that once such cleaner fuel is required for 
transportation use, it will in practice become the fuel used in mining 
as well--directly reducing

[[Page 5746]]

engine particulate emissions, increasing the efficiency of 
aftertreatment devices, and eventually through the introduction of new 
generation of cleaner equipment. Mayer states that reducing sulfur 
content, decreasing aromatic components and increasing the Cetane index 
of diesel fuel can generally result in a 5% to 15% reduction in total 
particulate emissions.
    Meyer reports the test by VERT of a special synthetic fuel 
containing neither sulfur nor bound nitrogen nor aromatics, with a very 
high Cetane index. The fuel performed very well, but produced only abut 
10% fewer particulates than low sulfur diesel fuel, nor did it have the 
slightest improvement in diminishing nonparticulate emissions.
    NIOSH provided information on the work that has been done with 
Biodiesel fuel. Biodiesel fuel is a registered fuel and fuel additive 
with the EPA and meets clean diesel standards established by the 
California Air Resources Board. NIOSH stated that the undisputed 
consensus among the research conducted is that the use of biodiesel 
will significantly reduce dpm and other harmful emissions in 
underground mines. MSHA agrees that biodiesel fuel is an option that 
mine operators can use from the toolbox to meet the dpm standards.
    Cabs. A cab is an enclosure around the operator installed on a 
piece of mobile equipment. It can provide the same type of protection 
as a booth at a crusher station. While cabs are not available for all 
mining equipment, they are available for much of the larger equipment 
that also has application in the construction industry.
    Even though cabs are not the type of control device that is bolted 
onto the exhaust of the diesel engine to reduce emissions, cabs can 
protect miners from environmental exposures to dpm. Both cabs and 
control booths are discussed in the context of reducing miners 
exposures to dpm.
    To be effective, a cab should be tightly sealed with windows and 
doors must be closed. Rubber seals around doors and windows should be 
in good conditions. Door and window latches should operate properly. In 
addition to being well sealed, the cab should have an air filtration 
and space pressurizing system. Air intake should be located away from 
engine exhaust. The airflow should provide one air change per minute 
for the cab and should pressurize the cab to 0.20 inches of water. 
While these are not absolute requirements, they do provide a guideline 
of how a cab should be designed. If a cab does not have an air 
filtration and pressurizing system, the diesel particulate 
concentration inside the cab will be similar to the diesel particulate 
concentration outside the cab.
    MSHA has evaluated the efficiency of cab filters for diesel 
particulate reduction (Commercial Stone Study, PS&HTC-DD-98-346, 
Commercial Stone Study, PS&HTC-DD-99-402 and Homestake Mine Study, 
PS&HTC-DD-00-505.) Several different types of filter media have been 
tested in underground mines. Depending on the filter media, cabs can 
reduce diesel particulate exposures by 45 to 90 percent.

(7) MSHA's Diesel Safety Rule for Underground Coal Mines and its Effect 
on dpm

    MSHA's proposed rule to limit the concentration of dpm in 
underground metal and nonmetal mines included a number of elements 
which have already proven successful in helping to reduce dpm 
concentrations in the coal sector. Accordingly, this section provides 
some background on the substance of the rules that have been in effect 
in underground coal mines (for more information on the history of 
rulemaking in the coal sector, please refer to section 9 of this Part). 
It should be noted, however, that not all of the requirements discussed 
here are going to be required for underground metal and nonmetal mines; 
see Part IV of this preamble for details on what is included in the 
final rule.
    Diesel Equipment Rule in Underground Coal Mines. On October 25, 
1996, MSHA promulgated standards for the ``Approval, Exhaust Gas 
Monitoring, and Safety Requirements for the Use of Diesel-Powered 
Equipment in Underground Coal Mines,'' sometimes referred to as the 
``diesel equipment rule'' (61 FR 55412; the history of this rulemaking 
is briefly discussed in section 9 of this Part). The diesel equipment 
rule focuses on the safe use of diesels in underground coal mines. 
Integrated requirements are established for the safe storage, handling, 
and transport of diesel fuel underground, training of mine personnel, 
minimum ventilating air quantities for diesel powered equipment, 
monitoring of gaseous diesel exhaust emissions, maintenance 
requirements, incorporation of fire suppression systems, and design 
features for nonpermissible machines.
    MSHA Approval Requirements for Engines Used in Underground Coal 
Mines. MSHA requires that all diesel engines used in underground coal 
mines be ``approved'' by MSHA for such use, and be maintained by 
operators in approved condition. Among other things, approval of an 
engine by MSHA ensures that engines exceeding certain pollutant 
standards are not used in underground coal mines. MSHA sets the 
standards for such approval, establishes the testing criteria for the 
approval process, and administers the tests. The costs to obtain 
approval of an engine are usually borne by the engine manufacturer or 
equipment manufacturer. MSHA's 1996 diesel equipment rule made some 
significant changes to the consequences of approval. The new rule 
required the whole underground coal fleet to convert to approved 
engines no later than November 1999.
    The new rule also required that during the approval process the 
agency determine the particulate index (PI) for the engine. The 
particulate index (or PI), calculated under the provisions of 30 CFR 
7.89, indicates the air quantity necessary to dilute the diesel 
particulate in the engine exhaust to 1 milligram of diesel particulate 
matter per cubic meter of air.
    The PI does not appear on the engine's approval plate. (61 FR 
55421). Furthermore, the particulate index of an engine is not, under 
the diesel equipment rule, used to determine whether or not the engine 
can be used in an underground coal mine.
    At the time the equipment rule was issued, MSHA explicitly deferred 
the question of whether to require engines used in mining environments 
to meet a particular PI. (61 FR 55420-21, 55437). While there was some 
discussion of using it in this fashion during the diesel equipment 
rulemaking, the approach taken in the final rule was to adopt, instead, 
the multi-level approach recommended by the Diesel Advisory Committee. 
This multi-level approach included the requirement to use clean fuel, 
low emission engines, equipment design, maintenance, and ventilation, 
all of which appear in the final rule. The requirement for determining 
the particulate index was included in the diesel equipment rule in 
order to provide information to the mining community in purchasing 
equipment--so that mine operators can compare the particulate levels 
generated by different engines. Mine operators and equipment 
manufacturers can use the information along with consideration of the 
type of machine the engines would power and the area of the mine in 
which it would be used to make decisions concerning the engine's 
contribution of diesel particulate to the mine's total respirable dust. 
Equipment manufactures can use the particulate index to design and 
install exhaust after-treatments. (61 FR 55421). So that the PI for any 
engine is

[[Page 5747]]

known to the mining community, MSHA reports the index in the approval 
letter, posted the PI and ventilating air requirement for all approved 
engines on its website, and publishes the index with its lists of 
approved engines.
    Gas Monitoring. As discussed in section 5, there are limitations on 
the exposure of miners to various gases emitted from diesel engines in 
both underground coal mines and underground metal and nonmetal mines.
    The 1996 diesel equipment rule for underground coal mines 
supplemented these protections in that sector by providing for the 
monitoring and control of gaseous diesel exhaust emissions. (30 CFR 
part 70; 61 FR 55413). The rule requires that underground coal mine 
operators take samples of carbon monoxide and nitrogen dioxide as part 
of existing onshift workplace examinations. Samples exceeding an action 
level of 50 percent of the threshold limits set forth in 30 CFR 75.322 
trigger corrective action by the mine operator.
    Engine Maintenance. The diesel equipment rule also requires that 
diesel-powered equipment be maintained in safe and approved condition. 
As explained in the preamble, maintenance requirements were included 
because of MSHA's recognition that inadequate equipment maintenance 
can, among other things, result in increased levels of harmful gaseous 
and particulate components from diesel exhaust.
    Among other things, the rule requires the weekly examination of 
diesel-powered equipment in underground coal mines. To determine if 
more extensive maintenance is required, the rule further requires that 
a weekly check of the gaseous CO emission levels on permissible and 
heavy duty outby machines be made. The CO check requires that the 
engine be operated at a repeatable loaded condition and the CO 
measured. The carbon monoxide concentration in the exhaust provides a 
good indication of engine condition. If the CO measurement increases to 
a higher concentration than what was normally measured during the past 
weekly checks, then a maintenance person would know that a problem has 
developed that requires further investigation. In addition, underground 
coal mine operators are required to establish programs to ensure that 
those performing maintenance on diesel equipment are qualified.
    Fuel. The diesel equipment rule also requires that underground coal 
mine operators use diesel fuel with a sulfur content of 0.05% (500 ppm) 
or less. Some types of exhaust aftertreatment technology designed to 
lower hazardous diesel emissions work more effectively when the sulfur 
content of the fuel is low. More effective aftertreatment devices will 
result in reduced hydrocarbons, carbon monoxide, and particulate 
levels. Low sulfur fuel also greatly reduces the sulfate production 
from the catalytic converters currently in use in underground coal 
mines, thereby decreasing exhaust particulate. To further reduce 
miners' exposure to diesel exhaust, the final rule prohibits operators 
from unnecessarily idling diesel-powered equipment.
    Ventilation. The diesel equipment rule requires that as part of the 
approval process, ventilating air quantities necessary to maintain the 
gaseous emissions of diesel engines within existing required ambient 
limits be set. The ventilating air quantities are required to appear on 
the engine's approval plate. The rule also requires that mine operators 
maintain the approval plate quantity minimum airflow in areas of 
underground coal mines where diesel-powered equipment is operated. The 
engine's approval plate air quantity is also used to determine the 
minimum air quantity in areas where multiple units of diesel powered 
equipment are being operated. The minimum ventilating air quantity 
where multiple units of diesel powered equipment are operated on 
working sections and in areas where mechanized mining equipment is 
being installed or removed, must be the sum of 100 percent of the 
approval plate quantities of all of the equipment. As set forth in the 
preamble of the diesel equipment rule, MSHA believes that effective 
mine ventilation is a key component in the control of miners' exposure 
to gasses and particulate emissions generated by diesel equipment.
    Impact of the diesel equipment rule on dpm levels in underground 
coal mines. The diesel equipment rule has many features which, by 
reducing the emission and concentration of harmful diesel emissions in 
underground coal mines, will indirectly reduce particulate emissions.
    In developing the diesel equipment rule, however, MSHA did not 
explicitly consider the risks to miners of a working lifetime of dpm 
exposure at very high levels, nor the actions that could be taken to 
specifically reduce dpm exposure levels in underground coal mines. It 
was understood that the agency would be taking a separate look at the 
health risks of dpm exposure. For example, the agency explicitly 
deferred discussion of whether to make operators use only equipment 
that complied with a specific Particulate Index.

(8) Information on How Certain States are Restricting Occupational 
Exposure to DPM.

    As noted earlier in this part, the Federal government has long been 
involved in efforts to restrict diesel particulate emissions into the 
environment--both through ambient air quality standards, and through 
restrictions on diesel engine emissions. While MSHA's actions to limit 
the concentration of dpm in underground mines are the first effort by 
the Federal government to deal with the special risks faced by workers 
exposed to diesel exhaust on the job, several states have already taken 
actions in this regard with respect to underground coal mines.
    This section reviews some of these actions, as they were the 
subject of considerable discussion and comment during this rulemaking.
    Pennsylvania. As indicated in section 1, Pennsylvania essentially 
had a ban on the use of diesel-powered equipment in underground coal 
mines for many years. As noted by one commenter, diesel engines were 
permitted provided the request was approved by the Secretary of the 
Department of Environmental Protection.
    In 1995, one company in the State submitted a plan for approval and 
started negotiations with its local union representatives. This led to 
statewide discussions and the adoption of a new law in the State that 
permits the use of diesel-powered equipment in deep coal mines under 
certain circumstances specified in the law (Act 182). As further noted 
by this commenter, the drafters of the law completed their work before 
the issuance of MSHA's new regulation on the safe use of diesel-powered 
equipment in underground coal mines. The Pennsylvania law, unlike 
MSHA's diesel equipment rule, specifically addresses diesel 
particulate. The State did not set a limit on the exposure of miners to 
dpm, nor did it establish a limit on the concentration of dpm in deep 
coal mines. Rather, it approached the issue by imposing controls that 
will limit dpm emissions at the source.
    First, all diesel engines used in underground deep coal mines in 
Pennsylvania must be MSHA-approved engines with an ``exhaust emissions 
control and conditioning system'' that meets certain tests. (Article 
II-A, Section 203-A, Exhaust Emission Controls). Among these are dpm 
emissions from each engine no greater than ``an average concentration 
of 0.12 mg/m\3\ diluted by fifty percent of the MSHA approval plate 
ventilation for that diesel engine.'' In addition, any exhaust 
emissions

[[Page 5748]]

control and conditioning system must include a ``Diesel Particulate 
Matter (DPM) filter capable of an average of ninety-five percent or 
greater reduction of dpm emissions.'' It also requires the use of an 
oxidation catalytic converter. Thus, the Pennsylvania statute requires 
the use of low-emitting engines, and then the use of aftertreatment 
devices that significantly reduce the particulates emitted from these 
engines.
    The Pennsylvania law also has a number of other requirements for 
the safe use of diesel-powered equipment in the particularly hazardous 
environments of underground coal mines. Many of these parallel the 
requirements in MSHA's diesel equipment rule. Like MSHA's requirements, 
they too can result in reducing miner exposure to diesel particulate--
e.g., regular maintenance of diesel engines by qualified personnel and 
equipment operator examinations. The requirements in the Pennsylvania 
law take into account the need to maintain the aftertreatment devices 
required to control diesel particulate.
    While both mine operators and labor supported this approach, it 
remains controversial. During the hearings on this rulemaking, one 
commenter indicated that at the time the standards were established, it 
would have taken a 95% filter to reduce dpm from certain equipment to 
the 0.12 mg/m\3\ emissions standard because 0.25 sulfur fuel was being 
utilized. This test reported by the commenter was completed prior to 
MSHA promulgating the diesel equipment rule that required the use of 
.05% sulfur fuel. Another commenter pointed out that as operators in 
the state began considering the use of newer, less polluting engines, 
achieving an efficiency of 95% reduction of the emissions from any such 
engines would become even more difficult. There was some disagreement 
among the commenters as to whether existing technology would permit 
operators to meet the 0.12 mg/m\3\ emission standard in many 
situations. One commenter described efforts to get a small outby unit 
approved under Pennsylvania law. Accordingly, the industry has 
indicated that it would seek changes to the Pennsylvania diesel law. 
Commenters representing miners indicated that they were involved in 
these discussions.
    West Virginia. Until 1997, West Virginia law banned the use of 
diesel-powered equipment in underground coal mines. In that year, the 
State created the joint labor-management West Virginia Diesel Equipment 
Commission (Commission) and charged it with developing regulations to 
permit and govern diesel engine use in underground coal mines. As 
explained by several commenters, the Commission, in collaboration with 
West Virginia University (WVU), developed a protocol for testing diesel 
engine exhaust controls, and the legislature appropriated more than 
$150,000 for WVU to test diesel exhaust controls and an array of diesel 
particulate filters.
    There were a number of comments received by MSHA on the test 
protocols and results. These are discussed in part IV this preamble. 
One commenter noted that various manufacturers of products have been 
very interested in how their products compare to those of other 
manufacturers tested by the WVU. Another asserted that mine operators 
had been slowing the scheduling of tests by WVA.
    Pursuant to the West Virginia law establishing the Commission, the 
Commission was given only a limited time to determine the applicable 
rules for the use of diesel engines underground, or the matter was 
required to be referred to an arbitrator for resolution. One commenter 
during the hearings noted that the Commission had not been able to 
reach resolution and that indeed arbitration was the next step. Other 
commenters described the proposal of the industry members of the 
Commission--0.5mg/m\3\ for all equipment, as configured, before 
approval is granted. In this regard, the industry members of the West 
Virginia Commission said:

    ``We urge you to accelerate the finalization of * * * these 
proposed rules. We believe that will aid our cause, as well as the 
other states that currently don't use diesel.'' (Id)

    Virginia. According to one commenter, diesel engine use in 
underground mining was legalized in Virginia in the mid-1980s. It 
was originally used on some heavy production equipment, but the haze 
it created was so thick it led to a drop in production. Thereafter, 
most diesel equipment has been used outby (805 pieces). The current 
state regulations consist of requiring that MSHA approved engines be 
used, and that the ``most up-to-date, approved, available diesel 
engine exhaust aftertreatment package'' be utilized. There are no 
distinctions between types of equipment. The commenter noted that 
more hearings were planned soon. Under a directive from the governor 
of Virginia, the state is reviewing its regulations and making 
recommendations for revisions to sections of its law on diesels.
    Ohio. The record of this rulemaking contains little specific 
information on the restrictions on the underground use of diesel-
powered equipment in Ohio. MSHA understands, however, that in 
practice it is not used. According to a communication with the 
Division of Mines and Reclamation of the Ohio Division of Natural 
Resources, this outcome stems from a law enacted on October 29, 
1995, now codified as section 1567.35 of Ohio Revised Code Title 15, 
which imposes strict safety restrictions on the use of various fuels 
underground.

(9) History of this Rulemaking.

    As discussed throughout this part, the Federal government has 
worked closely with the mining community to ascertain whether and 
how diesel-powered equipment might be used safely and healthfully in 
this industry. As the evidence began to grow that exposure to diesel 
exhaust might be harmful to miners, particularly in underground 
mines, formal agency actions were initiated to investigate this 
possibility and to determine what, if any, actions might be 
appropriate. These actions, including a number of non-regulatory 
initiatives taken by MSHA, are summarized here in chronological 
sequence.
    Activities Prior to Proposed Rulemaking on DPM. In 1984, the 
National Institute for Occupational Safety and Health (NIOSH) 
established a standing Mine Health Research Advisory Committee to 
advise it on matters involving or related to mine health research. 
In turn, that standing body established the Mine Health Research 
Advisory Committee Diesel Subgroup to determine if:
    * * * there is a scientific basis for developing a 
recommendation on the use of diesel equipment in underground mining 
operations and defining the limits of current knowledge, and 
recommending areas of research for NIOSH, if any, taking into 
account other investigators' ongoing and planned research. (49 FR 
37174).

    In 1985, MSHA established an Interagency Task Group with NIOSH and 
the former Bureau of Mines (BOM) to assess the health and safety 
implications of the use of diesel-powered equipment in underground coal 
mines.
    In April 1986, in part as a result of the recommendation of the 
Task Group, MSHA began drafting proposed regulations on the approval 
and use of diesel-powered equipment in underground coal mines. Also in 
1986, the Mine Health Research Advisory Committee Diesel Subgroup 
(which, as noted above, was created by a standing NIOSH committee) 
summarized the evidence available at that time as follows:

    It is our opinion that although there are some data suggesting a 
small excess risk of adverse health effects associated with exposure 
to diesel exhaust, these data are not compelling enough to exclude 
diesels from underground mines. In cases where diesel equipment is 
used in mines, controls should be employed to minimize exposure to 
diesel exhaust.

    On October 6, 1987, pursuant to Section 102(c) of the Mine Act, 30 
U.S.C. 812(c), which authorizes MSHA to appoint advisory committees as 
he deems appropriate, the agency appointed an advisory committee ``to 
provide advice on the complex issues concerning the use of diesel-
powered

[[Page 5749]]

equipment in underground coal mines.'' (52 FR 37381). MSHA appointed 
nine members to this committee, officially known as The Mine Safety and 
Health Administration Advisory Committee on Standards and Regulations 
for Diesel-Powered Equipment in Underground Coal Mines (hereafter the 
MSHA Diesel Advisory Committee). As required by section 101(a)(1) of 
the Mine Act, MSHA provided the MSHA Diesel Advisory Committee with 
draft regulations on the approval and use of diesel-powered equipment 
in underground coal mines. The draft regulations did not include 
standards setting specific limitations on diesel particulate, nor had 
MSHA at that time determined that such standards would be promulgated.
    In July 1988, the MSHA Diesel Advisory Committee completed its work 
with the issuance of a report entitled ``Report of the Mine Safety and 
Health Administration Advisory Committee on Standards and Regulations 
for Diesel-Powered Equipment in Underground Coal Mines.'' It also 
recommended that MSHA promulgate standards governing the approval and 
use of diesel-powered equipment in underground coal mines. The MSHA 
Diesel Advisory Committee recommended that MSHA promulgate standards 
limiting underground coal miners' exposure to diesel exhaust.
    With respect to diesel particulate, the MSHA Diesel Advisory 
Committee recommended that MSHA ``set in motion a mechanism whereby a 
diesel particulate standard can be set.'' (MSHA, 1988). In this regard, 
the MSHA Diesel Advisory Committee determined that because of 
inadequacies in the data on the health effects of diesel particulate 
matter and inadequacies in the technology for monitoring the amount of 
diesel particulate matter at that time, it could not recommend that 
MSHA promulgate a standard specifically limiting the level of diesel 
particulate matter in underground coal mines (Id. 64-65). Instead, the 
MSHA Diesel Advisory Committee recommended that MSHA ask NIOSH and the 
former Bureau of Mines to prioritize research in the development of 
sampling methods and devices for diesel particulate.
    The MSHA Diesel Advisory Committee also recommended that MSHA 
request a study on the chronic and acute effects of diesel emissions 
(Id). In addition, the MSHA Diesel Advisory Committee recommended that 
the control of diesel particulate ``be accomplished through a 
combination of measures including fuel requirements, equipment design, 
and in-mine controls such as the ventilation system and equipment 
maintenance in conjunction with undiluted exhaust measurements.'' The 
MSHA Diesel Advisory Committee further recommended that particulate 
emissions ``be evaluated in the equipment approval process and a 
particulate emission index reported.'' (Id. at 9).
    In addition, the MSHA Diesel Advisory Committee recommended that 
``the total respirable particulate, including diesel particulate, 
should not exceed the existing two milligrams per cubic meter 
respirable dust standard.'' (Id. at 9.) It should be noted that section 
202(b)(2) of the Mine Act requires that coal mine operators maintain 
the average concentration of respirable dust at their mines at or below 
two milligrams per cubic meter which effectively prohibits diesel 
particulate matter in excess of two milligrams per cubic meter (30 
U.S.C. 842(b)(2)).
    As noted, the MSHA Diesel Advisory Committee issued its report in 
1988. During that year, NIOSH issued a Current Intelligence Bulletin 
recommending that whole diesel exhaust be regarded as a potential 
carcinogen and controlled to the lowest feasible exposure level (NIOSH, 
1988). In its bulletin, NIOSH concluded that although the excess risk 
of cancer in diesel exhaust exposed workers has not been quantitatively 
estimated, it is logical to assume that reductions in exposure to 
diesel exhaust in the workplace would reduce the excess risk. NIOSH 
stated that ``[g]iven what we currently know, there is an urgent need 
for efforts to be made to reduce occupational exposures to DEP [dpm] in 
mines.''
    Consistent with the MSHA Diesel Advisory Committee's research 
recommendations, MSHA, in September 1988, formally requested NIOSH to 
perform a risk assessment for exposure to diesel particulate. (57 FR 
500). MSHA also requested assistance from NIOSH and the former BOM in 
developing sampling and analytical methodologies for assessing exposure 
to diesel particulate in mining operations. (Id.). In part, as a result 
of the MSHA Diesel Advisory Committee's recommendation, MSHA also 
participated in studies on diesel particulate sampling methodologies 
and determination of underground occupational exposure to diesel 
particulate.
    On October 4, 1989, MSHA published a Notice of Proposed Rulemaking 
on approval requirements, exposure monitoring, and safety requirements 
for the use of diesel-powered equipment in underground coal mines. (54 
FR 40950). The proposed rule followed the MSHA Diesel Advisory 
Committee's recommendation that MSHA promulgate regulations requiring 
the approval of diesel engines.
    On January 6, 1992, MSHA published an Advance Notice of Proposed 
Rulemaking (ANPRM) (57 FR 500). In the ANPRM, MSHA, among other things, 
sought comment on specific reports on diesel particulate prepared by 
NIOSH and the former BOM. MSHA also sought comment on reports on diesel 
particulate which were prepared by or in conjunction with MSHA. The 
ANPRM also sought comments on the health effects, technological and 
economic feasibility, and provisions which should be considered for 
inclusion in a diesel particulate rule. The notice also identified five 
specific areas where the agency was particularly interested in 
comments, and about which it asked a number of detailed questions: (1) 
Exposure limits, including the basis thereof; (2) the validity of the 
NIOSH risk assessment model and the validity of various types of 
studies; (3) information about non-cancer risks, non-lung routes of 
entry, and the confounding effects of tobacco smoking; (4) the 
availability, accuracy and proper use of sampling and monitoring 
methods for diesel particulate; and (5) the technological and economic 
feasibility of various types of controls, including ventilation, diesel 
fuel, engine design, aftertreatment devices, and maintenance by 
mechanics with specialized training. The notice also solicited specific 
information from the mining community on ``the need for a medical 
surveillance or screening program and on the use of respiratory 
equipment.'' (57 FR 500). The comment period on the ANPRM closed on 
July 10, 1992.
    While MSHA was completing a ``comprehensive analysis of the 
comments and any other information received'' in response to the ANPRM 
(57 FR 501), it took also several actions to encourage the mining 
community to begin to deal with the problems identified.
    In 1995, MSHA sponsored three workshops ``to bring together in a 
forum format the U.S. organizations who have a stake in limiting the 
exposure of miners to diesel particulate (including) mine operators, 
labor unions, trade organizations, engine manufacturers, fuel 
producers, exhaust aftertreatment manufacturers, and academia.'' 
(McAteer, 1995). The sessions provided an overview of the literature 
and of diesel particulate exposures in the mining industry, state-of-
the-art technologies available for reducing diesel particulate levels, 
presentations on engineering technologies toward that end, and 
identification of possible

[[Page 5750]]

strategies whereby miners' exposure to diesel particulate matter can be 
limited both practically and effectively.
    The first workshop was held in Beckley, West Virginia on September 
12 and 13, and the other two were held on October 6, and October 12 and 
13, 1995, in Mt Vernon, Illinois and Salt Lake City, Utah, 
respectively. A transcript was made. During a speech early the next 
year, the Deputy Assistant Secretary for MSHA characterized what took 
place at these workshops:

    The biggest debate at the workshops was whether or not diesel 
exhaust causes lung cancer and whether MSHA should move to regulate 
exposures. Despite this debate, what emerged at the workshops was a 
general recognition and agreement that a health problem seems to 
exist with the current high levels of diesel exhaust exposure in the 
mines. One could observe that while all the debate about the studies 
and the level of risk was going on, something else interesting was 
happening at the workshops: one by one miners, mining companies, and 
manufacturers began describing efforts already underway to reduce 
exposures. Many are actively trying to solve what they clearly 
recognize is a problem. Some mine operators had switched to low 
sulfur fuel that reduces particulate levels. Some had increased mine 
ventilation. One company had tried a soy-based fuel and found it 
lowered particulate levels. Several were instituting better 
maintenance techniques for equipment. Another had hired extra diesel 
mechanics. Several companies had purchased electronically 
controlled, cleaner, engines. Another was testing a prototype of a 
new filter system. Yet another was using disposable diesel exhaust 
filters. These were not all flawless attempts, nor were they all 
inexpensive. But one presenter after another described examples of 
serious efforts currently underway to reduce diesel emissions. 
(Hricko, 1996).

    In March of 1997, MSHA issued, in draft form, a publication 
entitled ``Practical Ways to Control Exposure to Diesel Exhaust in 
Mining--a Toolbox''. The draft publication was disseminated by MSHA to 
all underground mines known to use diesel equipment and posted on 
MSHA's Web site.
    As explained in the publication, the Toolbox was designed to 
disseminate to the mining community information gained through the 
workshops about methods being used to reduce miner exposures to dpm. 
MSHA's Toolbox provided specific information about nine types of 
controls that can reduce dpm exposures: low emission engines; fuels; 
aftertreatment devices; ventilation; enclosed cabs; engine maintenance; 
work practices and training; fleet management; and respiratory 
protective equipment. Some of these approaches reduce emissions from 
diesel engines; others focus on reducing miner exposure to whatever 
emissions are present. Quotations from workshop participants were used 
to illustrate when and how such controls might be helpful.
    As it clearly stated in its introductory section entitled ``How to 
Use This Publication,'' the Toolbox was not designed as a guide to 
existing or pending regulations. As MSHA noted in that regard:
    ``While the (regulatory) requirements that will ultimately be 
implemented, and the schedule of implementation, are of course 
uncertain at this time, MSHA encourages the mining community not to 
wait to protect miners' health. MSHA is confident that whatever the 
final requirements may be, the mining community will find this Toolbox 
information of significant value.''
    On October 25, 1996, MSHA published a final rule addressing 
approval, exhaust monitoring, and safety requirements for the use of 
diesel-powered equipment in underground coal mines (61 FR 55412). The 
final rule addresses, and in large part is consistent with, the 
specific recommendations made by the MSHA Diesel Advisory Committee for 
limiting underground coal miners' exposure to diesel exhaust. As noted 
in section 7 of this part, the diesel safety rule was implemented in 
steps concluding in late 1999. Aspects of this diesel safety rule had a 
significant impact on this rulemaking.
    In the Fall of 1997, following comment, MSHA's Toolbox was 
finalized and disseminated to the mining community. At the same time, 
MSHA made available to the mining community a software modeling tool 
developed by the Agency to facilitate dpm control. This model enables 
an operator to evaluate the effect which various alternative 
combinations of controls would have on the dpm concentration in a 
particular mine--before making the investment. MSHA refers to this 
model as ``the Estimator.'' The Estimator is in the form of a template 
that can be used on standard computer spreadsheet programs. As 
information about a new combination of controls is entered, the results 
are promptly displayed.
    On April 9, 1998, MSHA published a proposed rule to ``reduce the 
risks to underground coal miners of serious health hazards that are 
associated with exposure to high concentrations of diesel particulate 
matter'' (63 FR 17492). In order to further facilitate participation by 
the mining community, MSHA developed as an introduction to its preamble 
explaining the proposed rule, a dozen ``plain language'' questions and 
answers.
    The proposed rule to limit the concentration of dpm in underground 
coal mines (63 FR 17578) focused on the exclusive use of aftertreatment 
filters on permissible and heavy duty nonpermissible equipment to limit 
the concentration of dpm in underground coal mines. In its Questions 
and Answers, however, and throughout the preamble, MSHA presented 
considerable information on a number of other approaches that might 
have merit in limiting the concentration of dpm in underground coal 
mines, and drew special attention to the fact that the text of the rule 
being proposed represented only one of the approaches on which the 
agency was interested in receiving comment. Training of miners in the 
hazards of dpm was also proposed.
    The Proposed Rule to Limit DPM Concentrations in Underground Metal 
and Nonmetal Mines and Related Actions. On October 29, 1998 (63 FR 
58104), MSHA published a proposed rule establishing new health 
standards for underground metal and nonmetal mines that use equipment 
powered by diesel engines.
    In order to further facilitate participation by the mining 
community, MSHA developed as an introduction to its preamble explaining 
the proposed rule, 30 ``plain language'' questions and answers.
    The notice of proposed rulemaking reviewed and discussed the 
comments received in response to the ANPRM, including information on 
such control approaches as fuel type, fuel additives, and maintenance 
practices (63 FR 58134). For the convenience of the mining community, a 
copy of MSHA's Toolbox was also reprinted as an Appendix at the end of 
the notice of proposed rulemaking (63 FR 58223). A complete description 
of the Estimator, and several examples, were also presented in the 
preamble of the proposed rule.
    MSHA proposed to adopt (63 FR 58104) a different rule to address 
dpm exposure in underground metal and nonmetal mines.
    MSHA proposed a limit on the concentration of dpm to which 
underground metal and nonmetal miners would be exposed.
    The proposed rule would have limited dpm concentrations in 
underground metal and nonmetal mines to about 200 micrograms per cubic 
meter of air. Operators would have been able to select whatever 
combination of engineering and work practice controls they wanted to 
keep the dpm concentration in the mine below this limit.

[[Page 5751]]

    The concentration limit would have been implemented in two stages: 
an interim limit that would go into effect following 18 months of 
education and technical assistance by MSHA, and a final limit after 5 
years. MSHA sampling would be used to determine compliance.
    The proposal would also have required that all underground metal 
and nonmetal mines using diesel-powered equipment observe a set of 
``best practices'' to reduce engine emissions--e.g., to use low-sulfur 
fuel.
    Additionally, the Agency also considered alternatives that would 
have led to a significantly lower-cost proposal, e.g., establishing a 
less stringent concentration limit in underground metal and nonmetal 
mines, or increasing the time for mine operators to come into 
compliance. However, MSHA concluded at that time that such approaches 
would not be as protective, and that the approach proposed was both 
economically and technologically feasible.
    MSHA also explored whether to permit the use of administrative 
controls (e.g., rotation of personnel) and personal protective 
equipment (e.g., respirators) to reduce the diesel particulate exposure 
of miners. It is generally accepted industrial hygiene practice, 
however, to eliminate or minimize hazards at the source before 
resorting to personal protective equipment. Moreover, such a practice 
is generally not considered acceptable in the case of carcinogens since 
it merely places more workers at risk. Accordingly, the proposal 
explicitly prohibited the use of such approaches, except in those 
limited cases where MSHA approves, due to technological constraints, a 
2-year extension for an underground metal and nonmetal mine on the time 
to comply with the final concentration limit.
    MSHA sought comments from the mining community on the proposed 
regulatory text as well as throughout the entire preamble.
    In addition, the Agency specifically requested comments on the 
following issues:
    (a) Assessment of Risk/Benefits of the Rule. The Agency welcomed 
comments on the significance of the material already in the record, and 
any information that could supplement the record. For example, 
information on the health risks associated with exposure to dpm--
especially observations by trained observers or studies of acute or 
chronic effects of exposure to known levels of dpm or fine particles in 
general, information about pre-existing health conditions in individual 
miners or miners as a group that might affect their reactions to 
exposures to dpm or other fine particles; information about how dpm 
affects human health; information on the costs to miners, their 
families and their employers of the various health problems linked to 
dpm exposure, and the assumptions and approach to use in quantifying 
the benefits to be derived from this rule.
    (b) Proposed rule. MSHA sought comments on specific alternative 
approaches discussed in Part V. The options discussed included: 
adjusting the concentration limit for dpm; adjusting the phase-in time 
for the concentration limit; and requiring that specific technology be 
used in lieu of establishing a concentration limit.
    The Agency also requested comments on the composition of the diesel 
fleet, what controls cannot be utilized due to special conditions, and 
any studies of alternative controls using the computer spreadsheet 
described in the Appendix to Part V of the proposed rule preamble. The 
Agency also requested information about the availability and costs of 
various control technologies being developed (e.g., high-efficiency 
ceramic filters), experience with the use of available controls, and 
information that would help the Agency evaluate alternative approaches 
for underground metal and nonmetal mines. In addition, the Agency 
requested comments from the underground coal sector on the 
implementation to date of diesel work practices (like the rule limiting 
idling, and the training of those who provide maintenance) to help 
evaluate related proposals for the underground metal and nonmetal 
sector. The Agency also asked for information about any unusual 
situations that might warrant the application of special provisions.
    (c) Compliance Guidance. The Agency solicited comments on any 
topics on which initial guidance ought to be provided as well as any 
alternative practices which MSHA should accept for compliance before 
various provisions of the rule go into effect; and
    (d) Minimizing Adverse Impact of the Proposed Rule. The Agency set 
forth assumptions about impacts (e.g., costs, paperwork, and impact on 
smaller mines in particular) in some detail in the preamble and in the 
PREA. We sought comments on the methodology, and information on current 
operator equipment replacement planning cycles, tax, State 
requirements, or other information that might be relevant to purchasing 
new engines or control technology. The Agency also welcomed comments on 
the financial situation of the underground metal and nonmetal sector, 
including information that may be relevant to only certain commodities.
    From this point on, the actions taken on the rulemakings in 
underground coal mines and underground metal and nonmetal mines began 
to overlap in chronology. There is considerable overlap between the 
coal and metal/nonmetal communities, and so their participation in 
these separate rulemakings was often intertwined.
    In November 1998, MSHA held hearings on the proposed rule for 
underground coal mines in Salt Lake City, Utah and Beckley, West 
Virginia. In December 1998, hearings were held in Mt. Vernon, Illinois, 
and Birmingham, Alabama.
    Hearings concerning the proposed rule for underground coal mines 
were well attended, including representatives from both the coal and 
metal and nonmetal sectors. Testimony was presented by individual 
miners, representatives of miners, mine operators, mining industry 
associations, representatives of engine and equipment manufacturers, 
and one individual manufacturer. Members of the mining community 
participating had an extensive opportunity to hear and respond to 
alternative views; some participated in several hearings. They also had 
an opportunity to exchange in direct dialogues with the members of 
MSHA's dpm rulemaking committee--responding to questions and asking 
questions of their own. There was extensive comment not only about the 
provisions of the proposed rule itself, but also about the need for 
diesel powered equipment in this sector, the risks associated with its 
use, the need for regulation in this sector, alternative approaches 
including those on which MSHA sought comment, and the technological and 
economic feasibility of various alternatives.
    On February 12, 1999, (64 FR 7144) MSHA published a notice in the 
Federal Register announcing: (1) The availability of three additional 
studies applicable to the proposals; (2) the extension of the post-
hearing comment period and close of record on the proposed rule for 
underground coal mines for 60 additional days, until April 30, 1999; 
(3) the extension of the comment period on the proposed rule for metal 
and nonmetal mines for an additional 60 days, until April 30, 1999; and 
(4) an announcement that the Agency would hold public hearings on the 
metal and nonmetal proposal.
    On March 24, 1999, (64 FR 14200) MSHA published a notice in the 
Federal Register announcing the dates, time, and location of four 
public hearings for the metal and nonmetal proposed rule.

[[Page 5752]]

The notice also announced that the close of the post-hearing comment 
period would be on July 26, 1999.
    On April 27, 1999, (64 FR 22592) in response to requests from the 
public, MSHA extended the post-hearing comment period and close of 
record on the proposed rule for underground coal for 90 additional 
days, until July 26, 1999.
    In May 1999, hearings on the metal and nonmetal proposed rule were 
held in Salt Lake City, Ut; Albuquerque, NM; St. Louis, MO and 
Knoxville, TN.
    Hearings were well attended and testimony was presented by both 
labor (miners) and industry (mining associations, coal companies) and 
government (NIOSH). Testimony was presented by individual mining 
companies, mining industry associations, mining industry consultants 
and the National Institute of Occupational Safety and Health. The 
hearings were held for MSHA to obtain specific comments on the proposed 
rule for diesel particulate matter exposure of metal and nonmetal 
miners; additional information on existing and projected exposures to 
diesel particulate matter and to other fine particulates in various 
mining operations; information on the health risk associated with 
exposure to diesel particulate matter; information on the cost to 
miners, their families and their employers of the various health 
problems linked to diesel particulate matter; and information on 
additional benefits to be expected from reducing diesel particulate 
matter exposure.
    Members of the mining community participating, had an extensive 
opportunity to hear and respond to alternative views; some participated 
in several of the hearings. They also had an opportunity to exchange in 
direct dialogues with members of MSHA's dpm rulemaking committee--
responding to questions and asking questions of their own. There was 
extensive comment not only about the provisions of the proposed rule 
itself, but also about potential interferences with the method used to 
measure dpm, the studies that MSHA used to document the risk associated 
with exposure to dpm, the cost estimates derived by MSHA for industry 
implementation, and the technology and economic feasibility of various 
alternatives (specifically, industry use of a tool box approach without 
accountability for an exposure limit).
    One commenter, at the Knoxville hearing, specifically requested 
that the credentials and experience (related to the medical field, 
epidemiology, metal and nonmetal mining, mining engineering, and diesel 
engineering) of the hearing panelists be made a part of the public 
record. The commenter was informed by one of the panelists at the 
hearing that if this information was wanted it should be requested 
under the Freedom Of Information Act (FOIA). Such a request was 
submitted to MSHA by the commenter and appropriately responded to by 
the Agency.
    On July 8, 1999, (64 FR 36826) MSHA published a notice in the 
Federal Register correcting technical errors in the preamble discussion 
on the Diesel Emission Control Estimator formula in the Appendix to 
Part V of the proposed rulemaking notice, and correcting Figure V-5 of 
the preamble. Comments on these changes were solicited. (The Estimator 
model was subsequently published in the literature (Haney, R.A. and 
Saseen, G.P., ``Estimation of diesel particulate concentrations in 
underground mines'', Mining Engineering, Volume 52, Number 5, April 
2000)).
    The rulemaking records of both rules closed on July 26, 1999, nine 
months after the date the proposed rule on metal and nonmetal mines was 
published for public notice. The post-hearing comments, like the 
hearings, reflected extensive participation in this effort by the full 
range of interests in the mining community and covered a full range of 
ideas and alternatives.
    On June 30, 2000, the rulemaking record was reopened for 30 days in 
order to obtain public comment on certain additional documents which 
the agency determined should be placed in the rulemaking record. Those 
documents were the verification studies concerning NIOSH Method 5040 
mentioned in section 3 of this Part. In addition, the notice provided 
an opportunity for comment on additional documents being placed in the 
rulemaking record for the related rulemaking for underground coal mines 
(paper filter verification investigation and recent hot gas filter test 
results from VERT), and an opportunity to comment on some additional 
documents on risk being placed in both records. In this regard, the 
notice reassured the mining community that any comments filed on risk 
in either rulemaking proceeding would be placed in both records, since 
the two rulemakings utilize the same risk assessment.

Part III. Risk Assessment

Introduction
1. Exposures of U.S. Miners
    a. Underground Coal Mines
    b. Underground Metal and Nonmetal Mines
    c. Surface Mines
    d. Miner Exposures Compared to Exposures of Other Groups
2. Health Effects Associated with dpm Exposures
    a. Relevancy Considerations
    i. Animal Studies
    ii. Reversible Health Effects
    iii. Health Effects Associated with PM2.5 in Ambient 
Air
    b. Acute Health Effects
    i. Symptoms Reported by Exposed Miners
    ii. Studies Based on Exposures to Diesel Emissions
    iii. Studies Based on Exposures to Particulate Matter in Ambient 
Air
    c. Chronic Health Effects
    i. Studies Based on Exposures to Diesel Emissions
    (1) Chronic Effects other than Cancer
    (2) Cancer
    (a) Lung Cancer
    (i) Evaluation Criteria
    (ii) Studies Involving Miners
    (iii) Best Available Epidemiologic Evidence
    (iv) Counter-Evidence
    (v) Summation
    (b) Bladder Cancer
    ii. Studies Based on Exposures to PM2.5 in Ambient 
Air
    d. Mechanisms of Toxicity
    i. Agent of Toxicity
    ii. Deposition, Clearance, and Retention
    iii. Effects other than Cancer
    iv. Lung Cancer
    (1) Genotoxicity Studies
    (2) Animal Inhalation Studies
3. Characterization of Risk
    a. Material Impairments to Miners' Health or Functional Capacity
    i. Sensory Irritations and Respiratory Symptoms (including 
allergenic responses)
    ii. Premature Death from Cardiovascular, Cardiopulmonary, or 
Respiratory Causes
    iii. Lung Cancer
    (1) Summary of Collective Epidemiologic Evidence
    (a) Consistency of Epidemiologic Results
    (b) Best Available Epidemiologic Evidence
    (c) Studies with Quantitative or Semiquantitative Exposure 
Assessments
    (d) Studies Involving Miners
    (2) Meta-Analyses
    (3) Potential Systematic Biases
    (4) Causality
    (5) Other Interpretations of the Evidence
    b. Significance of the Risk of Material Impairment to Miners
    i. Meaning of Significant Risk
    (1) Legal Requirements
    (2) Standards and Guidelines for Risk Assessment
    ii. Significance of Risk for Underground Miners Exposed to Dpm
    (1) Sensory Irritations and Respiratory Symptoms (including 
allergenic responses)
    (2) Premature Death from Cardiovascular, Cardiopulmonary, or 
Respiratory Causes
    (3) Lung Cancer
    (a) Risk Assessment Based on Studies Involving Miners
    (b) Risk Assessment Based on Miners' Cumulative Exposure
    (i) Exposure-Response Relationships from Studies Outside Mining

[[Page 5753]]

    (ii) Exposure-Response Relationships from Studies on Miners
    (iii) Excess Risk at Specific Dpm Exposure Levels
    c. The Rule's Expected Impact on Risk
4. Conclusions

Introduction

    MSHA has reviewed the scientific literature to evaluate the 
potential health effects of occupational dpm exposures at levels 
encountered in the mining industry. This part of the preamble presents 
MSHA's review of the currently available information and MSHA's 
assessment of health risks associated with those exposures. All 
material submitted during the public comment periods was considered 
before MSHA drew its final conclusions.
    The risk assessment begins, in Section III.1, with a discussion of 
dpm exposure levels observed by MSHA in the mining industry. This is 
followed by a review, in Section III.2, of information available to 
MSHA on health effects that have been studied in association with dpm 
exposure. Finally, in Section III.3 entitled ``Characterization of 
Risk,'' the Agency considers three questions that must be addressed for 
rulemaking under the Mine Act and relates the available information 
about risks of dpm exposure at current levels to the regulatory 
requirements.
    A risk assessment must be technical enough to present the evidence 
and describe the main controversies surrounding it. At the same time, 
an overly technical presentation could cause stakeholders to lose sight 
of the main points. MSHA is guided by the first principle the National 
Research Council established for risk characterization, that the 
approach be:

    [a] decision driven activity, directed toward informing choices 
and solving problems * * * Oversimplifying the science or skewing 
the results through selectivity can lead to the inappropriate use of 
scientific information in risk management decisions, but providing 
full information, if it does not address key concerns of the 
intended audience, can undermine that audience's trust in the risk 
analysis.

    Although the final rule covers only one sector, this portion of the 
preamble was intended to enable MSHA and other interested parties to 
assess risks throughout the coal and M/NM mining industries. 
Accordingly, the risk assessment includes information pertaining to all 
sectors of the mining industry. All public comments on the exposures of 
miners and the health effects of dpm exposure--whether submitted 
specifically for the coal rulemaking or for the metal/nonmetal 
rulemaking--were incorporated into the record for each rulemaking and 
have been considered for this assessment.
    MSHA had an earlier version of this risk assessment independently 
peer reviewed. The risk assessment as proposed incorporated revisions 
made in accordance with the reviewers' recommendations, and the final 
version presented here contains clarifications and other responses to 
public comments. With regard to the risk assessment as published in the 
proposed preamble, the reviewers stated that:

* * * principles for identifying evidence and characterizing risk 
are thoughtfully set out. The scope of the document is carefully 
described, addressing potential concerns about the scope of 
coverage. Reference citations are adequate and up to date. The 
document is written in a balanced fashion, addressing uncertainties 
and asking for additional information and comments as appropriate. 
(Samet and Burke, Nov. 1997).

    Some commenters generally agreed with this opinion. Dr. James 
Weeks, representing the UMWA, found the proposed risk assessment to be 
``balanced, thorough, and systematic.'' Dr. Paul Schulte, representing 
NIOSH, stated that ``MSHA has prepared a thorough review of the health 
effects associated with exposure to high concentrations of dpm, and 
NIOSH concurs with the published [proposed] characterization of risks 
associated with these exposures.'' Dr. Michael Silverstein, 
representing the Washington State Dept. of Labor and Industries, found 
MSHA's ``regulatory logic * * * thoroughly persuasive.'' He commented 
that ``the best available scientific evidence shows that diesel 
particulate exposure is associated with serious material impairment of 
health * * * the evidence * * * is particularly strong and certainly 
provides a sufficient basis for regulatory action.''
    Many commenters, however, vigorously criticized various aspects of 
the proposed assessment and some of the scientific studies on which it 
was based. MSHA's final assessment, published here, was modified to 
respond to all of these criticisms. Also, in response to commenters' 
suggestions, this assessment incorporates some research studies and 
literature reviews not covered or inadequately discussed in the 
previous version.
    Some commenters expressed the opinion that the proposed risk 
assessment should have been peer-reviewed by a group representing 
government, labor, industry, and independent scientists. Since the 
rulemaking process included a pre-hearing comment period, eight public 
hearings (four for coal and four for M/NM), and two post-hearing 
comment periods, these constituencies had ample opportunity to review 
and comment upon MSHA's proposed risk assessment. The length of the 
comment period for the Coal Dpm proposal was 15 months. The length of 
the comment period for the Metal/Nonmetal Dpm proposal was nine months.

1. Exposures of U.S. Miners

    Information about U.S. miner exposures comes from published studies 
and from additional mine investigations conducted by MSHA since 
1993.\1\ Previously published studies of exposures to dpm among U.S. 
miners are: Watts (1989, 1992), Cantrell (1992, 1993), Haney (1992), 
and Tomb and Haney (1995). MSHA has also conducted investigations 
subsequent to the period covered in Tomb and Haney (1995), and the 
previously unpublished data through mid-1998 are included here. Both 
the published and unpublished studies were placed in the record with 
the proposal, giving MSHA's stakeholders the opportunity to analyze and 
comment on all of the exposure data considered.
---------------------------------------------------------------------------

    \1\ MSHA has only limited information about miner exposures in 
other countries. Based on 223 personal and area samples, average 
exposures at 21 Canadian noncoal mines were reported to range from 
170 to 1300 g/m3 (respirable combustible dust), 
with maximum measurements ranging from 1020 to 3100 g/
m3 (Gangel and Dainty, 1993). Among 622 full shift 
measurements collected since 1989 in German underground noncoal 
mines, 91 (15%) exceeded 400 g/m3 (total carbon) 
(Dahmann et al., 1996). As explained elsewhere in this preamble, 400 
g/m3 (total carbon) corresponds to approximately 
500 g/m3 dpm.
---------------------------------------------------------------------------

    MSHA's field studies involved measuring dpm concentrations at a 
total of 50 mines: 27 underground metal and nonmetal (M/NM) mines, 12 
underground coal mines, and 11 surface mining operations (both coal and 
M/NM). At all surface mines and all underground coal mines, dpm 
measurements were made using the size-selective method, based on 
gravimetric determination of the amount of submicrometer dust collected 
with an impactor. With few exceptions, dpm measurements at underground 
M/NM mines were made using the Respirable Combustible Dust (RCD) method 
(with no impactor). At two of the underground M/NM mines, measurements 
were made using the total carbon (TC) method, and at one, RCD 
measurements were made in one year and TC measurements in another. 
Measurements at the two remaining underground M/NM mines were made 
using the size-selective method, as in

[[Page 5754]]

coal and surface mines.\2\ Weighing errors inherent in the gravimetric 
analysis required for both size-selective and RCD methods become 
statistically insignificant at the relatively high dpm concentrations 
observed.
---------------------------------------------------------------------------

    \2\ The various methods of measuring dpm are explained in 
section 3 of Part II of the preamble to the proposed rule. This 
explanation, along with additional information on these methods, is 
also provided in section 3 of Part II of the preamble to the final 
M/NM rule.
---------------------------------------------------------------------------

    According to MSHA's experience, the dpm samples reflect exposures 
typical of mines known to use diesel equipment for face haulage in the 
U.S. However, they do not constitute a random sample of mines, and care 
was taken in the proposed risk assessment not to characterize results 
as necessarily representing conditions in all mines. Several commenters 
objected to MSHA's use of these exposure measurements in making 
comparisons to exposures reported in other industries and, for M/NM, in 
estimating the proposed rule's impact. These objections are addressed 
in Sections III.1.d and III.3.b.ii(3)(c) below. Comments related to the 
measurement methods used in underground coal and M/NM mines are 
addressed, respectively, in Sections III.1.b and III.1.c.
    Each underground study typically included personal dpm exposure 
measurements for approximately five production workers. Also, area 
samples were collected in return airways of underground mines to 
determine diesel particulate emission rates.\3\ Operational information 
such as the amount and type of equipment, airflow rates, fuel, and 
maintenance was also recorded. Mines were selected to obtain a wide 
range of diesel equipment usage and mining methods. Mines with greater 
than 175 horsepower and less than 175 horsepower production equipment 
were sampled. Single and multiple level mines were sampled. Mine level 
heights ranged from eight to one-hundred feet. In general, MSHA's 
studies focused on face production areas of mines, where the highest 
concentrations of dpm could be expected; but, since some miners do not 
spend their time in face areas, samples were collected in other areas 
as well, to get a more complete picture of miner exposure. Because of 
potential interferences from tobacco smoke in underground M/NM mines, 
samples were not collected on or near smokers.
---------------------------------------------------------------------------

    \3\ Since area samples in return airways do not necessarily 
represent locations where miners normally work or travel, they were 
excluded from the present analysis. A number of area samples were 
included, however, as described in Sections III.1.b and III.1.c. The 
included area samples were all taken in production areas and 
haulageways.
---------------------------------------------------------------------------

    Table III-1 summarizes key results from MSHA's studies. The higher 
concentrations in underground mines were typically found in the 
haulageways and face areas where numerous pieces of equipment were 
operating, or where airflow was low relative to the amount of equipment 
operating. In production areas and haulageways of underground mines 
where diesel powered equipment was used, the mean dpm concentration 
observed was 644 g/m3 for coal and 808 g/
m3 for M/NM. In travelways of underground mines where diesel 
powered equipment was used, the mean dpm concentration (based on 112 
area samples not included in Table III-1) was 517 g/
m3 for M/NM and 103 g/m3 for coal. In 
surface mines, the higher concentrations were generally associated with 
truck drivers and front-end loader operators. The mean dpm 
concentration observed was less than 200 g/m3 at 
all eleven of the surface mines in which measurements were made. More 
information about the dpm concentrations observed in each sector is 
presented in the material that follows.

 Table III-1.--Full-shift Diesel Particulate Matter Concentrations Observed in Production Areas and Haulageways
                                           of 50 Dieselized U.S. Mines
----------------------------------------------------------------------------------------------------------------
                                                                                        Standard
                                                                            Mean        error of      Exposure
                  Mine type                    Number of    Number of     exposure    mean (g/     m>g/m3)    (g/
                                                                             m3)                         m3)
----------------------------------------------------------------------------------------------------------------
Surface.....................................           11           45            88            11         9-380
Underground coal............................           12          226           644            41       0-3,650
Underground metal and nonmetal..............           27          355           808            39     10-5,570
----------------------------------------------------------------------------------------------------------------
 Note: Intake and return area samples are excluded.

a. Underground Coal Mines
    Approximately 145 out of the 910 existing underground coal mines 
currently utilize diesel powered equipment. Of these 145 mines, 32 
mines currently use diesel equipment for face coal haulage. The 
remaining mines use diesel equipment for transportation, materials 
handling and other support operations. MSHA focused its efforts in 
measuring dpm concentrations in coal mines on mines that use diesel 
powered equipment for face coal haulage. Twelve mines using diesel-
powered face haulage were sampled. Mines with diesel powered face 
haulage were selected because the face is an area with a high 
concentration of vehicles operating at a heavy duty cycle at the 
furthest end of the mine's ventilation system.
    Diesel particulate levels in underground mines depend on: (1) The 
amount, size, and workload of diesel equipment; (2) the rate of 
ventilation; and, (3) the effectiveness of whatever diesel particulate 
control technology may be in place. In the dieselized mines studied by 
MSHA, the sections used either two or three diesel coal haulage 
vehicles. In eastern mines, the haulage vehicles were equipped with a 
nominal 100 horsepower engine. In western mines, the haulage vehicles 
were equipped with a nominal 150 horsepower engine. Ventilation rates 
ranged from the approval plate requirement, based on the 100-75-50 
percent rule (Holtz, 1960), to ten times the approval plate 
requirement. In most cases, the section airflow was approximately twice 
the approval plate requirement. Other control technology included 
aftertreatment filters and fuel. Two types of aftertreatment filters 
were used. These filters included a disposable diesel emission filter 
(DDEF) and a Wire Mesh Filter (WMF). The DDEF is a commercially 
available product; the WMF was developed by and only used at one mine. 
Both low sulfur and high sulfur fuels were used.
    Figure III-1 displays the range of exposure measurements obtained 
by MSHA in the field studies it conducted in underground coal mines. A 
study normally consisted of collecting samples on the continuous miner 
operator and coal haulage vehicle

[[Page 5755]]

operators for two to three shifts, along with area samples in the 
haulageways. A total of 142 personal samples and 84 area samples were 
collected, excluding any area samples taken in intake or return 
airways.
[GRAPHIC] [TIFF OMITTED] TR19JA01.058

    As stated in the proposed risk assessment, no statistically 
significant difference was observed in mean dpm concentration between 
the personal and area samples.\4\ A total of 19 individual measurements 
exceeded 1500 g/m3, still excluding intake and 
return area samples. Although the three highest of these were from area 
samples, nine of the 19 measurements exceeding 1500 g/
m3 were from personal samples.
---------------------------------------------------------------------------

    \4\ One commenter (IMC Global) noted that MSHA had provided no 
data verifying this statement. For the 142 personal samples, the 
mean dpm concentration measurement was 608 g/m3, 
with a standard error of 42.5 g/m3. For the 84 
area samples, the mean was 705 g/m3, with a 
standard error of 82.1 g/m3. The significance 
level (p-value) of a t-test comparing these means is 0.29 using a 
separate-variance test or 0.25 using a pooled-variance test. 
Therefore, a difference in population means cannot be inferred at 
any confidence level greater than 75%.
    Here, and in other sections of this risk assessment, MSHA has 
employed standard statistical methods described in textbooks on 
elementary statistical inference.
---------------------------------------------------------------------------

    In six mines, measurements were taken both with and without use of 
disposable after-treatment filters, so that a total of eighteen 
studies, carried out in twelve mines, are displayed. Without use of 
after-treatment filters, average observed dpm concentrations exceeded 
500 g/m3 in eight of the twelve mines and exceeded 
1000 g/m3 in four.\5\ At five of the twelve mines, 
all dpm measurements were 300 g/m3 or greater in 
the absence of after-treatment filters.
---------------------------------------------------------------------------

    \5\ In coal mine E, the average as expressed by the mean 
exceeded 1000 g/m3, but the median did not.
---------------------------------------------------------------------------

    The highest dpm concentrations observed at coal mines were 
collected at Mine ``G.'' Eight of these samples were collected during 
employment of WMFs, and eight were collected while filters were not 
being employed. Without filters, the mean dpm concentration observed at 
Mine ``G'' was 2052 g/m3 (median = 2100 g/
m3). With employment of WMFs, the mean

[[Page 5756]]

dropped to 1241 g/m3 (median = 1235 g/
m3).
    Filters were employed during three of the four studies showing 
median dpm concentration at or below 200 g/m3. 
After adjusting for outby sources of dpm, exposures were found to be 
reduced by up to 95 percent in mines using the DDEF and by 
approximately 50 percent in the mine using the WMF.
    The higher dpm concentrations observed at the mine using the WMF 
(Mine ``G*'') are attributable partly to the lower section airflow. The 
only study without filters showing a median concentration at or below 
200 g/m \3\ was conducted in a mine (Mine ``A'') which had 
section airflow approximately ten times the nameplate requirement. The 
section airflow at the mine using the WMF was approximately the 
nameplate requirement.
    Some commenters [e.g., WV Coal Assoc and Energy West] objected to 
MSHA's presentation of underground coal mine exposures based on 
measurements made using the size-selective method (gravimetric 
determination of the amount of submicrometer dust collected with an 
impactor). These commenters argued that the data were ``* * * collected 
with emissions monitoring devices discredited by MSHA itself in the 
preamble * * *'' and that these measurements do not reliably ``* * * 
distinguish it [dpm] from other particles in coal mine dust, at the 
critical upper end range of submicron particles.''
    MSHA did not ``discredit'' use of the size-selective method for all 
purposes. As discussed elsewhere in this preamble, the size-selective 
method of measuring dpm was designed by the former BOM specifically for 
use in coal mines, and the size distribution of coal mine dust was 
taken into account in its development. Despite the recognized 
interference from a small fraction of coal mine dust particles, MSHA 
considers gravimetric size-selective measurements to be reasonably 
accurate in measuring dpm concentrations greater than 200 g/
m3, based on a full-shift sample, when coal mine dust 
concentrations are not excessive (i.e., not greater than 2.0 mg/
m3). Interference from submicrometer coal mine dust is 
counter-balanced, to some extent, by the fraction of larger size, 
uncaptured dpm. Coal mine dust concentrations were not excessive when 
MSHA collected its size-selective samples. Therefore, even if as much 
as 10 percent of the coal mine dust were submicrometer, this fraction 
would not have contributed significantly to the high concentrations 
observed at the sampled mines.
    At lower concentrations, or shorter sampling times, random 
variability in the gravimetric determination of weight gain becomes 
significant, compared to the weight of dust accumulated on the filter. 
For this reason, MSHA has rejected the use of the gravimetric size-
selective method for enforcement purposes.\6\ This does not mean, 
however, that MSHA has ``discredited'' this method for other purposes, 
including detection of very high dpm concentrations at coal mines 
(i.e., greater than 500 g/m3) and estimation of 
average dpm concentrations, based on multiple samples, when coal mine 
dust concentrations are not excessive. On the contrary, MSHA regards 
the gravimetric size-selective method as a useful tool for detecting 
and monitoring very high dpm concentrations and for estimating average 
exposures.
---------------------------------------------------------------------------

    \6\ MSHA has concluded that random weighing variability would 
make it impractical to use the size-selective method to enforce 
compliance with any dpm concentration limit less than about 300 
g/m3. MSHA believes that, at such levels, 
single-sample noncompliance determinations based on the size-
selective method could not be made at a sufficiently high confidence 
level.
---------------------------------------------------------------------------

b. Underground Metal and Nonmetal Mines
    Currently there are approximately 265 underground M/NM mines in the 
United States. Nearly all of these mines utilize diesel powered 
equipment, and 27 of those doing so were sampled by MSHA for dpm.\7\ 
The M/NM studies typically included measurements of dpm exposure for 
dieselized production equipment operators (such as truck drivers, roof 
bolters, haulage vehicles) on two to three shifts. A number of area 
samples were also collected. None of the M/NM mines studied were using 
diesel particulate afterfilters.
---------------------------------------------------------------------------

    \7\ The proposal discussed data from 25 underground M/NM mines. 
Studies at two additional mines, carried out too late to be included 
in the proposal, were placed into the public record along with the 
earlier studies. During the proceedings, MSHA provided copies of all 
of these studies to stakeholders requesting them.
---------------------------------------------------------------------------

    Figure III-2 displays the range of dpm concentrations measured by 
MSHA in the 27 underground M/NM mines studied. A total of 275 personal 
samples and 80 area samples were collected, excluding intake and return 
area samples. Personal exposures observed ranged from less than 100 
g/m\3\ to more than 3500 g/m\3\. Exposure 
measurements based on area samples ranged from less than 100 
g/m\3\ to more than 3000 g/m\3\. With the exception 
of Mine ``V'', personal exposures were for face workers. Mine ``V'' did 
not use dieselized face equipment.

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    As stated in the proposed risk assessment, no statistically 
significant difference was observed in mean dpm concentration between 
the personal and area samples.\8\ A total of 45 individual measurements 
exceeded 1500 g/m\3\, still excluding intake and return area 
samples. The three highest of these, all exceeding 3500 g/
m\3\, were from personal samples. Of the 45 measurements exceeding 1500 
g/m\3\, 30 were from personal samples and 15 were from area 
samples.
---------------------------------------------------------------------------

    \8\ One commenter (IMC Global) noted that MSHA had provided no 
data verifying this statement. For the 275 personal samples, the 
mean dpm concentration measurement was 770 g/m\3\, with a 
standard error of 42.8 g/m\3\. For the 80 area samples, the 
mean was 939 g/m\3\, with a standard error of 86.6 
g/m\3\. The significance level (p-value) of a t-test 
comparing these means is 0.08 using a separate-variance test or 0.07 
using a pooled-variance test. Therefore, a difference in population 
means cannot be inferred at a 95% confidence level.
---------------------------------------------------------------------------

    Average observed dpm concentrations exceeded 500 g/m\3\ in 
18 of the 27 underground M/NM mines and exceeded 1000 g/m\3\ 
in 12.\9\ At eight of the 27 mines, all dpm measurements exceeded 300 
g/m\3\. The highest dpm concentrations observed at M/NM mines 
were collected at Mine ``E''. Based on 16 samples, the mean dpm 
concentration observed at Mine ``E'' was 2008 g/m\3\ (median = 
1835 g/m\3\). Twenty-five percent of the dpm measurements at 
this mine exceeded 2400 g/m\3\. All four of these were based 
on personal samples.
---------------------------------------------------------------------------

    \9\ At M/NM mines C, I, J, P, and Z the average as expressed by 
the mean exceeded 1000 g/m\3\ but the median did not. At M/
NM mines H and S, the median exceeded 1000 g/m\3\ but the 
mean did not. At M/NM mine K, the mean exceeded 500 g/m\3\, 
but the median did not.
---------------------------------------------------------------------------

    As with underground coal mines, dpm levels in underground M/NM 
mines are related to the amount and size of equipment, to the 
ventilation rate, and to the effectiveness of the diesel particulate 
control technology employed. In the dieselized M/NM mines studied by 
MSHA, front-end-loaders were used either to load ore onto trucks or to 
haul and load ore onto belts. Additional pieces of diesel powered 
support equipment, such as bolters and mantrips, were also used at the 
mines. The typical piece of production equipment was rated at 150 to 
350 horsepower. Ventilation rates in the M/NM mines studied mostly 
ranged from 100 to 200 cfm per horsepower of equipment. In only a few 
of the mines inventoried did ventilation exceed 200 cfm/hp. For single-
level mines, working areas were ventilated in series (i.e., the exhaust 
air from one area became the intake for the next working area). For 
multi-level mines, each level typically had a separate fresh air 
supply. One or two working areas could be on a level. Control 
technology used to reduce diesel particulate emissions in mines 
inventoried included oxidation catalytic converters and engine 
maintenance programs. Both low sulfur and high sulfur fuel were used; 
some mines used aviation grade low sulfur fuel.
    Some commenters argued that, because of the limited number of 
underground M/NM mines sampled by MSHA, ``* * * results of MSHA's 
admittedly non-random sample cannot be extrapolated to other mines.'' 
[MARG] More specifically, IMC Global claimed that since only 25 [now 
27] of about 260 underground M/NM mines were sampled,\10\ then ``if the 
* * * measurements are correct, this information shows at best 
potential exposure problems to diesel particulate in only 10% of the 
miners working in the metal-nonmetal mining sector and then only for 
certain unlisted commodities.'' \11\ IMC Global went on to suggest that 
MSHA should ``perform sufficient additional exposure monitoring * * * 
to show that the diesel particulate exposures are representative of the 
entire industry before promulgating regulations that will be applicable 
to the entire industry.''
---------------------------------------------------------------------------

    \10\ Three underground M/NM mine surveys, carried out too late 
to be included in the discussion, were placed into the public record 
and provided to interested stakeholders. These surveys contained 
data from two additional underground M/NM mines (``Z'' and ``aa'') 
and additional data for a mine (``d'') that had previously been 
surveyed. The risk assessment has now been updated to include these 
data, representing a total of 27 underground M/NM mines.
    \11\ A breakdown by commodity is given at the end of this 
subsection.
---------------------------------------------------------------------------

    As mentioned earlier, MSHA acknowledges that the mines for which 
dpm measurements are available do not comprise a statistically random 
sample of all underground M/NM mines. MSHA also acknowledges that the 
results obtained for these mines cannot be extrapolated in a 
statistically rigorous way to the entire population of underground M/NM 
mines. According to MSHA's experience, however, the selected mines (and 
sampling locations within those mines) represent typical diesel 
equipment use condition at underground M/NM. MSHA believes that results 
at these mines, as depicted in Figure III-2, in fact fairly reflect the 
broad range of diesel equipment used by the industry, regardless of 
type of M/NM mine. Based on its extensive experience with underground 
mines, MSHA believes that this body of data better represents those 
diverse diesel equipment use conditions, with respect to dpm exposures, 
than any other body of data currently available.
    MSHA strongly disagrees with IMC Global's contention that, ``* * * 
this information shows at best potential exposure problems to diesel 
particulate in only 10% of the miners working in the metal-nonmetal 
mining sector.'' IMC Global apparently drew this conclusion from the 
fact that MSHA sampled approximately ten percent of all underground M/
NM mines. This line of argument, however, depends on an unwarranted and 
highly unrealistic assumption: namely, that all of the underground M/NM 
mines not included in the sampled group of 25 experience essentially no 
``potential [dpm] exposure problems.'' MSHA certainly did not go out 
and, by chance or design, pick for sampling just exactly those mines 
experiencing the highest dpm concentrations. IMC Global's argument 
fails to recognize that the sampled mines could be fairly 
representative without being randomly chosen.
    MSHA also disagrees with the premise that 27 [or 25 as in the 
proposal] is an inherently insufficient number of mines to sample for 
the purpose of identifying an industry-wide dpm exposure problem that 
would justify regulation. The between-mine standard deviation of the 27 
mean concentrations observed within mines was 450 g/m\3\. 
Therefore, the standard error of the estimated grand mean, based on the 
variability observed between mines, was 450/27 = 87 
g/m\3\.\12\ MSHA considers this degree of uncertainty to be 
acceptable, given that the overall mean concentration observed exceeded 
800 g/m\3\.
---------------------------------------------------------------------------

    \12\ This quantity, 87 g/m\3\, differs from the 
standard error of the mean of individual measurements for 
underground M/NM mines, presented in Table III-1. The tabled value 
is based on 355 measurements whose standard deviation is 
727g/m\3\. Therefore, the standard error of the mean of all 
individual measurements is 727/355 = 39 g/m\3\, as 
shown in the table. Similarly, the mean of all individual 
measurements (listed in Table III-1 as 808 g/m\3\) differs 
from the grand mean of individual mean concentrations observed 
within mines, which is 838 g/m\3\.
---------------------------------------------------------------------------

    Several commenters questioned MSHA's use of the RCD and size-
selective methods for measuring dpm exposures at underground M/NM 
mines. IMC Global indicated that MSHA's RCD measurements might 
systematically inflate the dpm concentrations presented in this 
section, because ``* * * estimates for the non-diesel particulate 
component of RCD actually vary between 10% to 50%, averaging 33%.''
    MSHA considers the size-selective, gravimetric method capable of 
providing reasonably accurate

[[Page 5759]]

measurements when the dpm concentration is greater than 200 g/
m\3\, interferences are adequately limited, and the measurement is 
based on a full-shift sample. Relatively few M/NM measurements were 
made using this method, and none at the mines showing the highest dpm 
concentrations. No evidence was presented that the size distribution of 
coal mine dust (for which the impactor was specifically developed) 
differs from that of other mineral dusts in a way that significantly 
alters the impactor's performance. Similarly, MSHA considers the RCD 
method, when properly applied, to be capable of providing reasonably 
accurate dpm measurements at concentrations greater than 200 
g/m\3\. As with the size selective method, however, random 
weighing errors can significantly reduce the precision of even full-
shift RCD measurements at lower dpm concentrations. For this reason, in 
order to maintain a sufficiently high confidence level for its 
noncompliance determinations, MSHA will not use the RCD method for 
enforcement purposes. This does not mean, however, that MSHA has 
``discredited'' the RCD measurements for all other purposes, including 
detection of very high dpm concentrations (i.e., greater than 300 
g/m\3\) and estimation of average concentrations based on 
multiple samples. On the contrary, MSHA considers the RCD method to be 
a useful tool for detecting and monitoring very high dpm concentrations 
in appropriate environments and for estimating average exposures when 
those exposures are excessive.
    MSHA did not employ an impactor in its RCD measurements, and it is 
true that some of these measurements may have been subject to 
interference from lubrication oil mists. However, MSHA believes that 
the high estimates sometimes made of the non-dpm component of RCD 
(cited by IMC Global) do not apply to the RCD measurements depicted in 
Figure III-2. MSHA has three reasons for believing these RCD 
measurements consisted almost entirely of dpm:
    (1) MSHA took special care to sample only environments where 
interferences would not be significant. No samples were taken near 
pneumatic drills or smoking miners.
    (2) There was no interference from carbonates. The RCD analysis was 
performed at 500 deg. C, and carbonates are not released below 
1000 deg. C. (Gangel and Dainty, 1993)
    (3) Although high sulphur fuel was used in some mines, thereby 
adding sulfates to the RCD measurement, these sulfates are considered 
part of the dpm, as explained in section 2 of Part II of this preamble. 
Sulfates should not be regarded as an interference in RCD measurements 
of dpm.
    Commenters presented no evidence that there were substantial 
interferences in MSHA's RCD measurements, and, as stated above, MSHA 
was careful to avoid them. Therefore, MSHA considers it reasonable, in 
the context of this risk assessment, to assume that all of the RCD was 
in fact dpm. Moreover, in the majority of underground M/NM mines 
sampled, even if the RCD measurements were reduced by \1/3\, the mine's 
average would still be excessive: it would still exceed the maximum 
exposure level reported for non-mining occupations presented in section 
III.1.d.
    The breakdown, as suggested by IMC Global, of sampled underground 
M/NM mines by commodity is as follows:

------------------------------------------------------------------------
                                                              Number of
                         Commodity                              mines
------------------------------------------------------------------------
Copper.....................................................            2
Gold.......................................................            1
Lead/Zinc..................................................            6
Limestone..................................................            6
Potash.....................................................            2
Salt.......................................................            6
Trona (soda ash)...........................................            2
Other Nonmetal.............................................            2
                                                            ------------
      Total................................................           27
------------------------------------------------------------------------

c. Surface Mines
    Currently, there are approximately 12,620 surface mining operations 
in the United States. The total consists of approximately 1,550 coal 
mines and 11,070 M/NM mines. Virtually all of these mines utilize 
diesel powered equipment.
    MSHA conducted dpm studies at eleven surface mining operations: 
eight coal mines and three M/NM mines. MSHA deliberately directed its 
surface sampling efforts toward occupations likely to experience high 
dpm concentrations. To help select such occupations, MSHA first made a 
visual examination (based on blackness of the filter) of surface mine 
respirable dust samples collected during a November 1994 study of 
surface coal mines. This preliminary screening of samples indicated 
that relatively high surface mine dpm concentrations are typically 
associated with front-end-loader operators and haulage-truck operators; 
accordingly, sampling focused on these operations. A total of 45 
samples was collected.
    Figure III-3 displays the range of dpm concentrations measured at 
the eleven surface mines. The average dpm concentration observed was 
less than 200 g/m\3\ at all mines sampled. The maximum dpm 
concentration observed was less than or equal to 200 g/m\3\ in 
8 of the 11 mines (73%). The surface mine studies suggest that even 
when sampling is performed at the areas of surface mines believed most 
likely to have high exposures, dpm concentrations are generally likely 
to be less than 200 g/m\3\.

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d. Miner Exposures Compared to Exposures of Other Groups
    Occupational exposure to diesel particulate primarily originates 
from industrial operations employing equipment powered with diesel 
engines. Diesel engines are used to power ships, locomotives, heavy 
duty trucks, heavy machinery, as well as a small number of light-duty 
passenger cars and trucks. NIOSH has estimated that approximately 1.35 
million workers are occupationally exposed to the combustion products 
of diesel fuel in approximately 80,000 workplaces in the United States. 
(NIOSH 1988) Workers who are likely to be exposed to diesel emissions 
include: mine workers; bridge and tunnel workers; railroad workers; 
loading dock workers; truck drivers; fork-lift drivers; farm workers; 
and, auto, truck, and bus maintenance garage workers (NIOSH, 1988). 
Besides miners, groups for which occupational exposures have been 
reported and health effects have been studied include loading dock 
workers, truck drivers, and railroad workers.
    As estimated by the reported geometric mean,\13\ the median site-
specific occupational exposures for loading dock workers operating or 
otherwise exposed to unfiltered diesel fork lift trucks ranged from 23 
to 55 g/m\3\, as measured by submicrometer elemental carbon 
(EC) (NIOSH, 1990). Reported geometric mean concentrations of 
submicrometer EC ranged from 2.0 to 7.0 g/m\3\ for truck 
drivers and from 4.8 to 28 g/m\3\ for truck mechanics, 
depending on weather conditions (Zaebst et al., 1991).
---------------------------------------------------------------------------

    \13\ Median concentrations were not reported. The geometric mean 
provides a smoothed estimate of the median.
---------------------------------------------------------------------------

    Because these exposure averages, unlike those for railroad workers 
and miners, were reported in terms of EC, it is necessary, for purposes 
of comparison, to convert them to estimates of total dpm. Watts (1995) 
states that ``elemental carbon generally accounts for about 40% to 60% 
of diesel particulate mass.'' Therefore, in earlier versions of this 
risk assessment, a 2.0 conversion factor was assumed for dock workers, 
truck drivers, and truck mechanics, based on the midpoint of the 40-60% 
range proposed by Watts.
    Some commenters objected to MSHA's use of this conversion factor. 
IMC Global, for example, asserted that Watts' ``* * * 40 to 60% 
relationship between elemental carbon and diesel particulate mass * * * 
applies only to underground coal mines where diesel haulage equipment 
is used.'' IMC Global, and other commenters, also objected to MSHA's 
use of a single conversion factor for ``* * * different types of diesel 
engines under different duty cycles with different fuels and different 
types of emission control devices (if any) subjected to varying degrees 
of maintenance.''
    MSHA's quotation from Watts (1995) was taken from the ``Summary'' 
section of his paper. That paper covers a variety of occupational 
environments, and the summary makes no mention of coal mines. The 
sentence immediately preceding the quoted passage refers to the 
``occupational environment'' in general, and there is no indication 
that Watts meant to restrict the 40- to 60-percent range to any 
specific environment. It seems clear that the 40-to 60-percent range 
refers to average values across a spectrum of occupational 
environments.
    IMC Global mistakenly attributed to MSHA ``the blanket statement'' 
that the same ratio of elemental carbon to dpm applies ``for all diesel 
engines in different industries for all patterns of use.'' MSHA made no 
such statement. On the contrary, MSHA agrees with Watts (and IMC 
Global) that ``the percentage of elemental carbon in total diesel 
particulate matter fluctuates'' depending on ``engine type, duty cycle, 
fuel, lube oil consumption, state of engine maintenance, and the 
presence or absence of an emission control device.'' (Watts, op cit.) 
Indeed, MSHA acknowledges that, because of these factors, the 
percentage on a particular day in a particular environment may 
frequently fall outside the stated range. But MSHA is not applying a 
single conversion factor to individual elemental carbon measurements 
and claiming knowledge of the total dpm corresponding to each separate 
measurement. Instead, MSHA is applying an average conversion factor to 
an average of measurements in order to derive an estimate of an average 
dpm exposure. Averages are always less widely dispersed than individual 
values.

[[Page 5762]]

    Still, MSHA agrees with IMC Global that better estimates of dpm 
exposure levels are attainable by applying conversion factors more 
specifically related to the separate categories within the trucking 
industry: dock workers, truck drivers, and truck mechanics. Based on a 
total of 63 field measurements, the mean ratios (in percent) of EC to 
total carbon (TC) reported for these three categories were 47.3, 36.6, 
and 34.2, respectively (Zaebst et al., 1991).\14\ As explained 
elsewhere in this preamble, TC amounts to approximately 80 percent, by 
weight, of total dpm. Therefore, each of these ratios must be 
multiplied by 0.8 in order to estimate the corresponding percentage of 
EC in dpm.
---------------------------------------------------------------------------

    \14\ MSHA calculated the ratio for truck drivers by taking a 
weighted average of the ratios reported for ``local drivers'' and 
``road drivers.''
---------------------------------------------------------------------------

    It follows that the median mass concentration of dpm can be 
estimated as 2.64 (i.e., 1/(0.473 x 0.8)) times the geometric mean EC 
reported for dock workers, 3.42 times the geometric mean EC for truck 
drivers, and 3.65 times the geometric mean EC for truck mechanics. 
Applying the 2.64 conversion factor to the range of geometric mean EC 
concentrations reported for dock workers (i.e, 23 to 55 g/
m3) results in an estimated range of 61 to 145 g/
m3 in median dpm concentrations at various docks. Similarly, 
the estimated range of median dpm concentrations is calculated to be 
6.8 to 24 g/m3 for truck drivers and 18 to 102 
g/m3 for truck mechanics. It should be noted that 
MSHA is using conversion factors only for those occupational groups 
whose geometric mean exposures have been reported in terms of EC 
measurements.
    Average exposures of railroad workers to dpm were estimated by 
Woskie et al. (1988) and Schenker et al. (1990). As measured by total 
respirable particulate matter other than cigarette smoke, Woskie et al. 
reported geometric mean concentrations for various occupational 
categories of exposed railroad workers ranging from 49 to 191 
g/m3.
    For comparison with the exposures reported for these other 
industries, median dpm exposures measured within sampled mines were 
calculated directly from the data described in subsections a, b, and c 
above. The median within each mine is shown as the horizontal ``belt'' 
plotted for the mine in Figures III-1, III-2, and III-3.
    Figure III-4 compares the range of median dpm concentrations 
observed for mine workers within different mines to a range of dpm 
exposure levels estimated for urban ambient air and to the ranges of 
median dpm concentrations estimated for loading dock workers operating 
or otherwise exposed to diesel fork lift trucks, truck drivers, truck 
mechanics, and railroad workers. The range for ambient air, 1 to 10 
g/m3, was obtained from Cass and Gray (1995). For 
dock workers, truck drivers, truck mechanics, and railroad workers, the 
estimated ranges of median dpm exposures are, respectively: 61 to 145 
g/m3, 6.8 to 24 g/m3, 18 to 102 
g/m3 and 49 to 191 g/m3. The 
range of median dpm concentrations observed at different underground 
coal mines is 55 to 2100 g/m3, with filters 
employed at mines showing the lower concentrations.\15\ For underground 
M/NM mines, the corresponding range is 68 to 1835 g/
m3, and for surface mines it is 19 to 160 g/
m3. Since each range plotted is a range of median values or 
(for ambient air) mean values, the plots do not encompass all of the 
individual measurements reported.
---------------------------------------------------------------------------

    \15\ One commenter misinterpreted the tops of the ranges plotted 
in Figure III-4. This commenter apparently mistook the top of the 
range depicted for underground coal mines as the mean or median dpm 
exposure concentration measured across all underground coal mines. 
The top of this range (at 2100 g/m3, actually 
represents the highest median concentration at any of the coal mines 
sampled. It corresponds to the ``belt'' plotted for Mine ``G'' (with 
no after-filters) in Figure III-1. The bottom of the same bar, at 55 
g/m3, corresponds to the ``belt'' plotted for 
Mine H* (with after-filters) in Figure III-1.

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    As shown in Figure III-4, some miners are exposed to far higher 
concentrations of dpm than are any other populations for which exposure 
data have been reported. Indeed, median dpm concentrations observed in 
some underground mines are up to 200 times as high as mean 
environmental exposures in the most heavily polluted urban areas,\16\ 
and up to 10 times as high as median exposures estimated for the most 
heavily exposed workers in other occupational groups.
---------------------------------------------------------------------------

    \16\ It should be noted, however, that 24-hour environmental 
exposures for a full lifetime are not directly comparable with 
workday exposures over an occupational lifetime. If it is assumed 
that air inhaled during a work shift comprises half the total air 
inhaled during a 24-hour day, then the amount of air inhaled over 
the course of a 70-year lifetime is approximately 4.7 times the 
amount inhaled over a 45-year occupational lifetime with 240 working 
days per year.
---------------------------------------------------------------------------

    Several commenters objected to Figure III-4 and, more generally, to 
MSHA's comparison of dpm exposure levels for miners against the levels 
reported for other occupations. The objections to MSHA's method of 
estimating ranges of median dpm exposure for job categories within the 
trucking industry have already been discussed and addressed above. 
Other objections to the comparison were based on claims of insufficient 
accuracy in the RCD and gravimetric size selective measurements MSHA 
used to measure dpm levels for miners. MSHA considers its use of these 
methods appropriate for purposes of this comparison and has responded 
to criticisms of the dpm measurements for miners in Subsections 1.a and 
1.b of this risk assessment.\17\
---------------------------------------------------------------------------

    \17\ One commenter pointed out that the measurements for miners 
included both area and personal samples but provided no evidence 
that this would invalidate the comparison. As pointed out in 
Subsections 1.a and 1.b, area samples did not dominate the upper end 
of MSHA's dpm measurements. Furthermore, Figure III-4 presents a 
comparison of medians rather than means or individual measurements, 
so inclusion of the area samples has very little impact on the 
results.
---------------------------------------------------------------------------

    Some commenters objected to MSHA's basing a characterization of dpm 
exposures to miners on data spanning a ten-year period. These 
commenters contended that, in at least some M/NM mines, dpm levels had 
improved substantially during that period. No data were submitted, 
however, to support the premise that dpm exposures throughout the 
mining industry have declined to the levels reported for other 
occupations. As stated in the proposal and emphasized above, MSHA's dpm 
measurements were not technically designed as a random or statistically 
representative sample of the industry. They do show, however, that very 
high exposures have recently occurred in some mines. For example, as 
shown in Figure III-2, more than 25 percent of MSHA's dpm measurements 
exceeded 2000 g/m3 at underground M/NM mines ``U'' 
and ``Z''--and these measurements were made in 1996-7. In M/NM mines 
where exposures are actually commensurate with other industries 
already, little or nothing would need to be changed to meet the 
exposure limits.
    IMC Global further objected to Figure III-4 on the grounds that ``* 
* * the assumptions that MSHA used to develop that figure are grossly 
inaccurate and do not do make sense in the context of a dose-response 
relationship between lung cancer and dpm exposure.'' IMC Global 
suggested that the comparison in Figure III-4 be deleted for this 
reason. MSHA believes that the comparison is informative and that 
empirical evidence should be used, when it is available, even though 
the evidence was not generated under ideal, theoretical dose-response 
model conditions. The issue of whether Figure III-4 is consistent with 
an exposure-response relationship for dpm is addressed in Subsection 
3.a.iii(4) of this risk assessment.

2. Health Effects Associated With DPM Exposures

    This section reviews the various health effects (of which MSHA is 
aware) that may be associated with dpm exposures. The review is divided 
into three main sections: acute effects, such as diminished pulmonary 
function and eye irritation; chronic effects, such as lung cancer; and 
mechanisms of toxicity. Prior to that review, however, the relevance of 
certain types of information will be considered. This discussion will 
address the relevance of health effects observed in animals, health 
effects that are reversible, and health effects associated with fine 
particulate matter in the ambient air.
    Several commenters described medical surveillance studies that 
NIOSH and/or the former Bureau of Mines had carried out in the late 
1970s and early 1980s on underground miners employed in western, 
dieselized coal mines. These commenters urged MSHA to make these 
studies available and to consider the results in this rulemaking. Some 
of these commenters also suggested that these data would provide a 
useful baseline for pulmonary function and lung diseases among miners 
exposed to dpm, and recommended that follow-up examinations now be 
conducted to evaluate the possible effects of chronic dpm exposure.
    In response to such comments presented at some of the public 
hearings, another commenter wrote:

    First of all, MSHA is not a research agency, it is a regulatory 
agency, so that it would be inappropriate for MSHA to initiate 
research. MSHA did request that NIOSH conduct a risk assessment on 
the health effects of diesel exhaust and encouraged NIOSH and is 
currently collaborating with NIOSH (and NCI) on research of other 
underground miners exposed to diesel exhaust. And third, research on 
the possible carcinogenicity of diesel particulate matter was not 
undertaken on coal miners in the West or anywhere else because of 
the confounding exposure to crystalline silica, also considered a 
carcinogen, because too few coal miners have been exposed, and for 
too short a time to conduct a valid study. It was not arbitrariness 
or indifference on MSHA's part that it did not initiate research on 
coal miners; it was not within their mandate and it is inappropriate 
in any event. [UMWA]

    Three reports summarizing and presenting results from these medical 
surveillance studies related to dpm exposures in coal mines were, in 
fact, utilized and cited in the proposed risk assessment (Ames et al., 
1982; Reger et al., 1982; Ames et al., 1984). Ames et al. (1982) 
evaluated acute respiratory effects, and their results are considered 
in Subsection 2.b.ii of this risk assessment. Reger et al. (1982) and 
Ames et al. (1984) evaluated chronic effects, and their results are 
considered in Subsection 2.c.i(1).
    A fourth report (Glenn et al., 1983) summarized results from the 
overall research program of which the coal mine studies were a part. 
This health and environmental research program included not only coal 
miners, but also workers at potash, trona, salt, and metal mines. All 
subjects were given chest radiographs and spirometric tests and were 
questioned about respiratory symptoms, smoking and occupational 
history. In conjunction with these medical evaluations, industrial 
hygiene surveys were conducted to characterize the mine environments 
where diesel equipment was used. Diesel exhaust exposure levels were 
characterized by area and personal samples of NO2 (and, in 
some cases, additional gasses), aldehydes, and both respirable and 
total dust. For the evaluations of acute effects, exposure measures 
were based on the shift concentrations to which the examined workers 
were exposed. For the evaluations of chronic effects, exposures were 
usually estimated by summing the products of time spent in various 
locations by each miner by concentrations estimated for the various 
locations. Results of studies on acute effects in salt mines were 
reported by Gamble et al. (1978) and are considered

[[Page 5765]]

in Subsection 2.b.ii of this risk assessment. Attfield (1979), Attfield 
et al. (1982), and Gamble et al. (1983) evaluated effects in M/NM 
mines, and their results are considered in Subsection 2.c.i(1). The 
general summary provided by Glenn et al. (1983) was among the reports 
that one commenter (MARG) listed as having received inadequate 
attention in the proposed risk assessment. In that context, the general 
results summarized in this report are discussed, under the heading of 
``Counter-Evidence,'' in Subsection 2.c.i(2)(a) of this risk 
assessment.
a. Relevancy Considerations
    i. Animal Studies. Since the lungs of different species may react 
differently to particle inhalation, it is necessary to treat the 
results of animal studies with some caution. Evidence from animal 
studies can nevertheless be valuable--both in helping to identify 
potential human health hazards and in providing a means for studying 
toxicological mechanisms. Respondents to MSHA's ANPRM who addressed the 
question of relevancy urged consideration of all animal studies related 
to the health effects of diesel exhaust.
    Unlike humans, laboratory animals are bred to be homogeneous and 
can be randomly selected for either non-exposure or exposure to varying 
levels of a potentially toxic agent. This permits setting up 
experimental and control groups of animals that exhibit relatively 
little biological variation prior to exposure. The consequences of 
exposure can then be determined by comparing responses in the 
experimental and control groups. After a prescribed duration of 
deliberate exposure, laboratory animals can also be sacrificed, 
dissected, and examined. This can contribute to an understanding of 
mechanisms by which inhaled particles may exert their effects on 
health. For this reason, discussion of the animal evidence is placed in 
the section entitled ``Mechanisms of Toxicity'' below.
    Animal evidence also can help isolate the cause of adverse health 
effects observed among humans exposed to a variety of potentially 
hazardous substances. If, for example, the epidemiologic data are 
unable to distinguish between several possible causes of increased risk 
of disease in a certain population, then controlled animal studies may 
provide evidence useful in suggesting the most likely explanation--and 
provide that information years in advance of definitive evidence from 
human observations.
    Furthermore, results from animal studies may also serve as a check 
on the credibility of observations from epidemiologic studies of human 
populations. If a particular health effect is observed in animals under 
controlled laboratory conditions, this tends to corroborate 
observations of similar effects in humans.
    One commenter objected to MSHA's reference to using animal studies 
as a ``check'' on epidemiologic studies. This commenter emphasized that 
animal studies provide far more than just corroborative information and 
that researchers use epidemiologic and animal studies ``* * * to help 
understand different aspects of the carcinogenic process.'' \18\ MSHA 
does not dispute the utility of animal studies in helping to provide an 
understanding of toxicological processes and did not intend to belittle 
their importance for this purpose. In fact, MSHA places the bulk of its 
discussion of these studies in a section entitled ``Mechanisms of 
Toxicity.'' However, MSHA considers the use of animal studies for 
corroborating epidemiologic associations to be also important--
especially with respect to ruling out potential confounding effects and 
helping to establish causal linkages. Animal studies make possible a 
degree of experimental design and statistical rigor that is not 
attainable in human studies.
    Other commenters disputed the relevance of at least some animal 
data to human risk assessment. For example, The West Virginia Coal 
Association indicated the following comments by Dr. Peter Valberg:

    * * * scientists and scientific advisory groups have treated the 
rat bioassay for inhaled particles as unrepresentative of human 
lung-cancer risks. For example, the Presidential/Congressional 
Commission on Risk Assessment and Risk Management (``CCRARM'') noted 
that the response of rat lungs to inhaled particulate in general is 
not likely to be predictive of human cancer risks. More specific to 
dpm, the Clean Air Scientific Advisory Committee (``CASAC''), a 
peer-review group for the U.S. EPA, has commented on two drafts 
(1995 and 1998) of the EPA's Health Assessment Document on Diesel 
Exhaust. On both occasions, CASAC emphasized that the data from rats 
are not relevant for human risk assessment. Likewise, the Health 
Effects Institute also has concluded that rat data should not be 
used for assessing human lung cancer risk.

Similarly, the NMA commented that the 1998 CASAC review ``makes it 
crystal clear that the rat studies cited by MSHA should not be relied 
upon as legitimate indicators of the carcinogenicity of Dpm in 
humans.'' The Nevada Mining Association, endorsing Dr. Valberg's 
comments, added:
---------------------------------------------------------------------------

    \18\ This risk assessment is not limited to cancer effects, but 
the commenter's point can be generalized.

    * * * to the extent that MSHA wishes to rest its case on rat 
studies, Dr. Valberg, among others, has impressively demonstrated 
that these studies are worthless for human comparison because of 
rats' unique and species-specific susceptibility to inhaled 
---------------------------------------------------------------------------
insoluble particles.

However, neither Dr. Valberg nor the Nevada Mining Association provided 
evidence that rats' susceptibility to inhaled insoluble particles was 
``unique'' and that humans, for example, were not also susceptible to 
lung overload at sufficiently high concentrations of fine particles. 
Even if (as has apparently been demonstrated) some species (such as 
hamsters) do not exhibit susceptibility similar to rats, this by no 
means implies that rats are the only species exhibiting such 
susceptibility.
    These commenters appear at times to be saying that, because studies 
of lung cancer in rats are (in the commenters' view) irrelevant to 
humans, MSHA should completely ignore all animal studies related to 
dpm. To the extent that this was the position advocated, the 
commenters' line of reasoning neglects several important points:
    1. The animal studies under consideration are not restricted to 
studies of lung cancer responses in rats. They include studies of 
bioavailability and metabolism as well as studies of immunological and 
genotoxic responses in a variety of animal species.
    2. The context for the determinations cited by Dr. Valberg was risk 
assessment at ambient levels, rather than the much higher dpm levels to 
which miners are exposed. The 1995 HEI report to which Dr. Valberg 
alludes acknowledged a potential mechanism of lung overload in humans 
at dpm concentrations exceeding 500 g/m\3\ (HEI, 1995). Since 
miners may concurrently be exposed to concentrations of mineral dusts 
significantly exceeding 500 g/m\3\, evidence related to the 
consequences of lung overload has special significance for mining 
environments.
    3. The scientific authorities cited by Dr. Valberg and other 
commenters objected to using existing animal studies for quantitative 
human risk assessment. MSHA has not proposed doing that. There is an 
important distinction between extrapolating results from the rat 
studies to human populations and using them to confirm epidemiologic 
findings and to identify and explore potential mechanisms of toxicity.

[[Page 5766]]

    MSHA by no means ``wishes to rest its case on rat studies,'' and it 
has no intention of doing so. MSHA does believe, however, that 
judicious consideration of evidence from animal studies is appropriate. 
The extent to which MSHA utilizes such evidence to help draw specific 
conclusions will be clarified below in connection with those 
conclusions.
    ii. Reversible Health Effects. Some reported health effects 
associated with dpm are apparently reversible--i.e., if the worker is 
moved away from the source for a few days, the symptoms dissipate. A 
good example is eye irritation.
    In response to the ANPRM, questions were raised as to whether so-
called ``reversible'' effects can constitute a ``material'' impairment. 
For example, a predecessor constituent of the National Mining 
Association (NMA) argued that ``it is totally inappropriate for the 
agency to set permissible exposure limits based on temporary, 
reversible sensory irritation'' because such effects cannot be a 
``material'' impairment of health or functional capacity within the 
definition of the Mine Act (American Mining Congress, 87-0-21, 
Executive Summary, p. 1, and Appendix A).
    MSHA does not agree with this categorical view. Although the 
legislative history of the Mine Act is silent concerning the meaning of 
the term ``material impairment of health or functional capacity,'' and 
the issue has not been litigated within the context of the Mine Act, 
the statutory language about risk in the Mine Act is similar to that 
under the OSH Act. A similar argument was dispositively resolved in 
favor of the Occupational Safety and Health Administration (OSHA) by 
the 11th Circuit Court of Appeals in AFL-CIO v. OSHA, 965 F.2d 962, 974 
(1992).
    In that case, OSHA proposed new limits on 428 diverse substances. 
It grouped these into 18 categories based upon the primary health 
effects of those substances: e.g., neuropathic effects, sensory 
irritation, and cancer. (54 FR 2402). Challenges to this rule included 
the assertion that a ``sensory irritation'' was not a ``material 
impairment of health or functional capacity'' which could be regulated 
under the OSH Act. Industry petitioners argued that since irritant 
effects are transient in nature, they did not constitute a ``material 
impairment.'' The Court of Appeals decisively rejected this argument.
    The court noted OSHA's position that effects such as stinging, 
itching and burning of the eyes, tearing, wheezing, and other types of 
sensory irritation can cause severe discomfort and be seriously 
disabling in some cases. Moreover, there was evidence that workers 
exposed to these sensory irritants could be distracted as a result of 
their symptoms, thereby endangering other workers and increasing the 
risk of accidents. (Id. at 974). This evidence included information 
from NIOSH about the general consequences of sensory irritants on job 
performance, as well as testimony by commenters on the proposed rule 
supporting the view that such health effects should be regarded as 
material health impairments. While acknowledging that ``irritation'' 
covers a spectrum of effects, some of which can be minor, OSHA had 
concluded that the health effects associated with exposure to these 
substances warranted action--to ensure timely medical treatment, reduce 
the risks from increased absorption, and avoid a decreased resistance 
to infection (Id at 975). Finding OSHA's evaluation adequate, the Court 
of Appeals rejected petitioners' argument and stated the following:

    We interpret this explanation as indicating that OSHA finds that 
although minor irritation may not be a material impairment, there is 
a level at which such irritation becomes so severe that employee 
health and job performance are seriously threatened, even though 
those effects may be transitory. We find this explanation adequate. 
OSHA is not required to state with scientific certainty or precision 
the exact point at which each type of sensory or physical irritation 
becomes a material impairment. Moreover, section 6(b)(5) of the Act 
charges OSHA with addressing all forms of ``material impairment of 
health or functional capacity,'' and not exclusively ``death or 
serious physical harm'' or ``grave danger'' from exposure to toxic 
substances. See 29 U.S.C. 654(a)(1), 655(c). [Id. at 974].

    In its comments on the proposed rule, the NMA claimed that MSHA had 
overstated the court's holding. In making this claim, the NMA 
attributed to MSHA an interpretation of the holding that MSHA did not 
put forth. In fact, MSHA agrees with the NMA's interpretation as stated 
in the following paragraph and takes special note of the NMA's 
acknowledgment that transitory or reversible effects can sometimes be 
so severe as to seriously threaten miners' health and safety:

    NMA reads the Court's decision to mean (as it stated) that 
``minor irritation may not be a material impairment'' * * * but that 
irritation can reach ``a level at which [it] becomes so severe that 
employee health and job performance are seriously threatened even 
though those effects may be transitory.'' * * * AMC in 1992 and NMA 
today are fully in accord with the view of the 11th Circuit that 
when health effects, transitory or otherwise, become so ``severe'' 
as to ``seriously threaten'' a miner's health or job performance, 
the materiality threshold has been met.

    The NMA, then, apparently agrees with MSHA that sensory irritations 
and respiratory symptoms can be so severe that they cross the material 
impairment threshold, regardless of whether they are ``reversible.'' 
Therefore, as MSHA has maintained, such health effects are highly 
relevant to this risk assessment--especially since impairments of a 
miner's job performance in an underground mining environment could 
seriously threaten the safety of both the miner and his or her co-
workers. Sensory irritations may also impede miners' ability to escape 
during emergencies.
    The NMA, however, went on to emphasize that ``* * * federal appeals 
courts have held that `mild discomfort' or even `moderate irritation' 
do not constitute `significant' or `material' health effects'':

    In International Union v. Pendergrass, 878 F. 2d 389 (1989), the 
D.C. Circuit upheld OSHA's formaldehyde standard against a challenge 
that it did not adequately protect against significant 
noncarcinogenic health effects, even though OSHA had found that, at 
the permissible level of exposure, ``20% of workers suffer `mild 
discomfort', while 30% more experience `slight discomfort','' Id. at 
398. Likewise, in Texas Independent Ginners Ass'n. v. Marshall, 630 
F, 2d 398 (1980), the Fifth Circuit Court of Appeals held that minor 
reversible symptoms do not constitute material impairment unless 
OSHA shows that those effects might develop into chronic disease. 
Id. at 408-09.

    MSHA is fully aware of the distinction that courts have made 
between mild discomfort or irritation and transitory health effects 
that can seriously threaten a miner's health and safety. MSHA's 
position, after reviewing the scientific literature, public testimony, 
and comments, is that all of the health effects considered in this risk 
assessment fall into the latter category.
    iii. Health Effects Associated with PM2.5 in Ambient 
Air. There have been many studies in recent years designed to determine 
whether the mix of particulate matter in ambient air is harmful to 
health. The evidence linking particulates in air pollution to health 
problems has long been compelling enough to warrant direction from the 
Congress to limit the concentration of such particulates (see part II, 
section 5 of this preamble). In recent years, the evidence of harmful 
effects due to airborne particulates has increased, suggesting that 
``fine'' particulates (i.e., particles less than 2.5 m in 
diameter) are more strongly associated than ``coarse'' respirable 
particulates (i.e., particles greater than 2.5 m but less

[[Page 5767]]

than 10 m in diameter) with the adverse health effects 
observed (EPA, 1996).
    MSHA recognizes that there are two difficulties involved in 
utilizing the evidence from such studies in assessing risks to miners 
from occupational dpm exposures. First, although dpm is a fine 
particulate, ambient air also contains fine particulates other than 
dpm. Therefore, health effects associated with exposures to fine 
particulate matter in air pollution studies are not associated 
specifically with exposures to dpm or any other one kind of fine 
particulate matter. Second, observations of adverse health effects in 
segments of the general population do not necessarily apply to the 
population of miners. Since, due to age and selection factors, the 
health of miners differs from that of the public as a whole, it is 
possible that fine particles might not affect miners, as a group, to 
the same degree as the general population.
    Some commenters reiterated these two points, recognized by MSHA in 
the proposal, without addressing MSHA's stated reasons for including 
health effects associated with fine particulates in this risk 
assessment. There are compelling reasons why MSHA considered this body 
of evidence in this rulemaking.
    Since dpm is a type of respirable particle, information about 
health effects associated with exposures to respirable particles, and 
especially to fine particulate matter, is certainly relevant, even if 
difficult to apply directly to dpm exposures. Adverse health effects in 
the general population have been observed at ambient atmospheric 
particulate concentrations well below the dpm concentrations studied in 
occupational settings. The potency of dpm differs from the total fine 
particulate found in ambient air. This makes it difficult to establish 
a specific exposure-response relationship for dpm that is based on fine 
particle results. However, this does not mean that these results should 
be ignored in a dpm risk assessment. The available evidence of adverse 
health effects associated with fine particulates is still highly 
relevant for dpm hazard identification. Furthermore, as shown in 
Subsection 3.c.ii of this risk assessment, the fine particle research 
findings can be used to construct a rough exposure-response 
relationship for dpm, showing significantly increased risks of material 
impairment among exposed miners. MSHA's estimates are based on the best 
available epidemiologic evidence and show risks high enough to warrant 
regulatory action.
    Moreover, extensive scientific literature shows that occupational 
dust exposures contribute to the development of Chronic Obstructive 
Pulmonary Diseases (COPD), thereby compromising the pulmonary reserve 
of some miners. Miners experience COPD at a significantly higher rate 
than the general population (Becklake 1989, 1992; Oxman 1993; NIOSH 
1995). In addition, many miners also smoke tobacco. This places 
affected miners in subpopulations specifically identified as 
susceptible to the adverse health effects of respirable particle 
pollution (EPA, 1996). Some commenters (e.g., MARG) repeated MSHA's 
observation that the population of miners differs from the general 
population but failed to address MSHA's concern for miners' increased 
susceptibility due to COPD incidence and/or smoking habits. The Mine 
Act requires that standards ``* * * most adequately assure on the basis 
of the best available evidence that no miner suffer material impairment 
of health or functional capacity * * *'' (Section 101(a)(6), emphasis 
added). This most certainly authorizes MSHA to protect miners who have 
COPD and/or smoke tobacco.
    MARG also submitted the opinion that if ``* * * regulation of fine 
particulate matter is necessary, it [MSHA] should propose a rule 
dealing specifically with the issue of concern, rather than a rule that 
limits total airborne carbon or arbitrarily singles out diesel exhaust 
* * *.'' MSHA's concern is not with ``total airborne carbon'' but with 
dpm, which consists mostly of submicrometer airborne carbon. At issue 
here, however, are the adverse health effects associated with dpm 
exposure. Dpm is a type of fine particulate, and there is no evidence 
to suggest that the dpm fraction contributes less than other fine 
particulates to adverse health effects linked to exposures in ambient 
air.
    For this reason, and because miners may be especially susceptible 
to fine particle effects, MSHA has concluded, after considering the 
public comments, that the body of evidence from air pollution studies 
is highly relevant to this risk assessment. The Agency is, therefore, 
taking that evidence fully into account.
b. Acute Health Effects
    Information pertaining to the acute health effects of dpm includes 
anecdotal reports of symptoms experienced by exposed miners, studies 
based on exposures to diesel emissions, and studies based on exposures 
to particulate matter in the ambient air. These will be discussed in 
turn. Subsection 2.a.iii of this risk assessment addressed the 
relevance to dpm of studies based on exposures to particulate matter in 
the ambient air.
    Only the evidence from human studies will be addressed in this 
section. Data from genotoxicity studies and studies on laboratory 
animals will be discussed later, in Subsection 2.d on mechanisms of 
toxicity. Section 3.a and 3.b contain MSHA's interpretation of the 
evidence relating dpm exposures to acute health hazards.
    i. Symptoms Reported by Exposed Miners. Miners working in mines 
with diesel equipment have long reported adverse effects after exposure 
to diesel exhaust. For example, at the dpm workshops conducted in 1995, 
a miner reported headaches and nausea experienced by several operators 
after short periods of exposure (dpm Workshop; Mt. Vernon, IL, 1995). 
Another miner reported that smoke from poorly maintained equipment, or 
from improper fuel use, irritates the eyes, nose, and throat. ``We've 
had people sick time and time again * * * at times we've had to use 
oxygen for people to get them to come back around to where they can 
feel normal again.'' (dpm Workshop; Beckley, WV, 1995). Other miners 
(dpm Workshops; Beckley, WV, 1995; Salt Lake City, UT, 1995), reported 
similar symptoms in the various mines where they worked.
    At the 1998 public hearings on MSHA's proposed dpm rule for coal 
mines, one miner, with work experience in a coal mine utilizing diesel 
haulage equipment at the face, testified that

    * * * unlike many, I have not experienced the headaches, the 
watering of the eyes, the cold-like symptoms and walking around in 
this cloud of smoke. Maybe it's because of the maintenance programs. 
Maybe it's because of complying with ventilation. * * * after 25 
years, I have not shown any effects. [SLC, 1998].

    Other miners working at dieselized coal mines testified at those 
hearings that they had personally experienced eye irritation and/or 
respiratory ailments immediately after exposure to diesel exhaust, and 
they attributed these ailments to their exposure. For example, one 
miner attributed a case of pneumonia to a specific episode of unusually 
high exposure. (Birm., 1998) The safety and training manager of the 
mining company involved noted that ``there had been a problem 
recognized in review with that exhaust system on that particular piece 
of equipment'' and that the pneumonia may have developed due to 
``idiosyncracy of his lungs that respond to any type of a respiratory 
irritant.'' The manager suggested that this incident should not

[[Page 5768]]

be generalized to other situations but provided no evidence that the 
miner's lungs were unusually susceptible to irritation.\19\
---------------------------------------------------------------------------

    \19\ MSHA realizes the incidents related in this subsection are 
anecdotal and draws no statistical conclusions from them. Since they 
pertain to specific experiences, however, they can be useful in 
identifying a potential hazard.
---------------------------------------------------------------------------

    Another miner, who had worked at the same underground mine before 
and after diesel haulage equipment was introduced, indicated that he 
and his co-workers began experiencing acute symptoms after the diesel 
equipment was introduced. This miner suggested that these effects were 
linked to exposure, and referring to a co-worker stated:

    * * * had respiratory problems, after * * * diesel equipment was 
brought into that mine--he can take off for two weeks vacation, come 
back--after that two weeks, he felt pretty good, his respiratory 
problems would straighten up, but at the very instant that he gets 
back in the face of diesel-powered equipment, it starts up again, 
his respiratory problems will flare up again, coughing, sore throat, 
numerous problems in his chest. (Birm., 1998).

    Several other underground miners asserted there was a correlation 
between diesel exposure levels and the frequency and/or intensity of 
respiratory symptoms, eye irritations, and chest ailments. One miner, 
for example, stated:

    I've experienced [these symptoms] myself. * * * other miners 
experience the same kind of distresses * * * Some of the stresses 
you actually can feel--you don't need a gauge to measure this--your 
burning eyes, nose, throat, your chest irritation. The more you're 
exposed to, the higher this goes. This includes headaches and nausea 
and some lasting congestion, depending on how long you've been 
exposed per shift or per week.
    The men I represent have experienced more cold-like symptoms, 
especially over the past, I would say, eight to ten years, when 
diesel has really peaked and we no longer really use much of 
anything else. [SLC, 1998].

    Kahn et al. (1988) conducted a study of the prevalence and 
seriousness of such complaints, based on United Mine Workers of America 
records and subsequent interviews with the miners involved. The review 
involved reports at five underground coal mines in Utah and Colorado 
between 1974 and 1985. Of the 13 miners reporting symptoms: 12 reported 
mucous membrane irritation, headache and light-headiness; eight 
reported nausea; four reported heartburn; three reported vomiting and 
weakness, numbness, and tingling in extremities; two reported chest 
tightness; and two reported wheezing (although one of these complained 
of recurrent wheezing without exposure). All of these incidents were 
severe enough to result in lost work time due to the symptoms (which 
subsided within 24 to 48 hours).
    In comments submitted for this rulemaking, the NMA pointed out, as 
has MSHA, that the evidence presented in this subsection is anecdotal. 
The NMA, further, suggested that the cited article by Kahn et al. 
typified this kind of evidence in that it was ``totally devoid of any 
correlation to actual exposure levels.'' A lack of concurrent exposure 
measurements is, unfortunately, not restricted to anecdotal evidence; 
and MSHA must base its evaluation on the available evidence. MSHA 
recognizes the scientific limitations of anecdotal evidence and has, 
therefore, compiled and considered it separately from more formal 
evidence. MSHA nevertheless considers such evidence potentially 
valuable for identifying acute health hazards, with the understanding 
that confirmation requires more rigorous investigation.\20\
---------------------------------------------------------------------------

    \20\ MSHA sees potential value in anecdotal evidence when it 
relates to immediate experiences. MSHA regards anecdotal evidence to 
be less appropriate for identifying chronic health effects, since 
chronic effects cannot readily be linked to specific experiences. 
Accordingly, this risk assessment places little weight on anecdotal 
evidence for the chronic health hazards considered.
---------------------------------------------------------------------------

    With respect to the same article (Kahn et al., 1988), and 
notwithstanding the NMA's claim that the article was totally devoid of 
any correlation to exposure levels, the NMA also stated that MSHA:

    * * * neglects to include in the preamble the article's 
description of the conditions under which the ``overexposures'' 
occurred, e.g., ``poor engine maintenance, poor maintenance of 
emission controls, prolonged idling of machinery, engines pulling 
heavy loads, use of equipment during times when ventilation was 
disrupted (such as during a move of longwall machinery), use of 
several pieces of equipment exhausting into the fresh-air intake, 
and use of poor quality fuel.

The NMA asserted that these conditions, cited in the article, ``have 
been addressed by MSHA's final standards for diesel equipment in 
underground coal mines issued October 25, 1996.'' \21\ Furthermore, 
despite its reservations about anecdotal evidence:
---------------------------------------------------------------------------

    \21\ The 1996 regulations to which the NMA was referring do not 
apply to M/NM mines.

    NMA is mindful of the testimony of several miners in the coal 
proceeding who complained of transient irritation owing to exposure 
to diesel exhaust * * *  the October 1996 regulations together with 
the phased-in introduction of catalytic converters on all outby 
equipment and the introduction of such devices on permissible 
equipment when such technology becomes available will address the 
---------------------------------------------------------------------------
complaints raised by the miners.

    The NMA provided no evidence, however, that elimination of the 
conditions described by Kahn et al., or implementation of the 1996 
diesel regulations for coal mines, would reduce dpm levels sufficiently 
to prevent the sensory irritations and respiratory symptoms described. 
Nor did the NMA provide evidence that these are the only conditions 
under which complaints of sensory irritations and respiratory symptoms 
occur, or explain why eliminating them would reduce the need to prevent 
excessive exposure under other conditions.
    In the proposal for the present rule, MSHA requested additional 
information about such effects from medical personnel who have treated 
miners. IMC Global submitted letters from four healthcare practitioners 
in Carlsbad, NM, including three physicians. None of these 
practitioners attributed any cases of respiratory problems or other 
acute symptoms to dpm exposure. Three of the four practitioners noted 
that they had observed respiratory symptoms among exposed miners but 
attributed these symptoms to chronic lung conditions, smoking, or other 
factors. One physician stated that ``[IMC Global], which has used 
diesel equipment in its mining operations for over 20 years, has never 
experienced a single case of injury or illness caused by exposures to 
diesel particulates.''
    ii. Studies Based on Exposures to Diesel Emissions. Several 
experimental and statistical studies have been conducted to investigate 
acute effects of exposure to diesel emissions. These more formal 
studies provide data that are more scientifically rigorous than the 
anecdotal evidence presented in the preceding subsection. Unless 
otherwise indicated, diesel exhaust exposures were determined 
qualitatively.
    In a clinical study (Battigelli, 1965), volunteers were exposed to 
three concentrations of diluted diesel exhaust and then evaluated to 
determine the effects of exposure on pulmonary resistance and the 
degree of eye irritation. The investigators stated that ``levels 
utilized for these controlled exposures are comparable to realistic 
values such as those found in railroad shops.'' No statistically 
significant change in pulmonary function was detected, but exposure for 
ten minutes to diesel exhaust diluted to the middle level produced 
``intolerable'' irritation in some subjects while the average 
irritation score was midway between ``some'' irritation and a 
``conspicuous but tolerable'' irritation level. Diluting

[[Page 5769]]

the concentration by 50% substantially reduced the irritation. At the 
highest exposure level, more than 50 percent of the volunteers 
discontinued the experiment before 10 minutes because of 
``intolerable'' eye irritation.
    A study of underground iron ore miners exposed to diesel emissions 
found no difference in spirometry measurements taken before and after a 
work shift (Jorgensen and Svensson 1970). Similarly, another study of 
coal miners exposed to diesel emissions detected no statistically 
significant relationship between exposure and changes in pulmonary 
function (Ames et al. 1982). However, the authors noted that the lack 
of a statistically significant result might be due to the low 
concentrations of diesel emissions involved.
    Gamble et al. (1978) observed decreases in pulmonary function over 
a single shift in salt miners exposed to diesel emissions. Pulmonary 
function appeared to deteriorate in relation to the concentration of 
diesel exhaust, as indicated by NO2; but this effect was 
confounded by the presence of NO2 due to the use of 
explosives.
    Gamble et al. (1987a) assessed response to diesel exposure among 
232 bus garage workers by means of a questionnaire and before- and 
after-shift spirometry. No significant relationship was detected 
between diesel exposure and change in pulmonary function. However, 
after adjusting for age and smoking status, a significantly elevated 
prevalence of reported symptoms was found in the high-exposure group. 
The strongest associations with exposure were found for eye irritation, 
labored breathing, chest tightness, and wheeze. The questionnaire was 
also used to compare various acute symptoms reported by the garage 
workers and a similar population of workers at a lead acid battery 
plant who were not exposed to diesel fumes. The prevalence of work-
related eye irritations, headaches, difficult or labored breathing, 
nausea, and wheeze was significantly higher in the diesel bus garage 
workers, but the prevalence of work-related sneezing was significantly 
lower.
    Ulfvarson et al. (1987) studied effects over a single shift on 47 
stevedores exposed to dpm at particle concentrations ranging from 130 
g/m 3 to 1000 g/m 3. Diesel 
particulate concentrations were determined by collecting particles on 
glass fiber filters of unspecified efficiency. A statistically 
significant loss of pulmonary function was observed, with recovery 
after 3 days of no occupational exposure.
    To investigate whether removal of the particles from diesel exhaust 
might reduce the ``acute irritative effect on the lungs'' observed in 
their earlier study, Ulfvarson and Alexandersson (1990) compared 
pulmonary effects in a group of 24 stevedores exposed to unfiltered 
diesel exhaust to a group of 18 stevedores exposed to filtered exhaust, 
and to a control group of 17 occupationally unexposed workers. The 
filters used were specially constructed from 144 layers of glass fiber 
with ``99.97% degrees of retention of dioctylphthalate mist with 
particle size 0.3 m.'' Workers in all three groups were 
nonsmokers and had normal spirometry values, adjusted for sex, age, and 
height, prior to the experimental workshift.
    In addition to confirming the earlier observation of significantly 
reduced pulmonary function after a single shift of occupational 
exposure, the study found that the stevedores in the group exposed only 
to filtered exhaust had 50-60% less of a decline in forced vital 
capacity (FVC) than did those stevedores who worked with unfiltered 
equipment. Similar results were observed for a subgroup of six 
stevedores who were exposed to filtered exhaust on one shift and 
unfiltered exhaust on another. No loss of pulmonary function was 
observed for the unexposed control group. The authors suggested that 
these results ``support the idea that the irritative effect of diesel 
exhausts [sic] to the lungs is the result of an interaction between 
particles and gaseous components and not of the gaseous components 
alone.'' They concluded that ``* * * it should be a useful practice to 
filter off particles from diesel exhausts in work places even if 
potentially irritant gases remain in the emissions'' and that ``removal 
of the particulate fraction by filtering is an important factor in 
reducing the adverse effect of diesel exhaust on pulmonary function.''
    Rudell et al. (1996) carried out a series of double-blind 
experiments on 12 healthy, non-smoking subjects to investigate whether 
a particle trap on the tailpipe of an idling diesel engine would reduce 
acute effects of diesel exhaust, compared with exposure to unfiltered 
exhaust. Symptoms associated with exposure included headache, 
dizziness, nausea, tiredness, tightness of chest, coughing, and 
difficulty in breathing. The most prominent symptoms were found to be 
irritation of the eyes and nose, and a sensation of unpleasant smell. 
Among the various pulmonary function tests performed, exposure was 
found to result in significant changes only as measured by increased 
airway resistance and specific airway resistance. The ceramic wall flow 
particle trap reduced the number of particles by 46 percent, but 
resulted in no significant attenuation of symptoms or lung function 
effects. The authors concluded that diluted diesel exhaust caused 
increased irritant symptoms of the eyes and nose, unpleasant smell, and 
bronchoconstriction, but that the 46-percent reduction in median 
particle number concentration observed was not sufficient to protect 
against these effects in the populations studied.
    Wade and Newman (1993) documented three cases in which railroad 
workers developed persistent asthma following exposure to diesel 
emissions while riding immediately behind the lead engines of trains 
having no caboose. None of these workers were smokers or had any prior 
history of asthma or other respiratory disease. Asthma diagnosis was 
based on symptoms, pulmonary function tests, and measurement of airway 
hyperreactivity to methacholine or exercise.
    Although MSHA is not aware of any other published report directly 
relating diesel emissions exposures to the development of asthma, there 
have been a number of recent studies indicating that dpm exposure can 
induce bronchial inflammation and respiratory immunological allergic 
responses in humans. Studies published through 1997 are reviewed in 
Peterson and Saxon (1996) and Diaz-Sanchez (1997).
    Diaz-Sanchez et al.(1994) challenged healthy human volunteers by 
spraying 300 g dpm into their nostrils.\22\ Immunoglobulin E 
(IgE) binds to mast cells where it binds antigen leading to secretion 
of biologically active amines (e.g., histamine) causing dilation and 
increased permeability of blood vessels. These amines are largely 
responsible for clinical manifestations of such allergic reactions as 
hay fever, asthma, and hives. Enhanced IgE levels were found in nasal 
washes in as little as 24 hours, with peak production observed 4 days 
after the dpm was administered.\23\ No effect was observed on the 
levels of other immunoglobulin proteins. The selective enhancement of 
local IgE production was demonstrated by a dramatic increase in IgE-
secreting cells. The authors suggested that dpm may augment human 
allergic disease

[[Page 5770]]

responses by enhancing the production of IgE antibodies. Building on 
these results, Diaz-Sanchez et al.(1996) measured cytokine production 
in nasal lavage cells from healthy human volunteers challenged with 150 
g dpm sprayed into each nostril. Based on the responses 
observed, including a broad increase in cytokine production, along with 
the results of the 1994 paper, the authors concluded that dpm exposure 
contributes to enhanced local IgE production and thus plays a role in 
allergic airway disease.
---------------------------------------------------------------------------

    \22\ Assuming that a working miner inhales approximately 1.25 
m3 of air per hour, this dose corresponds to a 1-hour 
exposure at a dpm concentration of 240 g/m3.
    \23\ IgE is one of five types of immunoglobulin, which are 
proteins produced in response to allergens. Cytokine (mentioned 
later) is a substance involved in regulating IgE production.
---------------------------------------------------------------------------

    Salvi et al. (1999) exposed healthy human volunteers to diluted 
diesel exhaust at a dpm concentration of 300 g/m3 
for one hour with intermittent exercise. Although there were no changes 
in pulmonary function, there were significant increases in various 
markers of allergic response in airway lavage fluid. Bronchial biopsies 
obtained six hours after exposure also showed significant increases in 
markers of immunologic response in the bronchial tissue. Significant 
increases in other markers of immunologic response were also observed 
in peripheral blood following exposure. A marked cellular inflammatory 
response in the airways was reported. The authors concluded that ``at 
high ambient concentrations, acute short-term DE [diesel exhaust] 
exposure produces a well-defined and marked systemic and pulmonary 
inflammatory response in healthy human volunteers, which is 
underestimated by standard lung function measurements.''
    iii. Studies Based on Exposures to Particulate Matter in Ambient 
Air. Due to an incident in Belgium's industrial Meuse Valley, it was 
known as early as the 1930s that large increases in particulate air 
pollution, created by winter weather inversions, could be associated 
with large simultaneous increases in mortality and morbidity. More than 
60 persons died from this incident, and several hundred suffered 
respiratory problems. The mortality rate during the episode was more 
than ten times higher than normal, and it was estimated that over 3,000 
sudden deaths would occur if a similar incident occurred in London. 
Although no measurements of pollutants in the ambient air during the 
episode are available, high PM levels were obviously present (EPA, 
1996).
    A significant elevation in particulate matter (along with 
SO2 and its oxidation products) was measured during a 1948 
incident in Donora, PA. Of the Donora population, 42.7 percent 
experienced some acute adverse health effect, mainly due to irritation 
of the respiratory tract. Twelve percent of the population reported 
difficulty in breathing, with a steep rise in frequency as age 
progressed to 55 years (Schrenk, 1949).
    Approximately as projected by Firket (1931), an estimated 4,000 
deaths occurred in response to a 1952 episode of extreme air pollution 
in London. The nature of these deaths is unknown, but there is clear 
evidence that bronchial irritation, dyspnea, bronchospasm, and, in some 
cases, cyanosis occurred with unusual prevalence (Martin, 1964).
    These three episodes ``left little doubt about causality in regard 
to the induction of serious health effects by very high concentrations 
of particle-laden air pollutant mixtures'' and stimulated additional 
research to characterize exposure-response relationships (EPA, 1996). 
Based on several analyses of the 1952 London data, along with several 
additional acute exposure mortality analyses of London data covering 
later time periods, the U.S. Environmental Protection Agency (EPA) 
concluded that increased risk of mortality is associated with exposure 
to combined particulate and SO2 levels in the range of 500-
1000 g/m 3. The EPA also concluded that relatively 
small, but statistically significant increases in mortality risk exist 
at particulate (but not SO2) levels below 500 g/m 
3, with no indications of a specific threshold level yet 
indicated at lower concentrations (EPA, 1986).
    Subsequently, between 1986 and 1996, increasingly sophisticated 
techniques of particulate measurement and statistical analysis have 
enabled investigators to address these questions more quantitatively. 
The studies on acute effects carried out since 1986 are reviewed in the 
1996 EPA Air Quality Criteria for Particulate Matter, which forms the 
basis for the discussion below (EPA, 1996).
    At least 21 studies have been conducted that evaluate associations 
between acute mortality and morbidity effects and various measures of 
fine particulate levels in the ambient air. These studies are 
identified in Tables III-2 and III-3. Table III-2 lists 11 studies that 
measured primarily fine particulate matter using filter-based optical 
techniques and, therefore, provide mainly qualitative support for 
associating observed effects with fine particles. Table III-3 lists 
quantitative results from 10 studies that reported gravimetric 
measurements of either the fine particulate fraction or of components, 
such as sulfates, that serve as indicators or surrogates of fine 
particulate exposures.

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    A total of 38 studies examining relationships between short-term 
particulate levels and increased mortality, including nine with fine 
particulate measurements, were published between 1988 and 1996 (EPA, 
1996). Most of these found statistically significant positive 
associations. Daily or several-day elevations of particulate 
concentrations, at average levels as low as 18-58 /m 
3, were associated with increased mortality, with stronger 
relationships observed in those with preexisting respiratory and 
cardiovascular disease. Overall, these studies suggest that an increase 
of 50 g/m 3 in the 24-hour average of 
PM10 is associated with a 2.5 to 5-percent increase in the 
risk of mortality in the general population, excluding accidents, 
suicides, and homicides. Based on Schwartz et al. (1996), the relative 
risk (RR) of mortality in the general population increases by about 2.6 
to 5.5 percent per 25 g/m 3 of fine particulate 
(PM2.5) (EPA, 1996). More specifically, Schwartz et al. 
(1996) reported significantly elevated risks of mortality due to 
pneumonia, chronic obstructive pulmonary disease (COPD), and ischemic 
heart disease (IHD). For these three causes of death, the estimated 
increases in risk per incremental increase of 10 g/m 
3 in the concentration of PM2.5 were 4.0 percent, 
3.3 percent, and 2.1 percent, respectively. Each of these three results 
was statistically significant at a 95-percent confidence level.
    A total of 22 studies were published on associations between short-
term particulate levels and hospital admissions, outpatient visits, and 
emergency room visits for respiratory disease, Chronic Obstructive 
Pulmonary Disease (COPD), pneumonia, and heart disease (EPA, 1996). 
Fifteen of these studies were focused on the elderly. Of the seven that 
dealt with all ages (or in one case, persons less than 65 years old), 
all showed positive results. All of the five studies relating fine 
particulate measurements to increased hospitalization, listed in Tables 
III-2 and III-3, dealt with general age populations and showed 
statistically significant associations. The estimated increase in risk 
ranges from 3 to 16 percent per 25 g/m\3\ of fine particulate. 
Overall, these studies are indicative of acute morbidity effects being 
related to fine particulate matter and support the mortality findings.
    Most of the 14 published quantitative studies on ambient 
particulate exposures and acute respiratory diseases were restricted to 
children (EPA, 1996, Table 12-12). Although they generally showed 
positive associations, and may be of considerable biological relevance, 
evidence of toxicity in children is not necessarily applicable to 
adults. The few studies on adults have not produced statistically 
significant evidence of a relationship.
    Thirteen studies since 1982 have investigated associations between 
ambient particulate levels and loss of pulmonary function (EPA, 1996, 
Table 12-13). In general, these studies suggest a short term effect, 
especially in symptomatic groups such as asthmatics, but most were 
carried out on children only. In a study of adults with mild COPD, Pope 
and Kanner (1993) found a 2910 ml decrease in 1-second 
Forced Expiratory Volume (FEV1) per 50 g/m\3\ 
increase in PM10, which is similar in magnitude to the 
change generally observed in the studies on children. In another study 
of adults, with PM10 ranging from 4 to 137 g/m\3\, 
Dusseldorp et al. (1995) found 45 and 77 ml/sec decreases, 
respectively, for evening and morning Peak Expiratory Flow Rate (PEFR) 
per 50 g/m\3\ increase in PM10 (EPA, 1996). In the 
only study carried out on adults that specifically measured fine 
particulate (PM2.5), Perry et al. (1983) did not detect any 
association of exposure with loss of pulmonary function. This study, 
however, was conducted on only 24 adults (all asthmatics) exposed at 
relatively low concentrations of PM2.5 and,therefore, had 
very little power to detect any such association.
c. Chronic Health Effects
    During the 1995 dpm workshops, miners reported observable adverse 
health effects among those who have worked a long time in dieselized 
mines. For example, a miner (dpm Workshop; Salt Lake City, UT, 1995), 
stated that miners who work with diesel ``have spit up black stuff 
every night, big black--what they call black (expletive) * * * [they] 
have the congestion every night * * * the 60-year-old man working there 
40 years.'' Similarly, in comments submitted in response to MSHA's 
proposed dpm regulations, several miners reported cancers and chronic 
respiratory ailments they attributed to dpm exposure.
    Scientific investigation of the chronic health effects of dpm 
exposure includes studies based specifically on exposures to diesel 
emissions and studies based more generally on exposures to fine 
particulate matter in the ambient air. Only the evidence from human 
studies will be addressed in this section of the risk assessment. Data 
from genotoxicity studies and studies on laboratory animals will be 
discussed later, in Subsection 2.d on mechanisms of toxicity. 
Subsection 3.a(iii) contains MSHA's interpretation of the evidence 
relating dpm exposures to one chronic health hazard: lung cancer.
    i. Studies Based on Exposures to Diesel Emissions. The discussion 
will (1) summarize the epidemiologic literature on chronic effects 
other than cancer, and then (2) concentrate on the epidemiology of 
cancer in workers exposed to dpm.
(1) Chronic Effects other than Cancer
    A number of epidemiologic studies have investigated relationships 
between diesel exposure and the risk of developing persistent 
respiratory symptoms (i.e., chronic cough, chronic phlegm, and 
breathlessness) or measurable loss in lung function. Three studies 
involved coal miners (Reger et al., 1982; Ames et al., 1984; Jacobsen 
et al., 1988); four studies involved metal and nonmetal miners 
(Jorgenson & Svensson, 1970; Attfield, 1979; Attfield et al., 1982; 
Gamble et al., 1983). Three studies involved other groups of workers--
railroad workers (Battigelli et al., 1964), bus garage workers (Gamble 
et al., 1987), and stevedores (Purdham et al., 1987).
    Reger et al. (1982) examined the prevalence of respiratory symptoms 
and the level of pulmonary function among more than 1,600 underground 
and surface U.S. coal miners, comparing results for workers (matched 
for smoking status, age, height, and years worked underground) at 
diesel and non-diesel mines. Those working at underground dieselized 
mines showed some increased respiratory symptoms and reduced lung 
function, but a similar pattern was found in surface miners who 
presumably would have experienced less diesel exposure. Miners in the 
dieselized mines, however, had worked underground for less than 5 years 
on average.
    In a study of 1,118 U.S. coal miners, Ames et al. (1984) did not 
detect any pattern of chronic respiratory effects associated with 
exposure to diesel emissions. The analysis, however, took no account of 
baseline differences in lung function or symptom prevalence, and the 
authors noted a low level of exposure to diesel-exhaust contaminants in 
the exposed population.
    In a cohort of 19,901 British coal miners investigated over a 5-
year period, Jacobsen et al. (1988) found increased work absence due to 
self-reported chest illness in underground workers exposed to diesel 
exhaust, as compared to surface workers, but found no correlation with 
their estimated level of exposure.

[[Page 5774]]

    Jorgenson & Svensson (1970) found higher rates of chronic 
productive bronchitis, for both smokers and nonsmokers, among Swedish 
underground iron ore miners exposed to diesel exhaust as compared to 
surface workers at the same mine. No significant difference was found 
in spirometry results.
    Using questionnaires collected from 4,924 miners at 21 U.S. metal 
and nonmetal mines, Attfield (1979) evaluated the effects of exposure 
to silica dust and diesel exhaust and obtained inconclusive results 
with respect to diesel exposure. For both smokers and non-smokers, 
miners occupationally exposed to diesel for five or more years showed 
an elevated prevalence of persistent cough, persistent phlegm, and 
shortness of breath, as compared to miners exposed for less than five 
years, but the differences were not statistically significant. Four 
quantitative indicators of diesel use failed to show consistent trends 
with symptoms and lung function.
    Attfield et al. (1982) reported on a medical surveillance study of 
630 white male miners at 6 U.S. potash mines. No relationships were 
found between measures of diesel use or exposure and various health 
indices, based on self-reported respiratory symptoms, chest 
radiographs, and spirometry.
    In a study of U.S. salt miners, Gamble and Jones (1983) observed 
some elevation in cough, phlegm, and dyspnea associated with mines 
ranked according to level of diesel exhaust exposure. No association 
between respiratory symptoms and estimated cumulative diesel exposure 
was found after adjusting for differences among mines. However, since 
the mines varied widely with respect to diesel exposure levels, this 
adjustment may have masked a relationship.
    Battigelli et al. (1964) compared pulmonary function and complaints 
of respiratory symptoms in 210 U.S. railroad repair shop employees, 
exposed to diesel for an average of 10 years, to a control group of 154 
unexposed railroad workers. Respiratory symptoms were less prevalent in 
the exposed group, and there was no difference in pulmonary function; 
but no adjustment was made for differences in smoking habits.
    In a study of workers at four diesel bus garages in two U.S. 
cities, Gamble et al. (1987b) investigated relationships between job 
tenure (as a surrogate for cumulative exposure) and respiratory 
symptoms, chest radiographs, and pulmonary function. The study 
population was also compared to an unexposed control group of workers 
with similar socioeconomic background. After indirect adjustment for 
age, race, and smoking, the exposed workers showed an increased 
prevalence of cough, phlegm, and wheezing, but no association was found 
with job tenure. Age- and height-adjusted pulmonary function was found 
to decline with duration of exposure, but was elevated on average, as 
compared to the control group. The number of positive radiographs was 
too small to support any conclusions. The authors concluded that the 
exposed workers may have experienced some chronic respiratory effects.
    Purdham et al. (1987) compared baseline pulmonary function and 
respiratory symptoms in 17 exposed Canadian stevedores to a control 
group of 11 port office workers. After adjustment for smoking, there 
was no statistically significant difference in self-reported 
respiratory symptoms between the two groups. However, after adjustment 
for smoking, age, and height, exposed workers showed lower baseline 
pulmonary function, consistent with an obstructive ventilatory defect, 
as compared to both the control group and the general metropolitan 
population.
    In a review of these studies, Cohen and Higgins (1995) concluded 
that they did not provide strong or consistent evidence for chronic, 
nonmalignant respiratory effects associated with occupational exposure 
to diesel exhaust. These reviewers stated, however, that ``several 
studies are suggestive of such effects * * * particularly when viewed 
in the context of possible biases in study design and analysis.'' Glenn 
et al (1983) noted that the studies of chronic respiratory effects 
carried out by NIOSH researchers in coal, salt, potash, and trona mines 
all ``revealed an excess of cough and phlegm in the diesel exposed 
group.'' IPCS (1996) noted that ``[a]lthough excess respiratory 
symptoms and reduced pulmonary function have been reported in some 
studies, it is not clear whether these are long-term effects of 
exposure.'' Similarly, Morgan et al. (1997) concluded that while there 
is ``some evidence that the chronic inhalation of diesel fumes leads to 
the development of cough and sputum, that is chronic bronchitis, it is 
usually impossible to show a cause and effect relationship * * *.'' 
MSHA agrees that these dpm studies considers them to be suggestive of 
adverse chronic, non-cancerous respiratory effects.
(2) Cancer
    Because diesel exhaust has long been known to contain carcinogenic 
compounds (e.g., benzene in the gaseous fraction and benzopyrene and 
nitropyrene in the dpm fraction), a great deal of research has been 
conducted to determine if occupational exposure to diesel exhaust 
actually results in an increased risk of cancer. Evidence that exposure 
to dpm increases the risk of developing cancer comes from three kinds 
of studies: human studies, genotoxicity studies, and animal studies. In 
this risk assessment, MSHA has placed the most weight on evidence from 
the human epidemiologic studies and views the genotoxicity and animal 
studies as lending support to the epidemiologic evidence.
    In the epidemiologic studies, it is generally impossible to 
disassociate exposure to dpm from exposure to the gasses and vapors 
that form the remainder of whole diesel exhaust. However, the animal 
evidence shows no significant increase in the risk of lung cancer from 
exposure to the gaseous fraction alone (Heinrich et al., 1986, 1995; 
Iwai et al., 1986; Brightwell et al., 1986). Therefore, dpm, rather 
than the gaseous fraction of diesel exhaust, is usually assumed to be 
the agent associated with any excess prevalence of lung cancer observed 
in the epidemiologic studies. Subsection 2.d of this risk assessment 
contains a summary of evidence supporting this assumption.
(a) Lung Cancer
    MSHA evaluated 47 epidemiologic studies examining the prevalence of 
lung cancer within groups of workers occupationally exposed to dpm. 
This includes four studies not included in MSHA's risk assessment as 
originally proposed.\24\ The earliest of these studies was published in 
1957 and the latest in 1999. The most recent published reviews of these 
studies are by Mauderly (1992), Cohen and Higgins (1995), Muscat and 
Wynder (1995), IPCS (1996), Stober and Abel (1996), Cox (1997), Morgan 
et al. (1997), Cal-EPA (1998), ACGIH (1998), and U.S. EPA (1999). In 
response to both the ANPRM and the 1998 proposals, several commenters 
also provided MSHA with their own reviews of many of these studies. In 
arriving at its conclusions, MSHA considered all of these reviews,

[[Page 5775]]

including those of the commenters, as well as the 47 source studies 
available to MSHA.
---------------------------------------------------------------------------

    \24\ One of these studies (Christie et al., 1995) was cited in 
the discussion on mechanisms of toxicity but not considered in 
connection with studies involving dpm exposures. Several commenters 
advocated that it be considered. The other three were published in 
1997 or later. Johnston et al. (1997) was introduced to these 
proceedings in 64FR7144. Saverin et al. (1999) is the published 
English version of a German study submitted as part of the public 
comments by NIOSH on May 27, 1999. The remaining study is Bruske-
Hohlfeld et al. (1999).
---------------------------------------------------------------------------

    In addition, MSHA relied on two comprehensive statistical ``meta-
analyses'' \25\ of the epidemiologic literature: Lipsett and Campleman 
(1999)\26\ and Bhatia et al. (1998).\27\ These meta-analyses, which 
weight, combine, and analyze data from the various epidemiologic 
studies, were themselves the subject of considerable public comment and 
are discussed primarily in Subsection 3.a.iii of this risk assessment. 
The present section tabulates results of the studies and addresses 
their individual strengths and weaknesses. Interpretation and 
evaluation of the collective evidence, including discussion of 
potential publication bias or any other systematic biases, is deferred 
to Subsection 3.a.iii.
---------------------------------------------------------------------------

    \25\ MSHA restricts the term ``meta-analysis'' to formal, 
statistical analyses of the pooled data taken from several studies. 
Some commenters (and Cox in the article itself) referred to the 
review by Cox (op.cit.) as a meta-analysis. Although this article 
seeks to identify characteristics of the individual studies that 
might account for the general pattern of results, it performs no 
statistical analysis on the pooled epidemiologic data. For this 
reason, MSHA does not regard the Cox article as a meta-analysis in 
the same sense as the two studies so identified. MSHA does, however, 
recognize that the Cox article evaluates and rejects the collective 
evidence for causality, based on the common characteristics 
identified. In that context, Cox's arguments and conclusions are 
addressed in Subsection 3.a.iii. Cox also presents a statistical 
analysis of data from one of the studies, and that portion of the 
article is considered here, along with his observations about other 
individual studies.
    \26\ MSHA's risk assessment as originally proposed cited an 
unpublished version, attributed to Lipsett and Alexexeff (1998), of 
essentially the same meta-analysis. Both the 1999 and 1998 versions 
are now in the public record.
    \27\ Silverman (1998) reviewed the meta-analysis by Bhatia et 
al. (op cit.) and discussed, in general terms, the body of available 
epidemiologic evidence on which it is based. Some commenters stated 
that MSHA had not sufficiently considered Silverman's views on the 
limitations of this evidence. MSHA has thoroughly considered these 
views and addresses them in Subsection 3.a.(iii).
---------------------------------------------------------------------------

    Tables III-4 (27 cohort studies) and III-5 (20 case-control 
studies) identify all 47 known epidemiologic studies that MSHA 
considers relevant to an assessment of lung cancer risk associated with 
dpm exposure.\28\ These tables include, for each of the 47 studies 
listed, a brief description of the study and its findings, the method 
of exposure assessment, and comments on potential biases or other 
limitations. Presence or absence of an adjustment for smoking habits is 
highlighted, and adjustments for other potentially confounding factors 
are indicated when applicable. Although MSHA constructed these tables 
based primarily on its own reading of the 48 source publications, the 
tables also incorporate strengths and weaknesses noted in the 
literature reviews and/or in the public comments submitted.
---------------------------------------------------------------------------

    \28\ For simplicity, the epidemiologic studies considered here 
are placed into two broad categories. A cohort study compares the 
health of persons having different exposures, diets, etc. A case-
control study starts with two defined groups known to differ in 
health and compares their exposure characteristics.
---------------------------------------------------------------------------

    Some degree of association between occupational dpm exposure and an 
excess prevalence of lung cancer was reported in 41 of the 47 studies 
reviewed by MSHA: 22 of the 27 cohort studies and 19 of the 20 case-
control studies. Despite some commenters' use of conflicting 
terminology, which will be addressed below, MSHA refers to these 41 
studies as ``positive.'' The 22 positive cohort studies in Table III-4 
are identified as those reporting a relative risk (RR) or standardized 
mortality ratio (SMR) exceeding 1.0. The 19 positive case-control 
studies in Table III-5 are identified as those reporting an RR or odds 
ratio (OR) exceeding 1.0. A study does not need to be statistically 
significant (at the 0.05 level) or meet all criteria described, in 
order to be considered a ``positive'' study. The six remaining studies 
were entirely negative: they reported a deficit in the prevalence of 
lung cancer among exposed workers, relative to whatever population was 
used in the study as a basis for comparison. These six negative studies 
are identified as those reporting no relative risk (RR), standard 
mortality ratio (SMR), or odds ratio (OR) greater than 1.0.\29\
---------------------------------------------------------------------------

    \29\ The six entirely negative studies are: Kaplan (1959); 
DeCoufle et al. (1977); Waller (1981); Edling et al. (1987); Bender 
et al. (1989); Christie et al. (1995).
---------------------------------------------------------------------------

    MSHA recognizes that these 47 studies are not of equal importance 
for determining whether dpm exposure leads to an increased risk of lung 
cancer. Some of the studies provide much better evidence than others. 
Furthermore, since no epidemiologic study can be perfectly controlled, 
the studies exhibit various strengths and weaknesses, as described by 
both this risk assessment and a number of commenters. Several 
commenters, and some of the reviewers cited above, focused on the 
weaknesses and argued that none of the existing studies is conclusive. 
MSHA, in accordance with other reviewers and commenters, maintains: (1) 
that the weaknesses identified in both negative and positive studies 
mainly cause underestimation of risks associated with high occupational 
dpm exposure; (2) that it is legitimate to base conclusions on the 
combined weight of all available evidence and that, therefore, it is 
not necessary for any individual study to be conclusive; and (3) that 
even though the 41 positive studies vary a great deal in strength, 
nearly all of them contribute something to the weight of positive 
evidence.

                 Table III-4.--Summary of Information From 27 Cohort Studies on Lung Cancer and Occupational Exposure to Diesel Exhaust
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                      Number of     Follow-up      Exposure       Smk.                      Stat.
             Study                 Occupation         subjects        period      assessment      adj.      Findings a      sig.b         Comments
--------------------------------------------------------------------------------------------------------------------------------------------------------
Ahlberg et al. (1981).........  Male truck        35,883..........    1961-73  Occupation only.          RR = 1.33 for       (*)   Risk relative to
                                 drivers.                                                                 drivers of                males employed in
                                                                                                          ``ordinary''              trades thought to
                                                                                                          trucks.                   have no exposure to
                                                                                                                                    ``petroleum products
                                                                                                                                    or other
                                                                                                                                    chemicals.''
                                                                                                                                    Comparison
                                                                                                                                    controlled for age
                                                                                                                                    and province of
                                                                                                                                    residence (Sweden).
                                                                                                                                    Based on comparison
                                                                                                                                    of smoking habits
                                                                                                                                    between truck
                                                                                                                                    drivers and general
                                                                                                                                    Stockholm
                                                                                                                                    population, authors
                                                                                                                                    concluded that
                                                                                                                                    excess rate of lung
                                                                                                                                    cancer could not be
                                                                                                                                    entirely attributed
                                                                                                                                    to smoking.

[[Page 5776]]

 
Ahlman et al. (1991)..........  Underground       597.............    1968-86  Job histories             RR = 1.45                 Age-adjusted relative
                                 sulfide ore                                    from personnel            overall. RR =             risk compared to
                                 miners.                                        records.                  2.9 for 45-64             males living in same
                                                                                Measurements of           age group                 area of Finland. No
                                                                                alpha energy              (calculated by            excess observed
                                                                                concentration             MSHA).                    among 338 surface
                                                                                from radon                                          workers at same
                                                                                daughters at                                        mines, with similar
                                                                                each mine                                           smoking and alcohol
                                                                                worked.                                             consumption, based
                                                                                                                                    on questionnaire.
                                                                                                                                    Based on calculation
                                                                                                                                    of expected lung
                                                                                                                                    cancers due to
                                                                                                                                    radon, excess risk
                                                                                                                                    attributed by author
                                                                                                                                    partly to radon
                                                                                                                                    exposure and partly
                                                                                                                                    to diesel exhaust &
                                                                                                                                    silica exposure.
Balarajan & McDowall (1988)...  Professional      3,392...........    1950-84  Occupation only.          SMR = 0.86 for      (*)   Possibly higher rates
                                 drivers.                                                                 taxi drivers..            of smoking among bus
                                                                                                         SMR = 1.42 for             and truck drivers
                                                                                                          bus drivers..             than among taxi
                                                                                                         SMR = 1.59 for             drivers.
                                                                                                          truck drivers.
Bender et al. (1989)..........  Highway           4,849...........    1945-84  Occupation only.          SMR = 0.69                No adjustment for
                                 maintenance                                                                                        healthy worker
                                 workers.                                                                                           effect.
Boffetta et al. (1988)........  Railroad workers  2,973...........    1982-84  Occupation and            RR = 1.24 for             Risk relative to
                                Truck drivers...  16,208..........              diesel exposure           truck drivers.            reporting that they
                                Heavy Eq. Op's..  855.............              by                       RR = 1.59 for       (*)    never worked in
                                Miners..........  2,034...........              questionnaire.            railroad           (*)    these four
                                                                                                          workers.                  occupations and were
                                                                                                         RR = 2.60 for              never occupationally
                                                                                                          heavy Eq. Op's.           exposed to diesel
                                                                                                         RR = 2.67 for              exhaust. Adjusted
                                                                                                          miners.                   for age and smoking
                                                                                                                                    only.
    Do........................  All workers.....  476,648.........    1982-84  Occupation and            RR = 1.05 for 1-          Based on self-
                                                                                diesel exposure           15 years. RR =            reported exposure,
                                                                                by                        1.21 for 16+              relative to
                                                                                questionnaire.            years.                    unexposed workers.
                                                                                                                                    Adjusted for
                                                                                                                                    occupational
                                                                                                                                    exposures to
                                                                                                                                    asbestos, coal and
                                                                                                                                    stone dusts, coal
                                                                                                                                    tar & pitch, and
                                                                                                                                    gasoline exhaust (in
                                                                                                                                    addition to age and
                                                                                                                                    smoking). Possible
                                                                                                                                    biases due to
                                                                                                                                    volunteered
                                                                                                                                    participation and
                                                                                                                                    elevated lung cancer
                                                                                                                                    rate among 98,026
                                                                                                                                    subjects with
                                                                                                                                    unknown dpm
                                                                                                                                    exposure.
Christie et al. (1994, 1995)..  Coal miners.....  23,630..........    1973-92  Occupation only.          SMR = 0.76                No adjustment for
                                                                                                                                    healthy worker
                                                                                                                                    effect. Cohort
                                                                                                                                    includes workers who
                                                                                                                                    entered workforce up
                                                                                                                                    through 1992. SMR
                                                                                                                                    reported to be
                                                                                                                                    greater than for
                                                                                                                                    occupationally
                                                                                                                                    unexposed petroleum
                                                                                                                                    workers.
Dubrow & Wegman (1984)........  Truck & tractor   Not reported....    1971-73  Occupation only.          sMOR = 1.73         (*)   Excess cancers
                                 drivers.                                                                 based on 176              observed over the
                                                                                                          deaths.                   entire respiratory
                                                                                                                                    system and upper
                                                                                                                                    alimentary tract.
Edling et al. (1987)..........  Bus workers.....  694.............    1951-83  Occupation only.          SMR = 0.7 for             Small size of cohort
                                                                                                          overall cohort.           lacks statistical
                                                                                                                                    power to detect
                                                                                                                                    excess risk of lung
                                                                                                                                    cancer. No
                                                                                                                                    adjustment for
                                                                                                                                    healthy worker
                                                                                                                                    effect.
Garshick et al. (1988, 1991)..  Railroad workers  55,395 (1991        1959-80  Job in 1959 &             RR = 1.31 for 1-    (*)   Adjusted for attained
                                                   report).                     years of diesel           4 years.                  age (1991 report).
                                                                                exposure since           RR = 1.28 for 5-    (*)    Cumulative diesel
                                                                                1959.                     9 years..                 exposure-years
                                                                                                         RR = 1.19 for 10-   (*)    lagged by 5 years.
                                                                                                          14 years..           .    Subjects with likely
                                                                                                         RR = 1.40 for 15           asbestos exposure
                                                                                                          or more years..           excluded from
                                                                                                                                    cohort.
                                                                                                                                    Statistically
                                                                                                                                    significant results
                                                                                                                                    corroborated if
                                                                                                                                    12,872 shopworkers
                                                                                                                                    and hostlers
                                                                                                                                    possibly exposed to
                                                                                                                                    asbestos are also
                                                                                                                                    excluded. Missing
                                                                                                                                    12% of death
                                                                                                                                    certificates.
                                                                                                                                    Cigarette smoking
                                                                                                                                    judged to be
                                                                                                                                    uncorrelated with
                                                                                                                                    diesel exposure
                                                                                                                                    within cohort.
                                                                                                                                    Higher RR for each
                                                                                                                                    exposure group if
                                                                                                                                    shopworkers and
                                                                                                                                    hostlers are
                                                                                                                                    excluded.
Guberan et al (1992)..........  Professional      1,726...........    1961-86  Occupation only.          SMR = 1.50......    (*)   Approximately 1/3 to
                                 drivers.                                                                                           1/4 of cohort
                                                                                                                                    reported to be long-
                                                                                                                                    haul truck drivers.
                                                                                                                                    SMR based on
                                                                                                                                    regional lung cancer
                                                                                                                                    mortality rate.
Gustafsson et al. (1986)......  Dock workers....  6,071...........    1961-80  Occupation only.          SMR = 1.32          (*)   No adjustment for
                                                                                                          (mortality).              healthy worker
                                                                                                         SMR = 1.68          (*)    effect.
                                                                                                          (morbidity).

[[Page 5777]]

 
Gustavsson et al. (1990)......  Bus garage        708.............    1952-86  Semi-                     SMR = 1.22 for            Lack of statistical
                                 workers.                                       quantitative,             overall cohort.           significance may be
                                                                                based on job              SMR = 1.27 for            attributed to small
                                                                                history &                 highest-exposed           size of cohort.
                                                                                exposure                  subgroup.
                                                                                intensity
                                                                                estimated for
                                                                                each job.
Hansen (1993).................  Truck drivers...  14,225..........    1970-80  Occupation only.           SMR = 1.60 for     (*)   Compared to unexposed
                                                                                                          overall cohort.           control group of
                                                                                                          Some indication           38,301 laborers
                                                                                                          of increasing             considered to
                                                                                                          SMR with age              ``resemble the group
                                                                                                          (i.e., greater            of truck drivers in
                                                                                                          cumulative                terms of work-
                                                                                                          exposure).                related demands on
                                                                                                                                    physical strength
                                                                                                                                    and fitness,
                                                                                                                                    educational
                                                                                                                                    background, social
                                                                                                                                    class, and life
                                                                                                                                    style.'' Correction
                                                                                                                                    for estimated
                                                                                                                                    differences in
                                                                                                                                    smoking habits
                                                                                                                                    between cohort and
                                                                                                                                    control group
                                                                                                                                    reduces SMR from
                                                                                                                                    1.60 to 1.52.
                                                                                                                                    Results judged
                                                                                                                                    ``unlikely *** [to]
                                                                                                                                    have been seriously
                                                                                                                                    confounded by
                                                                                                                                    smoking habit
                                                                                                                                    differences.''
Howe et al. (1983)............  Railroad workers  43,826..........    1965-77  Jobs classified           RR = 1.20 for       (*)   Risk is relative to
                                                                                by diesel                 ``possibly                unexposed subgroup
                                                                                exposure.                 exposed.''.               of cohort. Similar
                                                                                                                                    results obtained for
                                                                                                                                    coal dust exposure.
                                                                                                         RR = 1.35 for       (*)   Possible confounding
                                                                                                          ``probably                with asbestos and
                                                                                                          exposed.''.               coal dust.
Johnston et al. (1997)........  Underground coal  18,166..........    1950-85  Quantitative,             Mine-adjusted             Risk is relative to
                                 miners.                                        based on                  model: RR =               unexposed workers in
                                                                                detailed job              1.156 per g-hr/           coal miners based on
                                                                                history &                 m \3\.                    cohort. Adjusted for
                                                                                surrogate dpm                                       age, smoking habit &
                                                                                measurements.                                       intensity, mine
                                                                                                                                    site, and cohort
                                                                                                                                    entry date. Mine
                                                                                                                                    site highly
                                                                                                                                    correlated with dpm
                                                                                                                                    exposure.
                                                                                                         Mine-unadjusted           Both models lag
                                                                                                          model: RR =               exposure by 15
                                                                                                          1.227 per g-hr/           years.
                                                                                                          m \3\.
Kaplan (1959).................  Railroad workers  Approx. 32000...    1953-58  Jobs classified           SMR=0.88 for              No adjustment for
                                                                                by diesel                 operationally             healthy worker
                                                                                exposure.                 exposed.                  effect. Clerks (in
                                                                                                                                    rarely exposed
                                                                                                                                    group) found more
                                                                                                                                    likely to have had
                                                                                                                                    urban residence than
                                                                                                                                    occupationally
                                                                                                                                    exposed workers.
                                                                                                         SMR = 0.72 for            No attempt to
                                                                                                          somewha exposed           distinguish between
                                                                                                          SMR = 0.80 for            diesel and coal-
                                                                                                          rarely exposed.           fired locomotives.
                                                                                                                                    Results may be
                                                                                                                                    attributable to
                                                                                                                                    short duration of
                                                                                                                                    exposure and/or
                                                                                                                                    inadequate follow-up
                                                                                                                                    time.
 Leupker & Smith (1978).......  Truck drivers...  183,791.........  May-July,  Occupation only.          SMR = 1.21......          Lack of statistical
                                                                         1976                                                       significance may be
                                                                                                                                    due to inadequate
                                                                                                                                    follow-up period.
                                                                                                                                    Retirees excluded
                                                                                                                                    from cohort, so lung
                                                                                                                                    cancers occurring
                                                                                                                                    after retirement
                                                                                                                                    were not included.
 Lindsay et al. (1933)........  Truck drivers...  Not reported....    1965-79  Occupation only.          SMR = 1.15......    (*)
 Menck & Henderson (1976).....  Truck drivers...  34,800 estimated    1968-73  Occupation only.          SMR = 1.65......    (*)   Number of subjects in
                                                                                                                                    cohort estimated
                                                                                                                                    from census data.
 Raffle (1957)................  Transport         2,666 estimated     1950-55  Occupation only.          SMR = 1.42......          SMR calculated by
                                 engineers.        from manyears                                                                    combining data
                                                   at risk.                                                                         presented for four
                                                                                                                                    quadrants of London.
                                                                                                                                    Excluded from most
                                                                                                                                    retirees and lung
                                                                                                                                    cancers occurring
                                                                                                                                    after retirement.
 Rafnsson & Gunnarsdottir       Truck drivers...  868.............    1951-88  Occupation only.          SMR = 2.14......    (*)   No trend of
 (1991).                                                                                                                            increasing risk with
                                                                                                                                    increased duration
                                                                                                                                    of employment or
                                                                                                                                    increased follow-up
                                                                                                                                    time. Based on
                                                                                                                                    survey of smoking
                                                                                                                                    habits in cohort
                                                                                                                                    compared to general
                                                                                                                                    male population, and
                                                                                                                                    fact that there were
                                                                                                                                    fewer than expected
                                                                                                                                    deaths from
                                                                                                                                    respiratory disease,
                                                                                                                                    authors concluded
                                                                                                                                    that differences in
                                                                                                                                    smoking habits were
                                                                                                                                    unlikely to be
                                                                                                                                    enough to explain
                                                                                                                                    excess rate of lung
                                                                                                                                    cancer. However, not
                                                                                                                                    all trucks were
                                                                                                                                    diesel prior to
                                                                                                                                    1951, and there is
                                                                                                                                    possible confounding
                                                                                                                                    by asbestos
                                                                                                                                    exposure.

[[Page 5778]]

 
 Rushton et al. (1983)........  Bus maintenamce   8,480...........    5.9 yrs  Occupation only.          SMR = 1.01 for      (*)   Short follow-up
                                 workers.                              (mean)                             overall cohort.           period. SMR based on
                                                                                                          SMR = 1.33 for            comparison to
                                                                                                          ``general                 national rates, with
                                                                                                          hand'' subgroup.          no adjustment for
                                                                                                                                    regional or
                                                                                                                                    socioeconomic
                                                                                                                                    differences, which
                                                                                                                                    could account for
                                                                                                                                    excess lung cancers
                                                                                                                                    observed among
                                                                                                                                    general hands. No
                                                                                                                                    adjustment for
                                                                                                                                    healthy worker
                                                                                                                                    effect.
 Saverin et al. (1999)........  Underground       5,536...........    1970-94  Quantitative,             RR = 2.17 for             Based on routine
                                 potash miners.                                 based on TC               highest                   measurements, miners
                                                                                measurements &            compared to               determined to have
                                                                                detailed job              least exposed             had no occupational
                                                                                history.                  categories.               exposure to radon
                                                                                                         RR = 1.03 to               progeny. Authors
                                                                                                          1.225 per mg-yr/          judged asbestos
                                                                                                          m3, depending             exposure minor, with
                                                                                                          on statistical            negligible effects.
                                                                                                          model &                   Cigarette smoking
                                                                                                          inclusion                 determined to be
                                                                                                          criteria.                 uncorrelated with
                                                                                                                                    cumulative TC
                                                                                                                                    exposure within
                                                                                                                                    cohort.
 Schenker et al. (1984).......  Railroad workers  2,519...........    1967-79  Job histories,            RR = 1.50 for             Risk relative to
                                                                                with exposure             low exposure              unexposed subgroup.
                                                                                classified as             subgroup.                 Jobs considered to
                                                                                unexposed,               RR = 2.77 for              have similar
                                                                                high, low, or             high exposure             socioeconomic
                                                                                undefined.                subgroup.                 status. Differences
                                                                                                                                    in smoking
                                                                                                                                    calculated to be
                                                                                                                                    insufficient to
                                                                                                                                    explain findings.
                                                                                                                                    Possible confounding
                                                                                                                                    by asbestos
                                                                                                                                    exposure.
 Waller (1981)................  Bus workers.....  16,828 Est. from    1950-74  Occupation only.          SMR = 0.79 for            Lung cancers
                                                   manyears at                                            overall cohort.           occurring after
                                                   risk.                                                                            retirement or
                                                                                                                                    resignation from
                                                                                                                                    London Transport
                                                                                                                                    Authority were not
                                                                                                                                    counted. No
                                                                                                                                    adjustment for
                                                                                                                                    healthy worker
                                                                                                                                    effect.
 Waxweiler et al. (1973)......  Potash miners...  3,886...........    1941-67  Miners                    SMR = 1.1 for             No adjustment for
                                                                                classified as             both                      healthy worker
                                                                                underground or            underground and           effect. SMR based on
                                                                                surface.                  surface miners.           national lung cancer
                                                                                                                                    mortality, which is
                                                                                                                                    about 1/3 higher
                                                                                                                                    than lung cancer
                                                                                                                                    mortality rate in
                                                                                                                                    New Mexico, where
                                                                                                                                    miners resided.
                                                                                                                                    Authors judged this
                                                                                                                                    to be balanced by
                                                                                                                                    smoking among
                                                                                                                                    miners. A
                                                                                                                                    substantial
                                                                                                                                    percentage of the
                                                                                                                                    underground subgroup
                                                                                                                                    may have had little
                                                                                                                                    or no occupational
                                                                                                                                    exposure to diesel
                                                                                                                                    exhaust.
                                                                                                         SMR = 0.99 for
                                                                                                          overall cohort.
                                                                                                         SMR = 1.07 for
                                                                                                          20
                                                                                                          yr member
                                                                                                         SMR = 1.12 for
                                                                                                          20
                                                                                                          yr. latency.
 Wong et al. (1973)...........  Heavy equipment   34,156..........    1964-78  Job histories,            SMR = 1.30 for      (*)   Increasing trend in
                                 operators.                                     latency, &                4,075                     SMR with latency and
                                                                                years of union            ``normal''                (up to 15 yr) with
                                                                                membership.               retirees.                 duration of union
                                                                                                                                    membership. No
                                                                                                                                    adjustment for
                                                                                                                                    healthy worker
                                                                                                                                    effect.
                                                                                                         SMR = 3.43 for      (*)
                                                                                                          ``high
                                                                                                          exposure''
                                                                                                          dozer operators
                                                                                                          with 15-19 yr
                                                                                                          union
                                                                                                          membership &
                                                                                                          20
                                                                                                          yr latency.
--------------------------------------------------------------------------------------------------------------------------------------------------------
\a\ RR = Relative Risk; SMR = Standardized Mortality Ratio. Values greater than 1.0 indicate excess prevalence of lung cancer associated with diesel
  exposure.
\b\ An asterisk (*) indicates statistical significance based on 2-tailed test at confidence level of at least 95%.


                                    Table III-5.--Summary of Published Information From 20 Case-Control Studies on Lung Cancer and Exposure to Diesel Exhaust
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                            Number   Number                                 Matching
              Study                      Cases              Controls          of       of          Exposure      ------------------------------     Findings a        Stat.        Comments
                                                                            cases   controls      assessment        Smk.        Additional                            sig.b
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Benhamou et al. (1988)..........  Histologically       Non-tobacco           1,625     3,091  Occupational            diagnosis,                                        reported.
                                   cancers.                                                    questionnaire.                hospital,
                                                                                                                             interviewer.

[[Page 5779]]

 
                                                                                                                                                RR=1.42 for             (*)   No evidence of an
                                                                                                                                                 professional                  increase in risk
                                                                                                                                                 drivers.                      with duration of
                                                                                                                                                                               exposure.
Boffetta et al. (1990)..........  Hospitalized males   Hospitalized males    2,584     5,099  Occupation              yr, hospital,       jobs with                     asbestos
                                   histologically       related disease.                       probability of                year of interview.  probable exposure.            exposure,
                                   confirmed lung                                              diesel exposure.                                 OR=1.49 for more               education, &
                                   cancer.                                                                                                       than 30 yr in                 number of
                                                                                                                                                 ``probable'' jobs.            cigarettes per
                                                                                                                                                                               day.
    Do..........................                                               477       846  Occupational                                self-reported
                                                                                               duration of                                       diesel exposure.
                                                                                               diesel exposure                                  OR=2.39 for more
                                                                                               by interview.                                     than 30 yr of
                                                                                                                                                 self-reported
                                                                                                                                                 diesel exposure..
Bruske-Hohlfeld et al. (1999)...  Cytologically and/   Randomly selected     3,498     3,541  Occupational            of residence.       occupational           (*)    cumulative
                                   confirmed lung       registries of                          interview; total                                  diesel exposure        (*)    smoking &
                                   cancers.             residents.                             duration of                                       during lifetime.       (*)    asbestos
                                                                                               diesel exposure                                  OR=1.56 for West        (*)    exposure. All
                                                                                               compiled from                                     German                        interviews
                                                                                               individual job                                    professional                  conducted
                                                                                               episodes.                                         drivers post-1955.            directly with
                                                                                                                                                OR=2.88 for > 20               cases and
                                                                                                                                                 yr in ``traffic-              controls. Lack of
                                                                                                                                                 related'' jobs                elevated risk for
                                                                                                                                                 other than                    East German
                                                                                                                                                 driving.                      professional
                                                                                                                                                OR=6.81 for > 30               drivers
                                                                                                                                                 yr as full-time               attributed to
                                                                                                                                                 driver of farm                relatively low
                                                                                                                                                 tractors.                     traffic density &
                                                                                                                                                OR=4.30 for > 20               low proportion of
                                                                                                                                                 yr as heavy                   vehicles with
                                                                                                                                                 equipment                     diesel engines in
                                                                                                                                                 operator.                     East Germany. Non-
                                                                                                                                                                               driving ``traffic-
                                                                                                                                                                               related jobs''
                                                                                                                                                                               include switchmen
                                                                                                                                                                               & operators of
                                                                                                                                                                               diesel
                                                                                                                                                                               locomotives &
                                                                                                                                                                               forklifts.
Buiatti et al. (1985)...........  Histologically       Patients at same        376       892  Occupational            admission date.     drivers.                      current and past
                                   cancers.                                                    interview.                                                                      smoking patterns
                                                                                                                                                                               and for asbestos
                                                                                                                                                                               exposure.
Coggon et al. (1984)............  Lung cancer deaths   Deaths from other       598     1,180  Occupation from               Sex, death year,    RR=1.3 for all          (*)   Only most recent
                                   of males under 40.   causes in males                        death                         region, and birth   jobs with diesel              full-time
                                                        under 40.                              certificate,                  year (approx.).     exposure.                     occupation
                                                                                               classified as                                    RR=1.1 for jobs                recorded on death
                                                                                               high, low, or no                                  classified as                 certificate.
                                                                                               diesel exposure.                                  high exposure.
Damber & Larsson (1985).........  Male patients with   One living and one      604     1,071  Job, with tenure,       age, municipality.  smoking truck                 not smoke for at
                                                        lung cancer.                           questionnaire.                                    drivers aged 70               least last 10
                                                                                                                                                 yr.                           years included
                                                                                                                                                RR=4.5 for non-                with non-smokers.
                                                                                                                                                 smoking truck
                                                                                                                                                 drivers aged 70 yr.

[[Page 5780]]

 
DeCoufle et al. (1997)..........  Male patients with   Non-neoplastic        6,434       (c)  Occupation only,                            taxi, and truck               occupation
                                                                                               questionnaire.                                    drivers.                      compared to
                                                                                                                                                RR=0.94 for                    clerical workers.
                                                                                                                                                 locomotive                    Positive
                                                                                                                                                 engineers.                    associations
                                                                                                                                                                               found before
                                                                                                                                                                               smoking
                                                                                                                                                                               adjustment.
Emmelin et al. (1993)...........  Deaths from primary  Dock workers             50       154  Semi-quantitative       port, and           ``medium''                    relative risk
                                   dock workers.        cancer.                                & records of                  survival to         duration of                   also observed
                                                                                               diesel fuel usage.            within 2 years of   exposure..                    using exposure
                                                                                                                             case's diagnosis   RR = 2.9 for                   estimates based
                                                                                                                             of lung cancer.     ``high'' duration             on machine usage
                                                                                                                                                 of exposure.                  & diesel fuel
                                                                                                                                                                               consumption.
                                                                                                                                                                               Confounding from
                                                                                                                                                                               asbestos may be
                                                                                                                                                                               significant.
                  Garshick et     Deaths with primary  Deaths from other     1,256     2,385  Job history and         death.              diesel-years in               asbestos
                                   railroad workers.    suicide,                               with current                                      workers aged  64 yr..               workers had
                                                        unknown causes.                        measured for each                                RR = 0.91 for 20+              relatively short
                                                                                               job.                                              diesel-years in               diesel exposure,
                                                                                                                                                 workers aged  65 yr.
Gustavsson et al. (1990)........  Deaths from lung     Non-cases within         20       120  Semi-quantitative             Born within two     RR = 1.34, 1.81,        (*)   Authors judged
                                   cancer among bus     cohort mortality                       based on job,                 years of case.      and 2.43 for                  smoking habits to
                                   garage workers.      study.                                 tenure, &                                         increasing                    be similar for
                                                                                               exposure class                                    cumulative diesel             different
                                                                                               for each job.                                     exposure                      exposure
                                                                                                                                                 categories,                   categories. RR
                                                                                                                                                 relative to                   did not increase
                                                                                                                                                 lowest exposure               with increasing
                                                                                                                                                 category.                     asbestos
                                                                                                                                                                               exposure.
Hall & Wynder (1984)............  Hospitalized males   Hospitalized males      502       502  Usual occupation        hospital, and       with diesel                   other
                                                        related diseases.                                                    hospital room       exposure..                    occupational
                                                                                                                             status.            RR = 1.9 for heavy             exposures
                                                                                                                                                 equipment                     possible.
                                                                                                                                                 operators &
                                                                                                                                                 repairmen.
Hayes et al. (1989).............  Lung cancer deaths   Various--lung         2,291     2,570  Occupational            either race or      thn-eq> 10 yr                 birth-year cohort
                                   studies.                                                    interview.                    area of residence.  truck driving. OR             and state of
                                                                                                                                                 = 2.1 for  10 yr                     NJ, or LA), in
                                                                                                                                                 operating heavy               addition to
                                                                                                                                                 equipment. OR =               average cigarette
                                                                                                                                                 1.7 for  10 yr bus                 for  10 yr in
                                                                                                                                                 driving.                      these jobs.
Lerchen et al. (1987)...........  New Mexico           Medicare                506       771  Occupational            ethnicity.          thn-eq> 1 yr                  cases and
                                   lung cancer.                                                industry, & self-                                 occupational                  controls in
                                                                                               reported                                          exposure to                   diesel-exposed
                                                                                               exposure, by                                      diesel exhaust..              jobs. Possibly
                                                                                               interview.                                       OR = 2.1 for                   insufficient
                                                                                                                                                 underground non-              exposure
                                                                                                                                                 uranium mining.               duration. Not
                                                                                                                                                                               matched on date
                                                                                                                                                                               of birth or
                                                                                                                                                                               death.

[[Page 5781]]

 
Milne et al. (1983).............  Lung cancer deaths.  Deaths from any         925     6,565  Occupation from               None..............  OR = 3.5 for bus        (*)   Inadequate latency
                                                        other cancer.                          death certificate.                                drivers. OR = 1.6             allowance.
                                                                                                                                                 for truck drivers.
Morabia et al. (1992)...........  Male lung cancer     Patients without      1,793     3,228  Job, with coal and      hospital, and      OR=1.1 for bus                 specified.
                                                        other tobacco-                         durations, by                 smoking history.    drivers..                     Potential
                                                        related condition.                     interview.                                       OR=1.0 for truck               confounding by
                                                                                                                                                 or tractor                    other
                                                                                                                                                 drivers.                      occupational
                                                                                                                                                                               exposures for
                                                                                                                                                                               miners.
Pfluger and Minder (1994).......  Professional         Workers in              284     1,301  Occupation only,              None..............  OR=1.48 for             (*)   Stratified by age.
                                   drivers.             occupational                           from death                                        professional                  Indirectly
                                                        categories with                        certificate.                                      drivers.                      adjusted for
                                                        no known excess                                                                                                        smoking, based on
                                                        lung cancer risk.                                                                                                      smoking-rate for
                                                                                                                                                                               occupation.
Siemiatycki et al. (1988).......  Squamous cell lung   Other cancer            359     1,523  Semi-quantitative,                          exposure;.                    socioeconomic
                                   type of lung                                                history by                                       OR=2.8 for mining.             status,
                                   cancer.                                                     interview, &                                                                    ethnicity, and
                                                                                               exposure class                                                                  blue- vs. white-
                                                                                               for each job.                                                                   collar job
                                                                                                                                                                               history.
                                                                                                                                                                               Examination of
                                                                                                                                                                               files indicated
                                                                                                                                                                               that most miners
                                                                                                                                                                               ``were exposed to
                                                                                                                                                                               diesel exhaust
                                                                                                                                                                               for short periods
                                                                                                                                                                               of time.'' Mining
                                                                                                                                                                               included
                                                                                                                                                                               quarrying, so
                                                                                                                                                                               result is likely
                                                                                                                                                                               to be confounded
                                                                                                                                                                               by silica
                                                                                                                                                                               exposure.
Steenland et al. (1990, 1992,     Deaths from lung CA  Deaths other than       996     1,085  Occupational            within 2 years.     truck drivers                 not necessarily
                                                        cancer or motor                        tenure from next-                                 with 1-24 yr                  all at main job
                                                        vehicle accidents.                     of-kin,                                           tenure..                      (i.e., diesel
                                                                                               supplemented by                                  OR=1.26 for diesel             truck driver). OR
                                                                                               IH data.                                          truck drivers                 adjusted for
                                                                                                                                                 with 25-34 yr                 asbestos
                                                                                                                                                 tenure..                      exposure.
                                                                                                                                                OR=1.89 for diesel
                                                                                                                                                 truck drivers
                                                                                                                                                 with 35 yr tenure..
                                                                                                                                                OR=1.50 for truck
                                                                                                                                                 mechanics with
                                                                                                                                                 18 yr
                                                                                                                                                 tenure after 1959.

[[Page 5782]]

 
Swanson et al. (1993) See also    Histologically       Colon or rectal     d 3,792   d 1,966  Occupational        e 5,935   e 3,956   history from             >                        truck drivers                 drivers & RR
                                   metro area lung                                             interview.                                        with 1-9 yr                   workers is for
                                   cancers.                                                                                                      tenure.                       white males,
                                                                                                                                                OR = 1.6 for heavy             relative to
                                                                                                                                                 truck drivers                 corresponding
                                                                                                                                                 with 10-19 yr                 group with  1 yr
                                                                                                                                                 tenure.                       tenure, adjusted
                                                                                                                                                OR = 2.5 for heavy             for age at
                                                                                                                                                 truck drivers                 diagnosis.
                                                                                                                                                 with 20 yr tenure.              increasing risk
                                                                                                                                                                               with duration of
                                                                                                                                                                               employment also
                                                                                                                                                                               reported for
                                                                                                                                                                               black male
                                                                                                                                                                               railroad workers,
                                                                                                                                                                               based on fewer
                                                                                                                                                                               cases. (1993
                                                                                                                                                                               report).
                                                                                                                                                OR = 1.2 for
                                                                                                                                                 railroad workers
                                                                                                                                                 with 1-9 yr
                                                                                                                                                 tenure.
                                                                                                                                                OR = 2.5 for
                                                                                                                                                 railroad workers       (*)
                                                                                                                                                 with ;10 yr
                                                                                                                                                 tenure.
                                                                                                                                                OR = 2.98 for           (*)   OR for mining
                                                                                                                                                 mining industry               machinery
                                                                                                                                                 workers.                      operators and
                                                                                                                                                OR = 5.03 for                  mining is for all
                                                                                                                                                 mining machinery       (*)    males, adjusted
                                                                                                                                                 operators.                    for race and age
                                                                                                                                                                               at diagnosis.
                                                                                                                                                                               Type of mining
                                                                                                                                                                               not reported.
                                                                                                                                                                               Potential
                                                                                                                                                                               confounding by.
Williams et al. (1977)..........  Male lung cancer     Other male cancer       432     2,817  Main lifetime                               truck drivers.                age, race,
                                                                                               interview.                                                                      alcohol use, and
                                                                                                                                                                               socioeconomic
                                                                                                                                                                               status.
                                                                                                                                                                               Unexplained
                                                                                                                                                                               discrepancies in
                                                                                                                                                                               reported number
                                                                                                                                                                               of controls.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
a RR = Relative Risk; OR = Odds Ratio. Values greater than 1.0 indicate excess prevalence of lung cancer associated with diesel exposure.
b An asterisk (*) indicates statistical significance based on 2-tailed test at confidence level of at least 95%.
c Not reported.    d Males.    e Total.

    (i) Evaluation Criteria. Several commenters contended that MSHA 
paid more attention to positive studies than to negative ones and 
indicated that MSHA had not sufficiently explained its reasons for 
discounting studies they regarded as providing negative evidence. MSHA 
used five principal criteria to evaluate the strengths and weaknesses 
of the individual studies:
    (1) power of the study to detect an exposure effect;
    (2) composition of comparison groups;
    (3) exposure assessment;
    (4) statistical significance; and
    (5) potential confounders.
    These criteria are consistent with those proposed by the HEI Diesel 
Epidemiology Expert Panel (HEI, 1999). To help explain MSHA's reasons 
for valuing some studies over others, these five criteria will now be 
discussed in turn.

Power of The Study

    There are several factors that contribute to a study's power, or 
ability to detect an increased risk of lung cancer in an exposed 
population. First is the study's size--i.e., the number of subjects in 
a cohort or the number of lung cancer cases in a case-control study. If 
few subjects or cases are included, then any statistical relationships 
are likely to go undetected. Second is the duration and intensity of 
exposure among members of the exposed group. The greater the exposure, 
the more likely it is that the study will detect an effect if it 
exists. Conversely, a study in which few members of the exposed group 
experienced cumulative exposures

[[Page 5783]]

significantly greater than the background level is unlikely to detect 
an exposure effect. Third is the length of time the study allows for 
lung cancer to exhibit a statistical impact after exposure begins. This 
involves a latency period, which is the time required for lung cancer 
to develop in affected individuals, or (mainly pertaining to cohort 
studies) a follow-up period, which is the time allotted, including 
latency, for lung cancers in affected individuals to show up in the 
study. It is generally acknowledged that lung cancer studies should, at 
the very minimum, allow for a latency period of at least 10 years from 
the time exposure begins and that it is preferable to allow for latency 
periods of at least 20 years. The shorter the latency allowance, the 
less power the study has to detect any increased risk of lung cancer 
that may be associated with exposure.
    As stated above, six of the 47 studies did not show positive 
results: One of these studies (Edling et al.) was based on a small 
cohort of 694 bus workers, thus having little statistical power. Three 
other of these studies (DeCoufle, Kaplan, and Christie) included 
exposed workers for whom there was an inadequate latency allowance 
(i.e., less than 10 years). The entire period of follow-up in the 
Kaplan study was 1953-1958. The Christie study was designed in such a 
way as to provide for neither a minimum period of exposure nor a 
minimum period of latency: the report covers lung cancers diagnosed 
only through 1992, but the ``exposed'' cohort includes workers who may 
have entered the work force (and thus begun their exposure) as late as 
Dec. 31, 1992. Such workers would not be expected to develop lung 
cancer during the study period. The remaining two negative studies 
(Bender, 1989 and Waller, 1981) appear to have been included a 
reasonably adequate number of exposed workers and to have allowed for 
an adequate latency period.
    Some of the 41 positive studies also had little power, either 
because they included relatively few exposed workers (e.g., Lerchen et 
al., 1987, Ahlman et al., 1991; Gustavsson et al., 1990) or an 
inadequate latency allowance or follow-up period (e.g., Leupker and 
Smith (1978); Milne, 1983; Rushton et al., 1983). In those based on few 
exposed workers, there is a strong possibility that the positive 
association arose merely by chance.\30\ The other studies, however, 
found increased prevalence of lung cancer despite the relatively short 
periods of latency and follow-up time involved. It should be noted 
that, for reasons other than lack of power, MSHA places very little 
weight on the Milne and Rushton studies. As mentioned in Table III-4, 
the Rushton study compared the cohort to the national population, with 
no adjustment for regional or socioeconomic differences. This may 
account for the excess rate of lung cancers reported for the exposed 
``general hand'' job category. The Milne study did not control for 
potentially important ``confounding'' variables, as explained below in 
MSHA's discussion of that criterion.
---------------------------------------------------------------------------

    \30\ As noted in Table III-4, the underground sulfide ore miners 
studied by Ahlman et al. (1991) were exposed to radon in addition to 
diesel emissions. The total number of lung cancers observed, 
however, was greater than what was attributable to the radon 
exposure, based on a calculation by the authors. Therefore, the 
authors attributed a portion of the excess risk to diesel exposure.
---------------------------------------------------------------------------

Composition of Comparison Groups

    This criterion addresses the question of how equitable is the 
comparison between the exposed and unexposed populations in a cohort 
study, or between the subjects with lung cancer (i.e., the ``cases'') 
and the subjects without lung cancer (i.e., the ``controls'') in a 
case-control study. MSHA includes bias due to confounding variables 
under this criterion if the groups differ systematically with respect 
to such factors as age or exposure to non-diesel carcinogens. For 
example, unless adequate adjustments are made, comparisons of 
underground miners to the general population may be systematically 
biased by the miners' greater exposure to radon gas. Confounding not 
built into a study's design or otherwise documented is considered 
potential rather than systematic and is considered under a separate 
criterion below. Other factors included under the present criterion are 
systematic (i.e., ``differential'') misclassification of those placed 
into the ``exposed'' and ``unexposed'' groups, selection bias, and bias 
due to the ``healthy worker effect.''
    In several of the studies, a group identified with diesel exposure 
may have systematically included workers who, in fact, received little 
or no occupational diesel exposure. For example, a substantial 
percentage of the ``underground miner'' subgroup in Waxweiler et al. 
(1973) worked in underground mines with no diesel equipment. This would 
have diluted any effect of dpm exposure on the group of underground 
miners as a whole.\31\ Similarly, the groups classified as miners in 
Benhamou et al. (1988), Boffetta et al. (1988), and Swanson et al. 
(1993) included substantial percentages of miners who were probably not 
occupationally exposed to diesel emissions. Potential effects of 
exposure misclassification are discussed further under the criterion of 
``Exposure Assessment'' below.
---------------------------------------------------------------------------

    \31\ Furthermore, as pointed out in comments submitted by Dr. 
Peter Valberg through the NMA, the subgroup of underground miners 
working at mines with diesel engines was small, and the exposure 
duration in one of the mines with diesel engines was only ten years. 
Therefore, the power of the study was inadequate to detect an excess 
risk of lung cancer for that subgroup by itself.
---------------------------------------------------------------------------

    Selection bias refers to systematic differences in characteristics 
of the comparison groups due to the criteria and/or methods used to 
select those included in the study. For example, three of the cohort 
studies (Raffle, 1957; Leupker and Smith, 1976; Waller, 1981) 
systematically excluded retirees from the cohort of exposed workers--
but not from the population used for comparison. Therefore, cases of 
lung cancer that developed after retirement were counted against the 
comparison population but not against the cohort. This artificially 
reduced the SMR calculated for the exposed cohort in these three 
studies.
    Another type of selection bias may occur when members of the 
control group in a case-control study are non-randomly selected. This 
happens when cases and controls are selected from the same larger 
population of patients or death certificates, and the controls are 
simply selected (prior to case matching) from the group remaining after 
those with lung cancer are removed. Such selection can lead to a 
control group that is biased with respect to occupation and smoking 
habits. Specifically, ``* * * a severely distorted estimate of the 
association between exposure to diesel exhaust and lung cancer, and a 
severely distorted picture of the direction and degree of confounding 
by cigarette smoking, can come from case-control studies in which the 
controls are a collection of `other deaths' '' when the cause of most 
`other deaths' is itself correlated with smoking or occupational choice 
(HEI, 1999). This selection bias can distort results in either 
direction.
    MSHA judged that seven of the 20 available case-control studies 
were susceptible to this type of selection bias because controls were 
drawn from a population of ``other deaths'' or ``other patients.'' \32\ 
These control groups were likely to have over-represented cases of 
cardiovascular disease, which is known to be highly correlated with 
smoking and is possibly also correlated with

[[Page 5784]]

occupation. The only case-control study not reporting a positive result 
(DeCoufle et al., 1977) fell into this group of seven. The remaining 13 
case-control studies all reported positive results.
---------------------------------------------------------------------------

    \32\ These were: Buiatti et al. (1985), Coggan et al. (1984), 
DeCoufle et al. (1977), Garshick et al. (1987), Hayes et al. (1989), 
Lerchen et al. (1987), and Steenland et al. (1990).
---------------------------------------------------------------------------

    It is ``well established that persons in the work force tend to be 
`healthier' than persons not employed, and therefore healthier than the 
general population. Worker mortality tends to be below average for all 
major causes of death.'' (HEI, 1999) Because workers tend to be 
healthier than non-workers, the prevalence of disease found among 
workers exposed to a toxic substance may be lower than the rate 
prevailing in the general population, but higher than the rate 
occurring in an unexposed population of similar workers. This 
phenomenon is called the ``healthy worker effect.''
    All five cohort studies reporting entirely negative results drew 
comparisons against the general population and made no adjustments to 
take the healthy worker effect into account. (Kaplan, 1959; Waller 
(1981); Edling et al. (1987); Bender et al. (1989); Christie et al. 
(1995). The sixth negative study (DeCoufle, 1977) was a case-control 
study in which vehicle drivers and locomotive engineers were compared 
to clerical workers. As mentioned earlier, this study did not meet the 
criterion for a minimum 10-year latency period. All other studies in 
which exposed workers were compared against similar but unexposed 
workers reported some degree of elevated lung cancer risk for exposed 
workers.
    Many of the 41 positive studies also drew comparisons against the 
general population with no compensating adjustment for the healthy 
worker effect. But the healthy worker effect can influence results even 
when the age-adjusted mortality or morbidity rate observed among 
exposed workers is greater than that found in the general population. 
In such studies, comparison with the general population tends to reduce 
the excess risk attributable to the substance being investigated. For 
example, Gustafsson et al. (1986), Rushton et al. (1983), and Wong et 
al. (1985) each reported an unadjusted SMR exceeding 1.0 for lung 
cancer in exposed workers and an SMR significantly less than 1.0 for 
all causes of death combined. Since the SMR for all causes is less than 
1.0, there is evidence of a healthy worker effect. Therefore, the SMR 
reported for lung cancer was probably lower than if the comparison had 
been made against a more similar population of unexposed workers. 
Bhatia et al. (1998) constructed a simple estimate of the healthy 
worker effect evident in these studies, based on the SMR for all causes 
of death except lung cancer. This estimate was then used to adjust the 
SMR reported for lung cancer. For the three positive studies mentioned, 
the adjustment raised the SMR from 1.29 to 1.48, from 1.01 to 1.23, and 
from 1.07 to 1.34, respectively.\33\
---------------------------------------------------------------------------

    \33\ A similar adjustment was applied to the SMR for lung cancer 
reported in one of the negative studies (Edling et al., 1987). This 
raised the SMR from 0.67 to 0.80. Because of insufficient data, 
Bhatia et al. did not carry out the adjustment for the three other 
studies they considered with potentially important healthy worker 
effects. (Bhatia et al., 1998)
---------------------------------------------------------------------------

Exposure Assessment

    Many commenters suggested that a lack of concurrent exposure 
measurements in available studies limits their utility for quantitative 
risk assessment (QRA). MSHA is fully aware of these limitations but 
also recognizes that less desirable surrogates of exposure must 
frequently be employed out of practical necessity. As stated by HEI's 
expert panel on diesel epidemiology:

    Quantitative measures of exposures are important in any 
epidemiologic study used for QRA. The greater the detail regarding 
specific exposure, including how much, for how long, and at what 
concentration, the more useful the study is for this purpose. 
Frequently, however, individual measurements are not available, and 
surrogate measures or markers are used. For example, the most 
general surrogate measures of exposure in occupational epidemiologic 
studies are job classification and work location. (HEI, 1999)

    It is important to distinguish, moreover, between studies used to 
identify a hazard (i.e., to establish that dpm exposure is associated 
with an excess risk of lung cancer) and studies used for QRA (i.e., to 
quantify the amount of excess risk corresponding to a given level of 
exposure). Although detailed exposure measurements are desirable in any 
epidemiologic study, they are more important for QRA than for 
identifying and characterizing a hazard. Conversely, epidemiologic 
studies can be highly useful for purposes of hazard identification and 
characterization even if a lack of personal exposure measurements 
renders them less than ideal for QRA.
    Still, MSHA agrees that the quality of exposure assessment affects 
the value of a study for even hazard identification. Accordingly, MSHA 
has divided the 47 studies into four categories, depending on the 
degree to which exposures were quantified for the specific workers 
included. This ranking refers only to exposure assessment and does not 
necessarily correspond to the overall weight MSHA places on any of the 
studies.
    The highest rank, with respect to this criterion, is reserved for 
studies having quantitative, concurrent exposure measurements for 
specific workers or for specific jobs coupled with detailed work 
histories. Only two studies (Johnston et al., 1997 and Saverin et al., 
1999) fall into this category.\34\ Both of these recent cohort studies 
took smoking habits into account. These studies both reported an excess 
risk of lung cancer associated with dpm exposure.
---------------------------------------------------------------------------

    \34\ The study of German potash miners by Saverin et al. was 
introduced by NIOSH at the Knoxville public hearing prior to 
publication. The study, as cited, was later published in English. 
Although the dpm measurements (total carbon) were all made in one 
year, the authors provide a justification for assuming that the 
mining technology and type of machinery used did not change 
substantially during the period miners were exposed (ibid., p.420).
---------------------------------------------------------------------------

    The second rank is defined by semi-quantitative exposure 
assessments, based on job history and an estimated exposure level for 
each job. The exposure estimates in these studies are crude, compared 
to those in the first rank, and they are subject to many more kinds of 
error. This severely restricts the utility of these studies for QRA 
(i.e., for quantifying the change in risk associated with various 
specified exposure levels). For purposes of hazard identification and 
characterization, however, crude exposure estimates are better than no 
exposure estimates at all. MSHA places two cohort studies and five 
case-control studies into this category.\35\ All seven of these studies 
reported an excess risk of lung cancer risk associated with diesel 
exposure. Thus, results were positive in all nine studies with 
quantitative or semi-quantitative exposure assessments.
---------------------------------------------------------------------------

    \35\ The cohort studies are Garshick et al. (1988) and 
Gustavsson et al. (1990). The case-control studies are Emmelin et 
al. (1993), Garshick et al. (1997), Gustavsson et al. (1990), 
Siemiatycki et al. (1988), and Steenland et al. (1990, 1992).
---------------------------------------------------------------------------

    The next rank belongs to those studies with only enough information 
on individual workers to construct estimates of exposure duration. 
Although these studies present no data relating excess risk to specific 
exposure levels, they do provide excess risk estimates for those 
working a specified minimum number of years in a job associated with 
diesel exposure. One cohort study and five case-control studies fall 
into this category, and all six of them reported an excess risk of lung 
cancer.\36\ With one exception

[[Page 5785]]

(Benhamou et al. 1988), these studies also presented evidence of 
increased age-adjusted risk for workers with longer exposures and/or 
latency periods.
---------------------------------------------------------------------------

    \36\ The cohort study is Wong et al. (1985). The case-control 
studies are Bruske-Hohlfeld et al. (1999), Benhamou et al. (1988), 
Boffetta et al. (1990), Hayes et al. (1989), and Swanson et al. 
(1993).
---------------------------------------------------------------------------

    The bottom rank, with respect to exposure assessment, consists of 
studies in which no exposure information was collected for individual 
workers. These studies used only job title to distinguish between 
exposed and unexposed workers. The remaining 32 studies, including five 
of the six with entirely negative results, fall into this category.
    Studies basing exposure assessments on only a current job title (or 
even a history of job titles) are susceptible to significant 
misclassification of exposed and unexposed workers. Unless the study is 
poorly designed, this misclassification is ``nondifferential''--i.e., 
those who are misclassified are no more and no less likely to develop 
lung cancer (or to have been exposed to carcinogens such as tobacco 
smoke) than those who are correctly classified. If workers are 
sometimes misclassified nondifferentially, then this will tend to mask 
or dilute any excess risk attributable to exposure. Furthermore, 
differential misclassification in these studies usually consists of 
systematically including workers with little or no diesel exposure in a 
job category identified as ``exposed.'' This too would generally mask 
or dilute any excess risk attributable to exposure. Therefore, MSHA 
assumes that in most of these studies, more rigorous and detailed 
exposure assessments would have resulted in somewhat higher estimates 
of excess risk.
    IMC Global, MARG, and some other commenters expressed special 
concern about potential exposure misclassification and suggested that 
such misclassification might be partly responsible for results showing 
excess risk. IMC Global, for example, quoted a textbook observation 
that, contrary to popular misconceptions, nondifferential exposure 
misclassification can sometimes bias results away from the null. MSHA 
recognizes that this can happen under certain special conditions. 
However, there is an important distinction between ``can sometimes'' 
and ``can frequently.'' There is an even more important distinction 
between ``can sometimes'' and ``in this case does.'' As noted by the 
HEI Expert Panel on Diesel Epidemiology (HEI, 1999, p. 48), ``* * * 
nondifferential misclassification most often leads to an overall 
underestimation of effect.'' Similarly, Silverman (1998) noted, 
specifically with respect to the diesel studies, that ``* * * this 
[exposure misclassification] bias is most likely to be nondifferential, 
and the effect would probably have been to bias point estimates [of 
excess risk] toward the null value.''

Statistical Significance

    A ``statistically significant'' finding is a finding unlikely to 
have arisen by chance in the particular group, or statistical sample, 
of persons being studied. An association arising by chance would have 
no predictive value for exposed workers outside the sample. However, a 
specific epidemiologic study may fail to achieve statistical 
significance for two very different reasons: (1) there may be no real 
difference in risk between the two groups being compared, or (2) the 
study may lack the power needed to detect whatever difference actually 
exists. As described earlier, a lack of sufficient power comes largely 
from limitations such as a small number of subjects in the sample, low 
exposure and/or duration of exposure, or too short a period of latency 
or follow-up time. Therefore, a lack of statistical significance in an 
individual study does not demonstrate that the results of that study 
were due merely to chance--only that the study (viewed in isolation) is 
statistically inconclusive.
    As explained earlier, MSHA classifies a reported RR, SMR, or OR 
(i.e., the point estimate of relative risk) as ``positive'' if it 
exceeds 1.0 and ``negative'' if it is less than or equal to 1.0. By 
common convention, a positive result is considered statistically 
significant if its 95-percent confidence interval does not overlap 1.0. 
If all other relevant factors are equal, then a statistically 
significant positive result provides stronger evidence of an underlying 
relationship than one that is not statistically significant. On the 
other hand, a study must meet two requirements in order to provide 
statistically significant evidence of no positive relationship: (1) the 
upper limit of its 95-percent confidence interval must not exceed 1.0 
by an appreciable amount \37\ and (2) it must have allowed for 
sufficient exposure, latency, and follow-up time to have detected an 
existing relationship.
---------------------------------------------------------------------------

    \37\ As a matter of practicality, MSHA places the threshold at 
1.05.
---------------------------------------------------------------------------

    As shown in Tables III-4 and III-5, statistically significant 
positive results were reported in 25 of the 47 studies: 11 of the 19 
positive case-control studies and 14 of the 22 positive cohort studies. 
In 16 of the 41 studies showing a positive association, the association 
observed was not statistically significant. Results in five of the six 
negative studies were not statistically significant. One of the six 
negative studies (Christie et al., 1995, in full version), reported a 
statistically significant deficit in lung cancer for miners. This 
study, however, provided for no minimum period of exposure or latency 
and, therefore, lacked the power necessary to provide statistically 
significant evidence.\38\
---------------------------------------------------------------------------

    \38\ More detailed discussion of this study appears later in 
this subsection.
---------------------------------------------------------------------------

    Whether or not a study provides statistically significant evidence 
is dependent upon many variables, such as study size, adequate follow-
up time (to account for enough exposure and latency), and adequate case 
ascertainment. In the ideal world, a sufficiently powerful study that 
failed to demonstrate a statistically significant positive relationship 
would, by its very failure, provide statistically significant evidence 
that an underlying relationship between an exposure and a specific 
disease was unlikely. It is important to note that MSHA regards a real 
10-percent increase in the risk of lung cancer (i.e., a relative risk 
of 1.1) as constituting a clearly significant health hazard. Therefore, 
``sufficiently powerful'' in this context means that the study would 
have to be of such scale and quality as to detect a 10-percent increase 
in risk if it existed. The outcome of such a study could plausibly be 
called ``negative'' even if the estimated RR slightly exceeded 1.0--so 
long as the lower confidence limit did not exceed 1.0 and the upper 
confidence limit did not exceed 1.05. Rarely does an epidemiological 
study fall into this ``ideal'' study category. MSHA reviewed the dpm 
epidemiologic studies to determine which of them could plausibly be 
considered to be negative.
    For example, one study (Waxweiller et al., 1973) reported positive 
but statistically non-significant results corresponding to an RR of 
about 1.1. Among the studies MSHA counts as positive, this is the one 
that is numerically closest to being ``negative''. This study, however, 
relied on a relatively small cohort containing an indeterminate but 
probably substantial percentage of occupationally unexposed workers. 
Furthermore, there was no minimum latency allowance for the exposed 
workers. Therefore, even if MSHA were to use 1.1 rather than 1.05 as a 
threshold for significant relative risk, the study had insufficient 
statistical power to merit ``negative'' status.
    One commenter (Dr. James Weeks, representing the UMWA) argued that 
``MSHA's reliance on * * * statistical

[[Page 5786]]

significance is somewhat misplaced. Results that are not significant 
statistically * * * can nevertheless indicate that the exposure in 
question caused the outcome.'' MSHA agrees that an otherwise sound 
study may yield positive (or negative) results that provide valuable 
evidence for (or against) an underlying relationship but fail, because 
of an insufficient number of exposed study subjects, to achieve 
statistical significance. In the absence of other evidence to the 
contrary, a single positive but not statistically significant result 
could even show that a causal relationship is more likely than not. By 
definition, however, such a result would not be conclusive at a high 
level of confidence. A finding of even very high excess risk in a 
single, well-designed study would be far from conclusive if based on a 
very small number of observed lung cancer cases or if it were in 
conflict with evidence from toxicity studies.
    MSHA agrees that evidence should not be ignored simply because it 
is not conclusive at a conventional but arbitrary 95-percent confidence 
level. Lower confidence levels may represent weaker but still important 
evidence. Nevertheless, to rule out chance effects, the statistical 
significance of individual studies merits serious consideration when 
only a few studies are available. That is not the case, however, for 
the epidemiology literature relating lung cancer to diesel exposure. 
Since many studies contribute to the overall weight of evidence, the 
statistical significance of individual studies is far less important 
than the statistical significance of all findings combined. Statistical 
significance of the combined findings is addressed in Subsection 
3.a.iii of this risk assessment.

Potential Confounders

    There are many variables, both known and unknown, that can 
potentially distort the results of an epidemiologic study. In studies 
involving lung cancer, the most important example is tobacco smoking. 
Smoking is highly correlated with the development of lung cancer. If 
the exposed workers in a study tend to smoke more (or less) than the 
population to which they are being compared, then smoking becomes what 
is called a ``confounding variable'' or ``confounder'' for the study. 
In general, any variable affecting the risk of lung cancer potentially 
confounds observed relationships between lung cancer and diesel 
exposure. Conspicuous examples are age, smoking habits, and exposure to 
airborne carcinogens such as asbestos or radon progeny. Diet and other 
lifestyle factors may also be potential confounders, but these are 
probably less important for lung cancer than for other forms of cancer, 
such as bladder cancer.
    There are two ways to avoid distortion of study results by a 
potential confounder: (1) design the study so that the populations 
being compared are essentially equivalent with respect to the 
potentially confounding variable; or (2) allow the confounding to take 
place, but adjust the results to compensate for its effects. Obviously, 
the second approach can be applied only to known confounders. Since no 
adjustment can be made for unknown confounders, it is important to 
minimize their effects by designing the comparison groups to be as 
similar as possible.
    The first approach requires a high degree of control over the two 
groups being compared (exposed and unexposed in a cohort study; with 
and without lung cancer in a case-control study). For example, the 
effects of age in a case-control study can be controlled by matching 
each case of lung cancer with one or more controls having the same year 
of birth and age in year of diagnosis or death. Matching on age is 
never perfect, because it is generally not feasible to match within a 
day or even a month. Similarly, the effects of smoking in a case-
control study can be imperfectly controlled by matching on smoking 
habits to the maximum extent possible.\39\ In a cohort study, there is 
no confounding unless the exposed cohort and the comparison group 
differ with respect to a potential confounder. For example, if both 
groups consist entirely of never-smokers, then smoking is not a 
confounder in the study. If both groups contain the same percentage of 
smokers, then smoking is still an important confounder to the extent 
that smoking intensity and history differ between the two groups. In an 
attempt to minimize such differences (along with potentially important 
differences in diet and lifestyle) some studies restrict comparisons to 
workers of similar socioeconomic status and area of residence. Studies 
may also explicitly investigate smoking habits and histories and forego 
any adjustment of results if these factors are found to be 
homogeneously distributed across comparison groups. In that case, 
smoking would not actually appear to function as a confounder, and a 
smoking adjustment might not be required or even desirable. 
Nevertheless, a certain amount of smoking data is still necessary in 
order to check or verify homogeneity. The study's credibility may also 
be an important consideration. Therefore, MSHA agrees with the HEI's 
expert panel that even when smoking appears not to be a confounder,
---------------------------------------------------------------------------

    \39\ If cases and controls cannot be closely matched on smoking 
or other potentially important confounder, then a hybrid approach is 
often taken. Cases and controls are matched as closely as possible, 
differences are quantified, and the study results are adjusted to 
account for the differences.

    * * * a study is open to criticism if no smoking data are 
collected and the association between exposure and outcome is weak. 
* * * When the magnitude of the association of interest is weak, 
uncontrolled confounding, particularly from a strong confounder such 
as cigarette smoking, can have a major impact on the study's results 
---------------------------------------------------------------------------
and on the credibility of their use. [HEI, 1999]

However, this does not mean that a study cannot, by means of an 
efficient study design and/or statistical verification of homogeneity, 
demonstrate adequate control for smoking without applying a smoking 
adjustment.
    The second approach to dealing with a confounder requires knowledge 
or estimation both of the differences in group composition with respect 
to the confounder and of the effect that the confounder has on lung 
cancer. Ideally, this would entail specific, quantitative knowledge of 
how the variable affects lung cancer risk for each member of both 
groups being compared. For example, a standardized mortality ratio 
(SMR) can be used to adjust for age differences when a cohort of 
exposed workers with known birth dates is compared to an unexposed 
reference population with known, age-dependent lung cancer rates.\40\ 
In practice, it is not usually possible to obtain detailed information, 
and the effects of smoking and other known confounders cannot be 
precisely quantified.
---------------------------------------------------------------------------

    \40\ Since these rates may vary by race, geographic region, or 
other factors, the validity of this adjustment depends heavily on 
choice of an appropriate reference population. For example, 
Waxweiler et al. (1973) based SMRs for a New Mexico cohort on 
national lung cancer mortality rates. Since the national age-
adjusted rate of lung cancer is about 1/3 higher than the New Mexico 
rate, the reported SMRs were roughly 3/4 of what they would have 
been if based on rates specific to New Mexico.
---------------------------------------------------------------------------

    Stoober and Abel (1996) argue, along with Morgan et al. (1997) and 
some commenters, that even in those epidemiologic studies that are 
adjusted for smoking and show a statistically significant association, 
the magnitude of relative or excess risk observed is too small to 
demonstrate any causal link between dpm exposure and cancer. Their 
reasoning is that in these studies, errors in the collection or 
interpretation of smoking data can create a bias in the results larger 
than any potential contribution attributable to diesel particulate. 
They propose that studies

[[Page 5787]]

failing to account for smoking habits should be disqualified from 
consideration, and that evidence of an association from the remaining, 
smoking-adjusted studies should be discounted because of potential 
confounding due to erroneous, incomplete, or otherwise inadequate 
characterization of smoking histories.
    It should be noted, first of all, that five of the six negative 
studies neither matched nor adjusted for smoking.\41\ But more 
importantly, MSHA concurs with IARC (1989), Cohen and Higgins (1995), 
IPCS (1996), CAL-EPA (1998), ACGIH (1998), Bhatia et al. (1998), and 
Lipsett and Campleman (1999) in not accepting the view that studies 
should automatically be disqualified from consideration because of 
potential confounders. MSHA recognizes that unknown exposures to 
tobacco smoke or other human carcinogens can distort the results of 
some lung cancer studies. MSHA also recognizes, however, that it is not 
possible to design a human epidemiologic study that perfectly controls 
for all potential confounders. It is also important to note that a 
confounding variable does not necessarily inflate an observed 
association. For example, if the exposed members of a cohort smoke less 
than the reference group to which they are compared, then this will 
tend to reduce the apparent effects of exposure on lung cancer 
development. In the absence of evidence to the contrary, it is 
reasonable to assume that a confounder is equally likely to inflate or 
to deflate the results.
---------------------------------------------------------------------------

    \41\ The exception is DeCoufle et al. (1977), a case-control 
study that apparently did not match or otherwise adjust for age.
---------------------------------------------------------------------------

    As shown in Tables III-4 and III-5, 18 of the published 
epidemiologic studies involving lung cancer did, in fact, control or 
adjust for exposure to tobacco smoke, and five of these 18 also 
controlled or adjusted for exposure to asbestos and other carcinogenic 
substances (Garshick et al., 1987; Boffetta et al., 1988; Steenland et 
al., 1990; Morabia et al., 1992; Bruske-Hohlfeld et al., 1999). These 
results are less likely to be confounded than results from most of the 
studies with no adjustment. All but one of these 18 studies reported 
some degree of excess risk associated with occupational exposure to 
diesel particulate, with statistically significant results reported in 
eight.
    In addition, several of the studies with no smoking adjustment took 
the first approach described above for preventing or substantially 
mitigating potential confounding by smoking habits: they drew 
comparisons against internal control groups or other control groups 
likely to have similar smoking habits as the exposed groups (e.g., 
Garshick et al., 1988; Gustavsson et al., 1990; Hansen, 1993; and 
Saverin et al., 1999). Therefore, MSHA places more weight on these 
studies than on studies drawing comparisons against dissimilar groups 
with no smoking controls or adjustments. This emphasis is in accordance 
with the conclusion by Bhatia et al. (1998) that smoking homogeneity 
typically exists within cohorts and is associated with a uniform 
lifestyle and social class. Although it was not yet available at the 
time Bhatia et al. performed their analysis, an analysis of smoking 
patterns by Saverin et al. (op cit.) within the cohort they studied 
also supports this conclusion.
    IMC Global and MARG objected to MSHA's position on potential 
confounders and submitted comments in general agreement with the views 
of Morgan et al. (op cit.) and Stobel and Abel (op cit.). Specifically, 
they suggested that studies reporting relative risks solely between 1.0 
and 2.0 should be discounted because of potential confounders. Of the 
41 positive studies considered by MSHA, 22 fall into this category (16 
cohort and 6 case-control). In support of their suggestion, IMC Global 
quoted Speizer (1986), Muscat and Wynder (1995), Lee (1989), WHO 
(1980), and NCI (1994). These authorities all urged great caution when 
interpreting the results of such studies, because of potential 
confounders. MSHA agrees that none of these studies, considered 
individually, is conclusive and that each result must be considered 
with due caution. None of the quoted authorities, however, proposed 
that such studies should automatically be counted as ``negative'' or 
that they could not add incrementally to an aggregate body of positive 
evidence.
    IMC Global also submitted the following reference to two Federal 
Court decisions pertaining to estimated relative risks less than 2.0:

The Ninth Circuit concluded in Daubert v. Merrell Dow 
Pharmaceuticals'' that ``for an epidemiologic study to show 
causation * * * the relative risk * * * arising from the 
epidemiologic data will, at a minimum, have to exceed 2.'' 
Similarly, a District Court stated in Hall v. Baxter Healthcare 
Corp. 49: The threshold for concluding that an agent was more likely 
the cause of the disease than not is relative risk greater than 2.0. 
Recall that a relative risk of 1.0 means that the agent has no 
affect on the incidence of disease. When the relative risk reaches 
2.0. the agent is responsible for an equal number of cases of 
disease as all other background causes. Thus a relative risk of 2.0 
implies a 50% likelihood that an exposed individual's disease was 
caused by the agent. [IMC Global]

    In contrast with the two cases cited, the purpose of this risk 
assessment is not to establish civil liabilities for personal injury. 
MSHA's concern is with reducing the risk of lung cancer, not with 
establishing the specific cause of lung cancer for an individual miner. 
The excess risk of an outcome, given an excessive exposure, is not the 
same thing as the likelihood that an excessive exposure caused the 
outcome in a given case. To understand the difference, it may be 
helpful to consider two analogies: (1) The likelihood that a given 
death was caused by a lightning strike is relatively low, yet exposure 
to lightning is rather hazardous; (2) a specific smoker may not be able 
to prove that his or her lung cancer was ``more likely than not'' 
caused by radon exposure, yet radon exposure significantly increases 
the risk--especially for smokers. Lung cancer has a variety of 
alternative causes, but this fact does not reduce the risk associated 
with any one of them.
    Furthermore, there is ample precedent for utilizing epidemiologic 
studies reporting relative risks less than 2.0 in making clinical and 
public policy decisions. For example, the following table contains the 
RR for death from cardiovascular disease associated with cigarette 
smoking reported in several prospective epidemiologic studies:

BILLING CODE 4510-43-P

[[Page 5788]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.064


BILLING CODE 4510-43-C

[[Page 5789]]

By IMC Global's rule of thumb, all but one or two of these studies 
would be discounted as evidence of increased risk attributable to 
smoking. These studies, however, have not been widely discounted by 
scientific authorities. To the contrary, they have been instrumental in 
establishing that cigarette smoking is a principal cause of heart 
disease.
    A second example is provided by the increased risk of lung cancer 
found to be caused by residential exposure to radon progeny. As in the 
case of dpm, tobacco smoking has been an important potential confounder 
in epidemiological studies used to investigate whether exposures to 
radon concentrations at residential levels can cause lung cancer. Yet, 
in the eight largest residential epidemiological studies used to help 
establish the reality of this now widely accepted risk, the reported 
relative risks were all less than 2.0. Based on a meta-analysis of 
these eight studies, the combined relative risk of lung cancer 
attributable to residential radon exposure was 1.14. This elevation in 
the risk of lung cancer, though smaller than that reported in most 
studies of dpm effects, was found to be statistically significant at a 
95-percent confidence level (National Research Council, 1999, Table G-
25).
    (ii) Studies Involving Miners. In the proposed risk assessment, 
MSHA identified seven epidemiologic studies reporting an excess risk of 
lung cancer among miners thought to have been exposed occupationally to 
diesel exhaust. As stated in the proposal, two of these studies 
specifically investigated miners, and the other five treated miners as 
a subgroup within a larger population of workers.\42\ MSHA placed two 
additional studies specific to exposed coal miners (Christie et al., 
1995; Johnston et al., 1997) into the public record with its Feb. 12, 
1999 Federal Register notice. Another study,\43\ investigating lung 
cancer in exposed potash miners, was introduced by NIOSH at the 
Knoxville public hearing on May 27, 1999 and later published as Saverin 
et al., 1999. Finally, one study reporting an excess risk of lung 
cancer for presumably exposed miners was listed in Table III-5 as 
originally published, and considered by MSHA in its overall assessment, 
but inadvertently left out of the discussion on studies involving 
miners in the previous version of this risk assessment.\44\ There are, 
therefore, available to MSHA a total of 11 epidemiologic studies 
addressing the risk of lung cancer for miners, and five of these 
studies are specific to miners.
---------------------------------------------------------------------------

    \42\ In the proposed risk assessment, the studies identified as 
specifically investigating miners were Waxweiler et al. (1973) and 
Ahlman et al. (1991). At the Albuquerque public hearing, Mr. Bruce 
Watzman, representing the NMA, asked a member of the MSHA panel (Mr. 
Jon Kogut) to list six studies involving miners that he had cited 
earlier in the hearing and to identify those that were specific to 
miners. In both his response to Mr. Watzman, and in his earlier 
remarks, Mr. Kogut noted that the studies involving miners were 
listed in Tables III-4 and III-5. However, he inadvertently 
neglected to mention Ahlman et al. (op cit.) and Morabia et al. 
(1992). (The latter study addressed miners as a subgroup of a larger 
population.)
    In his response to Mr. Watzman, Mr. Kogut cited Swanson et al. 
(1993) but not Burns and Swanson (1991), which he had mentioned 
earlier in the hearing in connection with the same study. These two 
reports are listed under a single entry in Table III-5 (Swanson et 
al.) because they both report findings based on the same body of 
data. Therefore, MSHA considers them to be two parts of the same 
study. The 5.03 odds ratio for mining machine operators mentioned by 
Mr. Kogut during the hearing was reported in Burns and Swanson 
(1991).
    Only the six studies specified by Mr. Kogut in his response to 
Mr. Watzman were included in separate critiques by Dr. Peter Valberg 
and Dr. Jonathan Borak later submitted by the NMA and by MARG, 
respectively. Dr. Valberg did not address Burns and Swanson (1991), 
and he addressed a different report by Siemiatycki than the one 
listed in Table III-5 and cited during the hearing (i.e., 
Siemiatycki et al., 1988). Neither Dr. Valberg nor Dr. Borak 
addressed Ahlman et al. (op cit.) or Morabia et al. (op cit.). Also 
excluded were two additional miner-specific studies placed into the 
record on Feb. 12, 1999 (Fed. Reg. 64:29 at 59258). Mr. Kogut did 
not include them in his response to Mr. Watzman, or in his prior 
remarks, because he was referring only to studies listed in Tables 
III-4 and III-5 of the published proposals. Mr. Kogut also did not 
include a study specific to German potash miners submitted by NIOSH 
at a subsequent public hearing, and this too was left out of both 
critiques. A published version of the study (Saverin et al., 1999) 
was placed into the record on June 30, 2000. All of the studies 
involving miners are in the public record and have been available 
for comment by interested parties throughout the posthearing comment 
periods.
    \43\ Some commenters suggested that MSHA ``overlooked'' a 
recently published study on NSW miners, Brown et al., 1997. This 
study evaluated the occurrence of forms of cancer other than lung 
cancer in the same cohort studied by Christie et al. (1995).
    \44\ This study was published in two separate reports on the 
same body of data: Burns and Swanson (1991) and Swanson et al. 
(1993). Both published reports are listed in Table III-5 under the 
entry for Swanson et al.
---------------------------------------------------------------------------

    Five cohort studies (Waxweiler et al., 1973; Ahlman et al., 1991; 
Christie et al., 1996; Johnston et al., 1997; Saverin et al., 1999) 
were performed specifically on groups of miners, and one (Boffetta et 
al., 1988) addressed miners as a subgroup of a larger population. 
Except for the study by Christie et al., the cohort studies all showed 
elevated lung cancer rates for miners in general or for the most highly 
exposed miners within a cohort. In addition, all five case-control 
studies reported elevated rates of lung cancer for miners (Benhamou et 
al., 1988; Lerchen et al., 1987; Siemiatycki et al., 1988; Morabia et 
al., 1992; Burns and Swanson, 1991).
    Despite the risk assessment's emphasis on human studies, some 
members of the mining community apparently believed that the risk 
assessment relied primarily on animal studies and that this was because 
studies on miners were unavailable. Canyon Fuels, for example, 
expressed concerns about relying on animal studies instead of studies 
on western diesel-exposed miners:

    Since there are over a thousand miners here in the West that 
have fifteen or more years of exposure to diesel exhaust, why has 
there been no study of the health status of those miners? Why must 
we rely on animal studies that are questionable and inconclusive?

Actually, western miners were involved in several studies of health 
effects other than cancer, as described earlier in this risk 
assessment. With respect to lung cancer, there are many reasons why 
workers from a particular group of mines might not be selected for 
study. Lung cancer often takes considerably more than 15 years to 
develop, and a valid study must allow not only for adequate duration of 
exposure but also for an adequate period of latency following exposure. 
Furthermore, many mines contain radioactive gases and/or respirable 
silica dust, making it difficult to isolate the effects of a potential 
carcinogen.
    Similarly, at the public hearing in Albuquerque on May 13, 1999, a 
representative of Getchell Gold stated that he thought comparing miners 
to rats was irrational and that ``there has not been a study on these 
miners as to what the effects are.'' To correct the impression that 
MSHA was basing its risk assessment primarily on laboratory animal 
studies, an MSHA panelist pointed out Tables III-4 and III-5 of the 
proposed preamble and identified six studies pertaining to miners that 
were listed in those tables. However, he placed no special weight on 
these studies and cited them only to illustrate the existence of 
epidemiologic studies reporting an elevated risk of lung cancer among 
miners.
    With their post-hearing comments, the NMA and MARG submitted 
critiques by Dr. Peter Valberg and Dr. Jonathan Borak of six reports 
involving miners (see Footnote 42). Drs. Valberg and Borak both noted 
that the six studies reviewed lacked information on diesel exposure and 
were vulnerable to confounders and exposure misclassification. For 
these reasons, Dr. Valberg judged them ``particularly poor in 
identifying what specific role, if any, diesel exhaust plays in lung 
cancer for miners.'' He concluded that they do not ``implicate diesel 
exposure per se as

[[Page 5790]]

strongly associated with lung cancer risk in miners.'' Similarly, Dr. 
Borak suggested that, since they do not relate adverse health effects 
in miners to any particular industrial exposure, ``the strongest 
conclusion that can be drawn from these six studies is that the miners 
in the studies had an increased risk of lung cancer.''
    MSHA agrees with Drs. Valberg and Borak that none of the studies 
they reviewed provides direct evidence of a link between dpm exposure 
and the excess risk of lung cancer reported for miners. (A few 
disagreements on details of the individual studies will be discussed 
below). As MSHA said at the Albuquerque hearing, the lack of exposure 
information on miners in these studies led MSHA to rely more heavily on 
associations reported for other occupations. MSHA also noted the 
limitations of these studies in the proposed risk assessment. MSHA 
explicitly stated that other epidemiologic studies exist which, though 
not pertaining specifically to mining environments, contain better 
diesel exposure information and are less susceptible to confounding by 
extraneous risk factors.
    Inconclusive as they may be on their own, however, even studies 
involving miners with only presumed or sporadic occupational diesel 
exposure can contribute something to the weight of evidence. They can 
do this by corroborating evidence of increased lung cancer risk for 
other occupations with likely diesel exposures and by providing results 
that are at least consistent with an increased risk of lung cancer 
among miners exposed to dpm. Moreover, two newer studies pertaining 
specifically to miners do contain dpm exposure assessments based on 
concurrent exposure measurements (Johnston et al., op cit.; Saverin et 
al., op cit.). The major limitations pointed out by Drs. Valberg and 
Borak with respect to other studies involving miners do not apply to 
these two studies.

Case-Control Studies

    Five case-control studies, all of which adjusted for smoking, found 
elevated rates of lung cancer for miners, as shown in Table III-5. The 
results for miners in three of these studies (Benhamou et al., 1988; 
Morabia et al., 1992; Siemiatycki et al., 1988) are given little 
weight, partly because of possible confounding by occupational exposure 
to radioactive gasses, asbestos, and silica dust. Also, Benhamou and 
Morabia did not verify occupational diesel exposure status for the 
miners. Siemiatycki performed a large number of multiple comparisons 
and reported that most of the miners ``were exposed to diesel exhaust 
for short periods of time,'' Lerchen et al. (1987) showed a marginally 
significant result for underground non-uranium miners, but cases and 
controls were not matched on date of birth or death, and the frequency 
of diesel exposure and exposure to known occupational carcinogens among 
these miners was not reported.
    Burns and Swanson (1991) \45\ reported elevated lung cancer risk 
for miners and especially mining machine operators, which the authors 
attributed to diesel exposure. Potential confounding by other 
carcinogens associated with mining make the results inconclusive, but 
the statistically significant odds ratio of 5.0 reported for mining 
machine operators is high enough to cause concern with respect to 
diesel exposures, especially in view of the significantly elevated 
risks reported in the same study for other diesel-exposed occupations. 
The authors noted that the ``occupation most likely to have high levels 
of continuous exposure to diesel exhaust and to experience that 
exposure in a confined area has the highest elevated risks: mining 
machine operators.''
---------------------------------------------------------------------------

    \45\ This report is listed in Table III-5 under Swanson et al. 
(1993), which provides further analysis of the same body of data.
---------------------------------------------------------------------------

Cohort Studies

    As shown in Table III-4, MSHA identified six cohort studies 
reporting results for miners likely to have been exposed to dpm. An 
elevated risk of lung cancer was reported in five of these six studies. 
These results will be discussed chronologically.
    Waxweiller (1973) investigated a cohort of underground and surface 
potash miners. The authors noted that potash ore ``is not embedded in 
siliceous rock'' and that the ``radon level in the air of potash mines 
is not significantly higher than in ambient air.'' Contrary to Dr. 
Valberg's review of this study, the number of lung cancer cases was 
reported to be slightly higher than expected, for both underground and 
surface miners, based on lung cancer rates in the general U.S. 
population (after adjustment for age, sex, race, and date of death). 
Although the excess was not statistically significant, the authors 
noted that lung cancer rates in the general population of New Mexico 
were about 25 percent lower than in the general U.S. population. They 
also noted that a higher than average percentage of the miners smoked 
and that this would ``tend to counterbalance'' the adjustment needed 
for geographic location. The authors did not, however, consider two 
other factors that would tend to obscure or deflate an excess risk of 
lung cancer, if it existed: (1) a healthy worker effect and (2) the 
absence of any occupational diesel exposure for a substantial 
percentage of the underground miners.
    MSHA agrees with Dr. Valberg's conclusion that ``low statistical 
power and indeterminate diesel-exhaust exposure render this study 
inadequate for assessing the effect of diesel exhaust on lung-cancer 
risk in miners.'' However, given the lack of any adjustment for a 
healthy worker effect, and the likelihood that many of the underground 
miners were occupationally unexposed, MSHA views the slightly elevated 
risk reported in this study as consistent with other studies showing 
significantly greater increases in risk for exposed workers.
    Boffetta et al. (1988) investigated mortality in a cohort of male 
volunteers who enrolled in a prospective study conducted by the 
American Cancer Society. Lung cancer mortality was analyzed in relation 
to self-reported diesel exhaust exposure and to employment in various 
occupations identified with diesel exhaust exposure, including mining. 
After adjusting for smoking patterns,\46\ there was a statistically 
significant excess of 167 percent (RR = 2.67) in lung cancers among 
2034 workers ever employed as miners, compared to workers never 
employed in occupations associated with diesel exposure. No analysis by 
type of mining was reported. Other findings reported from this study 
are discussed in the next subsection.
---------------------------------------------------------------------------

    \46\ During the public hearing on May 25, 1999, Mr. Mark 
Kaszniak of IMC Global incorrectly asserted that ``smoking was 
treated in a simplistic way in this study by using three categories: 
smokers, ex-smokers, and non-smokers.'' The study actually used five 
categories, dividing smokers into separate categories for 1-20 
cigarettes per day, 21 or more cigarettes per day, and exclusively 
pipe and/or cigar smoking.
---------------------------------------------------------------------------

    Although an adjustment was made for smoking patterns, the relative 
risk reported for mining did not control for exposures to radioactive 
gasses, silica dust, and asbestos. These lung carcinogens are probably 
present to a greater extent in mining environments than in most of the 
occupational environments used for comparison. Self-reported exposures 
to asbestos and stone dusts were taken into account in other parts of 
the study, but not in the calculation of excess lung cancer risks 
associated with specific occupations, including mining.

[[Page 5791]]

    Several commenters reiterated two caveats expressed by the study's 
authors and noted in Table III-4. These are (1) that the study is 
susceptible to selection biases because participants volunteered and 
because the age-adjusted mortality rates differed between those who 
provided exposure information and those who did not; and (2) that all 
exposure information was self-reported with no quantitative 
measurements. Since these caveats are not specific to mining and 
pertain to most of the study's findings, they will be addressed when 
this study's overall results are described in the next subsection.
    One commenter, however, (Mr. Mark Kaszniak of IMC Global) argued 
that selection bias due to unknown diesel exposure status played an 
especially important role in the RR calculated for miners. About 21 
percent of all participants provided no diesel exposure information. 
Mr. Kaszniak noted that diesel exposure status was unknown for an even 
larger percentage of miners and suggested that the RR calculated for 
miners was, therefore, inflated. He presented the following argument:

    In the miner category, this [unknown diesel exposure status] 
accounted for 44.2% of the study participants, higher than any other 
occupation studied. This is important as this group experienced a 
higher mortality for all causes as well as lung cancer than the 
analyzed remainder of the cohort. If these persons had been included 
in the ``no exposure to diesel exhaust group,'' their inclusion 
would have lowered any risk estimates from diesel exposure because 
of their higher lung cancer rates. [IMC Global post-hearing 
comments]

    This argument, which was endorsed by MARG, was apparently based on 
a misunderstanding of how the comparison groups used to generate the RR 
for mining were defined.\47\ Actually, persons with unknown diesel 
exposure status were included among the miners, but excluded from the 
reference population. Including sometime miners with unknown diesel 
exposure status in the ``miners'' category would tend to mask or reduce 
any strong association that might exist between highly exposed miners 
and an increased risk of lung cancer. Excluding persons with unknown 
exposure status from the reference population had an opposing effect, 
since they happened to experience a higher rate of lung cancer than 
cohort members who said they were unexposed. Therefore, removing 
``unknowns'' from the ``miner'' group and adding them to the reference 
group could conceivably shift the calculated RR for miners in either 
direction. However, the RR reported for persons with unknown diesel 
exposure status, compared to unexposed persons, was 1.4 (ibid., p. 
412)--which is smaller than the 2.67 reported for miners. Therefore, it 
appears more likely that the RR for mining was deflated than inflated 
on account of persons with unknown exposure status.
---------------------------------------------------------------------------

    \47\ During the public hearing on May 25, 1999, Mr. Kaszniak 
stated his belief that, for miners, the ``relative risk calculation 
excluded that 44% of folks who did not respond to the questionnaire 
with regards to diesel exposure.'' Contrary to Mr. Kaszniak's 
belief, however, the ``miners'' on which the 2.67 RR was based 
included all 2034 cohort members who had ever been a miner, 
regardless of whether they had provided diesel exposure information 
(see Boffetta et al., 1988, p. 409).
    Furthermore, the 44.2-percent nonrespondent figure is not 
pertinent to potential selection bias in the RR calculation reported 
for miners. The group of 2034 ``sometime'' miners used in that 
calculation was 65 percent larger than the group of 1233 ``mainly'' 
miners to which the 44-percent nonrespondent rate applies. The 
reference group used for comparison in the calculation consisted of 
all cohort members ``with occupation different from those listed 
[i.e., railroad workers, truck drivers, heavy equipment operators, 
and miners] and not exposed [to diesel exhaust].'' The overall 
nonrespondent rate for occupations in the reference group was about 
21 percent (calculated by MSHA from Table VII of Boffetta et al., 
1988).
---------------------------------------------------------------------------

    Although confounders and selection effects may have contributed to 
the 2.67 RR reported for mining, MSHA believes this result was high 
enough to support a dpm effect, especially since elevated lung cancer 
rates were also reported for the three other occupations associated 
with diesel exhaust exposure. Dr. Borak stated without justification 
that ``[the] association between dpm and lung cancer was confounded by 
age, smoking, and other occupational exposures * * *.'' He ignored the 
well-documented adjustments for age and smoking. Although it does not 
provide strong or direct evidence that dpm exposure was responsible for 
any of the increased risk of lung cancer observed among miners, the RR 
for miners is consistent with evidence provided by the rest of the 
study results.
    Ahlman et al. (1991) studied cohorts of 597 surface miners and 338 
surface workers employed at two sulfide ore mines using diesel powered 
front-end loaders and haulage equipment. Both of these mines (one 
copper and one zinc) were regularly monitored for alpha energy 
concentrations (i.e., due to radon progeny), which were at or below the 
Finish limit of 0.3 WL throughout the study period. The ore in both 
mines contained arsenic only as a trace element (less than 0.005 
percent). Lung cancer rates in the two cohorts were compared to rates 
for males in the same province of Finland. Age-adjusted excess 
mortality was reported for both lung cancer and cardiovascular disease 
among the underground miners, but not among the surface workers. None 
of the underground miners who developed lung cancer had been 
occupationally exposed to asbestos, metal work, paper pulp, or organic 
dusts. Based on the alpha energy concentration measurements made for 
the two mines, the authors calculated that not all of the excess lung 
cancer for the underground miners was attributable to radon exposure. 
Based on a questionnaire, the authors found similar underground and 
surface age-specific smoking habits and alcohol consumption and 
determined that ``smoking alone cannot explain the difference in lung 
cancer mortality between the [underground] miners and surface 
workers.'' Due to the small size of the cohort, the excess lung cancer 
mortality for the underground miners was not statistically significant. 
However, the authors concluded that the portion of excess lung cancer 
not attributable to radon exposure could be explained by the combined 
effects of diesel exhaust and silica exposure. Three of the ten lung 
cancers reported for underground miners were experienced by conductors 
of diesel-powered ore trains.
    Christie et al. (1994, 1995) studied mortality in a cohort of 
23,630 male Australian (New South Wales, NSW) coal mine workers who 
entered the industry after 1972. Although the majority of these workers 
were underground miners, most of whom were presumably exposed to diesel 
emissions, the cohort included office workers and surface (``open 
cut'') miners. The cohort was followed up through 1992. After adjusting 
for age, death rates were lower than those in the general male 
population for all major causes except accidents. This included the 
mortality rate for all cancers as a group (Christie et al., 1995, Table 
1). Lower-than-normal incidence rates were also reported for cancers as 
a group and for lung cancer specifically (Christie et al., 1994, Table 
10).
    The investigators noted that the workers included in the cohort 
were all subject to pre-employment physical examinations. They 
concluded that ``it is likely that the well known `healthy worker' 
effect * * * was operating'' and that, instead of comparing to a 
general population, ``a more appropriate comparison group is Australian 
petroleum industry workers.'' (Christie et al., 1995) In contrast to 
the comparison with the population of NSW, the all-cause standardized 
mortality ratio (SMR) for the cohort of coal miners was greater than 
for petroleum workers by a factor of over 20 percent--i.e., 0.76 vs. 
0.63 (ibid., p. 20). However, the investigators did not

[[Page 5792]]

compare the cohort to petroleum workers specifically with respect to 
lung cancer or other causes of death. Nor did they adjust for a healthy 
worker effect or make any attempt to compare mortality or lung cancer 
rates among workers with varying degrees of diesel exposure within the 
cohort.
    Despite the elevated SMR relative to petroleum workers, several 
commenters cited this study as evidence that exposure to diesel 
emissions was not causally associated with an increased risk of lung 
cancer (or with adverse health effects associated with fine 
particulates). These commenters apparently ignored the investigators' 
explanation that the low SMRs they reported were likely due to a 
healthy worker effect. Furthermore, since the cohort exhibited lower-
than-normal mortality rates due to heart disease and non-cancerous 
respiratory disease, as well as to cancer, there may well have been 
less tobacco smoking in the cohort than in the general population. 
Therefore, it is reasonably likely that the age-adjusted lung cancer 
rate would have been elevated, if it had been adjusted for smoking and 
for a healthy worker effect based on mortality from causes other than 
accidents or respiratory disease. In addition, the cohort SMR for 
accidents (other than motor vehicle accidents) was significantly above 
that of the general population. Since the coal miners experienced an 
elevated rate of accidental death, they had a lower-than-normal chance 
to die from other causes or to develop lung cancer. The investigators 
made no attempt to adjust for the competing, elevated risk of death due 
to occupational accidents.
    Given the lack of any adjustment for smoking, healthy worker 
effect, or the competing risk of accidental death, the utility of this 
study in evaluating health consequences of Dpm exposure is severely 
limited by its lack of any internal comparisons or comparisons to a 
comparable group of unexposed workers. Furthermore, even if such 
adjustments or comparisons were made, several other attributes of this 
study limit its usefulness for evaluating whether exposure to diesel 
emissions is associated with an increased risk of lung cancer. First, 
the study was designed in such a way as to allow inadequate latency for 
a substantial portion of the cohort. Although the cohort was followed 
up only through 1992, it includes workers who entered the workforce at 
the end of 1992. Therefore, there is no minimum duration of 
occupational exposure for members of the cohort. Approximately 30 
percent of the cohort was employed in the industry for less than 10 
years, and the maximum duration of employment and latency combined was 
20 years. Second, average age for members of the cohort was only 40 to 
50 years (Christie et al., p. 7), and the rate of lung cancer was based 
on only 29 cases. The investigators acknowledged that ``it is a 
relatively young cohort'' and that ``this means a small number of 
cancers available for analysis, because cancer is more common with 
advancing age * * *.'' They further noted that ``* * * the number of 
cancers available for analysis is increasing very rapidly. As a 
consequence, every year that passes makes the cancer experience of the 
cohort more meaningful in statistical terms.'' (ibid., p. 27) Third, 
miners's work history was not tracked in detail, beyond identifying the 
first mine in which a worker was employed. Some of these workers may 
have been employed, for various lengths of time, in both underground 
and surface operations at very different levels of diesel exposure. 
Without detailed work histories, it is not possible to construct even 
semi-quantitative measures of diesel exposure for making internal 
comparisons within the cohort.
    One commenter (MARG) claimed that this (NSW) study ``* * * reflects 
the latest and best scientific evidence, current technology, and the 
current health of miners'' and that it ``is not rational to predicate 
regulations for the year 2000 and beyond upon older scientific studies 
* * *.'' For the reasons stated above, MSHA believes, to the contrary, 
that the NSW study contributes little or no information on the 
potential health effects of long-term dpm exposures and that whatever 
information it does contribute does not extend to effects, such as 
cancer, expected in later life.
    Furthermore, three even more recent studies are available that MSHA 
regards as far more informative for the purposes of the present risk 
assessment. Unlike the NSW study, these directly address dpm exposure 
and the risk of lung cancer. Two of these studies (Johnston et al., 
1997; Saverin et al., 1999), both incorporating a quantitative dpm 
exposure assessment, were carried out specifically on mining cohorts 
and will be discussed next. The third (Bruske-Hohlfeld et al., 1999) is 
a case-control study not restricted to miners and will be discussed in 
the following subsection. In accordance with MARG's emphasis on the 
timeliness of scientific studies, MSHA places considerable weight on 
the fact that all three--the most recent epidemiologic studies 
available--reported an association between diesel exposure and an 
increased risk of lung cancer.
    Johnston et al. (1997) studied a cohort of 18,166 coal miners 
employed in ten British coal mines over a 30-year period. Six of these 
coal mines used diesel locomotives, and the other four were used for 
comparison. Historical NOX and respirable dust concentration 
measurements were available, having routinely been collected for 
monitoring purposes. Two separate approaches were taken to estimate dpm 
exposures, leading to two different sets of estimates. The first 
approach was based on NOX measurements, combined with 
estimated ratios between dpm and NOX. The second approach 
was based on complex calculations involving measurements of total 
respirable dust, ash content, and the ratio of quartz to dust for 
diesel locomotive drivers compared to the ratio for face workers 
(ibid., Figure 4.1 and pp. 25-46). These calculations were used to 
estimate dpm exposure concentrations for the drivers, and the estimates 
were then combined with traveling times and dispersion rates to form 
estimates of dpm concentration levels for other occupational groups. In 
four of the six dieselized mines, the NOX-based and dust-
based estimates of dpm were in generally good agreement, and they were 
combined to form time-independent estimates of shift average dpm 
concentration for individual seams and occupational groups within each 
mine. In the fifth mine, the PFR measurements were judged unreliable 
for reasons extensively discussed in the report, so the NOX-
based estimates were used. There was no NOX exposure data 
for the sixth mine, so they used dust-based estimates of dpm exposure.
    Final estimates of shift-average dpm concentrations ranged from 44 
g/m\3\ to 370 g/m\3\ for locomotive drivers and from 
1.6 g/m\3\ to 40 g/m\3\ for non-drivers at various 
mines and work locations (ibid., Tables 8.3 and 8.6, respectively). 
These were combined with detailed work histories, obtained from 
employment records, to provide an individual estimate of cumulative dpm 
exposure for each miner in the cohort. Although most cohort members 
(including non-drivers) had estimated cumulative exposures less than 1 
g-hr/m\3\, some members had cumulative exposures that ranged as high as 
11.6 g-hr/m\3\ (ibid., Figure 9.1 and Table 9.1).
    A statistical analysis (time-dependent proportional hazards 
regression) was performed to examine the relationship between lung 
cancer risk and each miner's estimated cumulative dpm exposure 
(unlagged and lagged by 15 years), attained age, smoking habit,

[[Page 5793]]

mine, and cohort entry date. Smoking habit was represented by non-
smoker, ex-smoker, and smoker categories, along with the average number 
of cigarettes smoked per day for the smokers. Pipe tobacco consumption 
was expressed by an equivalent number of cigarettes per day.
    In their written comments, MARG and the NMA both mischaracterized 
the results of this study, apparently confusing it with a preliminary 
analysis of the same cohort. The preliminary analysis (one part of what 
Johnston et al. refer to as the ``wider mortality study'') was 
summarized in Section 1.2 (pp. 3-5) of the 105-page report at issue, 
which may account for the confusion by MARG and the NMA.\48\
---------------------------------------------------------------------------

    \48\ Since MARG and the NMA both stressed the importance of a 
quantitative exposure assessment, it is puzzling that they focused 
on a crude SMR from the preliminary analysis and ignored the 
quantitative results from the subsequent analysis. Johnston et al. 
noted that SMRs from the preliminary analysis were consistent ``with 
other studies of occupational cohorts where a healthy worker effect 
is apparent.'' But even the preliminary analysis explored a possible 
surrogate exposure-response relationship, rather than simply relying 
on SMRs. Unlike the analysis by Johnston et al., the preliminary 
analysis used travel time as a surrogate measure of dpm exposure and 
made no attempt to further quantify dpm exposure concentrations. 
(ibid., p. 5)
---------------------------------------------------------------------------

    Contrary to the MARG and NMA characterization, Johnston et al. 
found a positive, quantitative relationship between cumulative dpm 
exposure (lagged by 15 years) and an excess risk of lung cancer, after 
controlling for age, smoking habit, and cohort entry date. For each 
incremental g-hr/m\3\ of cumulative occupational dpm exposure, the 
relative risk of lung cancer was estimated to increase by a factor of 
22.7 percent. Adjusting for mine-to-mine differences that may account 
for a portion of the elevated risk reduced the estimated RR factor to 
15.6 percent. Therefore, with the mine-specific adjustment, the 
estimated RR was 1.156 per g-hr/m\3\ of cumulative dpm exposure. It 
follows that, based on the mine-adjusted model, the estimated RR for a 
specified cumulative exposure is 1.156 raised to a power equal to that 
exposure. For example, RR = (1.156)3.84 = 1.74 for a 
cumulative dpm exposure of 3.84 g-hr/m\3\, and RR = 
(1.156)7.68 = 3.04 for a cumulative dpm exposure of 7.68 g-
hr/m\3\.\49\ Estimates of RR based on the mine-unadjusted model would 
substitute 1.227 for 1.156 in these calculations.
---------------------------------------------------------------------------

    \49\ Assuming an average dpm concentration of 200 g/
m\3\ and 1920 work hours per year, 3.84 g-hr/m\3\ and 7.68 g-hr/m\3\ 
correspond to 10 and 20 years of occupational exposure, 
respectively.
---------------------------------------------------------------------------

    Two limitations of this study weaken the evidence it presents of an 
increasing exposure-response relationship. First, although the exposure 
assessment is quantitative and carefully done, it is indirect and 
depends heavily on assumptions linking surrogate measurements to dpm 
exposure levels. The authors, however, analyzed sources of inaccuracy 
in the exposure assessment and concluded that ``the similarity between 
the estimated * * * [dpm] exposure concentrations derived by the two 
different methods give some degree of confidence in the accuracy of the 
final values * * *.'' (ibid., pp.71-75) Second, the highest estimated 
cumulative dpm exposures were clustered at a single coal mine, where 
the SMR was elevated relative to the regional norm. Therefore, as the 
authors pointed out, this one mine greatly influences the results and 
is a possible confounder in the study. The investigators also noted 
that this mine was ``* * * found to have generally the higher exposures 
to respirable quartz and low level radiation.'' Nevertheless, MSHA 
regards it likely that the relatively high dpm exposures at this mine 
were responsible for at least some of the excess mortality. There is no 
apparent way, however, to ascertain just how much of the excess 
mortality (including lung cancer) at this coal mine should be 
attributed to high occupational dpm exposures and how much to 
confounding factors distinguishing it (and the employees working there) 
from other mines in the study.
    The RR estimates based on the mine-unadjusted model assume that the 
excess lung cancer observed in the cohort is entirely attributable to 
dpm exposures, smoking habits, and age distribution. If some of the 
excess lung cancer is attributed to other differences between mines, 
then the dpm effect is estimated by the lower RR based on the mine-
adjusted model.
    For purposes of comparison with the findings of Saverin et al. 
(1999), it will be useful to calculate the RR for a cumulative dpm 
exposure of 11.7 g-hr/m\3\ (i.e., the approximate equivalent of 4.9 mg-
yr/m\3\ TC).\50\ At this exposure level, the mine-unadjusted model 
produces an estimated RR = (1.227)\11.7\ = 11, and the mine-adjusted 
model produces an estimated RR = (1.156)\11.7\ = 5.5.
---------------------------------------------------------------------------

    \50\ This value represents 20 years of cumulative exposure for 
the most highly exposed category of workers in the cohort studied by 
Saverin et al.
    As explained elsewhere in this preamble, TC constitutes 
approximately 80 percent of total dpm. Therefore, the TC value of 
4.9 mg-yr/m\3\ presented by Saverin et al. must first be divided by 
0.8 to produce a corresponding dpm value of 6.12 mg-yr/m\3\. To 
convert this result to the units used by Johnston et al., it is then 
multiplied by 1920 work hours per year and divided by 1000 mg/g to 
yield 11.7 g-hr/m\3\. This is nearly identical to the maximum 
cumulative dpm exposure estimated for locomotive drivers in the 
study by Johnston et al. (See Johnston et al., op cit., Table 9.1.)
---------------------------------------------------------------------------

    Saverin et al. (1999) studied a cohort of male potash miners in 
Germany who had worked underground for at least one year after 1969, 
when the mines involved began converting to diesel powered vehicles and 
loading equipment. Members of the cohort were selected based on company 
medical records, which also provided bi-annual information on work 
location for each miner and, routinely after 1982, the miner's smoking 
habits. After excluding miners whose workplace histories could not be 
reconstructed from the medical records (5.5 percent) and miners lost to 
follow-up (1.9 percent), 5,536 miners remained in the cohort. Within 
this full cohort, the authors defined a sub-cohort consisting of 3,258 
miners who had ``worked underground for at least ten years, held one 
single job during at least 80% of their underground time, and held not 
more than three underground jobs in total.''
    The authors divided workplaces into high, medium, and low diesel 
exposure categories, respectively corresponding to production, 
maintenance, and workshop areas of the mine. Each of these three 
categories was assigned a representative respirable TC concentration, 
based on an average of measurements made in 1992. These averages were 
390 g/m3 for production, 230 g/
m3 for maintenance, and 120 g/m3 for 
workshop. Some commenters expressed concern about using average 
exposures from 1992 to represent exposure throughout the study. The 
authors justified using these measurement averages to represent 
exposure levels throughout the study period because ``the mining 
technology and the type of machinery used did not change substantially 
after 1970.'' This assumption was based on interviews with local 
engineers and industrial hygienists.
    Thirty-one percent of the cohort consented to be interviewed, and 
information from these interviews was used to validate the work history 
and smoking data reconstructed from the medical records. The TC 
concentration assigned to each work location was combined with each 
miner's individual work history to form an estimate of cumulative 
exposure for each member of the cohort. Mean duration of exposure was 
15 years. As of the end of follow-up in 1994, average age was 49 years, 
average time since first exposure was 19 years, and average cumulative 
exposure was 2.70 mg-y/m3.

[[Page 5794]]

    The authors performed an analysis (within each TC exposure 
category) of smoking patterns compared with cumulative TC exposure. 
They also analyzed smoking misclassification as estimated by comparing 
information from the interviews with medical records. From these 
analyses, the authors determined that the cohort was homogeneous with 
respect to smoking and that a smoking adjustment was neither necessary 
nor desirable for internal comparisons. However, they did not entirely 
rule out the possibility that smoking effects may have biased the 
results to some extent. On the other hand, the authors concluded that 
asbestos exposure was minor and restricted to jobs in the workshop 
category, with negligible effects. The miners were not occupationally 
exposed to radon progeny, as documented by routine measurement records.
    As compared to the general male population of East Germany, the 
cohort SMR for all causes combined was less than 0.6 at a 95-percent 
confidence level. The authors interpreted this as demonstrating a 
healthy worker effect, noting that ``underground workers are heavily 
selected for health and sturdiness, making any surface control group 
incomparable.'' Accordingly, they performed internal comparisons within 
the cohort of underground miners. The RR reported for lung cancer among 
miners in the high-exposure production category, compared to those in 
the low-exposure workshop category, was 2.17. The corresponding RR was 
not elevated for other cancers or for diseases of the circulatory 
system.
    Two statistical methods were used to investigate the relationship 
between lung cancer RR and each miner's age and cumulative TC exposure: 
Poisson regression and time-dependent proportional hazards regression. 
These two statistical methods were applied to both the full cohort and 
the subcohort, yielding four different estimates characterizing the 
exposure-response relationship. Although a high confidence level was 
not achieved, all four of these results indicated that the RR increased 
with increasing cumulative TC exposure. For each incremental mg-yr/
m3 of occupational TC exposure, the relative risk of lung 
cancer was estimated to increase by the following multiplicative 
factor: \51\
---------------------------------------------------------------------------

    \51\ MSHA determined these values by calculating the antilog, to 
the base e, of each corresponding estimate of  reported by 
Saverin et al. (op cit.) in their Tables III and IV. The cumulative 
exposure unit of mg-yr/m3 refers to the average TC 
concentration experienced over a year's worth of 8-hour shifts.

------------------------------------------------------------------------
                                                RR per mg-yr/m \3\
                 Method                  -------------------------------
                                            Full cohort      Subcohort
------------------------------------------------------------------------
Poisson.................................           1.030           1.139
Proportional Hazards....................           1.112           1.225
------------------------------------------------------------------------

Based on these estimates, the RR for a specified cumulative TC exposure 
(X) can be calculated by raising the tabled value to a power equal to 
X. For example, using the proportional hazards analysis of the 
subcohort, the RR for X = 3.5 mg-yr/m3 is 
(1.225) 3.5 = 2.03.\52\
---------------------------------------------------------------------------

    \52\ This is the estimated risk relative not to miners in the 
workshop category but to a theoretical age-adjusted baseline risk 
for cohort members accumulating zero occupational TC exposure.
---------------------------------------------------------------------------

    The authors calculated the RR expected for a cumulative TC exposure 
of 4.9 mg-yr/m3, which corresponds to 20 years of occupational exposure 
for miners in the production category of the cohort. These miners were 
exposed for five hours per 8-hour shift at an average TC concentration 
of 390 g/m.\3\ The resulting RR values were reported as 
follows:

------------------------------------------------------------------------
                                              RR for 4.9 mg-yr/m \3\
                 Method                  -------------------------------
                                            Full cohort      Subcohort
------------------------------------------------------------------------
Poisson.................................            1.16            1.89
Proportional Hazards....................            1.68            2.70
------------------------------------------------------------------------

    This study has two important limitations that weaken the evidence 
it presents of a positive correlation between cumulative TC exposure 
and the risk of lung cancer. These are (1) potential confounding due to 
tobacco smoking and (2) a significant probability (i.e., greater than 
10 percent) that a correlation of the magnitude found could have arisen 
simply by chance, given that it were based on a relatively small number 
of lung cancer cases.
    Although data on smoking habits were compiled from medical records 
for approximately 80 percent of the cohort, these data were not 
incorporated into the statistical regression models. The authors 
justified their exclusion of smoking from these models by showing that 
the likelihood of smoking was essentially unrelated to the cumulative 
TC exposure for cohort members. Based on the portion of the cohort that 
was interviewed, they also determined that the average number of 
cigarettes smoked per day was the same for smokers in the high and low 
TC exposure categories (production and workshop, respectively). 
However, these same interviews led them to question the accuracy of the 
smoking data that had been compiled from medical records. Despite the 
cohort's apparent homogeneity with respect to smoking, the authors 
noted that smoking was potentially such a strong confounder that ``even 
small inaccuracies in smoking data could cause effects comparable in 
size to the weak carcinogenic effect of diesel exhaust.'' Therefore, 
they excluded the smoking data from the analysis and stated they could 
not entirely rule out the possibility of a smoking bias. MSHA agrees 
with the authors of this report and the HEI Expert Panel (op cit.) that 
even a high degree of cohort homogeneity does not rule out the 
possibility of a spurious correlation due to residual smoking effects. 
Nevertheless, because of the cohort's homogeneity, the authors 
concluded that ``the results are unlikely to be substantially biased by 
confounding,'' and MSHA accepts this conclusion.
    The second limitation of this study is related to the fact that the 
results are based on a total of only 38 cases of lung cancer for the 
full cohort and 21 cases for the subcohort. In their description of

[[Page 5795]]

this study at the May 27, 1999, public hearing, NIOSH noted that the 
``lack of [statistical] significance may be a result of the study 
having a small cohort (approximately 5,500 workers), a limited time 
from first exposure (average of 19 years), and a young population 
(average age of 49 years at the end of follow-up).'' More cases of lung 
cancer may be expected to occur within the cohort as its members grow 
older. The authors of the study addressed statistical significance as 
follows:

    * * * the small number of lung cancer cases produced wide 
confidence intervals for all measures of effect and substantially 
limited the study power. We intend to extend the follow-up period in 
order to improve the statistical precision of the exposure-response 
relationship. [Saverin et al., op cit.]

    Some commenters stated that due to these limitations, data from the 
Saverin et al. study should not be the basis of this rule. On the other 
hand, NIOSH commented that ``[d]espite the limitations discussed * * * 
the findings from the Saverin et al. (1999) study should be used as an 
alternative source of data for quantifying the possible lung cancer 
risks associated with Dpm exposures.'' As stated earlier, MSHA is not 
relying on any single study but, instead, basing its evaluation on the 
weight of evidence from all available data.
    (iii) Best Available Epidemiologic Evidence. Based on the 
evaluation criteria described earlier, and after considering all the 
public comment that was submitted, MSHA has identified four cohort 
studies (including two from U.S.) and four case-control studies 
(including three from U.S.) that provide the best currently available 
epidemiologic evidence relating dpm exposure to an increased risk of 
lung cancer. Three of the 11 studies involving miners fall into this 
select group. MSHA considers the statistical significance of the 
combined evidence far more important than confidence levels for 
individual studies. Therefore, in choosing the eight most informative 
studies, MSHA placed less weight on statistical significance than on 
the other criteria. The basis for MSHA's selection of these eight 
studies is summarized as follows:

----------------------------------------------------------------------------------------------------------------
                                      Statistical                                                 Controls on
             Study                Significance (at 95%   Comparison groups       Exposure          potential
                                         Conf.)                                 assessment        confounding
----------------------------------------------------------------------------------------------------------------
Boffetta et al. 1988 (cohort)..  Yes..................  Internal            Job history and    Adjustments for
                                                         Comparison.         self-reported      age, smoking,
                                                                             duration of        and, in some
                                                                             occupational       analyses, for
                                                                             diesel exposure.   occupational
                                                                                                exposures to
                                                                                                asbestos, coal &
                                                                                                stone dusts,
                                                                                                coal tar &
                                                                                                pitch, and
                                                                                                gasoline
                                                                                                exhaust.
Boffetta et al. 1990 (case-      No...................  Matched within      Job history and    Adjustments for
 control).                                               hospital on         self-reported      age, smoking
                                                         smoking, age,       duration of        habit and
                                                         year of interview.  occupational       intensity,
                                                                             diesel exposure.   asbestos
                                                                                                exposure, race,
                                                                                                and education.
Bruske-Hohlfeld et al. 1999      Yes..................  Matched on sex,     Total duration of  Adjustments for
 (case-control).                                         age, and region     occupational       current and past
                                                         of residence of     diesel exposure    smoking
                                                         residence.          based on           patterns,
                                                                             detailed job       cumulative
                                                                             history.           amount smoked
                                                                                                (packyears), and
                                                                                                asbestos
                                                                                                exposure.
Garshick et al. 1987 (case-      Yes..................  Matched within      Semi-              Adjustments for
 control).                                               cohort on dates     quantitative,      lifetime smoking
                                                         of birth and        based on job       and asbestos
                                                         death.              history and        exposure.
                                                                             tenure combined
                                                                             with exposure
                                                                             status
                                                                             established
                                                                             later for each
                                                                             job.
Garshick et al. 1988, 1991       Yes..................  Internal            Semi-              Subjects with
 (cohort).                                               Comparison.         quantitative,      likely or
                                                                             based on job       possible
                                                                             history and        asbestos
                                                                             tenure combined    exposure
                                                                             with exposure      excluded from
                                                                             status             cohort.
                                                                             established        Cigarette
                                                                             later for each     smoking
                                                                             job.               determined to be
                                                                                                uncorrelated
                                                                                                with diesel
                                                                                                exposure within
                                                                                                cohort.
Johnston et al. 1997 (cohort)..  No (marginal)........  Internal            Quantitative,      Adjustments for
                                                         Comparison.         based on           age, smoking
                                                                             surrogate          habit &
                                                                             exposure           intensity, mine
                                                                             measurements and   site, and cohort
                                                                             detailed           entry date.
                                                                             employment
                                                                             records.
Saverin et al. 1999 (cohort)...  No...................  Internal            Quantitative,      Adjustment for
                                                         Comparison.         based on TC        age. Cigarette
                                                                             exposure           smoking
                                                                             measurements and   determined to be
                                                                             detailed           uncorrelated
                                                                             employment         with cumulative
                                                                             records.           TC exposure
                                                                                                within cohort.
Steenland et al. 1990, 1992,     Yes..................  Matached within     Semi-              Adjustments for
 1998 (case-control).                                    cohort on date of   quantitative,      age, smoking,
                                                         death within 2      based on job       and asbestos
                                                         years.              history and        exposure.
                                                                             subsequent EC      Dietary
                                                                             measurements.      covariates were
                                                                                                tested and found
                                                                                                not to confound
                                                                                                the analysis.
----------------------------------------------------------------------------------------------------------------

    Six entirely negative studies were identified earlier in this risk 
assessment. Several commenters objected to MSHA's treatment of the 
negative studies, indicating that they had been discounted without 
sufficient justification. To put this in proper perspective, the six 
negative studies should be compared to those MSHA has identified as the 
best available epidemiologic evidence, with respect to the same 
evaluation criteria. (It should be noted that the statistical 
significance of a negative study is best represented by its power.) In 
accordance with those criteria, MSHA discounts the evidentiary 
significance of these six studies for the following reasons:

----------------------------------------------------------------------------------------------------------------
                                                                                                  Controls on
              Study                      Power         Comparison groups       Exposure            potential
                                                                              assessment          confounding
----------------------------------------------------------------------------------------------------------------
Bender et al. 1989 (cohort).....  Relative small      External            Job only: highway   Disparate
                                   cohort (N=4849).    comparison; No      maintenance         comparison groups
                                                       adjustment for      workers.            with no smoking
                                                       healthy worker                          adjustment.
                                                       effect.

[[Page 5796]]

 
Christie et al. 1996 (cohort)...  Inadequate latency  External            Industry only:      Disparate
                                   allowance.          comparison; No      combined all        comparison groups
                                                       adjustment for      underground and     with no smoking
                                                       healthy worker      surface workers     adjustment.
                                                       effect.             at coal mines.
DeCoufle et al. 1977 (case-       Inadequate latency  Cases not matched   Job only: (1)       Age differences
 control).                         allowance.          with controls.      Combined bus,       not taken into
                                                                           taxi, and truck     account.
                                                                           drivers; (2)
                                                                           locomotive
                                                                           engineers.
Edling et al. 1987 (cohort).....  Small cohort        External            Job only: bus       Disparate
                                   (N=694).            comparison; No      workers.            comparison groups
                                                       adjustment for                          with no smoking
                                                       healthy worker                          adjustment.
                                                       effect.
Kaplan 1959 (cohort)............  Inadequate latency  External            Jobs classified by  Disparate
                                   allowance.          comparison; No      diesel exposure.    comparison groups
                                                       adjustment for      No attempt to       with no smoking
                                                       healthy worker      differentiate       adjustment.
                                                       effect.             between diesel
                                                                           and coal-fired
                                                                           locomotives.
Waller 1981 (cohort)............  Acceptable........  External            Job only: bus       Disparate
                                                       comparison; No      workers.            coparison groups
                                                       adjustment for                          with no smoking
                                                       healthy worker                          adjustment.
                                                       effect; Selection
                                                       bias due to
                                                       excluding
                                                       retirees from
                                                       cohort.
----------------------------------------------------------------------------------------------------------------

    Other studies proposed as counter-evidence by some commenters will 
be addressed in the next subsection of this risk assessment.
    The eight studies MSHA identified as representing the best 
available epidemiologic evidence all reported an elevated risk of lung 
cancer associated with diesel exposure. The results from these studies 
will now be reviewed, along with MSHA's response to public comments as 
appropriate.

Boffetta et al., 1988

    The structure of this cohort study was summarized in the preceding 
subsection of this risk assessment. The following table contains the 
main results. The relative risks listed for duration of exposure were 
calculated with reference to all members of the cohort reporting no 
diesel exposure, regardless of occupation, and adjusted for age, 
smoking pattern, and other occupational exposures (asbestos, coal and 
stone dusts, coal tar and pitch, and gasoline exhausts). The relative 
risks listed for occupations were calculated for cohort members that 
ever worked in the occupation, compared to cohort members never working 
in any of the four occupations listed and reporting no diesel exposure. 
These four relative risks were adjusted for age and smoking pattern 
only. Smoking pattern was coded by 5 categories: never smoker; current 
1-20 cigarettes per day; current 21 or more cigarettes per day; ex-
smoker of cigarettes; current or past pipe and/or cigar smoker.

                 Main Results From Boffetta et al., 1988
   [RRs by duration adjusted for age, smoking, and other occupational
     exposures; Occupational RRs adjusted for age and smoking only]
------------------------------------------------------------------------
                                                      Lung    95-percent
   Self-reported duration of exposure to diesel      cancer   confidence
                      exhaust                          RR      interval
------------------------------------------------------------------------
Years:
  1 to 15.........................................     1.05    0.80-1.39
  16 or more......................................     1.21    0.94-1.56
Occupation:
  Truck Drivers...................................     1.24    0.93-1.66
  Railroad Workers................................     1.59    0.94-2.69
  Heavy Equipment Operators.......................     2.60    1.12-6.06
  Miners..........................................     2.67    1.63-4.37
------------------------------------------------------------------------

    In addition to comments (addressed earlier) on the RR for miners in 
this study, IMC Global submitted several comments pertaining to the RR 
calculated for persons who explicitly stated that they had been 
occupationally exposed to diesel emissions. This RR was 1.18 for 
persons reporting any exposure (regardless of duration) compared to all 
subjects reporting no exposure. MSHA considers the most important issue 
raised by IMC Global to be that 20.6 percent of all cohort members did 
not answer the question about occupational diesel exhaust exposure 
during their lifetimes, and these subjects experienced a higher age-
adjusted mortality rate than the others. As the authors of this study 
acknowledged, this ``could introduce a substantial bias in the estimate 
of the association.'' (Boffetta et al., 1988, p. 412).
    To show that the impact of this bias could indeed be substantial, 
the authors of the study addressed one extreme possibility, in which 
all ``unknowns'' were actually unexposed. Under this scenario, 
excluding the ``unknowns'' would have biased the calculated RR upward 
by a sufficient amount to explain the entire 18-percent excess in RR. 
This would not, however, explain the higher RR for persons reporting 
more than 16 years exposure, compared to the RR for persons reporting 1 
to 15 years. Moreover, the authors did not discuss the opposite 
extreme: if all or most of the ``unknowns'' who experienced lung cancer 
were actually exposed, then excluding them would have biased the 
calculated RR downward. There is little basis for favoring one of these 
extremes over the other.
    Another objection to this study raised by IMC Global was:

    All exposure information in the study was self-reported and not 
validated. The authors of the study have no quantitative data or 
measurements of actual diesel exhaust exposures.

    MSHA agrees with IMC Global and other commenters that a lack of 
quantitative exposure measurements limits the strength of the evidence 
this study presents. MSHA believes, however, that the evidence 
presented is nevertheless substantial. The possibility of random 
classification errors due to self-reporting of exposures does not 
explain why persons reporting 16 or more years of exposure would 
experience a higher relative risk of lung cancer than persons reporting 
1 to 15 years of exposure. This difference is not statistically 
significant, but random exposure misclassification would tend to make 
the effects of exposure less

[[Page 5797]]

conspicuous. Nor can self-reporting explain why an elevated risk of 
lung cancer would be observed for four occupations commonly associated 
with diesel exposure.
    Furthermore, the study's authors did perform a rough check on the 
accuracy of the cohort's exposure information. First, they confirmed 
that, after controlling for age, smoking, and other occupational 
exposures, a statistically significant relationship was found between 
excess lung cancer and the cohort's self-reported exposures to 
asbestos. Second they found no such association for self-reported 
exposure to pesticides and herbicides, which they considered unrelated 
to lung cancer (ibid., pp. 410-411).
    IMC Global also commented that the ``* * * study may suffer from 
volunteer bias in that the cohort was healthier and less likely to be 
exposed to important risk factors, such as smoking or alcohol.'' They 
noted that this possibility ``is supported by the U.S. EPA in their 
draft Health Assessment Document for Diesel Emissions.''
    The study's authors noted that enrollment in the cohort was 
nonrandom and that participants tended to be healthier and less exposed 
to various risk factors than the general population. These differences, 
however, would tend to reduce any relative risk for the cohort 
calculated in comparison to the external, general population. The 
authors pointed out that external comparisons were, therefore, 
inappropriate; but ``the internal comparisons upon which the foregoing 
analyses are based are not affected strongly by selection biases.'' 
(ibid.)
    Although the 1999 EPA draft notes potential volunteer bias, it 
concludes: ``Given the fact that all diesel exhaust exposure 
occupations * * * showed elevated lung cancer risk, this study is 
suggestive of a causal association.'' \53\ (EPA, 1999, p. 7-13) No 
objection to this conclusion was raised in the most recent CASAC review 
of the EPA draft (CASAC, 2000).
---------------------------------------------------------------------------

    \53\ In his review of this study for the NMA, Dr. Peter Valberg 
stated: ``This last sentence reveals EPA's bias; the RRs for truck 
drivers and railroad workers were not statistically elevated.'' 
Contrary to Dr. Valberg's statement, the RRs were greater than 1.0 
and, therefore, were ``statistically elevated.'' Although the 
elevation for these two occupations was not statistically 
significant at a 95-percent confidence level, the EPA made no claim 
that it was. Under a null hypothesis of no real association, the 
probability should be \1/2\ that the RR would exceed 1.0 for an 
occupation associated with diesel exposure. Therefore, under the 
null hypothesis, the probability that the RR would exceed 1.0 for 
all four such occupations is (1/2)4 = 0.06. This 
corresponds to a 94-percent confidence level for rejecting the null 
hypothesis.
---------------------------------------------------------------------------

Boffetta et al., 1990

    This case-control study was based on 2,584 male hospital patients 
with histologically confirmed lung cancer, matched with 5099 male 
patients with no tobacco-related diseases. Cases and controls were 
matched within each of 18 hospitals by age (within two years) and year 
of interview. Information on each patient, including medical and 
smoking history, occupation, and alcohol and coffee consumption, was 
obtained at the time of diagnosis in the hospital, using a structured 
questionnaire. For smokers, smoking data included the number of 
cigarettes per day. Prior to 1985, only the patient's usual job was 
recorded. In 1985, the questionnaire was expanded to include up to five 
other jobs and the length of time worked in each job. After 1985, 
information was also obtained on dietary habits, vitamin consumption, 
and exposure to 45 groups of chemicals, including diesel exhaust.
    The authors categorized all occupations into three groups, 
representing low, possible, and probable diesel exhaust exposure. The 
``low exposure'' group was used as the reference category for 
calculating odds ratios for the ``possible'' and ``probable'' job 
groups. These occupational comparisons were based on the full cohort of 
patients, enrolled both before and after 1985. A total of 35 cases and 
49 controls (all enrolled after the questionnaire was expanded in 1985) 
reported a history of diesel exposure. The reference category for self-
reported diesel exposure consisted of a corresponding subset of 442 
cases and 897 controls reporting no diesel exposure on the expanded 
questionnaire. The authors made three comparisons to rule out bias due 
to self-reporting of exposure: (1) No difference was found between the 
average number of jobs reported by cases and controls; (2) the 
association between self-reported asbestos exposure was in agreement 
with previously published estimates; and (3) no association was found 
for two exposures (pesticides and fuel pumping) considered unrelated to 
lung cancer (ibid., p. 584).
    Stober and Abel (1996) identified this study as being ``of eminent 
importance owing to the care taken in including the most influential 
confounding factors and analyses of dose-effect relationships.'' The 
main findings are presented in the following table. All of these 
results were obtained using logistic regression, factoring in the 
estimated effects of age, race, years of education, number of 
cigarettes per day, and asbestos exposure (yes or no). An elevated risk 
of lung cancer was reported for workers with more than 30 years of 
either self-reported or ``probable'' diesel exposure. The authors 
repeated the occupational analysis using ``ever'' rather than ``usual'' 
employment in jobs classified as ``probable'' exposure, with 
``remarkably similar'' results (ibid., p. 584).

                 Main Results From Boffetta et al., 1990
   [Adjusted for age, race, education, smoking, and asbestos exposure]
------------------------------------------------------------------------
                                                            95-percent
  Self-reported duration of exposure to     Lung cancer     confidence
             diesel exhaust                 odds ratio       interval
------------------------------------------------------------------------
Years:
    1 to 15.............................            0.90       0.40-1.99
    16 to 30............................            1.04       0.44-2.48
    31 or more..........................            2.39       0.87-6.57
Likelihood of Exposure:
    19 jobs with ``possible'' exposure..            0.92       0.76-1.10
    13 jobs with ``probable'' exposure..            0.95       0.78-1.16
    1 to 15 years in ``probable'' jobs..            0.52       0.15-1.86
    16 to 30 years in ``probable'' jobs.            0.70       0.34-1.44
    31 or more years in ``probable''                1.49       0.72-3.11
     jobs...............................
------------------------------------------------------------------------


[[Page 5798]]

    The study's authors noted that most U.S. trucks did not have diesel 
engines until the late 1950s or early 1960s and that many smaller 
trucks are still powered by gasoline engines. Therefore, they performed 
a separate analysis of truck drivers cross-classified by self-reported 
diesel exposure ``to compare presumptive diesel truck drivers with 
nondiesel drivers.'' After adjusting for smoking, the resulting OR for 
diesel drivers was 1.25, with a 95-percent confidence interval of 0.85 
to 2.76 (ibid., p. 585).

Bruske-Hohlfeld et al., 1999

    This was a pooled analysis of two case-control studies on lung 
cancer in Germany. The data pool consisted of 3,498 male cases with 
histologically or cytologically confirmed lung cancer and 3,541 male 
controls randomly drawn from the general population. Cases and controls 
were matched for age and region of residence. For the pooled analysis, 
information on demographic characteristics, smoking, and detailed job 
and job-task history was collected by personal interviews with the 
cases and controls, using a standardized questionnaire.
    Over their occupational lifetimes, cases and controls were employed 
in an average of 2.9 and 2.7 different jobs, respectively. Jobs 
considered to have had potential exposure to diesel exhaust were 
divided into four groups: Professional drivers (including trucks, 
buses, and taxis), other ``traffic-related'' jobs (including switchmen 
and operators of diesel locomotives or diesel forklift trucks), full-
time drivers of farm tractors, and heavy equipment operators. Within 
these four groups, each episode of work in a particular job was 
classified as being exposed or not exposed to diesel exhaust, based on 
the written description of job tasks obtained during the interview. 
This exposure assessment was done without knowledge of the subject's 
case or control status. Each subject's lifetime duration of 
occupational exposure was compiled using only the jobs determined to 
have been diesel-exposed. There were 264 cases and 138 controls who 
accumulated diesel exposure exceeding 20 years, with 116 cases and 64 
controls accumulating more than 30 years of occupational exposure.
    For each case and control, detailed smoking histories from the 
questionnaire were used to establish smoking habit, including 
consumption of other tobacco products, cumulative smoking exposure 
(expressed as packyears), and years since quitting smoking. Cumulative 
asbestos exposure (expressed as the number of exposed working days) was 
assessed based on 17 job-specific questionnaires that supplemented the 
main questionnaire.
    The main findings of this study, all adjusted for cumulative 
smoking and asbestos exposure, are presented in the following table. 
Although the odds ratio for West German professional drivers was a 
statistically significant 1.44, as shown, the odds ratio for East 
German professional drivers was not elevated. As a possible 
explanation, the authors noted that after 1960, the number of vehicles 
(cars, busses, and trucks) with diesel engines per unit area was about 
five times higher in West Germany than in East Germany. Also, the 
higher OR shown for professional drivers first exposed after 1955, 
compared to earlier years of first exposure, may have resulted from the 
higher density of diesel traffic in later years.

BILLING CODE 4510-43-P

[[Page 5799]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.065


BILLING CODE 4510-43-C
    As the authors noted, a strength of this study is the good 
statistical power resulting from having a significant number of workers 
exposed to diesel emissions for more than 30 years. Another strength is 
the statistical treatment of potential confounders, using quantitative 
measures of cumulative smoking and asbestos exposures.
    Although they did not rely solely on job title, and differentiated 
between diesel-exposed and unexposed work periods, the authors 
identified limitations in the assessment of diesel exposure, ``under 
these circumstances leading to an odds ratio that is biased towards one 
and an underestimation of the true [relative] risk of lung cancer.'' A 
more quantitative assessment of diesel exposure would tend to remove 
this bias, thereby further elevating the relative risks. Therefore, the 
authors concluded that their study ``showed a statistically significant 
increase in lung cancer risk for workers occupationally exposed to 
[diesel exhaust] in Germany with the exception of professional drivers 
in East Germany.'' Garshick et al., 1987
    This case-control study was based on 1,256 primary lung cancer 
deaths and 2,385 controls whose cause of death was not cancer, suicide, 
accident, or unknown. Cases and controls were drawn from records of the 
U.S. Railroad Retirement Board (RRB) and matched within 2.5 years of 
birth date and 31 days of death date. Selected jobs, with and without 
regular diesel exposure, were identified by a review of job titles and 
duties and classified as ``exposed'' or ``unexposed'' to diesel 
exhaust. For

[[Page 5800]]

39 jobs, this exposure classification was confirmed by personal 
sampling of current respirable dust concentrations, adjusted for 
cigarette smoke, at four different railroads. Jobs for which no 
personal sampling was available were classified based on similarities 
in location and activity to sampled jobs.
    A detailed work history for each case and control was obtained from 
an annual report filed with the RRB. This was combined with the 
exposure classification for each job to estimate the lifetime total 
diesel exposure (expressed as ``diesel-years'') for each subject. Years 
spent not working for a railroad, or for which a job was not recorded, 
were considered to be unexposed. This amounted to 2.4% of the total 
worker-years from 1959 to death or retirement.
    Because of the transition from steam to diesel locomotives in the 
1950s, occupational lifetime exposures were accumulated beginning in 
1959. Since many of the older workers retired not long after 1959 and 
received little or no diesel exposure, separate analyses were carried 
out for subjects above and below the age of 65 years at death. The 
group of younger workers was considered to be less susceptible to 
exposure misclassification.
    Detailed smoking histories, including years smoked, cigarettes per 
day, and years between quitting and death, were obtained from next of 
kin. Based on job history, each case and control was also classified as 
having had regular, intermittent, or no occupational asbestos exposure.
    The main results of this study, adjusted for smoking and asbestos 
exposure, are presented in the following table for workers aged less 
than 65 years at the time of their death. All of these results were 
obtained using logistic regression, conditioned on dates of birth and 
death. The odds ratio presented in the shaded cell for 20 years of 
unlagged exposure was derived from an analysis that modeled diesel-
years as a continuous variable. All of the other odds ratios in the 
table were derived from analyses that modeled cumulative exposure 
categorically, using workers with less than five diesel-years of 
exposure as the reference group. Statistically significant elevations 
of lung cancer risk were reported for the younger workers with at least 
20 diesel-years of exposure or at least 15 years accumulated five years 
prior to death. No elevated risk of lung cancer was observed for the 
older workers, who were 65 or more years old at the time of their 
death. The authors attributed this to the fact, mentioned above, that 
many of these older workers retired shortly after the transition to 
diesel-powered locomotives and, therefore, experienced little or no 
occupational diesel exposure. Based on the results for younger workers, 
they concluded that ``this study supports the hypothesis that 
occupational exposure to diesel exhaust increases lung cancer risk.''

 Main Results From Garshick et al., 1987, for Workers Aged Less Than 65
                             Years at Death
[Controlled for dates of birth and death; adjusted for cigarette smoking
                         and asbestos exposure]
------------------------------------------------------------------------
                                  Lung cancer    95-percent confidence
        Diesel exposure           odds ratio            interval
------------------------------------------------------------------------
No lag:
    0-4 diesel-years...........             1      N/A (reference group)
    5-19 diesel-years..........          1.02                  0.72-1.45
    20 diesel-years (diesel              1.41                  1.06-1.88
     exposure modeled as
     continuous variable)......
    20 or more diesel-years....          1.64                  1.18-2.29
Accumulated at least 5 years
 before death:
    0-4 diesel-years...........             1      N/A (reference group)
    5-14 diesel-years..........          1.07                 0.69- 1.66
    15 or more diesel-years....          1.43                 1.06- 1.94
------------------------------------------------------------------------

    In its 1999 draft Health Assessment Document for Diesel Emissions, 
the U.S. EPA noted various limitations of this study but concluded that 
``compared with previous studies [i.e., prior to 1987] * * *, [it] 
provides the most valid evidence that occupational diesel exhaust 
emission exposure increases the risk of lung cancer.'' (EPA, 1999, p. 
7-33) No objection to this conclusion was raised in the most recent 
CASAC review of the EPA draft (CASAC, 2000).
    The EMA objected to this study's determination of smoking frequency 
based on interviews with next of kin, stating that such determination 
``generally results in an underestimate, as it has been shown that 
cigarette companies manufacture 60% more product than public surveys 
indicate are being smoked.''
    A tendency to mischaracterize smoking frequency would have biased 
the study's reported results if the degree of under- or over-estimation 
varied systematically with diesel exposure. The EMA, however, submitted 
no evidence that the smoking under-estimate, if it existed at all, was 
in any way correlated with cumulative duration of diesel exposure. In 
the absence of such evidence, MSHA finds no reason to assume 
differential mis-reporting of smoking frequency.
    Even more importantly, the EMA failed to distinguish between 
``public surveys'' of the smokers themselves (who may be inclined to 
understate their habit) and interviews with next of kin. The 
investigators specifically addressed the accuracy of smoking data 
obtained from next of kin, citing two studies on the subject. Both 
studies reported a tendency for surrogate respondents to overestimate, 
rather than underestimate, cigarette consumption. The authors concluded 
that ``this could exaggerate the contribution of cigarette smoking to 
lung cancer risk if the next of kin of subjects dying of lung cancer 
were more likely to report smoking histories than were those of 
controls.'' (ibid, p.1246)
    IMC Global, along with Cox (1997) objected to several 
methodological features of this study. MSHA's response to each of these 
criticisms appears immediately following a summary quotation from IMC 
Global's written comments:

    (A) The regression models used to analyze the data assumed 
without justification that an excess risk at any exposure level 
implied an excess risk at all exposure levels.

    The investigators did not extrapolate their regression models 
outside the range supported by the data. Furthermore, MSHA is using 
this study only for purposes of hazard identification at exposure 
levels at least as high as those experienced by workers in the study. 
Therefore, the possibility of a threshold effect at much lower levels 
is irrelevant.

    (B) The regression model used did not specify that the exposure 
estimates were

[[Page 5801]]

imperfect surrogates for true exposures. As a result, the regression 
coefficients do not bear any necessary relationship to the effects 
that they try to measure.

    As noted by Cox (op cit.), random measurement errors for exposures 
in an univariate regression model will tend to bias results in the 
direction of no apparent association, thereby masking or reducing any 
apparent effects of exposure. The crux of Cox's criticism, however, is 
that, for statistical analysis of the type employed in this study, 
random errors in a mutivariate exposure (such as an interdependent 
combination of smoking, asbestos, and diesel exposure) can potentially 
bias results in either direction. This objection fails to consider the 
fact that a nearly identical regression result was obtained for the 
effect of diesel exposure when smoking and asbestos exposure were 
removed from the model: OR = 1.39 instead of 1.41. Furthermore, even 
with a multivariate exposure, measurement errors in the exposure being 
evaluated typically bias the estimate of relative risk downward toward 
a null result. Relative risk is biased upwards only when the various 
exposures are interrelated in a special way. No evidence was presented 
that the data of this study met the special conditions necessary for 
upward bias or that any such bias would be large enough to be of any 
practical significance.

    (C) The * * * analysis used regression models without presenting 
diagnostics to show whether the models were appropriate for the 
data.

    MSHA agrees that regression diagnostics are a valuable tool in 
assuring the validity of a statistical regression analysis. There is 
nothing at all unusual, however, about their not having been mentioned 
in the published report of this study. Regression diagnostics are 
rarely, if ever, published in epidemiologic studies making use of 
regression analysis. This does not imply that such diagnostics were not 
considered in the course of identifying an appropriate model or 
checking how well the data conform to a given model's underlying 
assumptions. Evaluation of the validity of any statistical analysis is 
(or should be) part of the peer-review process prior to publication.

    (D) The * * * risk models assumed that 1959 was the effective 
year when DE exposure started for each worker. Thus, the analysis 
ignored the potentially large differences in pre-1959 exposures 
among workers. This modeling assumption makes it impossible to 
interpret the results of the study with confidence.

    MSHA agrees that the lack of diesel exposure information on 
individual workers prior to 1959 represents an important limitation of 
this study. This limitation, along with a lack of quantitative exposure 
data even after 1959, may preclude using it to determine, with 
reasonable confidence, the shape or slope of a quantitative exposure-
response relationship. Neither of these limitations, however, 
invalidates the study's finding of an elevated lung cancer risk for 
exposed workers. MSHA is not basing any quantitative risk assessment on 
this study and is relying on it, in conjunction with other evidence, 
only for purposes of hazard identification.

    (E) The risk regression models * * * assume, without apparent 
justification, that all exposed individuals have identical dose-
response model parameters (despite the potentially large differences 
in their pre-1959 exposure histories). This assumption was not 
tested against reasonable alternatives, e.g., that individuals born 
in different years have different susceptibilities * * *


    Cases and controls were matched on date of birth to within 2.5 
years, and separate analyses were carried out for the two groups of 
younger and older workers. Furthermore, it is not true that the 
investigators performed no tests of reasonable alternatives even to the 
assumption that younger workers shared the same model parameters. They 
explored and tested potential interactions between smoking intensity 
and diesel exposure, with negative results. The presence of such 
interactions would have meant that the response to diesel exposure 
differed among individuals, depending on their smoking intensity.
    One other objection that Cox (op cit.) raised specifically in 
connection with this study was apparently overlooked by IMC Global. To 
illustrate what he considered to be an improper evaluation of 
statistical significance when more than one hypothesis is tested in a 
study, Cox noted the finding that for workers aged less than 65 years 
at time of death, the odds ratio for lung cancer was significantly 
elevated at 20 diesel-years of exposure. He then asserted that this 
finding was merely

    * * * an instance of a whole family of statements of the form 
``Workers who were A years or younger at the time of death and who 
were exposed to diesel exhaust for Y years had a significantly 
increased relative odds ratios for lung cancer. The probability of 
at least one false positive occurring among the multiple hypotheses 
in this family corresponding to different combinations of A (e.g., 
no more than 54, 59, 64, 69, 74, 79, etc. years old at death) and 
durations of exposure (e.g., Y = 5, 10, 15, 20, 25, etc. years) is 
not limited to 5% when each combination of A and Y values is tested 
at a p = 5% significance level. For example, if 30 different (A, Y) 
combinations are considered, each independently having a 5% 
probability of a false positive (i.e., a reported 5% significance 
level), then the probability of at least one false positive 
occurring in the study as a whole is p = 1-(1-0.05) 30 = 78%. This 
p-value for the whole study is more than 15 times greater than the 
reported significance level of 5%.

    MSHA is evaluating the cumulative weight of evidence from many 
studies and is not relying on the level of statistical significance 
attached to any single finding or study viewed in isolation. 
Furthermore, Cox's analysis of the statistical impact of multiple 
comparisons or hypothesis tests is flawed on several counts, especially 
with regard to this study in particular. First, the analysis relies on 
a highly unrealistic assumption that when several hypotheses are tested 
within the same study, the probabilities of false positives are 
statistically independent. Second, Cox fails to distinguish between 
those hypotheses or comparisons suggested by exploration of the data 
and those motivated by prior considerations. Third, Cox ignores the 
fact that the result in question was based on a statistical regression 
analysis in which diesel exposure duration was modeled as a single 
continuous variable. Therefore, this particular result does not depend 
on multiple hypothesis-testing with respect to exposure duration. 
Fourth, and most importantly, Cox assumes that age and exposure 
duration were randomly picked for testing from a pool of 
interchangeable possibilities and that the only thing distinguishing 
the combination of ``65 years of age'' and ``20 diesel-years of 
exposure'' from other random combinations was that it happened to yield 
an apparently significant result. This is clearly not the case. The 
investigators divided workers into only two age groups and explained 
that this division was based on the history of dieselization in the 
railroad industry--not on the results of their data analysis. 
Similarly, the result for 20 diesel-years of exposure was not favored 
over shorter exposure times simply because 20 years yielded a 
significant result and the shorter times did not. Lengthy exposure and 
latency periods are required for the expression of increased lung 
cancer risks, and this justifies a focus on the longest exposure 
periods for which sufficient data are available.

Garshick et al., 1988; Garshick, 1991

    In this study, the investigators assessed the risk of lung cancer 
in a cohort of 55,407 white male railroad workers, aged 40 to 64 years 
in 1959,

[[Page 5802]]

who had begun railroad work between 1939 and 1949 and were employed in 
one of 39 jobs later surveyed for exposure. Workers whose job history 
indicated likely occupational exposure to asbestos were excluded. Based 
on the subsequent exposure survey, each of the 39 jobs represented in 
the cohort was classified as either exposed or unexposed to diesel 
emissions. The cohort was followed through 1980, and 1,694 cases of 
death due to lung cancer were identified.
    As in the 1987 study by the same investigators, detailed railroad 
job histories from 1959 to date of death or retirement were obtained 
from RRB records and combined with the exposure classification for each 
job to provide the years of diesel exposure accumulated since 1959 for 
each worker in the cohort. Using workers classified as ``unexposed'' 
within the cohort to establish a baseline, time-dependent proportional 
hazards regression models were employed to evaluate the relative risk 
of lung cancer for exposed workers. Although the investigators believed 
they had excluded most workers with significant past asbestos exposures 
from the cohort, based on job codes, they considered it possible that 
some workers classified as hostlers or shop workers may have been 
included in the cohort even if occupationally exposed to asbestos. 
Therefore, they carried out statistical analyses with and without shop 
workers and hostlers included.
    The main results of this study are presented in the following 
table. Statistically significant elevations of lung cancer risk were 
found regardless of whether or not shop workers and hostlers were 
included. The 1988 analysis adjusted for age in 1959, and the 1991 
analysis adjusted, instead, for age at death or end of follow-up (i.e., 
end of 1980).\54\ In the 1988 analysis, any work during a year counted 
as a diesel-year if the work was in a diesel-exposed job category, and 
the results from the 1991 analysis presented here are based on this 
same method of compiling exposure durations. Exposure durations 
excluded the year of death and the four prior years, thereby allowing 
for some latency in exposure effects. Results for the analysis 
excluding shop workers and hostlers were not presented in the 1991 
report, but the report stated that ``similar results were obtained.'' 
Using either method of age adjustment, a statistically significant 
elevation of lung cancer risk was associated with each exposure 
duration category. Using ``attained age,'' however, there was no strong 
indication that risk increased with increasing exposure duration. The 
1991 report concluded that ``there appears to be an effect of diesel 
exposure on lung cancer mortality'' but that ``because of weaknesses in 
exposure ascertainment * * *, the nature of the exposure-response 
relationship could not be found in this study.''
---------------------------------------------------------------------------

    \54\ Also, the 1991 analysis excluded 12 members of the cohort 
due to discrepancies between work history and reported year of 
death, leaving 55,395 railroad workers included in the analysis.

                           Main Results From Garshick et al., 1988 and Garshick, 1991
----------------------------------------------------------------------------------------------------------------
                                                                     Full cohort         Shopworkers & hostlers
                                                             --------------------------         excluded
   Exposure duration (diesel-years, last 5 years excluded)                             -------------------------
                                                                Relative    95% conf.     Relative    95% conf.
                                                                  risk         int.         risk         int.
----------------------------------------------------------------------------------------------------------------
1-4.........................................................         1.20    1.01-1.44         1.34    1.08-1.65
                                                                     1.31    1.09-1.57         N.R.         N.R.
5-9.........................................................         1.24    1.06-1.44         1.33    1.12-1.58
                                                                     1.28    1.09-1.49         N.R.         N.R.
10-14.......................................................         1.32    1.13-1.56         1.33    1.10-1.60
                                                                     1.19   1.002-1.41         N.R.         N.R.
15 or more..................................................         1.72    1.27-2.33         1.82    1.30-2.55
                                                                     1.40    1.03-1.90         N.R.        N.R.
----------------------------------------------------------------------------------------------------------------
Top entry within each cell is from 1988 analysis, adjusted for age in 1959. Bottom entry is from 1991 analysis,
  adjusted for age at death or end of follow-up (``attained age''). N.R. means ``not reported.''

    Some commenters noted that removing the shop workers and hostlers 
from the analysis increased the relative risk estimates. Dr. Peter 
Valberg found this ``paradoxical,'' since workers in these categories 
had later been found to experience higher average levels of diesel 
exposure than other railroad workers.
    This so-called paradox is likely to have resulted simply from 
exposure misclassification for a significant portion of the shop 
workers. The effect was explained by Garshick (1991) as follows:

    * * * shop workers who worked in the diesel repair shops shared 
job codes with workers in non-diesel shops where there was no diesel 
exhaust * * *. Apparent exposure as a shop worker based on the job 
code was then diluted with workers with the same job code but 
without true exposure, making it less likely to see an effect in the 
shop worker group. In addition, workers in the shop worker group of 
job codes tended to have less stable career paths * * * compared to 
the other diesel exposure categories.

    So although many of the shopworkers may have been exposed to 
relatively high dpm concentrations, many others were among the lowest-
exposed workers or were even unexposed because they spent their entire 
occupational lifetimes in unexposed locations. This could readily 
account for the increase in relative risks calculated when shop workers 
were excluded from the analysis.
    Dr. Valberg also noted that, according to Crump (1999), mortality 
rates for cirrhosis of the liver and heart disease were significantly 
elevated for ``train riders,'' who were exposed to diesel emissions, as 
compared to other members of the cohort, who were less likely to be 
exposed. It is also the train riders who account, primarily, for the 
elevated risk of lung cancer associated with diesel exposure in the 
overall cohort. Dr. Valberg interpreted this as suggesting that 
``lifestyle'' factors such as diet or smoking habits, rather than 
diesel exposure, were responsible for the increased risk of lung cancer 
observed among the diesel-exposed workers.
    Dr. Valberg presented no evidence that, apart from diesel exposure, 
the train riders differed systematically from the other workers in 
their smoking habits or in other ways that would be expected to affect 
their risk of lung cancer. Therefore, MSHA views the suggestion of such 
a bias as speculative. Even if lifestyle factors associated with

[[Page 5803]]

train ridership were responsible for an increased risk of cirrhosis of 
the liver or heart disease, this would not necessarily mean that the 
same factors were also responsible for the increased risk of lung 
cancer. Still, it is hypothetically possible that systematic 
differences, other than diesel exposure, between train riders and other 
railroad workers could account for some or even all of the increased 
lung cancer risk. That is why MSHA does not rely on this, or any other, 
single study in isolation.
    Some commenters, including the NMA, objected to this study on 
grounds that it failed to control for potentially confounding factors, 
principally smoking. The NMA stated that this ``has rendered its 
utility questionable at best.'' As explained earlier, there is more 
than one way in which a study can control for smoking or other 
potential confounders. One of the ways is to make sure that groups 
being compared do not differ with respect to the potential confounder. 
In this study, workers with likely asbestos exposure were excluded from 
the cohort, stability of workers within job categories was well 
documented, and similar results were reported when job categories 
subject to asbestos exposure misclassification were excluded. In their 
1988 report, the investigators provided the following reasons to 
believe that smoking did not seriously affect their findings:

    * * * the cohort was selected to include only blue-collar workers 
of similar socioeconomic class, a known correlate of cigarette smoking 
* * *, in our case-control study [Garshick et al., 1987], when 
cigarette smoking was considered, there was little difference in the 
crude or adjusted estimates of diesel exhaust effects. Finally, in the 
group of 517 current railroad workers surveyed by us in 1982 * * *, we 
found no difference in cigarette smoking prevalence between workers 
with and without potential diesel exhaust exposure. [Garshick et al., 
1988]
    Since relative risks were based on internal comparisons, and the 
cohort appears to have been fairly homogeneous, MSHA regards it as 
unlikely that the association of lung cancer with diesel exposure in 
this study resulted entirely from uncontrolled asbestos or smoking 
effects. Nevertheless, MSHA recognizes that differential smoking 
patterns may have affected, in either direction, the degree of 
association reported in each of the exposure duration categories.
    Cox (1997) re-analyzed the data of this study using exploratory, 
nonparametric statistical techniques. As quoted by IMC Global, Cox 
concluded that ``these methods show that DE [i.e., dpm] concentration 
has no positive causal association with lung cancer mortality risk.'' 
MSHA believes this quotation (taken from the abstract of Cox's article) 
overstates the findings of his analysis. At most, Cox confirmed the 
conclusion by Garshick (1991) that these data do not support a positive 
exposure-response relationship. Specifically, Cox determined that 
inter-relationships among cumulative diesel exposure, age in 1959, and 
retirement year make it ``impossible to prove causation by eliminating 
plausible rival hypotheses based on this dataset.'' (Cox, 1997; p. 826) 
Even if Cox's analysis were correct, it would not follow that there is 
no underlying causal connection between dpm exposure and lung cancer. 
It would merely mean that the data do not contain internal evidence 
implicating dpm exposure as the cause, rather than one or more of the 
variables with which exposure is correlated. Cox presented no evidence 
that any ``rival hypotheses'' were more plausible than causation by dpm 
exposure. Furthermore, it may simply be, as Garshick suggested, that an 
underlying exposure-response relationship is not evident ``because of 
weaknesses in exposure ascertainment.'' (Garshick, 1991, op cit.) None 
of this negates the fact that, after adjusting for either age in 1959 
or ``attained'' age, lung cancer was significantly more prevalent among 
the exposed workers.
    Along similar lines, many commenters pointed out that an HEI expert 
panel examined the data of this study (HEI, 1999) and found that it had 
very limited use for quantitative risk assessment (QRA). Several of 
these commenters mischaracterized the panel's findings. The NMA, for 
example, drew the following unjustified conclusion from the panel's 
report: ``In short, * * * the correct interpretation of the Garshick 
study is that any occupational increase in lung cancer among train 
workers was not due to diesel exposures.''
    Contrary to the NMA's characterization, the HEI Expert Panel's 
report stated that the data are

* * * consistent with findings of a weak association between death 
from lung cancer and occupational exposure to diesel exhaust. 
Although the secondary exposure-response analyses * * * are 
conflicting, the overall risk of lung cancer was elevated among 
diesel-exposed workers. [Ibid., p. 25]

    The panel agreed with Garshick (1991) and Cox (1997) that the data 
of this study do not support a positive exposure-response relationship. 
Like Garshick and unlike Cox, however, the panel explicitly recognized 
that problems with the data could mask such a relationship and that 
this does not negate the statistically significant finding of elevated 
risk among exposed workers. Indeed, the panel even identified several 
factors, in addition to weak exposure assessment as suggested by 
Garshick, that could mask a positive relationship: unmeasured 
confounding variables such as cigarette smoking, previous occupational 
exposures, or other sources of pollution; a ``healthy worker survivor 
effect''; and differential misclassification or incomplete 
ascertainment of lung cancer deaths. (HEI, 1999; p. 32)
    Positive exposure-response relationships based on these data were 
reported by the California EPA (OEHHA, 1998). MSHA recognizes that 
those findings were sensitive to various assumptions and that other 
investigators have obtained contrary results. The West Virginia Coal 
Association, paraphrasing Dr. Peter Valberg, concluded that although 
the two studies by Garshick et al. ``* * * may represent the best in 
the field, they fail to firmly support the proposition that lung cancer 
risk in workers derives from exposure to dpm.'' At least one commenter 
(IMC Global) apparently reached a considerably stronger conclusion that 
they were of no value whatsoever, and urged MSHA to ``discount their 
results and not consider them in this rulemaking.'' On the other hand, 
in response to the ANPRM, a consultant to the National Coal Association 
who was critical of all other studies available at the time 
acknowledged that these two:

[* * * have successfully controlled for severally [sic] potentially 
important confounding factors * * *. Smoking represents so strong a 
potential confounding variable that its control must be nearly 
perfect if an observed association between cancer and diesel exhaust 
is * * * [inferred to be causal]. In this regard, two observations 
are relevant. First, both case-control [Garshick et al., 1987] and 
cohort [Garshick et al., 1988] study designs revealed consistent 
results. Second, an examination of smoking related causes of death 
other than lung cancer seemed to account for only a fraction of the 
association observed between diesel exposure and lung cancer. A high 
degree of success was apparently achieved in controlling for smoking 
as a potentially confounding variable. [Robert A. Michaels, RAM TRAC 
Corporation, submitted by National Coal Association].

    To a limited extent, MSHA agrees with Dr. Valberg and the West 
Virginia Coal Association: these two studies--like every real-life 
epidemiologic study--are not ``firmly'' conclusive when viewed in 
isolation. Nevertheless, MSHA believes that they provide important 
contributions to the overall body of evidence. Whether or not they

[[Page 5804]]

can be used to quantify an exposure-response relationship, these 
studies--among the most comprehensive and carefully controlled 
currently available--do show statistically significant increases in the 
risk of lung cancer among diesel-exposed workers. Johnston et al. 
(1997)
    Since it focused on miners, this study has already been summarized 
and discussed in the previous subsection of this risk assessment. The 
main results are presented in the following table. The tabled relative 
risk estimates presented for cumulative exposures greater than 1000 mg-
hr/m\3\ (i.e., 1 g-hr/m\3\) were calculated by MSHA based on the 
regression coefficients reported by the authors. The conversion from 
mg-hr/m\3\ to mg-yr/m\3\ assumes 1,920 occupational exposure hours per 
year. Although 6.1 mg-yr/m\3\ Dpm roughly equals the cumulative 
exposure estimated for the most highly exposed locomotive drivers in 
the study, the relative risk associated with this exposure level is 
presented primarily for purposes of comparison with findings of Saverin 
et al. (1999).

                                                         Main Results From Johnston et al., 1997
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                    Mine-adjusted model (15-yr lag)                         Mine-unadjusted model (15-yr lag)
                                      ------------------------------------------------------------------------------------------------------------------
       Cumulative dpm exposure           Relative                                                  Relative
                                           risk                  95% conf. interval                  risk                  95% conf. interval
--------------------------------------------------------------------------------------------------------------------------------------------------------
1000 mg-hr/m\3\ (= 0.521 mg-yr/m\3\).         1.156  0.90-1.49.................................         1.227  1.00-1.50.
1920 mg-hr/m\3\ (= 1 mg-yr/m\3\).....         1.321  Not reported..............................         1.479  Not reported
11,700 mg-hr/m\3\ ( 6.1 mg-yr/m\3\)..         5.5    Not reported..............................        11.0    Not reported
--------------------------------------------------------------------------------------------------------------------------------------------------------

    In its post-hearing comments, MARG acknowledged that this study 
``found a `weak association' between lung cancer and respiratory diesel 
particulate exposure'' but failed to note that the estimated relative 
risk increased with increasing exposure. MARG also stated that the 
association was ``deemed non-significant by the researchers'' and that 
``no association was found among men with different exposures working 
in the same mines.'' Although the mine-adjusted model did not support 
95-percent confidence for an increasing exposure-response relationship, 
the mine-unadjusted model yielded a statistically significant positive 
slope at this confidence level. Furthermore, since the mine-adjusted 
model adjusts for differences in lung cancer rates between mines, the 
fact that relative risk increased with increasing exposure under this 
model indicates (though not at a 95-percent confidence level) that the 
risk of lung cancer increased with exposure among men with different 
exposures working in the same mines. Saverin et al. (1999)
    Since this study, like the one by Johnston et al., was carried out 
on a cohort of miners, it too was summarized and discussed in the 
previous subsection of this risk assessment. The main results are 
presented in the following table. The relative risk estimates and 
confidence intervals at the mean exposure level of 2.7 mg-yr/
m3 TC (total carbon) were calculated by MSHA, based on 
values of  and corresponding confidence intervals presented in 
Tables III and IV of the published report (ibid., p. 420). The 
approximate equivalency between 4.9 mg-yr/m\3\ TC and 6.1 mg-yr/m\3\ 
Dpm assumes that, on average, TC comprises 80 percent of Dpm.

                 Main results from Saverin et al., 1999
------------------------------------------------------------------------
                                                                 95%
                                                   Relative   confidence
                                                     risk      interval
------------------------------------------------------------------------
Highest compared to least exposed worker category     2.17     0.79-5.99
------------------------------------------------------------------------


----------------------------------------------------------------------------------------------------------------
                                                                Proportional hazards         Poisson mode *
                                                                    (Cox) Model *      -------------------------
              Cumulative total carbon exposure               --------------------------
                                                                Relative    95% conf.     Relative    95% conf.
                                                                  risk       interval       risk       interval
----------------------------------------------------------------------------------------------------------------
2.7 mg-yr/m\3\ TC (i.e., cohort mean).......................         1.33    0.67-2.64         1.08    0.59-1.99
                                                                     1.73    0.70-4.30         1.42    0.65-3.92
4.9 mg-yr/m3 TC (6.1 mg-yr/m\3\ dpm)........................         1.68     0.49-5.8         1.16     0.38-3.3
                                                                     2.70    0.52-14.1         1.89   0.46-11.9
----------------------------------------------------------------------------------------------------------------
* Top entry in each cell is based on full cohort; bottom entry is based on subcohort, which was restricted to
  miners who worked underground at least ten years, with at least 80 percent of employment in same job, etc.

    These results are not statistically significant at the conventional 
95-percent confidence level. However, the authors noted that the 
relative risk calculated for the subcohort was consistently higher than 
that calculated for the full cohort. They also considered the subcohort 
to have a superior exposure assessment and a better latency allowance 
than the full cohort. According to the authors, these factors provide 
``some assurance that the observed risk elevation was not entirely due 
to chance since improving the exposure assessment and allowing for 
latency effects should, in general, enhance exposure effects.''

Steenland et al., (1990, 1992, 1998)

    The basis for the analyses in this series was a case-control study 
comparing the risk of lung cancer for diesel-exposed and unexposed 
workers who had belonged to the Teamsters Union for at least twenty 
years (Steenland et al., 1990). Drawing from union records, 996 cases 
of lung cancer

[[Page 5805]]

were identified among more than 10,000 deaths in 1982 and 1983. For 
comparison to these cases, a total of 1,085 controls was selected 
(presumably at random) from the remaining deaths, restricted to those 
who died from causes other than lung cancer, bladder cancer, or motor 
vehicle accident. Information on work history, duration and intensity 
of cigarette smoking, diet, and asbestos exposure was obtained from 
next of kin. Detailed work histories were also obtained from pension 
applications on file with the Teamsters Union.
    Both data sources were used to classify cases and controls 
according to a job category in which they had worked the longest. Based 
on the data obtained from next of kin, the job categories were diesel 
truck drivers, gasoline truck drivers, drivers of both truck types, 
truck mechanics, and dock workers. Based on the pension applications, 
the principal job categories were long-haul drivers, short-haul or city 
drivers, truck mechanics, and dock workers. Of the workers identified 
by next of kin as primarily diesel truck drivers, 90 percent were 
classified as long-haul drivers according to the Teamster data. The 
corresponding proportions were 82 percent for mechanics and 81 percent 
for dock workers. According to the investigators, most Teamsters had 
worked in only one exposed job category. However, because of the 
differences in job category definitions, and also because the next of 
kin data covered lifetimes whereas the pension applications covered 
only time in the Teamsters Union, the investigators found it 
problematic to fully evaluate the concordance between the two data 
sources.
    In the 1990 report, separate analyses were conducted for each 
source of data used to compile work histories. The investigators noted 
that ``many trucking companies (where most study subjects worked) had 
completed most of the dieselization of their fleets by 1960, while 
independent drivers and nontrucking firms may have obtained diesel 
trucks later * * *'' Therefore, they specifically checked for 
associations between increased risk of lung cancer and occupational 
exposure after 1959 and, separately, after 1964. In the 1992 report, 
the investigators presented, for the Union's occupational categories 
used in the study, dpm exposure estimates based on subsequent 
measurements of submicrometer elemental carbon (EC) as reported by 
Zaebst et al. (1991). In the 1998 report, cumulative dpm exposure 
estimates for individual workers were compiled by combining the 
individual work histories obtained from the Union's records with the 
subsequently measured occupational exposure levels, along with an 
evaluation of historical changes in diesel engine emissions and 
patterns of diesel usage. Three alternative sets of cumulative exposure 
estimates were considered, based on alternative assumptions about the 
extent of improvement in diesel engine emissions between 1970 and 1990. 
A variety of statistical models and techniques were then employed to 
investigate the relationship between estimated cumulative dpm exposure 
(expressed as EC) and the risk of lung cancer. The authors pointed out 
that the results of these statistical analyses depended heavily on 
``very broad assumptions'' used to generate the estimates of cumulative 
dpm exposure. While acknowledging this limitation, however, they also 
evaluated the sensitivity of their results to various changes in their 
assumptions and found these changes to have little impact on the 
results.
    The investigators also identified and addressed several other 
limitations of this study as follows:

    (1) possible misclassification smoking habits by next of kin, 
(2) misclassification of exposure by next of kin, (3) a relatively 
small non-exposed group (n = 120) which by chance may have had a low 
lung cancer risk, and (4) lack of sufficient latency (time since 
first exposure) to observe a lung cancer excess. On the other hand, 
next-of-kin data on smoking have been shown to be reasonably 
accurate, non-differential misclassification of exposure * * * would 
only bias our findings toward * * * no association, and the trends 
of increased risk with increased duration of employment in certain 
jobs would persist even if the non-exposed group had a higher lung 
cancer risk. Finally, the lack of potential latency would only make 
any positive results more striking. (Steenland et al., 1990)

    The main results from the three reports covering this study are 
summarized in the following table. All of the analyses were controlled 
for age, race, smoking (five categories), diet, and asbestos exposure 
as reported by next of kin. Odds ratios for the occupations listed were 
calculated relative to the odds of lung cancer for occupations other 
than truck driver (all types), mechanic, dock worker, or other 
potentially diesel exposed jobs (Steenland et al., 1990, Appendix A). 
The exposure-response analyses were carried out using logistic 
regression. Although the investigators performed analyses under three 
different assumptions for the rate of engine emissions (gm/mile) in 
1970, they considered the intermediate value of 4.5 gm/mile to be their 
best estimate, and this is the value on which the results shown here 
are based. Under this assumption, cumulative occupational EC exposure 
for all workers in the study was estimated to range from 0.45 to 2,440 
g-yr/m3, with a median value of 373 g-yr/
m3. The estimates of relative risk (expressed as odds 
ratios) presented for EC exposures of 373 g-yr/m3, 
1000 g-yr/m3, and 2450 g-yr/m3 
were calculated by MSHA based on the regression coefficients reported 
by the authors for five-year lagged exposures (Steenland et al. 1998, 
Table II).

BILLING CODE 4510-43-P

[[Page 5806]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.066


BILLING CODE 4510-43-C
    Under the assumption of a 4.5 gm/mile emissions rate in 1970, the 
cumulative EC exposure of 2450 g-yr/m\3\ ( 6.1 mg-
yr/m\3\ dpm) shown in the table closely corresponds to the upper limit 
of the range of data on which the regression analyses were based 
(Steenland et al., 1998, p. 224). However, the relative risks (i.e., 
odds ratios) calculated for this level of occupational exposure are 
presented primarily for purposes of comparison with the findings of 
Johnston et al. (1997) and Saverin et al. (1999). At a cumulative dpm 
exposure of approximately 6.1 mg-yr/m\3\, it is evident that the 
Johnston models predict a far greater elevation in lung cancer risk 
than either the Saverin or Steenland models. A possible explanation for 
this is that the Johnston data included exposures of up to 30 years in 
duration, and the statistical models showing an exposure-response 
relationship allowed for a 15-year lag in exposure effects. The other 
two studies were based on generally shorter diesel exposures and 
allowed less time for latent effects. In Subsection 3.b.ii(3) of this 
risk assessment, the quantitative results of these three studies will 
be further compared with respect to

[[Page 5807]]

exposure levels found in underground mines.
    Several commenters noted that the HEI Expert Panel (HEI, 1999) had 
identified uncertainties in the diesel exposure assessment as an 
important limitation of the exposure-response analyses by Steenland et 
al. (1998) and had recommended further investigation before the 
quantitative results of this study were accepted as conclusive. In 
addition, Navistar International Transportation (NITC) raised a number 
of objections to the methods by which diesel exposures were estimated 
for the period between 1949 and 1990 (NITC, 1999). In general, the 
thrust of these objections was that exposures to diesel engine 
emissions had been overestimated, while potentially relevant exposures 
to gasoline engine emissions had been underestimated and/or unduly 
discounted.\55\
---------------------------------------------------------------------------

    \55\ Many of the issues NITC raised in its critique of this 
study depend on a peculiar identification of dpm exclusively with 
elemental carbon. For example, NITC argued that ``more than 65 
percent of the total carbon to which road drivers (and mechanics) 
were exposed consisted of organic (i.e., non-diesel) carbon, further 
suggesting that some other etiology caused or contributed to excess 
lung cancer mortality in these workers.'' (NITC, 1999, p. 16) Such 
lines of argument, which depend on identifying organic carbon as 
``non-diesel,'' ignore the fact that dpm contains a large measure of 
organic carbon compounds (and also some sulfates), as well as 
elemental carbon. Any adverse health effects due to the organic 
carbon or sulfate constituents of dpm would nonetheless be due to 
dpm exposures.
---------------------------------------------------------------------------

    As mentioned above, the investigators recognized that these 
analyses rely on ``broad assumptions rather than actual [concurrent] 
measurements,'' and they proposed that the ``results should be regarded 
with appropriate caution.'' While agreeing with both the investigators 
and the HEI Expert Panel that these results should be interpreted with 
appropriate caution, MSHA also agrees with the Panel ``* * * that 
regulatory decisions need to be made in spite of the limitations and 
uncertainties of the few studies with quantitative data currently 
available.'' (HEI, 1999, p. 39) In this context, MSHA considers it 
appropriate to regard the 1998 exposure-response analyses as 
contributing to the weight of evidence that dpm exposure increases the 
risk of lung cancer, even if the results are not conclusive when viewed 
in isolation.
    Some commenters also noted that the HEI Expert Panel raised the 
possibility that the method for selecting controls in this study could 
potentially have biased the results in an unpredictable direction. Such 
bias could have occurred because deaths among some of the controls were 
likely due to diseases (such as cardiovascular disease) that shared 
some of the same risk factors (such as tobacco smoking) with lung 
cancer. The Panel presented hypothetical examples of how this might 
bias results in either direction. Although the possibility of such bias 
further demonstrates why the results of this study should be regarded 
with ``appropriate caution,'' it is important to distinguish between 
the mere possibility of a control-selection bias, evidence that such a 
bias actually exists in this particular study, and the further evidence 
required to show that such bias not only exists but is of sufficient 
magnitude to have produced seriously misleading results. Unlike the 
commenters who cited the HEI Expert Panel on this issue, the Panel 
itself clearly drew this distinction, stating that ``no direct evidence 
of such bias is apparent'' and emphasizing that ``even though these 
examples [presented in HEI (1999), Appendix D] could produce misleading 
results, it is important to note that they are only hypothetical 
examples. Whether or not such bias is present will require further 
examination.'' (HEI, 1999, pp. 37-38) As the HEI showed in its 
examples, such bias (if it exists) could lead to underestimating the 
association between lung cancer and dpm exposure, as well as to 
overestimating it. Therefore, in the absence of evidence that control-
selection bias actually distorted the results of this study one way or 
the other, MSHA considers it prudent to accept the study's finding of 
an association at face value.
    One commenter (MARG) noted that information on cigarette smoking, 
asbestos exposure, and diet in the trucking industry study was obtained 
from next of kin and stated that such information was ``likely to be 
unreliable.'' By increasing random variability in the data, such errors 
could widen the confidence intervals around an estimated odds ratio or 
reduce the confidence level at which a positive exposure-response 
relationship might be established. However, unless such errors were 
correlated with diesel exposure or lung cancer in such a way as to bias 
the results, they would not, on average, inflate the estimated degree 
of association between diesel exposure and an increased risk of ling 
cancer. The commenter provided no reason to suspect that errors with 
respect to these factors were in any way correlated with diesel 
exposure or with the development of lung cancer.
    Some commenters pointed out that EC concentrations measured in 1990 
for truck mechanics were higher, on average, than for truck drivers, 
but the mechanics, unlike the drivers, showed no evidence of increasing 
lung cancer risk with increasing duration of employment. NITC referred 
to this as a ``discrepancy'' in the data, assuming that ``cumulative 
exposure increases with duration of employment such that mechanics who 
have been employed for 18 or more years would have greater cumulative 
exposure than workers who have been employed for 1-11 years.'' (NITC, 
1999)
    Mechanics were included in the logistic regression analyses 
(Steenland et al., 1998) showing an increase in lung cancer risk with 
increasing cumulative exposure. These analyses pooled the data for all 
occupations by estimating exposure for each worker based on the 
worker's occupation and the particular years in which the worker was 
employed. There are at least three reasons why, for mechanics viewed as 
a separate group, an increase in lung cancer risk with increasing dpm 
exposure may not have been reflected by increasing duration of 
employment.
    First, relatively few truck mechanics were available for analyzing 
the relationship between length of employment and the risk of lung 
cancer. Based on the union records, 50 cases and 37 controls were so 
classified; based on the next-of-kin data, 43 cases and 41 controls 
were more specifically classified as diesel truck mechanics (Steenland 
et al., 1990). In contrast, 609 cases and 604 controls were classified 
as long-haul drivers (union records). This was both the largest 
occupational category and the only one showing statistically 
significant evidence of increasing risk with increasing employment 
duration. The number of mechanics included in the study population may 
simply not have been sufficient to detect a pattern of increasing risk 
with increasing length of employment, even if such a pattern existed.
    The second part of the explanation as to why mechanics did not 
exhibit a pattern similar to truck drivers could be that the data on 
mechanics were more subject to confounding. After noting that ``the 
risk for mechanics did not appear to increase consistently with 
duration of employment,'' Steenland et al. (1990) further noted that 
the mechanics may have been exposed to asbestos when working on brakes. 
The data used to adjust for asbestos exposure may have been inadequate 
to control for variability in asbestos exposure among the mechanics.
    Third, as noted by NITC, the lung cancer risk for mechanics 
(adjusted for age, race, tobacco smoking, asbestos exposure, and diet) 
would be expected to increase with increasing duration of

[[Page 5808]]

employment only if the mechanics' cumulative dpm exposure corresponded 
to the length of their employment. None of the commenters raising this 
issue, however, provided any support for this assumption, which fails 
to consider the particular calendar years in which mechanics included 
in the study were employed. In compiling cumulative exposure for an 
individual worker, the investigators took into account historical 
changes in both diesel emissions and the proportion of trucks with 
diesel engines--so the exposure level assigned to each occupational 
category was not the same in each year. In general, workers included in 
the study neither began nor ended their employment in the same year. 
Consequently, workers with the same duration of employment in the same 
occupational category could be assigned different cumulative exposures, 
depending on when they were employed. Similarly, workers in the same 
occupational category who were assigned the same cumulative exposure 
may not have worked the same length of time in that occupation. 
Therefore, it should not be assumed that duration of employment 
corresponds very well to the cumulative exposure estimated for workers 
within any of the occupational categories. Furthermore, in the case of 
mechanics, there is an additional historical variable that is 
especially relevant to actual cumulative exposure but was not 
considered in formulating exposure estimates: the degree of ventilation 
or other means of protection within repair shops. Historical changes in 
shop design and work practices, as well as differences between shops, 
may have caused more exposure misclassification among mechanics than 
among long-haul or diesel truck drivers. Such misclassification would 
tend to further obscure any relationship between mechanics' risk of 
lung cancer and either duration of employment or cumulative exposure.
    (iv) Counter-Evidence. Several commenters stated that, in the 
proposal, MSHA had dismissed or not adequately addressed epidemiology 
studies showing no association between lung cancer and exposures to 
diesel exhaust. For example, the EMA wrote:

    MSHA's discussion of the negative studies generally consists of 
arguments to explain why those studies should be dismissed. For 
example, MSHA states that, ``All of the studies showing negative or 
statistically insignificant positive associations . . . lacked good 
information about dpm exposure . . .'' or showed similar 
shortcomings. 63 Fed. Reg. at 17533. The statement about exposure 
information is only partially true, for, in fact, very few of any of 
the cited studies (the ``positive'' studies as well) included any 
exposure measurements, and none included concurrent exposures.

    It should, first of all, be noted that the statement in question on 
dpm exposure referred to the issue of any diesel exposure--not to 
quantitative exposure measurements, which MSHA acknowledges are lacking 
in most of the available studies. In the absence of quantitative 
measurements, however, studies comparing workers known to have been 
occupationally exposed to unexposed workers are preferable to studies 
not containing such comparisons. Furthermore, two of the studies now 
available (and discussed above) utilize essentially concurrent exposure 
measurements, and both show a positive association (Johnston et al., 
1997; Saverin et al., 1999).
    MSHA did not entirely ``dismiss'' the negative studies. They were 
included in both MSHA's tabulation (see Tables III-4 and III-5) and (if 
they met the inclusion criteria) in the two meta-analyses cited both 
here and in the proposal (Lipsett and Campleman, 1999, and Bhatia et 
al., 1998). As noted by the commenter, MSHA presented reasons (such as 
an inadequate latency allowance) for why negative studies may have 
failed to detect an association. Similarly MSHA gave reasons for giving 
less weight to some of the positive studies, such as Benhamou et al. 
(1988), Morabia et al. (1992), and Siemiatycki et al., 1988. Additional 
reasons for giving less weight to the six entirely negative studies 
have been tabulated above, under the heading of ``Best Available 
Epidemiologic Evidence.'' The most recent of these negative studies 
(Christie et al., 1994, 1995) is discussed in detail under the heading 
of ``Studies Involving Miners.''
    One commenter (IMC Global) listed the following studies (all of 
which MSHA had considered in the proposed risk assessment) as 
``examples of studies that reported negative associations between [dpm] 
exposure and lung cancer risk'':
     Waller (1981). This is one of the six negative studies 
discussed earlier. Results were likely to have been biased by excluding 
lung cancers occurring after retirement or resignation from employment 
with the London Transit Authority. Comparison was to a general 
population, and there was no adjustment for a healthy worker effect. 
Comparison groups were disparate, and there was no adjustment for 
possible differences in smoking frequency or intensity.
     Howe et al. (1983). Contrary to the commenter's 
characterization of this study, the investigators reported 
statistically significant elevations of lung cancer risk for workers 
classified as ``possibly exposed'' or ``probably exposed'' to diesel 
exhaust. MSHA recognizes that these results may have been confounded by 
asbestos and coal dust exposures.
     Wong et al. (1985). The investigators reported a 
statistically insignificant deficit for lung cancer in the entire 
cohort and a statistically significant deficit for lung cancer in the 
less than 5-year duration group. However, since comparisons were to a 
general population, these deficits may be the result of a healthy 
worker effect, for which there was no adjustment. Because of the 
latency required for development of lung cancer, the result for ``less 
than 5-year duration'' is far less informative than the results for 
longer durations of employment and greater latency allowances. Contrary 
to the commenter's characterization of this study, the investigators 
reported statistically significant elevations of lung cancer risks for 
``normal'' retirees (SMR = 1.30) and for ``high exposure'' dozer 
operators with 15-19 years of union membership and a latency allowance 
of at least 20 years (SMR = 3.43).
     Edling et al. (1987). This is one of the six negative 
studies discussed earlier. The cohort consisted of only 694 bus workers 
and, therefore, lacked statistical power. Furthermore, comparison was 
to a general, external population with no adjustment for a healthy 
worker effect.
     Garshick (1988). The reason the commenter (IMC Global) 
gave for characterizing this study as negative was: ``That the sign of 
the association in this data set changes based on the models used 
suggests that the effect is not robust. It apparently reflects modeling 
assumptions more than data.'' Contrary to the commenter's 
characterization, however, the finding of increased lung cancer risk 
for workers classified as diesel-exposed did not change when different 
methods were used to analyze the data. What changed, depending on 
modeling assumptions, was the shape and direction of the exposure-
response relationship among exposed workers (Cal-EPA, 1998; Stayner et 
al., 1998; Crump, 1999; HEI, 1999). MSHA agrees that the various 
exposure-response relationships that have been derived from this study 
are highly sensitive to data modeling assumptions. This includes 
assumptions about historical patterns of exposure, as well as 
assumptions related to technical aspects of the statistical analysis. 
However, as noted by the HEI Expert Panel, the study provides evidence 
of a

[[Page 5809]]

positive association between exposure and lung cancer despite the 
conflicting exposure-response analyses. Even though different 
assumptions and methods of analysis have led to different conclusions 
about the utility of this study for quantifying an exposure-response 
relationship, ``the overall risk of lung cancer was elevated among 
diesel-exposed workers'' (HEI, 1999, p. 25).
    Another commenter (MARG) cited a number of studies (all of which 
had already been placed in the public record by MSHA) that, according 
to the commenter, ``reflect either negative health effects trends among 
miners or else failed to demonstrate a statistically significant 
positive trend correlated with dpm exposure.'' It should be noted that, 
as explained earlier, failure of an individual study to achieve 
statistical significance (i.e., a high confidence level for its 
results) does not necessarily prevent a study from contributing 
important information to a larger body of evidence. An epidemiologic 
study may fail to achieve statistical significance simply because it 
did not involve a sufficient number of subjects or because it did not 
allow for an adequate latency period. In addition to this general 
point, the following responses apply to the specific studies cited by 
the commenter.
     Ahlman et al. (1991). This study is discussed above, under 
the heading of ``Studies Involving Miners.'' MSHA agrees with the 
commenter that this study did not ``establish'' a relationship between 
diesel exposure and the excess risk of lung cancer reported among the 
miners involved. Contrary to the commenter's characterization, however, 
the evidence presented by this study does incrementally point in the 
direction of such a relationship. As mentioned earlier, none of the 
underground miners who developed lung cancer had been occupationally 
exposed to asbestos, metal work, paper pulp, or organic dusts. Based on 
measurements of the alpha energy concentration at the mines, and a 
comparison of smoking habits between underground and surface miners, 
the authors concluded that not all of the excess lung cancer for the 
underground miners was attributable to radon daughter exposures and/or 
smoking. A stronger conclusion may have been possible if the cohort had 
been larger.
     Ames et al. (1984). MSHA has taken account of this study, 
which made no attempt to evaluate cancer effects, under the heading of 
``Chronic Effects other than Cancer.'' The commenter repeated MSHA's 
statement (in the proposed risk assessment) that the investigators had 
not detected any association of chronic respiratory effects with diesel 
exposure, but ignored MSHA's observation that the analysis had failed 
to consider baseline differences in lung function or symptom 
prevalence. Furthermore, as acknowledged by the investigators, diesel 
exposure levels in the study population were low.
     Ames et al. (1983). As discussed later in this risk 
assessment, under the heading of ``Mechanisms of Toxicity,'' this study 
was among nine (out of 17) that did not find evidence of a relationship 
between exposure to respirable coal mine dust and an increased risk of 
lung cancer. Unlike the Australian mines studied by Christie et al. 
(1995), the coal mines included in this study were not extensively 
dieselized, and the investigators did not relate their findings to 
diesel exposures.
     Ames et al. (1982). As noted earlier under the heading of 
``Acute Health Effects,'' this study, which did not attempt to evaluate 
cancer or other chronic health effects, detected no statistically 
significant relationship between diesel exposure and pulmonary 
function. However, the authors noted that this might have been due to 
the low concentrations of diesel emissions involved.
     Armstrong et al. (1979). As discussed later in this risk 
assessment, this study was among nine (out of 17) that did not find 
evidence of a relationship between exposure to respirable coal mine 
dust and an increased risk of lung cancer. As pointed out by the 
commenter, comparisons were to a general population. Therefore, they 
were subject to a healthy worker effect for which no adjustment was 
made. The commenter further stated that ``diesel emissions were not 
found to be related to increased health risks.'' However, diesel 
emissions were not mentioned in the report, and the investigators did 
not attempt to compare lung cancer rates in exposed and unexposed 
miners.
     Attfield et al (1982). MSHA has taken the results of this 
study into account, under the heading of ``Chronic Effects other than 
Cancer.''
     Attfield (1979). MSHA has taken account of this study, 
which did not attempt to evaluate cancer effects, under the heading of 
``Chronic Effects other than Cancer.'' Although the results were not 
conclusive at a high confidence level, miners occupationally exposed to 
diesel exhaust for five or more years exhibited an increase in various 
respiratory symptoms, as compared to miners exposed for less than five 
years.
     Boffetta et al. (1988). This study is discussed in two 
places above, under the headings ``Studies Involving Miners'' and 
``Best Available Epidemiologic Evidence.'' The commenter stated that 
``the study obviously does not demonstrate risks from dpm exposure.'' 
If the word ``demonstrate'' is taken to mean ``conclusively prove,'' 
then MSHA would agree that the study, viewed in isolation, does not do 
this. As explained in the earlier discussion, however, MSHA considers 
this study to contribute to the weight of evidence that dpm exposure 
increases the risk of lung cancer.
     Costello et al. (1974). As discussed later in this risk 
assessment, this study was among nine (out of 17) that did not find 
evidence of a relationship between exposure to respirable coal mine 
dust and an increased risk of lung cancer. Since comparisons were to a 
general population, they were subject to a healthy worker effect for 
which no adjustment was made. Diesel emissions were not mentioned in 
the report.
     Gamble and Jones (1983). MSHA has taken account of this 
study, which did not attempt to evaluate cancer effects, under the 
heading of ``Chronic Effects other than Cancer.'' The commenter did not 
address MSHA's observation that the method of statistical analysis used 
by the investigators may have masked an association of respiratory 
symptoms with diesel exposure.
     Glenn et al. (1983). As summarized by the commenter, this 
report reviewed NIOSH medical surveillance on miners exposed to dpm and 
found that ``* * * neither consistent nor obvious trends implicating 
diesel exhaust in the mining atmosphere were revealed.'' The authors 
noted that ``results were rather mixed,'' but also noted that ``levels 
of diesel exhaust contaminants were generally low,'' and that ``overall 
tenure in these diesel equipped mines was fairly short.'' MSHA 
acknowledges the commenter's emphasis on the report's 1983 conclusion: 
``further research on this subject is needed.'' However, the authors 
also pointed out that ``all four of the chronic effects analyses 
revealed an excess of cough and phlegm among the diesel exposed group. 
In the potash, salt and trona groups, these excesses were 
substantial.'' The miners included in the studies summarized by this 
report would not have been exposed to dpm for sufficient time to 
exhibit a possible increase in the risk of lung cancer.
     Johnston et al. (1997). This study is discussed in two 
places above, under the headings ``Studies Involving Miners'' and 
``Best Available Epidemiologic Evidence.'' MSHA disagrees with the 
commenter's

[[Page 5810]]

assertion that ``the study does not support a health risk from dpm.'' 
This was not the conclusion drawn by the authors of the study. As 
explained in the earlier discussion, this study, one of the few 
containing quantitative estimates of cumulative dpm exposures, provides 
evidence of increasing lung cancer risk with increasing exposure.
     Jorgenson and Svensson (1970). MSHA discussed this study, 
which did not attempt to evaluate cancer effects, under the heading of 
``Chronic Effects other than Cancer.'' Contrary to the commenter's 
characterization, the investigators reported higher rates of chronic 
productive bronchitis, for both smokers and nonsmokers, among the 
underground iron ore miners exposed to diesel exhaust as compared to 
surface workers at the same mine.
     Kuempel (1995); Lidell (1973); Miller and Jacobsen (1985). 
As discussed later in this risk assessment, under the heading of 
``Mechanisms of Toxicity,'' these three studies were among the nine 
(out of 17) that did not find evidence of a relationship between 
exposure to respirable coal mine dust and an increased risk of lung 
cancer. The extent, if any, to which workers involved in these studies 
were occupationally exposed to diesel emissions was not documented, and 
diesel emissions were not mentioned in any of these reports.
     Morfeld et al. (1997). The commenter's summary of this 
study distorted the investigators' conclusions. Contrary to the 
commenter's characterization, this is one of eight studies that showed 
an increased risk of lung cancer for coal miners, as discussed later in 
this risk assessment under the heading of ``Mechanisms of Toxicity.'' 
For lung cancer, the relative SMR, which adjusts for the healthy worker 
effect, was 1.11. (The value of 0.70 cited by the commenter was the 
unadjusted SMR.) The authors acknowledged that the relative SMR 
obtained by the ``standard analysis'' (i.e., 1.11) was not 
statistically significant. However, the main object of the report was 
to demonstrate that the ``standard analysis'' is insufficient. The 
investigators presented evidence that the 1.11 value was biased 
downward by a ``healthy-worker-survivor-effect,'' thereby masking the 
actual exposure effects in these workers. They found that ``all the 
evidence points to the conclusion that a standard analysis suffers from 
a severe underestimate of the exposure effect on overall mortality, 
cancer mortality and lung cancer mortality.'' (Morfeld et al., 1997, p. 
350)
     Reger (1982). MSHA has taken account of this study, which 
made no attempt to evaluate cancer effects, under the heading of 
``Chronic Effects other than Cancer.'' As summarized by the commenter, 
``diesel-exposed miners were found to have more cough and phlegm, and 
lower pulmonary function,'' but the author found that ``the evidence 
would not allow for the rejection of the hypothesis of health equality 
between exposed and non-exposed miners.'' The commenter failed to note, 
however, that miners in the dieselized mines, had worked underground 
for less than 5 years on average.
     Rockette (1977). This is one of eight studies, discussed 
under ``Mechanisms of Toxicity,'' showing an increased risk of lung 
cancer for coal miners. As described by the commenter, the author 
reported SMRs of 1.12 for respiratory cancers and 1.40 for stomach 
cancer. MSHA agrees with the commenter that ``the study does not 
establish a dpm-related health risk,'' but notes that dpm effects were 
not under investigation. Diesel emissions were not mentioned in the 
report, and, given the study period, the miners involved may not have 
been occupationally exposed to diesel exhaust.
     Waxweiler (1972). MSHA's discussion of this study appears 
earlier in this risk assessment, under ``Studies Involving Miners.'' As 
noted by the commenter, the slight excess in lung cancer, relative to 
the general population of New Mexico, was not statistically 
significant. The commenter failed to note, however, that no adjustment 
was made for a healthy worker effect and that a substantial percentage 
of the underground miners were not occupationally exposed to diesel 
emissions.
    Summation. Limitations identified in both positive and negative 
studies include: lack of sufficient power, inappropriate comparison 
groups, exposure misclassification, statistically insignificant 
results, and potential confounders. As explained earlier, under 
``Evaluation Criteria,'' weaknesses of the first three of these types 
can reasonably be expected, for the most part, to artificially decrease 
the apparent strength of any observed association between diesel 
exposure and increased risk of lung cancer. Statistical insignificance 
and potential confounders may, in the absence of evidence to the 
contrary, be regarded as neutral on average. The weaknesses that have 
been identified in these studies are not unique to epidemiologic 
studies involving lung cancer and diesel exhaust. They are sources of 
uncertainty in virtually all epidemiologic research.
    Even when there is a strong possibility that the results of a study 
have been affected by confounding variables, it does not follow that 
the effect has been to inflate rather than deflate the results or that 
the study cannot contribute to the weight of evidence supporting a 
putative association. As cogently stated by Stober and Abel (op cit., 
p. 4), ``* * * associations found in epidemiologic studies can always 
be, at least in part, attributed to confounding.'' Therefore, an 
objection grounded on potential confounding can always be raised 
against any epidemiologic study. It is well known that this same 
objection was, in the past, raised against epidemiologic studies 
linking lung cancer and radon exposure, lung cancer and asbestos dust 
exposure, and even lung cancer and tobacco smoking.
    Some commenters, have now proposed that virtually every existing 
epidemiologic study relating lung cancer to dpm exposure be summarily 
discredited because of susceptibility to confounding or other perceived 
weaknesses. Given the practical difficulties of designing and executing 
an epidemiologic study, this is not so much an objection to any 
specific study as it is an attack on applied epidemiology in general. 
Indeed, in their review of these studies, Stober and Abel (1996) 
conclude that

    In this field * * * epidemiology faces its limits (Taubes, 
1995). * * * Many of these studies were doomed to failure from the 
very beginning.

    For important ethical reasons, however, tightly controlled lung 
cancer experiments cannot be performed on humans. Therefore, despite 
their inherent limitations, MSHA must rely on the weight of evidence 
from epidemiologic studies, placing greatest weight on the most 
carefully designed and executed studies available.
(b) Bladder Cancer
    With respect to cancers other than lung cancer, MSHA's review of 
the literature identified only bladder cancer as a possible candidate 
for a causal link to dpm. Cohen and Higgins (1995) identified and 
reviewed 14 epidemiologic case-control studies containing information 
related to dpm exposure and bladder cancer. All but one of these 
studies found elevated risks of bladder cancer among workers in jobs 
frequently associated with dpm exposure. Findings were statistically 
significant in at least four of the studies (statistical significance 
was not evaluated in three).

[[Page 5811]]

    These studies point quite consistently toward an excess risk of 
bladder cancer among truck or bus drivers, railroad workers, and 
vehicle mechanics. However, the four available cohort studies do not 
support a conclusion that exposure to dpm is responsible for the excess 
risk of bladder cancer associated with these occupations. Furthermore, 
most of the case-control studies did not distinguish between exposure 
to diesel-powered equipment and exposure to gasoline-powered equipment 
for workers having the same occupation. When such a distinction was 
drawn, there was no evidence that the prevalence of bladder cancer was 
higher for workers exposed to the diesel-powered equipment.
    This, along with the lack of corroboration from existing cohort 
studies, suggests that the excessive rates of bladder cancer observed 
may be a consequence of factors other than dpm exposure that are also 
associated with these occupations. For example, truck and bus drivers 
are subjected to vibrations while driving and may tend to have 
different dietary and sleeping habits than the general population. For 
these reasons, MSHA does not find that convincing evidence currently 
exists for a causal relationship between dpm exposure and bladder 
cancer. MSHA received no public comments objecting to this conclusion.
    ii. Studies Based on Exposures to PM2.5 in Ambient Air. 
Prior to 1990, the relationship between mortality and long-term 
exposure to particulate matter was generally investigated by means of 
cross-sectional studies, but unaddressed spatial confounders and other 
methodological problems inherent in such studies limited their 
usefulness (EPA, 1996).\56\ Two more recent prospective cohort studies 
provide better evidence of a link between excess mortality rates and 
exposure to fine particulate, although some of the uncertainties here 
are greater than with the short-term studies conducted in single 
communities. The two studies are the ``Six Cities'' study (Dockery et 
al., 1993), and the American Cancer Society (ACS) study (Pope et al., 
1995).\57\ The first study followed about 8,000 adults in six U.S. 
cities over 14 years; the second looked at survival data for half a 
million adults in 151 U.S. cities for 7 years. After adjusting for 
potential confounders, including smoking habits, the studies considered 
differences in mortality rates between the most polluted and least 
polluted cities.
---------------------------------------------------------------------------

    \56\ Unlike longitudinal studies, which examine responses at 
given locations to changes in conditions over time, cross-sectional 
studies compare results from locations with different conditions at 
a given point in time.
    \57\ A third such study, the California Seventh Day Adventist 
study (Abbey et al., 1991), investigated only TSP, rather than fine 
particulate. It did not find significant excess mortality associated 
with chronic TSP exposures.
---------------------------------------------------------------------------

    Both the Six Cities study and the ACS study found a significant 
association between chronically higher concentrations of 
PM2.5 (which includes dpm) and age-adjusted total 
mortality.\58\ The authors of the Six Cities Study concluded that the 
results suggest that exposures to fine particulate air pollution 
``contributes to excess mortality in certain U.S. cities.'' The ACS 
study, which not only controlled for smoking habits and various 
occupational exposures, but also, to some extent, for passive exposure 
to tobacco smoke, found results qualitatively consistent with those of 
the Six Cities Study.\59\ In the ACS study, however, the estimated 
increase in mortality associated with a given increase in fine 
particulate exposure was lower, though still statistically significant. 
In both studies, the largest increase observed was for cardiopulmonary 
mortality.
---------------------------------------------------------------------------

    \58\ The Six Cities study also found such relationships at 
elevated levels of PM10 and sulfates. The ACS study was 
designed to follow up on the fine particle results of the Six Cities 
Study, and also investigated sulfates separately. As explained 
earlier in this preamble, sulfates may be a significant constituent 
of dpm, depending on the type of diesel fuel used.
    \59\ The Six Cities study did not find a statistically 
significant increase in risk among non-smokers, suggesting that non-
smokers might be less sensitive than smokers to adverse health 
effects from fine particulate exposures; however, the ACS study, 
with more statistical power, did find significantly increased risk 
even for non-smokers.
---------------------------------------------------------------------------

    Both studies also showed an increased risk of lung cancer 
associated with increased exposure to fine particulate. Although the 
lung cancer results were not statistically significant, they are 
consistent with reports of an increased risk of lung cancer among 
workers occupationally exposed to diesel emissions (discussed above).
    The few studies on associations between chronic PM2.5 
exposure and morbidity in adults show effects that are difficult to 
separate from measures of PM10 and measures of acid 
aerosols. The available studies, however, show positive associations 
between particulate air pollution and adverse health effects for those 
with pre-existing respiratory or cardiovascular disease. This is 
significant for miners occupationally exposed to fine particulates such 
as dpm because, as mentioned earlier, there is a large body of evidence 
showing that respiratory diseases classified as COPD are significantly 
more prevalent among miners than in the general population. It also 
appears that PM exposure may exacerbate existing respiratory infections 
and asthma, increasing the risk of severe outcomes in individuals who 
have such conditions (EPA, 1996).
d. Mechanisms of Toxicity
    Four topics will be addressed in this section of the risk 
assessment: (i) the agent of toxicity, (ii) clearance and deposition of 
dpm, (iii) effects other than cancer, and (iv) lung cancer. The section 
on lung cancer will include discussions of the evidence from (1) 
genotoxicity studies (including bioavailability of genotoxins) and (2) 
animal studies.
    i. Agent of Toxicity. As described in Part II of this preamble, the 
particulate fraction of diesel exhaust is made up of aggregated soot 
particles, vapor phase hydrocarbons, and sulfates. Each soot particle 
consists of an insoluble, elemental carbon core and an adsorbed, 
surface coating of relatively soluble organic compounds, such as 
polycyclic aromatic hydrocarbons (PAHs). Many of these organic carbon 
compounds are suspected or known mutagens and/or carcinogens. For 
example, nitrated PAHs, which are present in dpm, are potent mutagens 
in microbial and human cell systems, and some are known to be 
carcinogenic to animals (IPCS, 1996, pp. 100-105).
    When released into an atmosphere, the soot particles formed during 
combustion tend to aggregate into larger particles. The total organic 
and elemental carbon in these soot particles accounts for approximately 
80 percent of the dpm mass. The remaining 20 percent consists mainly of 
sulfates, such as H2SO4 (sulfuric acid).
    Several laboratory animal studies have been performed to ascertain 
whether the effects of diesel exhaust are attributable specifically to 
the particulate fraction. (Heinrich et al., 1986, 1995; Iwai et al., 
1986; Brightwell et al., 1986). These studies compare the effects of 
chronic exposure to whole diesel exhaust with the effects of filtered 
exhaust containing no particles. The studies demonstrate that when the 
exhaust is sufficiently diluted to nullify the effects of gaseous 
irritants (NO2 and SO2), irritant vapors 
(aldehydes), CO, and other systemic toxicants, diesel particles are the 
prime etiologic agents of noncancer health effects. Exposure to dpm 
produced changes in the lung that were much more prominent than those 
evoked by the gaseous fraction alone. Marked differences in the effects 
of whole and filtered diesel exhaust were also evident from general 
toxicological

[[Page 5812]]

indices, such as body weight, lung weight, and pulmonary 
histopathology.
    These studies show that, when the exhaust is sufficiently diluted, 
it is the particles that are primarily responsible for the toxicity 
observed. However, the available studies do not completely settle the 
question of whether the particles might act additively or 
synergistically with the gases in diesel exhaust. Possible additivity 
or interaction effects with the gaseous portion of diesel exhaust 
cannot be completely ruled out.
    One commenter (MARG) raised an issue with regard to the agent of 
toxicity in diesel exhaust as follows:

    MSHA has not attempted to regulate exposure to suspected 
carcinogens contained in dpm, but has opted instead, in metal/non-
metal mines, to regulate total carbon (``TC'') as a surrogate for 
diesel exhaust, without any evidence of adverse health effects from 
TC exposure. * * * Nor does the mere presence of suspected 
carcinogens, in minute quantities, in diesel exhaust require a 95 
percent reduction of total diesel exhaust [sic] in coal mines. If 
there are small amounts of carcinogenic substances of concern in 
diesel exhaust, those substances, not TC, should be regulated 
directly on the basis of the risks (if any) posed by those 
substances in the quantities actually present in underground mines. 
[MARG]

    First, it should be noted that the ``suspected carcinogens'' in 
diesel exhaust to which the commenter referred are part of the organic 
fraction of the total carbon. Therefore, limiting the concentration of 
airborne total carbon attributable to dpm, or removing the soot 
particles from the diesel exhaust by filtration, are both ways of 
effectively limiting exposures to these suspected carcinogens. Second, 
the commenter seems to have assumed that cancer is the only adverse 
health effect of concern and that the only agents in dpm that could 
cause cancer are the ``suspected carcinogens'' in the organic fraction. 
This not only ignores non-cancer health effects associated with 
exposures to dpm and other fine particles, but also the possibility 
(discussed below) that, with sufficient deposition and retention, soot 
particles themselves could promote or otherwise increase the risk of 
lung cancer--either directly or by stimulating the body's natural 
defenses against foreign substances.
    The same commenter [MARG] also stated that ``* * * airborne carbon 
has not been shown to be harmful at levels currently established in 
MSHA's dust rules. If the problem is dpm, as MSHA asserts, then it is 
not rationally addressed by regulating airborne carbon.'' MSHA's intent 
is to limit dpm exposures in M/NM mines by regulating the submicrometer 
carbon from diesel emissions--not any and all airborne carbon. MSHA 
considers its approach a rational means of limiting dpm exposures 
because most of the dpm consists of carbon (approximately 80 percent by 
weight), and because using low sulfur diesel fuel will effectively 
reduce the sulfates comprising most of the remaining portion. The 
commenter offered no practical suggestion of a more direct, effective, 
and rational way of limiting airborne dpm concentrations in M/NM mines. 
Furthermore, direct evidence exists that the risk of lung cancer 
increases with increasing cumulative occupational exposure to dpm as 
measured by total carbon (Saverin et al., 1999, discussed earlier in 
this risk assessment).
    ii. Deposition, Clearance, and Retention. As suggested by Figure 
II-1 of this preamble, most of the aggregated particles making up dpm 
are no larger than one micrometer in diameter. Particles this small are 
able to penetrate into the deepest regions of the lungs, called 
alveoli. In the alveoli, the particles can mix with and be dispersed by 
a substance called surfactant, which is secreted by cells lining the 
alveolar surfaces.
    The literature on deposition of fine particles in the respiratory 
tract was reviewed in Green and Watson (1995) and U.S. EPA (1996). The 
mechanisms responsible for the broad range of potential particle-
related health effects varies depending on the site of deposition. Once 
deposited, the particles may be cleared from the lung, translocated 
into the interstitium, sequestered in the lymph nodes, metabolized, or 
be otherwise chemically or physically changed by various mechanisms. 
Clearance of dpm from the alveoli is important in the long-term effects 
of the particles on cells, since it may be more than two orders of 
magnitude slower than mucociliary clearance (IPCS, 1996).
    IARC (1989) and IPCS (1996) reviewed factors affecting the 
deposition and clearance of dpm in the respiratory tracts of 
experimental animals. Inhaled PAHs adhering to the carbon core of dpm 
are cleared from the lung at a significantly slower rate than 
unattached PAHs. Furthermore, there is evidence that inhalation of 
whole dpm may increase the retention of subsequently inhaled PAHs. IARC 
(op cit.) suggested that this can happen when newly introduced PAHs 
bind to dpm particles that have been retained in the lung.
    The evidence points to significant differences in deposition and 
clearance for different animal species (IPCS, 1996). Under equivalent 
exposure regimens, hamsters exhibited lower levels of retained dpm in 
their lungs than rats or mice and consequently less pulmonary function 
impairment and pulmonary pathology. These differences may result from a 
lower intake rate of dpm, lower deposition rate and/or more rapid 
clearance rate, or lung tissue that is less susceptible to the 
cytotoxicity of dpm. Observations of a decreased respiration in 
hamsters when exposed by inhalation favor lower intake and deposition 
rates.
    Retardation of lung clearance, called ``overload'' is not specific 
to dpm and may be caused by inhaling, at a sufficiently high rate, dpm 
in combination with other respirable particles, such as mineral dusts 
typical of mining environments. The effect is characterized by (1) an 
overwhelming of normal clearance processes, (2) disproportionately high 
retention and loading of the lung with particles, compared to what 
occurs at lower particle inhalation rates, (3) various pathological 
responses; generally including chronic inflammation, epithelial 
hyperplasia and metaplasia, and pulmonary fibrosis; and sometimes 
including lung tumors.
    In the proposed risk assessment, MSHA requested additional 
information, not already covered in the sources cited above, on fine 
particle deposition in the respiratory tract, especially as it might 
pertain to lung loading in miners exposed to a combination of diesel 
particulate and other dusts. In response to this request, NIOSH 
submitted a study that investigated rat lung responses to chronic 
inhalation of a combination of coal dust and diesel exhaust, compared 
to coal dust or dpm alone (Castranova et al., 1985). Although this 
report did not directly address deposition or clearance, the 
investigators reported that another phase of the study had shown that 
``particulate clearance, as determined by particulate accumulation in 
the lung, is inhibited after two years of exposure to diesel exhaust 
but is not inhibited by exposure to coal dust.''
    iii. Effects other than Cancer. A number of controlled animal 
studies have been undertaken to ascertain the toxic effects of exposure 
to diesel exhaust and its components. Watson and Green (1995) reviewed 
approximately 50 reports describing noncancerous effects in animals 
resulting from the inhalation of diesel exhaust. While most of the 
studies were conducted with rats or hamsters, some information was also 
available from studies conducted using cats, guinea pigs, and monkeys. 
The authors also

[[Page 5813]]

correlated reported effects with different descriptors of dose, 
including both gravimetric and non-gravimetric (e.g., particle surface 
area or volume) measures. From their review of these studies, Watson 
and Green concluded that:
    (a) Animals exposed to diesel exhaust exhibit a number of 
noncancerous pulmonary effects, including chronic inflammation, 
epithelial cell hyperplasia, metaplasia, alterations in connective 
tissue, pulmonary fibrosis, and compromised pulmonary function.
    (b) Cumulative weekly exposure to diesel exhaust of 70 to 80 
mg hr/m3 or greater are associated with the presence 
of chronic inflammation, epithelial cell proliferation, and depressed 
alveolar clearance in chronically exposed rats.
    (c) The extrapolation of responses in animals to noncancer 
endpoints in humans is uncertain. Rats were the most sensitive animal 
species studied.
    Subsequent to the review by Watson and Green, there have been a 
number of animal studies on allergic immune responses to dpm. Takano et 
al. (1997) investigated the effects of dpm injected into mice through 
an intratracheal tube and found manifestations of allergic asthma, 
including enhanced antigen-induced airway inflammation, increased local 
expression of cytokine proteins, and increased production of antigen-
specific immunoglobulins. The authors concluded that the study 
demonstrated dpm's enhancing effects on allergic asthma and that the 
results suggest that dpm is ``implicated in the increasing prevalence 
of allergic asthma in recent years.'' Similarly, Ichinose et al. 
(1997a) found that five different strains of mice injected 
intratracheally with dpm exhibited manifestations of allergic asthma, 
as expressed by enhanced airway inflammation, which were correlated 
with an increased production of antigen-specific immunoglobulin due to 
the dpm. The authors concluded that dpm enhances manifestations of 
allergic airway inflammation and that ``* * * the cause of individual 
differences in humans at the onset of allergic asthma may be related to 
differences in antigen-induced immune responses * * *.''
    The mechanisms that may lead to adverse health effects in humans 
from inhaling fine particulates are not fully understood, but potential 
mechanisms that have been hypothesized for non-cancerous outcomes are 
summarized in Table III-6. A comprehensive review of the toxicity 
literature is provided in U.S. EPA (1996).

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    Deposition of particulates in the human respiratory tract may 
initiate events leading to increased airflow obstruction, impaired 
clearance, impaired host defenses, or increased epithelial 
permeability. Airflow obstruction can result from laryngeal 
constriction or bronchoconstriction secondary to stimulation of 
receptors in extrathoracic or intrathoracic airways. In addition to 
reflex airway narrowing, reflex or local stimulation of mucus secretion 
can lead to mucus hypersecretion and, eventually, to mucus plugging in 
small airways.
    Pulmonary changes that contribute to cardiovascular responses 
include a variety of mechanisms that can lead to hypoxemia, including 
bronchoconstriction, apnea, impaired diffusion, and production of 
inflammatory mediators. Hypoxia can lead to cardiac arrhythmias and 
other cardiac electrophysiologic responses that, in turn, may lead to 
ventricular fibrillation and ultimately cardiac arrest. Furthermore, 
many respiratory receptors have direct cardiovascular effects. For 
example, stimulation of C-fibers leads to bradycardia and hypertension, 
and stimulation of laryngeal receptors can result in hypertension, 
cardiac arrhythmia, bradycardia, apnea, and even cardiac arrest. Nasal 
receptor or pulmonary J-receptor stimulation can lead to vagally-
mediated bradycardia and hypertension (Widdicombe, 1988).
    Some commenters mistakenly attributed the sensory irritant effects 
of diesel exhaust entirely to its gaseous components. The mechanism by 
which constituents of dpm can cause sensory irritations in humans is 
much better understood than the mechanisms for other adverse health 
effects due to fine particulates. In essence, sensory irritants are 
``scrubbed'' from air entering the upper respiratory tract, thereby 
preventing a portion from penetrating more deeply into the lower 
respiratory tract. However, the sensory irritants stimulate trigeminal 
nerve endings, which are located very close to the oro-nasal mucosa and 
also to the watery surfaces of the eye (cornea). This produces a 
burning, painful sensation. The intensity of the sensory irritant 
response is related to the irritant concentration and duration of 
exposure. Differences in relative potency are observed with different 
sensory irritants. Acrolein and formaldehyde are examples of highly 
potent sensory irritants which, along with others having low molecular 
weights (acids, aldehydes), are often found in the organic fraction of 
dpm (Nauss et al., 1995). They may be adsorbed onto the carbon-based 
core or released in a vapor phase. Thus, mixtures of sensory irritants 
in dpm may impinge upon the eyes and respiratory tract of miners and 
produce adverse health effects.
    It is also important to note that mixtures of sensory irritants in 
dpm may produce responses that are not predicted solely on the basis of 
the individual chemical constituents. Instead, these irritants may 
interact at receptor sites to produce additive, synergistic, or 
antagonistic effects. For example, because of synergism, dpm containing 
a mixture of sensory irritants at relatively low concentrations may 
produce intense sensory responses (i.e., responses far above those 
expected for the individual irritants). Therefore, the irritant effects 
of whole dpm cannot properly be evaluated by simply adding together the 
known effects of its individual components.
    As part of its public comments on the proposed preamble, NIOSH 
submitted a study (Hahon et al., 1985) on the effects of diesel 
emissions on mice infected with influenza virus. The object of this 
study was to determine if exposure to diesel emissions (either alone or 
in combination with coal dust) could affect resistance to pulmonary 
infections. The investigators exposed groups of mice to either coal 
dust, diesel emissions, a combination of both, or filtered air (control 
group) for various durations, after which they were infected with 
influenza. Although not reflected by excess mortality, the severity of 
influenza infection was found to be more pronounced in mice previously 
exposed to diesel emissions than in control animals. The effect was not 
intensified by inhalation of coal dust in combination with those 
emissions.
    In addition to possible acute toxicity of particles in the 
respiratory tract, chronic exposure to particles that deposit in the 
lung may induce inflammation. Inflammatory responses can lead to 
increased permeability and possibly diffusion abnormality. Furthermore, 
mediators released during an inflammatory response could cause release 
of factors in the clotting cascade that may lead to an increased risk 
of thrombus formation in the vascular system (Seaton, 1995). Persistent 
inflammation, or repeated cycles of acute lung injury and healing, can 
induce chronic lung injury. Retention of the particles may be 
associated with the initiation and/or progression of COPD.
    Takenaka et al. (1995) investigated mechanisms by which dpm may act 
to cause allergenic effects in human cell cultures. The investigators 
reported that application of organic dpm extracts over a period of 10 
to 14 days increased IgE production from the cells by a factor of up to 
360 percent. They concluded that enhanced IgE production in the human 
airway resulting from the organic fraction of dpm may be an important 
factor in the increasing incidence of allergic airway disease. 
Similarly, Tsien et al. (1997) investigated the effects of the organic 
fraction of dpm on IgE production in human cell cultures and found that 
application of the organic extract doubled IgE production after three 
days in cells already producing IgE.
    Sagai et al. (1996) investigated the potential role of dpm-induced 
oxygen radicals in causing pulmonary injuries. Repeated intratracheal 
instillation of dpm in mice caused marked infiltration of inflammatory 
cells, proliferation of goblet cells, increased mucus secretion, 
respiratory resistance, and airway constriction. The results indicated 
that oxygen radicals, induced by intratracheally instilled dpm, can 
cause responses characteristic of bronchial asthma.
    Lovik et al. (1997) investigated inflammatory and systemic IgE 
responses to dpm, alone and in combination with the model allergen 
ovalbumin (OA), in mice. To determine whether it was the elemental 
carbon core or substances in the organic fraction of dpm that were 
responsible for observed allergenic effects, they compared the effects 
of whole dpm with those of carbon black (CB) particles of comparable 
size and specific surface area. Although the effects were slightly 
greater for dpm, both dpm and CB were found to cause significant, 
synergistic increases in allergenic responses to the OA, as expressed 
by inflammatory responses of the local lymph node and OA-specific IgE 
production. The investigators concluded that both dpm and CB 
synergistically enhance and prolong inflammatory responses in the lymph 
nodes that drain the site of allergen deposition. They further 
concluded that the elemental carbon core contributes substantially to 
the adjuvant activity of dpm.
    Diaz-Sanchez et al. (1994, 1996, 1997) conducted a series of 
experiments on human subjects to investigate the effects of dpm on 
allergic inflammation as measured by IgE production. The studies by 
Takenaka et al. (op cit.) and Tsien et al. (op cit.) were also part of 
this series but were based on human cell cultures rather than live 
human volunteers. A principal objective of these experiments was to 
investigate the pathways and mechanisms by which dpm induces allergic 
inflammation. The investigators found that the organic fraction of dpm 
can enhance IgE production, but that the major

[[Page 5817]]

polyaromatic hydrocarbon in this fraction (phenanthrene) can enhance 
IgE without causing inflammation. On the other hand, when human 
volunteers were sprayed intranasally with carbon particles lacking the 
organic compounds, the investigators found a large influx of cells in 
the nasal mucosa but no increase in IgE. These results suggest that 
while the organic portion of dpm is not necessary for causing 
irritation and local inflammation, it is the organic compounds that act 
on the immune system to promote an allergic response.
    Salvi et al. (1999) investigated the impact of diesel exhaust on 
human airways and peripheral blood by exposing healthy volunteers to 
diesel exhaust at a concentration of 300 g/m\3\ for one hour 
with intermittent exercise. Following exposure, they found significant 
evidence of acute inflammatory responses in airway lavage and also in 
the peripheral blood. Some commenters expressed a belief that the 
gaseous, rather than particulate, components of diesel exhaust caused 
these effects. The investigators noted that the inflammatory responses 
observed could not be attributed to NO2 in the diesel 
exhaust because previous studies they had conducted, using a similar 
experimental protocol, had revealed no such responses in the airway 
tissues of volunteers exposed to a higher concentration of 
NO2, for a longer duration, in the absence of dpm. They 
concluded that ``[i]t therefore seems more likely that the particulate 
component of DE is responsible.''
    iv. Lung Cancer. (1) Genotoxicity Studies. Many studies have shown 
that diesel soot, or its organic component, can increase the likelihood 
of genetic mutations during the biological process of cell division and 
replication. A survey of the applicable scientific literature is 
provided in Shirname-More (1995). What makes this body of research 
relevant to the risk of lung cancer is that mutations in critical genes 
can sometimes initiate, promote, or advance a process of 
carcinogenesis.
    The determination of genotoxicity has frequently been made by 
treating diesel soot with organic solvents such as dichloromethane and 
dimethyl sulfoxide. The solvent removes the organic compounds from the 
carbon core. After the solvent evaporates, the mutagenic potential of 
the extracted organic material is tested by applying it to bacterial, 
mammalian, or human cells propagated in a laboratory culture. In 
general, the results of these studies have shown that various 
components of the organic material can induce mutations and chromosomal 
aberrations.
    One commenter (MARG) pointed out that ``even assuming diesel 
exhaust contains particular genotoxic substances, the bioavailability 
of these genotoxins has been questioned.'' As acknowledged in the 
proposed risk assessment, a critical issue is whether whole diesel 
particulate is mutagenic when dispersed by substances present in the 
lung. Since the laboratory procedure for extracting organic material 
with solvents bears little resemblance to the physiological environment 
of the lung, it is important to establish whether dpm as a whole is 
genotoxic, without solvent extraction. Early research indicated that 
this was not the case and, therefore, that the active genotoxic 
materials adhering to the carbon core of diesel particles might not be 
biologically damaging or even available to cells in the lung (Brooks et 
al., 1980; King et al., 1981; Siak et al., 1981). A number of more 
recent research papers, however, have shown that dpm, without solvent 
extraction, can cause DNA damage when the soot is dispersed in the 
pulmonary surfactant that coats the surface of the alveoli (Wallace et 
al., 1987; Keane et al., 1991; Gu et al., 1991; Gu et al., 1992). From 
these studies, NIOSH concluded in 1992 that:

    * * * the solvent extract of diesel soot and the surfactant 
dispersion of diesel soot particles were found to be active in 
procaryotic cell and eukaryotic cell in vitro genotoxicity assays. 
The cited data indicate that respired diesel soot particles on the 
surface of the lung alveoli and respiratory bronchioles can be 
dispersed in the surfactant-rich aqueous phase lining the surfaces, 
and that genotoxic material associated with such dispersed soot 
particles is biologically available and genotoxically active. 
Therefore, this research demonstrates the biological availability of 
active genotoxic materials without organic solvent interaction. 
[Cover letter to NIOSH response to ANPRM, 1992].

If this conclusion is correct, it follows that dpm itself, and not only 
its organic extract, can cause genetic mutations when dispersed by a 
substance present in the lung.
    One commenter (IMC Global) noted that Wallace et al. (1987) used 
aged dpm samples from scrapings inside an exhaust pipe and contended 
that this was not a realistic representation of dpm. The commenter 
further argued that the two studies cited by Gu et al. involved 
``direct application of an unusually high concentration gradient'' that 
does not replicate normal conditions of dpm exposure.
    MSHA agrees with this commenter's general point that conditions set 
up in such experiments do not duplicate actual exposure conditions. 
However, as a follow-up to the Wallace study, Keane et al. (op. cit.) 
demonstrated similar results with both exhaust pipe soot and particles 
obtained directly from an exhaust stream. With regard to the two Gu 
studies, MSHA recognizes that any well-controlled experiment serves 
only a limited purpose. Despite their limitations, however, these 
experiments provided valuable information. They avoided solvent 
extraction. By showing that solvent extraction is not a necessary 
condition of dpm mutagenicity, these studies provided incremental 
support to the hypothesis of bioavailability under more realistic 
conditions. This possibility was subsequently tested by a variety of 
other experiments, including experiments on live animals and humans.
    For example, Sagai et al. (1993) showed that whole dpm produced 
active oxygen radicals in the trachea of live mice, but that dpm 
stripped of organic compounds did not. Whole dpm caused significant 
damage to the lungs and also high mortality at low doses. According to 
the investigators, most of the toxicity observed appeared to be due to 
the oxygen radicals, which can also have genotoxic effects. 
Subsequently, Ichinose et al. (1997b) examined the relationship between 
tumor response and the formation of oxygen radicals in the lungs of 
mice injected with dpm. The mice were treated with sufficiently high 
doses of dpm to produce tumors after 12 months. As in the earlier 
study, the investigators found that the dpm generated oxygen radicals, 
even in the absence of biologically activating systems (such as 
macrophages), and that these oxygen radicals were implicated in the 
lung toxicity of the dpm. The authors concluded that ``oxidative DNA 
damage induced by the repeated DEP [i.e., dpm] treatment could be an 
important factor in enhancing the mutation rate leading to lung 
cancer.''
    The formation of DNA adducts is an important indicator of 
genotoxicity and potential carcinogenicity. Adduct formation occurs 
when molecules, such as those in dpm, attach to the cellular DNA. These 
adducts can negatively affect DNA transcription and/or cellular 
duplication. If DNA adducts are not repaired, then a mutation or 
chromosomal aberration can occur during normal mitosis (i.e., cell 
replication) eventually leading to cancer cell formation. IPCS (1996) 
contains a survey of animal experiments showing DNA adduct induction in 
the lungs of experimental animals exposed to diesel

[[Page 5818]]

exhaust.\60\ MSHA recognizes that such studies provide limited 
information regarding the bioavailability of organics, since positive 
results may well have been related to factors associated with lung 
particle overload. However, the bioavailability of genotoxic dpm 
components is also supported by human studies showing genotoxic effects 
of exposure to whole dpm. DNA adduct formation and/or mutations in 
blood cells following exposure to dpm, especially at levels 
insufficient to induce lung overload, can be presumed to result from 
organics diffusing into the blood.
---------------------------------------------------------------------------

    \60\ Some of these studies will be discussed in the next 
subsection of this risk assessment.
---------------------------------------------------------------------------

    Hemminki et al. (1994) found that DNA adducts were significantly 
elevated in lymphocytes of nonsmoking bus maintenance and truck 
terminal workers, as compared to a control group of hospital mechanics, 
with the highest adduct levels found among garage and forklift workers. 
Hou et al. (1995) reported significantly elevated levels of DNA adducts 
in lymphocytes of non-smoking diesel bus maintenance workers compared 
to a control group of unexposed workers. Similarly, Nielsen et al. 
(1996) found that DNA adducts were significantly increased in the blood 
and urine of bus garage workers and mechanics exposed to dpm as 
compared to a control group.
    One commenter (IMC Global) acknowledged that ``the studies 
conducted by Hemminiki [Hemminiki et al., 1994] showed elevations in 
lymphocyte DNA adducts in garage workers, bus maintenance workers and 
diesel forklift drivers'' but argued that ``these elevations were at 
the borderline of statistical significance.'' Although results at a 
higher level of confidence would have been more persuasive, this does 
not negate the value of the evidence as it stands. Furthermore, 
statistical significance in an individual study becomes less of an 
issue when, as in this case, the results are corroborated by other 
studies.
    IMC Global also acknowledged that the Nielsen study found 
significant differences in DNA adduct formation between diesel-exposed 
workers and controls but argued that ``the real source of genotoxins 
was unclear, and other sources of exposure, such as skin contact with 
lubricating oils could not be excluded.'' As is generally the case with 
studies involving human subjects, this study did not completely control 
for potential confounders. For this reason, MSHA considers it important 
that several human studies--not all subject to confounding by the same 
variables--found elevated adduct levels in diesel-exposed workers.
    IMC Global cited another human study (Qu et al., 1997) as casting 
doubt on the genotoxic effects of diesel exposure, even though this 
study (conducted on Australian coal miners) reported significant 
increases in DNA adducts immediately after a period of intense diesel 
exposure during a longwall move. As noted by the commenter, adduct 
levels of exposed miners and drivers were, prior to the longwall move, 
approximately 50% higher than for the unexposed control group; but 
differences by exposure category were not statistically significant. A 
more informative part of the study, however, consisted of comparing 
adducts in the same workers before and after a longwall move, which 
involved ``intensive use of heavy equipment, diesel powered in these 
mines, over a 2-3 week period.'' MSHA emphasizes that the comparison 
was made on the same workers, because doing so largely controlled for 
potentially confounding variables, such as smoking habits, that may be 
a factor when making comparisons between different persons. After the 
period of ``intensive'' exposure, statistically significant increases 
were observed in both total and individual adducts. Contrary to the 
commenter's characterization of this study, the investigators stated 
that their analysis ``provides results in which the authors have a high 
level of confidence.'' They concluded that ``given the * * * apparent 
increase in adducts during a period of intense DEE [i.e., diesel 
exhaust emissions] exposures it would be prudent to pay particular 
attention to keeping exposures as low as possible, especially during 
LWCO [i.e., `longwall change out'] operations.'' Although the commenter 
submitted this study as counter-evidence, it actually provides 
significant, positive evidence that high dpm exposures in a mining 
environment can produce genotoxic effects.
    The West Virginia Coal Association submitted an analysis by Dr. 
Peter Valberg, purporting to show that ``* * * the quantity of 
particle-bound mutagens that could potentially contact lung cells under 
human exposure scenarios is very small.'' According to Dr. Valberg's 
calculations, the dose of organic mutagens deposited in the lungs of a 
worker occupationally exposed (40 hours per week) to 500 g/
m\3\ of dpm would be equivalent in potency to smoking about one 
cigarette per month.\61\ Dr. Valberg indicated that a person smoking at 
this level would generally be classified a nonsmoker, but he made no 
attempt to quantify the carcinogenic effects. Nor did he compare this 
exposure level with levels of exposures to environmental tobacco smoke 
that have been linked to lung cancer.
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    \61\ The only details provided for this calculation pertained to 
adjusting 8-hour occupational exposures. Dr. Valberg adjusted the 
500 g/m\3\ concentration for an 8-hour occupational 
exposure to a supposedly equivalent 24-hour continuous concentration 
of 92 g/m\3\. This adjustment ignored differences in 
breathing rates between periods of sleep, leisure activities, and 
heavy work. Even under the unrealistic assumption of homogeneous 
breathing rates, the calculation appears to be erroneous, since (500 
g/m\3\)  x  (40 hours/week) is nearly 30 percent greater 
than (92 g/m\3\)  x  (168 hours/week). Also, Dr. Valberg 
stated that the calculation assumed a deposition fraction of 20 
percent for dpm but did not state what deposition fraction was being 
assumed for the particles in cigarette smoke.
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    Since the commenter did not provide details of Dr. Valberg's 
calculation, MSHA was unable to verify its accuracy or evaluate the 
plausibility of key assumptions. However, even if the equivalence is 
approximately correct, using it to discount the possibility that dpm 
increases the risk of lung cancer relies on several questionable 
assumptions. Although their precise role in the analysis is unclear 
because it was not presented in detail, these assumptions apparently 
include:
    (1) That there is a good correlation between genotoxicity dose-
response and carcinogenicity dose-response. Although genotoxicity data 
can be very useful for identifying a carcinogenic hazard, 
carcinogenesis is a highly complex process that may involve the 
interaction of many mutagenic, physiological, and biochemical 
responses. Therefore, the shape and slope of a carcinogenic dose-
response relationship cannot be readily predicted from a genotoxic 
dose-response relationship.
    (2) That only the organic fraction of dpm contributes to 
carcinogenesis. This contradicts the findings reported by Ichinose et 
al. (1997b) and does not take into account the contribution that 
inflammation and active oxygen radicals induced by the inorganic carbon 
core of dpm may have in promoting lung cancers. Multiple routes of 
carcinogenesis may operate in human lungs--some requiring only the 
various organic mutagens in dpm and others involving induction of free 
radicals by the elemental carbon core, either alone or in combination 
with the organics.
    (3) That the only mutagens in dpm are those that have been 
identified as mutagenic to bacteria and that the

[[Page 5819]]

mutagenic constituents of dpm have all been identified. One of the most 
potent of all known mutagens (3-nitrobenzanthrone) was only recently 
isolated and identified in dpm (Enya et al., 1997).
    (4) That the mutagenic components of dpm have the same combined 
potency as those in cigarette smoke. This ignores the relative potency 
and amounts of the various mutagenic constituents. If the calculation 
did not take into account the relative amounts and potencies of all the 
individual mutagens in dpm and cigarette smoke, then it oversimplified 
the task of making such a comparison.
    In sum, unlike the experimental findings of dpm genotoxicity 
discussed above, the analysis by Dr. Valberg is not based on empirical 
evidence from dpm experiments, and it appears to rely heavily on 
questionable assumptions. Moreover, the contention that active 
components of dpm are not available in sufficient quantities to cause 
significant mutagenic damage in humans appears to be directly 
contradicted by the empirical evidence of elevated DNA adduct levels in 
exposed workers (Hemminki et al., 1994; Hou et al., 1995; Nielsen et 
al., 1996; Qu et al., 1997).
    (2) Animal Inhalation Studies. When dpm is inhaled, a number of 
adverse effects that may contribute to carcinogenesis are discernable 
by microscopic and biochemical analysis. For a comprehensive review of 
these effects, see Watson and Green (1995). In brief, these effects 
begin with phagocytosis, which is essentially an attack on the diesel 
particles by cells called alveolar macrophages. The macrophages engulf 
and ingest the diesel particles, subjecting them to detoxifying 
enzymes. Although this is a normal physiological response to the 
inhalation of foreign substances, the process can produce various 
chemical byproducts injurious to normal cells. In attacking the diesel 
particles, the activated macrophages release chemical agents that 
attract neutrophils (a type of white blood cell that destroys 
microorganisms) and additional alveolar macrophages. As the lung burden 
of diesel particles increases, aggregations of particle-laden 
macrophages form in alveoli adjacent to terminal bronchioles, the 
number of Type II cells lining particle-laden alveoli increases, and 
particles lodge within alveolar and peribronchial tissues and 
associated lymph nodes. The neutrophils and macrophages release 
mediators of inflammation and oxygen radicals, which have been 
implicated in causing various forms of chromosomal damage, genetic 
mutations, and malignant transformation of cells (Weitzman and Gordon, 
1990). Eventually, the particle-laden macrophages are functionally 
altered, resulting in decreased viability and impaired phagocytosis and 
clearance of particles. This series of events may result in pulmonary 
inflammatory, fibrotic, or emphysematous lesions that can ultimately 
develop into cancerous tumors.
    IARC (1989), Mauderly (1992), Busby and Newberne (1995), IPCS 
(1996), Cal-EPA (1998), and US EPA (1999) reviewed the scientific 
literature relating to excess lung cancers observed among laboratory 
animals chronically exposed to filtered and unfiltered diesel exhaust. 
The experimental data demonstrate that chronic exposure to whole diesel 
exhaust increases the risk of lung cancer in rats and that dpm is the 
causative agent. This carcinogenic effect has been confirmed in two 
strains of rats and in at least five laboratories. Experimental results 
for animal species other than the rat, however, are either inconclusive 
or, in the case of Syrian hamsters, suggestive of no carcinogenic 
effect. In two of three mouse studies reviewed by IARC (1989), lung 
tumor formation (including adenocarcinomas) was increased in the 
exposed animals as compared to concurrent controls; in the third study, 
the total incidence of lung tumors was not elevated compared to 
historical controls. Two more recent mouse studies (Heinrich et al., 
1995; Mauderly et al., 1996) have both reported no statistically 
significant increase in lung cancer rates among exposed mice, as 
compared to contemporaneous controls. Monkeys exposed to diesel exhaust 
for two years did not develop lung tumors, but the short duration of 
exposure was judged inadequate for evaluating carcinogenicity in 
primates.
    Bond et al. (1990a) investigated differences in peripheral lung DNA 
adduct formation among rats, hamsters, mice, and monkeys exposed to dpm 
at a concentration of 8100 g/m\3\ for 12 weeks. Mice and 
hamsters showed no increase of DNA adducts in their peripheral lung 
tissue, whereas rats and monkeys showed a 60 to 80-percent increase. 
The increased prevalence of lung DNA adducts in monkeys suggests that, 
with respect to DNA adduct formation, the human lungs' response to dpm 
inhalation may more closely resemble that of rats than that of hamsters 
or mice.
    The conflicting carcinogenic effects of chronic dpm inhalation 
reported in studies of rats, mice, and hamsters may be due to non-
equivalent delivered doses or to differences in response among species. 
Indeed, monkey lungs have been reported to respond quite differently 
than rat lungs to both diesel exhaust and coal dust (Nikula, 1997). 
Therefore, the results from rat experiments do not, by themselves, 
establish that there is any excess risk due to dpm exposure for humans. 
However, the human epidemiologic and genotoxicity (DNA adduct) data 
indicate that humans comprise a species that, like rats, do suffer a 
carcinogenic response to dpm exposure. This would be consistent with 
the observation, mentioned above, that lung DNA adduct formation is 
increased among exposed rats but not among exposed hamsters or mice. 
Therefore, although MSHA recognizes that there are important 
differences between rats and humans (as there are also between rats and 
hamsters or mice), MSHA considers the rat studies relevant to an 
evaluation of human health risks.
    Reactions similar to those observed in rats inhaling dpm have also 
been observed in rats inhaling fine particles with no organic component 
(Mauderly et al., 1994; Heinrich et al., 1994, 1995; Nikula et al., 
1995). Rats exposed to titanium dioxide (TiO2) or pure 
carbon (``carbon black'') particles, which are not considered to be 
genotoxic, exhibited similar pathological responses and developed lung 
cancers at about the same rate as rats exposed to whole diesel exhaust. 
Carbon black particles were used in these experiments because they are 
physically similar to the inorganic carbon core of dpm but have 
negligible amounts of organic compounds adsorbed to their surface. 
Therefore, at least in some species, it appears that the lung cancer 
toxicity of dpm may result largely from a biochemical response to the 
core particle itself rather than from specific, genotoxic effects of 
the adsorbed organic compounds.\62\
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    \62\ NIOSH commented as follows: ``Data cited by MSHA in support 
of this statement are not comparable. Rats were exposed to dpm at 4 
mg/m\3\ for 2 years (Mauderly et al. 1987; Brightwell et al. 1989), 
in contrast to rats exposed to TiO2 at 250 mg/m\3\ for 
two years [reference to article (Lee et al. 1985) not cited by 
MSHA]. It is not apparent that the overload mechanism that is 
proposed to be responsible for tumors in the TiO2 exposed 
rats could also have been responsible for the tumors seen in the dpm 
exposed rats at 62-fold lower exposure concentrations.'' In the 
reports cited by MSHA, levels of TiO2 and/or carbon black 
were commensurate with dpm levels.
---------------------------------------------------------------------------

    One commenter stated that, in the proposed risk assessment, MSHA 
had neglected three additional studies suggesting that lung cancer 
risks in animals inhaling diesel exhaust are unrelated to genotoxic 
mechanisms. One of these studies (Mauderly et al., 1996) did not 
pertain to questions of

[[Page 5820]]

genotoxicity but has been cited in the discussion of mouse studies 
above. The other two studies (Randerath et al., 1995 and Belinsky et 
al., 1995) were conducted as part of the cancer bioassay described in 
the 1994 article by Mauderly et al. (cited in the preceding paragraph). 
In the Randerath study, the investigators found that no DNA adducts 
specific to either diesel exhaust or carbon black were induced in the 
lungs of rats exposed to the corresponding substance. However, after 
three months of exposure, the total level of DNA adducts and the levels 
of some individual adducts were significantly higher in the diesel-
exposed rats than in the controls. In contrast, multiple DNA adducts 
thought to be specific to diesel exhaust formed in the skin and lungs 
of mice treated topically with organic dpm extract. These results are 
consistent with the findings of Mauderly et al. (1994, op cit.). They 
imply that although the organic compounds of diesel exhaust are capable 
of damaging cellular DNA, they did not inflict such damage under the 
conditions of the inhalation experiment performed. The report noted 
that these results do not rule out the possibility of DNA damage by 
inhaled organics in ``other species or * * * [in] exposure situations 
in which the concentrations of diesel exhaust particles are much 
lower.'' In the Belinsky study, the investigators measured mutations in 
selected genes in the tumors of those rats that had developed lung 
cancer. This study did not succeed in elucidating the mechanisms by 
which dpm and carbon black cause lung tumors in rats. The authors 
concluded that ``until some of the genes involved in the 
carcinogenicity of diesel exhaust and carbon black are identified, a 
role for the organic compounds in tumor development cannot be 
excluded.''
    The carbon-black and TiO2 studies discussed above 
indicate that lung cancers in rats exposed to dpm may be induced by a 
mechanism that does not require the bioavailability of genotoxic 
organic compounds adsorbed on the elemental carbon particles. Some 
researchers have interpreted these studies as also suggesting that (1) 
the carcinogenic mechanism in rats depends on massive overloading of 
the lung and (2) that this may provide a mechanism of carcinogenesis 
involving a threshold effect specific to rats, which has not been 
observed in other rodents or in humans (Oberdorster, 1994; Watson and 
Valberg, 1996). Some commenters on the ANPRM cited the lack of a link 
between lung cancer and coal dust or carbon black exposure as evidence 
that carbon particles, by themselves, are not carcinogenic in humans. 
Coal mine dust, however, consists almost entirely of particles larger 
than those forming the carbon core of dpm or used in the carbon black 
and TiO2 rat studies. Furthermore, although there have been 
nine studies reporting no excess risk of lung cancer among coal miners 
(Liddell, 1973; Costello et al., 1974; Armstrong et al., 1979; Rooke et 
al., 1979; Ames et al., 1983; Atuhaire et al., 1985; Miller and 
Jacobsen, 1985; Kuempel et al., 1995; Christie et al., 1995), eight 
studies have reported an elevated risk of lung cancer for those exposed 
to coal dust (Enterline, 1972; Rockette, 1977; Howe et al., 1983; 
Correa et al., 1984; Levin et al., 1988; Morabia et al., 1992; Swanson 
et al., 1993; Morfeld et al., 1997). The positive results in five of 
these studies (Enterline, 1972; Rockette, 1977; Howe et al., 1983; 
Morabia et al., 1992; Swanson et al., 1993) were statistically 
significant. Morabia et al. (op cit.) reported increased risk 
associated with duration of exposure, after adjusting for cigarette 
smoking, asbestos exposure, and geographic area. Furthermore, excess 
lung cancers have been reported among carbon black production workers 
(Hodgson and Jones, 1985; Siemiatycki, 1991; Parent et al., 1996). 
After a comprehensive evaluation of the available scientific evidence, 
the World Health Organization's International Agency for Research on 
Cancer concluded: ``Carbon black is possibly carcinogenic to humans 
(Group 2B).'' (IARC, 1996).
    The carbon black and TiO2 animal studies cited above do 
not prove there is a threshold below which dpm exposure poses no risk 
of causing lung cancer in humans. They also do not prove that dpm 
exposure has no incremental, genotoxic effects. Even if the genotoxic 
organic compounds in dpm were biologically unavailable and played no 
role in human carcinogenesis, this would not rule out the possibility 
of a genotoxic route to lung cancer (even for rats) due to the presence 
of the particles themselves. For example, as a byproduct of the 
biochemical response to the presence of particles in the alveoli, free 
oxidant radicals may be released as macrophages attempt to digest the 
particles. There is evidence that dpm can both induce production of 
reactive oxygen agents and also depress the activity of naturally 
occurring antioxidant enzymes (Mori, 1996; Ichinose et al., 1997; Sagai 
et al., 1993). Oxidants can induce carcinogenesis either by reacting 
directly with DNA, or by stimulating cell replication, or both 
(Weitzman and Gordon, 1990). Salvi et al. (1999) reported acute 
inflammatory responses in the airways of human exposed to dpm for one 
hour at a concentration of 300 g/m\3\. Such inflammation is 
associated with the production of free radicals and could provide 
routes to lung cancer with even when normal lung clearance is 
occurring. It could also give rise to a ``quasi-threshold,'' or surge 
in response, corresponding to the exposure level at which the normal 
clearance rate becomes overwhelmed (lung overload).
    Oxidant activity is not the only mechanism by which dpm could exert 
carcinogenic effects in the absence of mutagenic activity by its 
organic fraction. In its commentary on the Randerath study discussed 
above, the HEI's Health Review Committee suggested that dpm could both 
cause genetic damage by inducing free oxygen radicals and also enhance 
cell division by inducing cytokines or growth hormones:

    It is possible that diesel exhaust exerts its carcinogenic 
effects through a mechanism that does not involve direct 
genotoxicity (that is, formation of DNA adducts) but involves 
proliferative responses such as chronic inflammation and hyperplasia 
arising from high concentrations of particles deposited in the lungs 
of the exposed rats. * * * Phagocytes (macrophages and neutrophils) 
released during inflammatory reactions ``produce reactive oxygen 
species that can damage DNA. * * * Particles (with or without 
adsorbed PAHs) may thus induce oxidative DNA damage via oxygen free 
radicals. * * * Alternatively, activated phagocytes may release 
cytokines or growth factors that are known to increase cell 
division. Increased cell division has been implicated in cancer 
causation. * * * Thus, in addition to oxidative DNA damage, 
increased cell proliferation may be an important mechanism by which 
diesel exhaust and other insoluble particles induce pulmonary 
carcinogenesis in the rat. [Randerath et al., 1995, p. 55]

    Even if lung overload were the primary or sole route by which dpm 
induced lung cancer, this would not mean that the high dpm 
concentrations observed in some mines are without hazard. It is 
noteworthy, moreover, that dpm exposure levels recorded in some mines 
have been almost as high as laboratory exposures administered to rats 
showing a clearly positive response. Intermittent, occupational 
exposure levels greater than about 500 g/m\3\ dpm may 
overwhelm the human lung clearance mechanism (Nauss et al., 1995). 
Therefore, concentrations at the even higher levels currently observed 
in some mines could be expected to cause overload in some humans, 
possibly inducing lung cancer by a mechanism

[[Page 5821]]

similar to what occurs in rats. In addition, a proportion of exposed 
individuals can always be expected to be more susceptible than normal 
to clearance impairments and lung overload. Inhalation at even moderate 
levels may significantly impair clearance, especially in susceptible 
individuals. Exposures to cigarette smoke and respirable mineral dusts 
may further depress clearance mechanisms and reduce the threshold for 
overload. Consequently, even at dpm concentrations far lower than 500 
g/m\3\ dpm, impaired clearance due to dpm inhalation may 
provide an important route to lung cancer in humans, especially if they 
are also inhaling cigarette smoke and other fine dusts simultaneously. 
(Hattis and Silver, 1992, Figures 9, 10, 11).
    Furthermore, as suggested above, lung overload is not necessarily 
the only route to carcinogenesis in humans. Therefore, dpm 
concentrations too low to cause overload still may present a hazard. In 
humans exposed over a working lifetime to doses insufficient to cause 
overload, carcinogenic mechanisms unrelated to overload may operate, as 
indicated by the human epidemiologic studies and the data on human DNA 
adducts cited in the preceding subsection of this risk assessment. It 
is possible that overload provides the dominant route to lung cancer at 
high concentrations of fine particulate, while other mechanisms emerge 
as more relevant for humans under lower-level exposure conditions.
    The NMA noted that, in 1998, the US EPA's Clean Air Scientific 
Advisory Committee (CASAC) concluded that there is ``no evidence that 
the organic fraction of soot played a role in rat tumorigenesis at any 
exposure level, and considerable evidence that it did not.'' According 
to the NMA, this showed ``* * * it is the rat data--not the hamster 
data--that lacks relevance for human health assessment.''
    It must first be noted that, in MSHA's view, all of the 
experimental animal data on health effects has relevance for human 
health risk assessment--whether the evidence is positive or negative 
and even if the positive results cannot be used to quantify human risk. 
The finding that different mammalian species exhibit important 
differences in response is itself relevant for human risk assessment. 
Second, the passage quoted from CASAC pertains to the route for 
tumorigenesis in rats and does not discuss whether this does or does 
not have relevance to humans exposed at high levels. The context for 
the CASAC deliberations was ambient exposure conditions in the general 
environment, rather than the higher occupational exposures that might 
impair clearance rates in susceptible individuals. Third, the comment 
assumes that only a finding of tumorigenesis attributable to the 
organic portion of dpm would elucidate mechanisms of potential health 
effects in humans. This ignores the possibility that a mechanism 
promoting tumors, but not involving the organics, could operate in both 
rats and humans. Induction of free oxygen radicals is an example. 
Fourth, although there may be little or no evidence that organics 
contributed to rat tumorigenesis in the studies performed, there is 
evidence that the organics contributed to increases in DNA adduct 
formation. This kind of activity could have tumorigenic consequences in 
humans who may be exposed for periods far longer than a rat's 3-year 
lifetime and who, as a consequence, have more time to accumulate 
genetic damage from a variety of sources.
    Bond et al. (1990b) and Wolff et al. (1990) investigated adduct 
formation in rats exposed to various concentrations of either dpm or 
carbon black for 12 weeks. At the highest concentration (10 mg/m\3\), 
DNA adduct levels in the lung were increased by exposure to either dpm 
or carbon black; but levels in the rats exposed to dpm were 
approximately 30 percent higher. Gallagher et al. (1994) exposed 
different groups of rats to diesel exhaust, carbon black, or 
TiO2 and detected no significant difference in DNA adduct 
levels in the lung. However, the level of one type of adduct, thought 
to be derived from a PAH, was elevated in the dpm-exposed rats but not 
found in the control group or in rats exposed to carbon black or 
TiO2.
    These studies indicate that the inorganic carbon core of dpm is not 
the only possible agent of genetic damage in rats inhaling dpm. After a 
review of these and other studies involving DNA adducts, IPCS (1996) 
concluded that ``Taken together, the studies of DNA adducts suggest 
that some organic chemicals in diesel exhaust can form DNA adducts in 
lung tissue and may play a role in the carcinogenic effects. * * 
*however, DNA adducts alone cannot explain the carcinogenicity of 
diesel exhaust, and other factors, such as chronic inflammation and 
cell proliferation, are also important.''
    Nauss et al. (1995, pp. 35-38) judged that the results observed in 
the carbon black and TiO2 inhalation studies on rats do not 
preclude the possibility that the organic component of dpm has 
important genotoxic effects in humans. More generally, they also do not 
prove that lung overload is necessary for dpm-induced lung cancer. 
Because of the relatively high doses administered in some of the rat 
studies, it is conceivable that an overload phenomenon masked or even 
inhibited other potential cancer mechanisms. At dpm concentrations 
insufficient to impair clearance, carcinogenesis may have followed 
other routes, some possibly involving the organic compounds. At these 
lower concentrations, or among rats for which overload did not occur, 
tumor rates for dpm, carbon black, and TiO2 may all have 
been too low to make statistically meaningful comparisons.
    The NMA argued that ``MSHA's contention that lung overload might 
``mask'' tumor production by lower doses of dpm has been convincingly 
rebutted by recognized experts in the field,'' but provided no 
convincing explanation of why such masking could not occur. The NMA 
went on to say:

    The [CASAC] Panel viewed the premises that: a) a small tumor 
response at low exposure was overlooked due to statistical power; 
and b) soot-associated organic mutagens had a greater effect at low 
than at high exposure levels to be without foundation. In the 
absence of supporting evidence, the Panel did not view derivation of 
a quantitative estimate of human lung cancer risk from the low-level 
rat data as appropriate.

MSHA is not attempting to ``derive a quantitative estimate of human 
lung cancer risk from the low-level rat data.''
    Dr. Peter Valberg, writing for the West Virginia Coal Association, 
provided the following argument for discounting the possibility of 
other carcinogenic mechanisms being masked by overload in the rat 
studies:

    Some regulatory agencies express concern about the mutagens 
bound to dpm. They hypothesize that, at high exposure levels, 
genotoxic mechanisms are overwhelmed (masked) by particle-overload 
conditions. However, they argue that at low-exposure concentrations, 
these organic compounds could represent a lung cancer risk. Tumor 
induction by mutagenic compounds would be characterized by a linear 
dose-response and should be detectable, given enough exposed rats. 
By using a ``meta-analysis'' type of approach and combining data 
from eight long-term rat inhalation studies, the lung tumor response 
can be analyzed. When all dpm-exposed rats from lifetime-exposure 
studies are combined, a threshold of response (noted above) occurs 
at approximately 600 g/m\3\ continuous lifetime exposure 
(approximately 2,500 g/m\3\ of occupational exposure). 
Additional statistical analysis of only those rats exposed to low 
concentrations of dpm confirms the absence of a tumorigenic effect 
below that threshold. Thus, even data in rats (the most sensitive 
laboratory species) do not support the hypothesis that particle-
bound organics cause tumors.


[[Page 5822]]


    MSHA finds that this analysis relies on several questionable and 
unsupported assumptions and that, for the following reasons, the 
possibility remains that organic compounds in inhaled dpm may, under 
the right exposure conditions, contribute to its carcinogenic effects:

    (1) The absence of evidence for an organic carbon effect is not 
equivalent to evidence of the absence of such an effect. Dr. Valberg 
did not demonstrate that enough rats were exposed, at levels 
insufficient to cause overload, to ensure detection of a 30- to 40-
percent increase in the risk of lung cancer. Also, the normal lifespan 
of a rat whose lung is not overloaded with particles may, because of 
the lower concentrations involved, provide insufficient time for the 
organic compounds to express carcinogenic effects. Furthermore, low 
bioavailability of the organics could further reduce the likelihood 
that a carcinogenic sequence of mutations would occur within a rat's 
relatively short lifespan (i.e., at particle concentrations too low to 
cause overload).
    (2) If the primary mechanism for carcinogenesis requires a reduced 
clearance rate (due to overload), then acute exposures are important, 
and it may not be appropriate to represent equivalent hazards by 
spreading an 8-hour occupational exposures over a 24-hour period. For 
example, eight hours at 600 g/m\3\ would have different 
implications for lung clearance than 24 hours at 200 g/m\3\.
    (3) Granting that the rat data cannot be used to extrapolate risk 
for humans, these data should also not be used to rule out mechanisms 
of carcinogenesis that may operate in humans but not in rats. 
Clearance, for example, may operate differently in humans than in rats, 
and there may be a gradual rather than abrupt change in human overload 
conditions with increasing exposure. Also, at least some of the organic 
compounds in dpm may be more biologically available to the human lung 
than to that of the rat.
    (4) For experimental purposes, laboratory rats are deliberately 
bred to be homogeneous. This is done, in part, to deliberately minimize 
differences in response between individuals. Therefore, individual 
differences in the threshold for lung overload would tend to be masked 
in experiments on laboratory rats. It is likely that human populations 
would exhibit, to a far greater extent than laboratory rats, a range of 
susceptibilities to lung overload. Also some humans, unlike the 
laboratory rats in these experiments, place additional burdens on their 
lung clearance by smoking.
    One commenter (MARG) concluded that ``[t]here is * * * no basis for 
extrapolating the rat results to human beings; the animal studies, 
taken together, do not justify MSHA's proposals.''
    MSHA is neither extrapolating the rat results to make quantitative 
risk estimates for humans nor using them, in isolation, as a 
justification for these regulations. MSHA does regard it as 
significant, however, that the evidence for an increased risk of lung 
cancer due to chronic dpm inhalation comes from both human and animal 
studies. MSHA agrees that the quantitative results observed for rats in 
existing studies should not be extrapolated to humans. Nevertheless, 
the fact that high dpm exposures for two or three years can induce lung 
cancer in rats enhances the epidemiologic evidence that much longer 
exposures to miners, at concentrations of the same order of magnitude, 
could also induce lung cancers.

3. Characterization of Risk

    After reviewing the evidence of adverse health effects associated 
with exposure to dpm, MSHA evaluated that evidence to ascertain whether 
exposure levels currently existing in mines warrant regulatory action 
pursuant to the Mine Act. The criteria for this evaluation are 
established by the Mine Act and related court decisions. Section 
101(a)(6)(A) provides that:

    The Secretary, in promulgating mandatory standards dealing with 
toxic materials or harmful physical agents under this subsection, 
shall set standards which most adequately assure on the basis of the 
best available evidence that no miner will suffer material 
impairment of health or functional capacity even if such miner has 
regular exposure to the hazards dealt with by such standard for the 
period of his working life.

    Based on court interpretations of similar language under the 
Occupational Safety and Health Act, there are three questions that need 
to be addressed: (a) Whether health effects associated with dpm 
exposure constitute a ``material impairment'' to miner health or 
functional capacity; (b) whether exposed miners are at significant 
excess risk of incurring any of these material impairments; and (c) 
whether the rule will substantially reduce such risks.
    Some commenters argued that the link between dpm exposure and 
material health impairments is questionable, and that MSHA should wait 
until additional scientific evidence becomes available before 
concluding that there are health risks due to such exposure warranting 
regulatory action. For example, MARG asserted that ``[c]ontrary to the 
suggestions in the [proposed] preamble, a link between dpm exposure and 
serious illness has never been established by reliable scientific 
evidence.'' \63\ MARG
---------------------------------------------------------------------------

    \63\ MARG supported this assertion by claiming that ``[t]he EPA 
reports which MSHA references in its preamble were found `not 
scientifically adequate for making regulatory decisions concerning 
the use of diesel-powered engines' by EPA's Clean Air Scientific 
Advisory Committee. [reference to CASAC (1998)]'' Contrary to MARG's 
claim, CASAC (1998) did not review any of the 20 EPA documents MSHA 
cited in the proposed preamble. Instead, the document reviewed by 
CASAC (1998) was an unpublished draft of a health risk assessment on 
diesel exhaust (EPA, 1998), to which MSHA made no reference. Since 
MSHA has not relied in any way on this 1998 draft document, its 
``scientific adequacy'' is entirely irrelevant to this rulemaking.
    In response to the 1998 CASAC review, EPA modified its draft 
risk assessment (EPA, 1999), and CASAC subsequently reviewed the 
1999 draft (CASAC, 2000). CASAC found the revised draft much 
improved over the previous version and agreed that even 
environmental exposure to diesel emissions is likely to increase the 
risk of lung cancer (CASAC, 2000). CASAC endorsed this conclusion 
for dpm concentrations in ambient air, which are lower, by a factor 
of more than 100, than the levels observed in some mines (see Fig. 
III-4).

---------------------------------------------------------------------------
continued as follows:

    Precisely because the scientific evidence * * * is inconclusive 
at best, NIOSH and NCI are now conducting a * * * [study] to 
determine whether diesel exhaust is linked to illness, and if so, at 
what level of exposure. * * * MARG is also funding an independent 
parallel study.
    * * * Until data from the NIOSH/NCI study, and the parallel MARG 
study, are available, the answers to these important questions will 
not be known. Without credible answers to these and other questions, 
MSHA's regulatory proposals * * * are premature * * *.''

    For reasons explained below, MSHA does not agree that the 
collective weight of scientific evidence is ``inconclusive at best.'' 
Furthermore, the criteria for evaluating the health effects evidence do 
not require scientific certainty. As noted by Justice Stevens in an 
important case on risk involving the Occupational Safety and Health 
Administration, the need to evaluate risk does not mean an agency is 
placed into a ``mathematical straitjacket.'' [Industrial Union 
Department, AFL-CIO v. American Petroleum Institute, 448 U.S. 607, 100 
S.Ct. 2844 (1980), hereinafter designated the ``Benzene'' case]. The 
Court recognized that regulation may be necessary even when scientific 
knowledge is not complete; and--

so long as they are supported by a body of reputable scientific 
thought, the Agency is free to use conservative assumptions in 
interpreting the data * * * risking error on the side of 
overprotection rather than underprotection. [Id. at 656].


[[Page 5823]]


    Moreover, the statutory criteria for evaluating health effects do 
not require MSHA to wait for incontrovertible evidence. In fact, MSHA 
is required to set standards based on the ``best available evidence'' 
(emphasis added).
a. Material Impairments to Miners' Health or Functional Capacity
    MSHA recognizes that there is considerable disagreement, among 
knowledgeable parties, in the interpretation of the overall body of 
scientific research and medical evidence related to human health 
effects of dpm exposures. One commenter for example, interpreted the 
collective evidence as follows:

    * * * the best available scientific evidence shows that diesel 
particulate exposure is associated with serious material impairment 
of health. * * * there is clear evidence that diesel particulate 
exposure can cause lung cancer (as well as other serious non-
malignant diseases) among workers in a variety of occupational 
settings. While no body of scientific evidence is ever completely 
definitive, the evidence regarding diesel particulate is 
particularly strong * * *. [Michael Silverstein, MD, State of 
Washington Dept. of Labor and Industries]

    Other commenters, including several national and regional 
organizations representing the mining industry, sharply disagreed with 
this interpretation. For example, one commenter stated that ``[i]n our 
opinion, the best available evidence does not provide substantial or 
credible support for the proposal.'' Several commenters argued that 
evidence from within the mining industry itself was especially 
weak.\64\ A representative of one mining company that had been using 
diesel equipment for many years commented: ``[t]o date, the medical 
history of our employees does not indicate a single case of lung 
cancer, chronic illness, or material impairment of health due to 
exposure to diesel exhaust. This appears to be the established norm 
throughout the U.S. coal mining industry.'' This commenter, however, 
submitted no evidence comparing the rate of lung cancer or other 
material impairment among exposed miners to the rate for unexposed 
miners (or comparable workers) of similar age, smoking habits, and 
geographic location.
---------------------------------------------------------------------------

    \64\ At the public hearing on May 11, 1999, a commenter 
representing MARG suggested there is evidence that miners exposed to 
dpm experience adverse health effects at lower-than-normal rates. 
According to this commenter, ``[s]ignificantly, the human studies 
conducted in the mining industry reveal a negative propensity for 
diesel particulate matter-related health effects.'' These studies 
drew comparisons against an external reference population and failed 
to adjust for the ``healthy worker effect.'' (See MSHA's discussion 
of this effect, especially as manifested in the study by Christie et 
al., 1995, in Subsection 2.c.i(2)(a) of this risk assessment.)
---------------------------------------------------------------------------

    With due consideration to all oral and written testimony, comments, 
and evidence submitted during the rulemaking proceedings, MSHA 
conducted a review of the scientific literature cited in Part III.2. 
Based on the combined weight of the best available evidence, MSHA has 
concluded that underground miners exposed to current levels of dpm are 
at excess risk of incurring the following three kinds of material 
impairment: (i) Sensory irritations and respiratory symptoms (including 
allergenic responses); (ii) premature death from cardiovascular, 
cardiopulmonary, or respiratory causes; and (iii) lung cancer. The next 
three subsections will respectively explain MSHA's basis for linking 
these effects with dpm exposure.
    i. Sensory Irritations and Respiratory Symptoms (including 
allergenic responses). Kahn et al. (1988), Battigelli (1965), Gamble et 
al. (1987a), and Rudell et al. (1996) identified a number of 
debilitating acute responses to diesel exhaust exposure. These 
responses included irritation of the eyes, nose and throat; headaches, 
nausea, and vomiting; chest tightness and wheeze. These symptoms were 
also reported by miners at the 1995 workshops and the public hearings 
held on these proceedings in 1998. In addition, Ulfvarson et al. (1987, 
1990) reported evidence of reduced lung function in workers exposed to 
dpm for a single shift. The latter study supports attributing a portion 
of the reduction to the dpm in diesel exhaust. After reviewing this 
body of literature, Morgan et al. (1997) concluded ``it is apparent 
that exposure to diesel fumes in sufficient concentrations may lead to 
[transient] eye and nasal irritation'' and ``a transient decline of 
ventilatory capacity has been noted following such exposures.''
    One commenter (Nevada Mining Association) acknowledged there was 
evidence that miners exposed to diesel exhaust experienced, as a 
possible consequence of their exposure, ``acute, short-term or 
`transitory' irritation, such as watering eyes, in susceptible 
individuals * * *''; but asserted that ``[a]ddressing any such 
transient irritant effects does not require the Agency's sweeping, 
stringent PEL approach [in M/NM mines].''
    Although there is evidence that such symptoms subside within one to 
three days of no occupational exposure, a miner who must be exposed to 
dpm day after day in order to earn a living may not have time to 
recover from such effects. Hence, the opportunity for a so-called 
``reversible'' health effect to reverse itself may not be present for 
many miners. Furthermore, effects such as stinging, itching and burning 
of the eyes, tearing, wheezing, and other types of sensory irritation 
can cause severe discomfort and can, in some cases, be seriously 
disabling. Also, workers experiencing sufficiently severe sensory 
irritations can be incapacitated or distracted as a result of their 
symptoms, thereby endangering themselves and other workers and 
increasing the risk of accidents. For these reasons, MSHA considers 
such irritations to constitute ``material impairments'' of health or 
functional capacity within the meaning of the Act, regardless of 
whether or not they are reversible. Further discussion of why MSHA 
believes reversible effects can constitute material impairments can be 
found above, in Subsection 2.a.2 of this risk assessment.
    The best available evidence also points to more severe respiratory 
consequences of exposure to dpm. Significant statistical associations 
have been detected between acute environmental exposures to fine 
particulates and debilitating respiratory impairments in adults, as 
measured by lost work days, hospital admissions, and emergency room 
visits (see Table III-3). Short-term exposures to fine particulates, or 
to particulate air pollution in general, have been associated with 
significant increases in the risk of hospitalization for both pneumonia 
and COPD (EPA, 1996).
    The risk of severe respiratory effects is exemplified by specific 
cases of persistent asthma linked to diesel exposure (Wade and Newman, 
1993). Glenn et al. (1983) summarized results of NIOSH health 
evaluations among coal, salt, trona, and potash miners and reported 
that ``all four of the chronic effects analyses revealed an excess of 
cough and phlegm among the diesel exposed group.'' There is persuasive 
evidence for a causal connection between dpm exposure and increased 
manifestations of allergic asthma and other allergic respiratory 
diseases, coming from recent experiments on animals and human cells 
(Takenaka et al., 1995; Lovik et al., 1997; Takano et al., 1997; 
Ichinose et al., 1997a). Based on controlled experiments on healthy 
human volunteers, Diaz-Sanchez et al. (1994, 1996, 1997), Peterson and 
Saxon (1996), and Salvi et al. (1999) reported significant increases in 
various markers of allergic response resulting from exposure to dpm.
    Peterson and Saxon (1996) reviewed the scientific literature on the 
relationship between PAHs and other products of fossil fuel combustion 
found

[[Page 5824]]

in dpm and trends in allergic respiratory disease. They found that the 
prevalences of allergic rhinitis (``hay fever'') and allergic asthma 
have significantly increased with the historical increase in fossil 
fuel combustion and that laboratory data support the hypothesis that 
certain organic compounds found in dpm ``* * * are an important factor 
in the long-term increases in the prevalence in allergic airway 
disease.'' Similarly, much of the research on allergenic responses to 
dpm was reviewed by Diaz-Sanchez (1997), who concluded that dpm 
pollution in the ambient environment ``may play an important role in 
the increased incidence of allergic airway disease.'' Morgan et al. 
(1997) noted that dpm ``* * * may be partly responsible for some of the 
exacerbations of asthma'' and that ``* * * it would be wise to err on 
the side of caution.'' Such health outcomes are clearly ``material 
impairments'' of health or functional capacity within the meaning of 
the Act.
    ii. Premature Death from Cardiovascular, Cardiopulmonary, or 
Respiratory Causes. The evidence from air pollution studies identifies 
death, largely from cardiovascular, cardiopulmonary, or respiratory 
causes, as an endpoint significantly associated with acute exposures to 
fine particulates (PM2.5--see Table III-3). The weight of 
epidemiologic evidence indicates that short-term ambient exposure to 
particulate air pollution contributes to an increased risk of daily 
mortality (EPA, 1996). Time-series analyses strongly suggest a positive 
effect on daily mortality across the entire range of ambient 
particulate pollution levels. Relative risk estimates for daily 
mortality in relation to daily ambient particulate concentration are 
consistently positive and statistically significant across a variety of 
statistical modeling approaches and methods of adjustment for effects 
of relevant covariates such as season, weather, and co-pollutants. The 
mortality effects of acute exposures appear to be primarily 
attributable to combustion-related particles in PM2.5 (such 
as dpm) and are especially pronounced for death due to pneumonia, COPD, 
and IHD (Schwartz et al., 1996). After thoroughly reviewing this body 
of evidence, the U.S. Environmental Protection Agency (EPA) concluded:

    It is extremely unlikely that study designs not yet employed, 
covariates not yet identified, or statistical techniques not yet 
developed could wholly negate the large and consistent body of 
epidemiologic evidence * * *. [EPA, 1996]

    There is also substantial evidence of a relationship between 
chronic exposure to fine particulates (PM2.5) and an excess 
(age-adjusted) risk of mortality, especially from cardiopulmonary 
diseases. The Six Cities and ACS studies of ambient air particulates 
both found a significant association between chronic exposure to fine 
particles and excess mortality. In some of the areas studied, 
PM2.5 is composed primarily of dpm; and significant 
mortality and morbidity effects were also noted in those areas. In both 
studies, after adjusting for smoking habits, a statistically 
significant excess risk of cardiopulmonary mortality was found in the 
city with the highest average concentration of PM2.5 as 
compared to the city with the lowest. Both studies also found excess 
deaths due to lung cancer in the cities with the higher average level 
of PM2.5, but these results were not statistically 
significant (EPA, 1996). The EPA concluded that--

    * * * the chronic exposure studies, taken together, suggest 
there may be increases in mortality in disease categories that are 
consistent with long-term exposure to airborne particles and that at 
least some fraction of these deaths reflect cumulative PM impacts 
above and beyond those exerted by acute exposure events * * * There 
tends to be an increasing correlation of long-term mortality with PM 
indicators as they become more reflective of fine particle levels. 
[EPA, 1996]

    Whether associated with acute or chronic exposures, the excess risk 
of death that has been linked to pollution of the air with fine 
particles like dpm is clearly a ``material impairment'' of health or 
functional capacity within the meaning of the Act.
    In a review, submitted by MARG, of MSHA's proposed risk assessment, 
Dr. Jonathan Borak asserted that ``MSHA appears to regard all 
particulates smaller than 2.5 g/m3 as equivalent.'' 
He argued that ``dpm and other ultra-fine particulates represents only 
a small proportion of ambient particulate samples,'' that ``chronic 
cough, chronic phlegm, and chronic wheezing reflect mainly 
tracheobronchial effects,'' and that tracheobronchial deposition is 
highly dependent on particle size distribution.
    No part of Dr. Borak's argument is directly relevant to MSHA's 
identification of the risk of death from cardiovascular, 
cardiopulmonary, or respiratory causes faced by miners exposed to high 
concentrations of dpm. First, MSHA does not regard all fine 
particulates as equivalent. However, dpm is a major constituent of 
PM2.5 in many of the locations where increased mortality has 
been linked to PM2.5 levels. MSHA regards dpm as presenting 
a risk by virtue of its comprising a type of PM2.5. Second, 
the studies MSHA used to support the existence of this risk 
specifically implicate fine particles (i.e., PM2.5), so the 
percentage of dpm in ``total suspended particulate emissions'' (which 
includes particles even larger than PM10) is not relevant. 
Third, the chronic respiratory symptoms listed by Dr. Borak are not 
among the material impairments that MSHA has identified from the 
PM2.5 studies. Much of the evidence pertaining to excess 
mortality is based on acute--not chronic--ambient exposures of 
relatively high intensity. In the preceding subsection of this risk 
assessment, MSHA identified various respiratory symptoms, including 
allergenic responses, but the evidence for these comes largely from 
studies on diesel emissions.
    As discussed in Section 2.a.iii of this risk assessment, many 
miners smoke tobacco, and miners experience COPD at a significantly 
higher rate than the general population. This places many miners in two 
of the groups that EPA (1996) identified as being at greatest risk of 
premature mortality due to particulate exposures.
    iii. Lung Cancer.  It is clear that lung cancer constitutes a 
``material impairment'' of health or functional capacity within the 
meaning of the Act. Therefore, the issue to be addressed in this 
section is whether there is sufficient evidence (i.e., enough to 
warrant regulatory action) that occupational exposure to dpm causes the 
risk of lung cancer to increase.
    In the proposed risk assessment, MSHA noted that various national 
and international institutions and governmental agencies had already 
classified diesel exhaust or particulate as a probable human 
carcinogen. Considerable weight was also placed on two comprehensive 
meta-analyses of the epidemiologic literature, which had both found 
that the combined evidence supported a causal link. MSHA also 
acknowledged, however, that some reviewers of the evidence disagreed 
with MSHA's conclusion that, collectively, it strongly supports a 
causal connection. As examples of the opposing viewpoint, MSHA cited 
Stober and Abel (1996), Watson and Valberg (1996), Cox (1997), Morgan 
et al. (1997), and Silverman (1998). As stated in the proposed risk 
assessment, MSHA considered the opinions of these reviewers and agreed 
that no individual study was perfect: even the strongest of the studies 
had limitations when viewed in isolation. MSHA nevertheless concluded 
(in the proposal) that the best available epidemiologic studies, 
supported by experimental data

[[Page 5825]]

showing toxicity, collectively provide strong evidence that chronic dpm 
exposure (at occupational levels) actually does increase the risk of 
lung cancer in humans.
    Although miners and labor representatives generally agreed with 
MSHA's interpretation of the collective evidence, many commenters 
representing the mining industry strongly objected to MSHA's 
conclusion. Some of these commenters also expressed dissatisfaction 
with MSHA's treatment, in the proposed risk assessment, of opposing 
interpretations of the collective evidence--saying that MSHA had 
dismissed these opposing views without sufficient explanation. Some 
commenters also submitted new critiques of the existing evidence and of 
the meta-analyses on which MSHA had relied. These commenters also 
emphasized the importance of two reports (CASAC, 1998 and HEI, 1999) 
that both became available after MSHA completed its proposed risk 
assessment.
    MSHA has re-evaluated the scientific evidence relating lung cancer 
to diesel emissions in light of the comments, suggestions, and detailed 
critiques submitted during these proceedings. Although MSHA has not 
changed its conclusion that occupational dpm exposure increases the 
risk of lung cancer, MSHA believes that the public comments were 
extremely helpful in identifying areas of MSHA's discussion of lung 
cancer needing clarification, amplification, and/or additional 
supportive evidence.
    Accordingly MSHA has re-organized this section of the risk 
assessment into five subsections. The first of these provides MSHA's 
summary of the collective epidemiologic evidence. Second is a 
description of results and conclusions from the only two existing peer-
reviewed and published statistical meta-analyses of the epidemiologic 
studies: Bhatia et al. (1998) and Lipsett and Campleman (1999). The 
third subsection contains a discussion of potential systematic biases 
that might tend to shift all study results in the same direction. The 
fourth evaluates the overall weight of evidence for causality, 
considering not only the collective epidemiologic evidence but also the 
results of toxicity experiments. Within each of these first four 
subsections, MSHA will respond to the relevant issues and criticisms 
raised by commenters in these proceedings, as well as by other outside 
reviewers. The final subsection will describe general conclusions 
reached by other reviewers of this evidence, and present some responses 
by MSHA about opposing interpretations of the collective evidence.
    (1) Summary of Collective Epidemiologic Evidence. As mentioned in 
Section III.2.c.i(2)(a) and listed in Tables III-4 and III-5, MSHA 
reviewed a total of 47 epidemiologic studies involving lung cancer and 
diesel exposure. Some degree of association between occupational dpm 
exposure and an excess rate of lung cancer was reported in 41 of these 
studies: 22 of the 27 cohort studies and 19 of the 20 case-control 
studies. Section III.2.c.1(2)(a) explains MSHA's criteria for 
evaluating these studies, summarizes those on which MSHA places 
greatest weight, and explains why MSHA places little weight on the six 
studies reporting no increased risk of lung cancer for exposed workers. 
It also contains summaries of the studies involving miners, addresses 
criticisms of individual studies by commenters and reviewers, and 
discusses studies that, according to some commenters, suggest that dpm 
exposure does not increase the risk of lung cancer.
    Here, as in the earlier, proposed version of the risk assessment, 
MSHA was careful to note and consider limitations of the individual 
studies. Several commenters interpreted this as demonstrating a 
corresponding weakness in the overall body of epidemiologic evidence. 
For example, one commenter [Energy West] observed that ``* * * by its 
own admission in the preamble * * * most of the evidence in [the 
epidemiologic] studies is relatively weak'' and argued that MSHA's 
conclusion was, therefore, unjustified.
    It should first be noted that the three most recent epidemiologic 
studies became available too late for inclusion in the risk assessment 
as originally written. These three (Johnston et al., 1997; Saverin et 
al., 1999; Bruske-Hohlfeld, 1999) rank among the strongest eight 
studies available (see Section III.2.c.1(2)(a)) and do not have the 
same limitations identified in many of the other studies. Even so, MSHA 
recognizes that no single one of the existing epidemiologic studies, 
viewed in isolation, provides conclusive evidence of a causal 
connection between dpm exposure and an elevated risk of lung cancer in 
humans. Consistency and coherency of results, however, do provide such 
evidence. An appropriate analogy for the collective epidemiologic 
evidence is a braided steel cable, which is far stronger than any of 
the individual strands of wire making it up. Even the thinnest strands 
can contribute to the strength of the cable.
(a) Consistency of Epidemiologic Results
    Although no epidemiologic study is flawless, studies of both cohort 
and case-control design have quite consistently shown that chronic 
exposure to diesel exhaust, in a variety of occupational circumstances, 
is associated with an increased risk of lung cancer. Furthermore, as 
explained earlier in this risk assessment, limitations such as small 
sample size, short latency, and (usually) exposure misclassification 
reduce the power of a study. These limitations make it more difficult 
to detect a relationship even when one exists. Therefore, the sheer 
number of studies showing a positive association readily distinguishes 
those studies criticized by Taubes (1995), where weak evidence is 
available from only a single study. With only rare exceptions, 
involving too few workers and/or observation periods too short to have 
a good chance of detecting excess cancer risk, the human studies have 
shown a greater risk of lung cancer among exposed workers than among 
comparable unexposed workers.
    Moreover, the fact that 41 out of 47 studies showed an excess risk 
of lung cancer for exposed workers may itself be a significant result, 
even if the evidence in most of those 41 studies is relatively weak. 
Getting ``heads'' on a single flip of a coin, or two ``heads'' out of 
three flips, does not provide strong evidence that there is anything 
special about the coin. However, getting 41 ``heads'' in 47 flips would 
normally lead one to suspect that the coin was weighted in favor of 
heads. Similarly, results reported in the epidemiologic literature lead 
one to suspect that the underlying relationship between diesel exposure 
and an increased risk of lung cancer is indeed positive.
    More formally, as MSHA pointed out in the earlier version of this 
risk assessment, the high proportion of positive studies is 
statistically significant according to the 2-tailed sign test. Under 
the ``null hypothesis'' that there is no systematic bias in one 
direction or the other, and assuming that the studies are independent, 
the probability of 41 or more out of 47 studies being either positive 
or negative is less than one per ten million. Therefore, the sign test 
rejects, at a very high confidence level, the null hypothesis that each 
study is equally likely to be positive or negative. This means that the 
collective results, showing increased risk for exposed workers, are 
statistically significant at a very high confidence level--regardless

[[Page 5826]]

of the statistical significance of any individual study.
    MSHA received no comments directly disputing its attribution of 
statistical significance to the collective epidemiologic evidence based 
the sign test. However, several commenters objected to the concept that 
a number of inconclusive studies can, when viewed collectively, provide 
stronger evidence than the studies considered in isolation. For 
example, the Engine Manufacturers Association (EMA) asserted that--

[j]ust because a number of studies reach the same conclusion does 
not make the collective sum of those studies stronger or more 
conclusive, particularly where the associations are admittedly weak 
and scientific difficulties exist in each. [EMA]

    Similarly, IMC Global stated that

* * * IMC Global does not consider cancer studies with a relative 
risk of less than 2.0 as showing evidence of a casual relationship 
between dpm exposure and lung cancer. * * * Thus while MSHA states 
[in the proposed risk assessment; now updated to 41 out of 47] that 
38 of 43 epidemiologic studies show some degree of association 
between occupational dpm exposures and lung cancer and considers 
that fact significant, IMC Global does not. [IMC Global]

    Although MSHA agrees that even statistically significant 
consistency of epidemiologic results is not sufficient to establish 
causality, MSHA believes that consistency is an important part of 
establishing that a suspected association is causal.\65\ Many of the 
commenters objecting to MSHA's emphasis on the collective evidence 
failed to distinguish the strength of evidence in each individual study 
from the strength of evidence in total.
---------------------------------------------------------------------------

    \65\ With respect to the IMC Global's blanket rejection of 
studies showing a relative risk less than 2.0, please see also the 
related discussions in Subsection 2.c.i(2)(a) above, under the 
heading of ``Potential Confounders,'' and in Subsection 3.a.iii(3) 
below, entitled ``Potential Systemic Biases.''
---------------------------------------------------------------------------

    Furthermore, weak evidence (from just one study) should not be 
confused with a weak effect. As Dr. James Weeks pointed out at the 
public hearing on Nov. 19, 1998, a 40-percent increase in lung cancer 
is a strong effect, even if it may be difficult to detect in an 
epidemiologic study.
    Explicable differences, or heterogeneity, in the magnitudes of 
relative risk reported from different studies should not be confused 
with inconsistency of evidence. For example, as described by Silverman 
(1998), one of the available meta-analyses (Bhatia et al., 1998) 
``examined the primary sources of heterogeneity among studies and found 
that a main source of heterogeneity is the variation in diesel exhaust 
exposure across different occupational groups.'' Figures III-5 and III-
6, taken from Cohen and Higgins (1995), respectively show relative 
risks reported for the two occupations on which the most studies are 
available: railroad workers and truck drivers.
    Each of these two charts compares results from studies that 
adjusted for smoking to results from studies that did not make such an 
adjustment. For each study, the point plotted is the estimated relative 
risk or odds ratio, and the horizontal line surrounding it represents a 
95-percent confidence interval. If the left endpoint of a confidence 
interval exceeds 1.0, then the corresponding result is statistically 
significant at a 95-percent confidence level.
    The two charts show that the risk of lung cancer has consistently 
been elevated for exposed workers and that the results are not 
significantly different within each occupational category. Differences 
in the magnitude and statistical significance of results within 
occupation are not surprising, since the groups studied differed in 
size, average exposure intensity and duration, and the time allotted 
for latent effects.

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    As documented in Subsection 2.c.i(2)(a) of this risk assessment, 
all of the studies showing negative associations were either based on 
relatively short observation or follow-up periods, lacked good 
information about dpm exposure, involved low duration or intensity of 
dpm exposure, or, because of inadequate sample size or latency 
allowance, lacked the power to detect effects of the magnitude found in 
the ``positive'' studies. Boffetta et al. (1988, p. 404) noted that, in 
addition, studies failing to show a statistically significant 
association--

    * * * often had low power to detect any association, had 
insufficient latency periods, or compared incidence or mortality 
rates among workers to national rates only, resulting in possible 
biases caused by the ``healthy worker effect.''

    Some commenters noted that limitations such as insufficient 
duration of exposure, inadequate latency allowance, small worker 
populations, exposure misclassification, and comparison to external 
populations with no adjustment for a healthy worker effect may explain 
why not all of the studies showed a statistically significant 
association between dpm exposure and an increased prevalence of lung 
cancer. According to these commenters, if an epidemiologic study shows 
a statistically significant result, this often occurs in spite of 
methodological weaknesses rather than because of them. MSHA agrees that 
limitations such as those listed make it more difficult to obtain a 
statistically significant result when a real relationship exists.
(b) Best Available Epidemiologic Evidence
    As explained above, it is statistically significant that 41 of the 
47 available epidemiologic studies reported an elevated risk of lung 
cancer for workers exposed to dpm. MSHA finds it even more informative, 
however, to examine the collective results of the eight studies 
identified in Section III.2.c.i(2)(a) as providing the best currently 
available epidemiologic evidence. These studies, selected using the 
criteria described earlier, are: Boffetta et al. (1988), Boffetta et 
al. (1990), Bruske-Hohlfeld et al. (1999), Garshick et al. (1987), 
Garshick et al. (1988, 1991), Johnston et al. (1997), Steenland et al. 
(90, 92, 98), and Saverin et al., (1999). All eight of these studies 
reported an increased risk of lung cancer for workers with the longest 
diesel exposures and for those most likely to have been exposed, 
compared to unexposed workers. Tables showing the results from each of 
these studies are provided in Section III.2.c.1(2)(a).
    The sign test of statistical significance can also be applied to 
the collective results of these eight studies. If there were no 
underlying association between exposure to diesel exhaust and an 
increased risk of lung cancer, or anything else systematically favoring 
a positive result, then there should be equal probabilities (equal to 
one-half) that any one of these eight studies would turn out positive 
or negative. Therefore, under the null hypothesis that positive and 
negative results are equally likely, the probability that all eight 
studies would show either a positive or a negative association is 
(0.5)\8\ = 0.0039, or 0.39 percent. This shows that the collective 
results of the eight studies comprising the best available 
epidemiologic evidence are statistically significant at a confidence 
level exceeding 99 percent (i.e., 100-2 x 0.39).
    When the risk of disease or death increases in response to higher 
cumulative exposures, this is described by a ``positive'' exposure-
response relationship. Like consistency of results, the existence of a 
positive exposure-response relationship is important in establishing 
that the exposures in question actually cause an increase in risk. 
Among the eight studies MSHA has identified as comprising the best 
available epidemiologic evidence, there are five that provide evidence 
of increasing lung cancer risk with increasing cumulative exposure: 
Boffetta, et al. (1990), Bruske-Hohlfeld et al. (1999), Johnston et al. 
(1997), Saverin et al. (1999), and Steenland et al. (1990, 1992, 1998). 
The results supporting such a relationship are provided in the table 
accompanying discussion of each of these studies in Section 
III.2.c.i(2)(a).
    Although some have interpreted the results from the two studies by 
Garshick et al. as also providing evidence of a positive exposure-
response relationship (e.g., Cal-EPA, 1998), this interpretation is 
highly sensitive to the statistical models and techniques used to 
analyze the data (HEI, 1999; Crump 1999). Therefore, for purposes of 
this risk assessment, MSHA is not relying on Garshick et al. (1987) or 
Garshick et. al (1988, 1991) to demonstrate the existence of a positive 
exposure-response relationship. MSHA used the study for purposes of 
hazard identification only. The Garshick studies contributed to the 
weight of evidence favoring a causal interpretation, since they show 
statistically significant excesses in lung cancer risk for the exposed 
workers.
    The relative importance of the five studies identified in 
demonstrating the existence of a positive exposure-response 
relationship varies with the quality of exposure assessment. Boffetta 
et al. (1990) and Bruske-Hohlfeld et al. (1999) were able to show such 
a relationship based on the estimated duration of occupational exposure 
for exposed workers, but quantitative measures of exposure intensity 
(i.e., dpm concentration) were unavailable. Although duration of 
exposure is frequently used as a surrogate of cumulative exposure, it 
is clearly preferable, as many commenters pointed out, to base 
estimates of cumulative exposure and exposure-response analyses on 
quantitative measurements of exposure levels combined with detailed 
work histories. Positive exposure-response relationships based on such 
data were reported in all three studies: Johnston et al. (1997), 
Steenland et al. (1998), and Saverin et al. (1999).
(c) Studies With Quantitative or Semiquantitative Exposure Assessments
    Several commenters stressed the fact that most of the available 
epidemiologic studies contained little or no quantitative information 
on diesel exposures and that those studies containing such information 
(such as Steenland et al., 1998) generated it using questionable 
assumptions. Some commenters also faulted MSHA for insufficiently 
addressing this issue. For example, one commenter stated:

    * * * the Agency fails to highlight the lack of acceptable (or 
any) exposure measurements concurrent with the 43 epidemiology 
studies cited in the Proposed Rule. * * * the lack of concurrent 
exposure data is a significant deficiency of the epidemiology 
studies at issue and is a major factor that prevents application of 
those epidemiology results to risk assessment. [EMA]

    MSHA agrees that the nature and quality of exposure information 
should be an important consideration in evaluating the strength of 
epidemiologic evidence. That is why MSHA included exposure assessment 
as one of the criteria used to evaluate and rank studies in Section 
2.c.1(2)(a) of this risk assessment. Two of the most recent studies, 
both conducted specifically on miners, utilize concurrent, quantitative 
exposure data and are included among the eight in MSHA's selection of 
best available epidemiologic evidence (Johnston et al., 1997 and 
Saverin et al., 1999). As a practical matter, however, epidemiologic 
studies rarely have concurrent exposure measurements; and, therefore, 
the commenter's line of

[[Page 5830]]

reasoning would exclude nearly all of the available studies from this 
risk assessment--including all six of the negative studies. Since 
Section 101(a)(6) of the Mine Act requires MSHA to consider the ``best 
available evidence'' (emphasis added), MSHA has not excluded studies 
with less-than-ideal exposure assessments, but, instead, has taken the 
quality of exposure assessment into account when evaluating them. This 
approach is also consistent with the recognition by the HEI Expert 
Panel on Diesel Emissions and Lung Cancer that ``regulatory decisions 
need to be made in spite of the limitations and uncertainties of the 
few studies with quantitative data currently available'' (HEI, 1999; 
p.39).
    The degree of quantification, however, is not the only relevant 
consideration in evaluating studies with respect to exposure 
assessment. MSHA also considered the likely effects of potential 
exposure misclassification. As expressed by another commenter:

    * * * [S]tudies that * * * have poor measures of exposure to 
diesel exhaust have problems in classification and will have weaker 
results. In the absence of information that misclassification is 
systematic or differential, in which case study results would be 
biased towards either positive or no-effect level, it is reasonable 
to assume that misclassification is random or nondifferentiated. If 
so, * * * study results are biased towards a risk ratio of 1.0, a 
ratio showing no association between diesel exhaust exposure and the 
occurrence of lung cancer. [Dr. James Weeks, representing UMWA]

In her review of Bhatia et al. (1998), Silverman (1998) proposed that 
``[o]ne approach to assess the impact of misclassification would be to 
exclude studies without quantitative or semiquantitative exposure 
data.'' According to Dr. Silverman, this would leave only four studies 
among those considered by Dr. Bhatia: Garshick et al. (1988), 
Gustavsson et al. (1990), Steenland et al. (1992), and Emmelin et al. 
(1993).\66\ All four of these studies showed higher rates of lung 
cancer for the workers estimated to have received the greatest 
cumulative exposure, as compared to workers who had accumulated little 
or no diesel exposure. Statistically significant results were reported 
in three of these four studies. Furthermore, the two more recent 
studies utilizing fully quantitative exposure assessments (Johnston et 
al., 1997; Saverin et al., 1999) were not evaluated or otherwise 
considered in the articles by Drs. Bhatia and Silverman. Like the other 
four studies, these too reported elevated rates of lung cancer for 
workers with the highest cumulative exposures. Specific results from 
all six of these studies are presented in Tables III-4 and III-5.
---------------------------------------------------------------------------

    \66\ Emmelin et al. (1993) was considered but excluded from the 
meta-analysis by Bhatia et al. (1998) for reasons explained by the 
authors.
---------------------------------------------------------------------------

    Once again, the sign test of statistical significance can be 
applied to the collective results of the four studies identified by Dr. 
Silverman plus the two more recent studies with quantitative exposure 
assessments. As before, under the null hypothesis of no underlying 
effect, the probability would equal one-half that any one of these six 
studies would turn out positive or negative. The probability that all 
six studies would show either a positive or a negative association 
would, under the null hypothesis, be (0.5) 6 = 0.0156, or 
1.56 percent. This shows that the collective results of these six 
studies, showing an elevated risk of lung cancer for workers estimated 
to have the greatest cumulative exposure, are statistically significant 
at a confidence level exceeding 96 percent (i.e., 100-2 x 1.56).
    As explained in the previous subsection, three studies showing 
evidence of increased risk with increasing exposure based on 
quantitative or semi-quantitative exposure assessments are included in 
MSHA's selection of best available epidemiologic evidence: Johnston et 
al. (1997), Steenland et al. (1998), and Saverin et al. (1999). Not 
only do these studies provide consistent evidence of elevated lung 
cancer risk for exposed workers, they also each provide evidence of a 
positive exposure-response relationship--thereby significantly 
strengthening the case for causality.
(d) Studies Involving Miners
    Eleven studies involving miners are summarized and discussed in 
Section 2.c.i(2)(a) of this risk assessment. Commenters' observations 
and criticisms pertaining to the individual studies in this group are 
also addressed in that section. Three of these studies are among the 
eight in MSHA's selection of best available epidemiologic evidence: 
(Boffetta et al., 1988; Johnston et al., 1997; Saverin et al., 1999). 
All three of these studies provide evidence of an increased risk of 
lung cancer for exposed miners. Although MSHA places less weight on the 
remaining eight studies, seven of them show some evidence of an excess 
lung cancer risk among the miners involved. The remaining study 
(Christie et al., 1995) reported a greater all-cause SMR for the coal 
miners involved than for a comparable population of petroleum workers 
but did not compare the miners to a comparable group of workers with 
respect to lung cancer.
    The NMA submitted a review of six of these studies by Dr. Peter 
Valberg, who concluded that ``[t]hese articles do not implicate diesel 
exhaust, per se, as strongly associated with lung cancer in miners * * 
* The reviewed studies do not form a consistent and cohesive picture 
implicating diesel exhaust as a major risk factor for miners.'' 
Similarly, Dr. Jonathan Borak reviewed six of the studies on behalf of 
MARG and concluded:

    [T]he strongest conclusion that can be drawn from these six 
studies is that the miners in those studies had an increased risk of 
lung cancer. These studies cannot relate such increased [risk] to 
any particular industrial exposure, lifestyle or combination of such 
factors.

Apparently, neither Dr. Valberg nor Dr. Borak disputed MSHA's 
observation that the miners involved in the studies they reviewed 
exhibited, overall, an excess risk of lung cancer. It is possible that 
any excess risk found in epidemiologic studies may be due to extraneous 
unknown or uncontrolled risk factors (i.e., confounding variables). 
However, neither Drs. Valberg or Borak, nor the NMA or MARG, offered 
evidence, beyond a catalog of speculative possibilities, that the 
excess lung cancer risk for these miners was due to anything other than 
dpm exposure.
    Nevertheless, MSHA agrees that the studies reviewed by Drs. Valberg 
and Borak do not, by themselves, conclusively implicate dpm exposure as 
the causal agent. Miners are frequently exposed to other occupational 
hazards associated with lung cancer, such as radon progeny, and it is 
not always possible to distinguish effects due to dpm exposure from 
effects due to these other occupational hazards. This is part of the 
reason why MSHA did not restrict its consideration of evidence to 
epidemiologic studies involving miners. What implicates exposure to 
diesel exhaust is the fact that diesel-exposed workers in a variety of 
different occupations, under a variety of different working conditions 
(including different types of mines), and in a variety of different 
geographical areas consistently exhibit an increased risk of lung 
cancer.
    Drs. Valberg and Borak did not review the two studies that utilize 
quantitative dpm exposure assessments: Johnston et al. (1997) and 
Saverin et al. (1999). In recently received comments Dr. Valberg, 
writing for the NMA brought up four issues on the Saverin et al. 1999. 
These issues were potential exposure misclassification, potential flaws 
in the sampling method, potential smoker

[[Page 5831]]

misclassification, and insufficient latency. Two of these issues have 
already been extensively discussed in section 2.c.i.2.a.ii and 
therefore will not be repeated here. Dr. Valberg suggested that the 
potential flaw in the sampling method would tend to over-estimate 
exposure and that there was insufficient latency. If, in fact, both of 
these issues are relevant, they would act to UNDERESTIMATE the lung 
cancer risk in this cohort instead of OVERESTIMATE it. MSHA regards 
these, along with Boffetta et al. (1988), Burns and Swanson (1991),\67\ 
and Lerchen et al. (1987) to be the most informative of the available 
studies involving miners. Results on miners from these five studies are 
briefly summarized in the following table, with additional details 
provided in Section 2.c.1(2)(a) and Tables III-4 and III-5 of this risk 
assessment. The cumulative exposures at which relative risks from the 
Johnston and Saverin studies are presented are equivalent, assuming 
that TC constitutes 80 percent of total dpm. The cumulative dpm 
exposure of 6.1 mg-yr/m 3 is the multiplicative product of 
exposure duration and dpm concentration for the most highly exposed 
workers in each of these two studies.
---------------------------------------------------------------------------

    \67\ Listed in Table III-5 under Swanson et al., 1993.

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    Although MSHA places less weight on the studies by Burns and 
Swanson and by Lerchen than on the other three, it is significant that 
the five best available studies involving miners all support an 
increased risk of lung cancer attributable to dpm exposure.
(2) Meta-Analyses
    MSHA recognizes that simply tabulating epidemiologic studies as 
positive or negative can sometimes be misleading. There are generally a 
variety of outcomes that could render a study positive or negative, 
some studies contain different analyses of related data sets, some 
studies involve multiple comparisons of various subgroups, and the 
studies differ widely in the reliability of their results. Therefore, 
MSHA is not limiting its assessment of the epidemiologic evidence to 
such a tabulation or relying only on the sign test described above. 
MSHA has also considered the results of two statistical meta-analyses 
covering most of the available studies (Lipsett and Campleman, 1999; 
Bhatia et al., 1998). These meta-analyses weighted and pooled 
independent results from those studies meeting certain inclusion 
requirements to form overall estimates of relative risk for exposed 
workers based on the combined body of data. In addition to forming 
pooled estimates of the effect of diesel exposure, both meta-analyses 
analyzed sources of heterogeneity in the individual results and 
investigated but rejected publication bias as an explanation for the 
generally positive results reported. Both meta-analyses derived a 
statistically significant increase of 30 to 40 percent in the risk of 
lung cancer, attributable to occupational dpm exposure.
    Lipsett and Campleman (1999) systematically analyzed and combined 
results from most of the studies summarized in Tables III-4 and III-5. 
Forty-seven studies published between 1957 and 1995 were identified for 
initial consideration. Some studies were excluded from the pooled 
analysis because they did not allow for a period of at least 10 years 
for the development of clinically detectable lung cancer. Others were 
excluded because of bias resulting from incomplete ascertainment of 
lung cancer cases in cohort studies or because they examined the same 
cohort population as another study. One study was excluded because 
standard errors could not be calculated from the data presented. The 
remaining 30 studies, contributing a total of 39 separate estimates of 
exposure effect (for distinct occupational groups within studies), were 
analyzed using a random-effects analysis of variance (ANOVA) model.
    Potential effects of publication bias (i.e., the likelihood that 
papers with positive results may be more likely to be published than 
those with negative results) were investigated by plotting the 
logarithm of relative risk estimated from each study against its 
estimated precision, as expressed by the inverse of its standard error. 
According to the authors, the resulting ``funnel plot'' was generally 
consistent with the absence of significant publication bias, although 
there were relatively few small-scale, statistically insignificant 
studies. The investigators performed a further check of potential 
publication bias by comparing results of the included studies with the 
only relevant unpublished report that became available to them during 
the course of their analysis. Smoking-adjusted relative risks for 
several diesel-exposed occupations in the unpublished study were, 
according to the investigators, consistent with those found in the 
studies included in the meta-analysis.
    Each of the 39 separate estimates of exposure effect was weighted 
by a factor proportional to its estimated precision. Sources of 
heterogeneity in results were investigated by subset analysis--using 
categorical variables to characterize each study's design, target 
population (general or industry-specific), occupational group, source 
of control or reference population, latency, duration of exposure, 
method of ascertaining occupation, location (North America or Europe), 
covariate adjustments (age, smoking, and/or asbestos exposure), and 
absence or presence of a clear healthy worker effect (as manifested by 
lower than expected all-cause mortality in the occupational population 
under study).
    Sensitivity analyses were conducted to evaluate the sensitivity of 
results to inclusion criteria and to various assumptions used in the 
analysis. This included (1) substitution of excluded ``redundant'' 
studies of the same cohort population for the included studies and (2) 
exclusion of studies involving questionable exposure to dpm. An 
influence analysis was also conducted to examine the effect of dropping 
one study at a time, to determine if any individual study had a 
disproportionate effect on results of the ANOVA.
    The pooled relative risk from all 39 exposure effects (estimated 
from 30 studies) was RR = 1.33, with a 95-percent confidence interval 
(CI) extending from 1.21 to 1.46. For the subgroup of 13 smoking-
adjusted exposure effects (nine studies) from populations ``most likely 
to have had substantial exposure'' to dpm, the pooled effect was RR = 
1.47, with a CI from 1.29 to 1.67. Based on the all of the various 
analyses they conducted, the authors concluded:

    Although substantial heterogeneity existed in the initial pooled 
analysis, stratification on several factors substantially reduced 
heterogeneity, producing subsets of studies with increased relative 
risk estimates that persisted through various influence and 
sensitivity analyses. * * *
    In studies that adjusted for confounding by cigarette smoking, 
not only did the positive association between diesel exhaust 
exposure and lung cancer persist but the pooled risk estimate showed 
a modest increase, with little evidence of heterogeneity.
    * * * [T]his meta-analysis provides quantitative evidence 
consistent with several prior reviews, which have concluded that the 
epidemiologic evidence supports a causal relationship between 
occupational exposure to diesel exhaust and lung cancer. [Lipsett 
and Campleman, 1999]

    The other meta-analysis was conducted by Bhatia et al. (1998) on 
epidemiologic studies published in peer-reviewed journals between 1957 
and 1993. In this analysis, studies were excluded if actual work with 
diesel equipment ``could not be confirmed or reliably inferred'' or if 
an inadequate latency period was allowed for cancer to develop, as 
indicated by less than 10 years from time of first exposure to end of 
follow-up. Studies of miners were also excluded, because of potential 
exposure to radon and silica. Likewise, studies were excluded if they 
exhibited selection bias or examined the same cohort population as a 
study published later. A total of 29 independent results on exposure 
effects from 23 published studies were identified as meeting the 
inclusion criteria.
    To address potential publication bias, the investigators identified 
several unpublished studies on truck drivers and noted that elevated 
risks for exposed workers observed in these studies were similar to 
those in the published studies utilized. Based on this and a ``funnel 
plot'' for the included studies, the authors concluded that there was 
no indication of publication bias.
    After assigning each of the 29 separate estimates of exposure 
effect a weight proportional to its estimated precision, Bhatia et al. 
(1998) used a fixed-effects ANOVA model to calculate pooled relative 
risks based on the following groupings: all 29 results; all case-
control studies; all cohort studies; cohort studies using internal 
reference populations; cohort studies making external comparisons; 
studies adjusted for smoking; studies not adjusted for smoking; and 
studies grouped by occupation (railroad workers,

[[Page 5834]]

equipment operators, truck drivers, and bus workers). Elevated risks of 
lung cancer were shown for exposed workers overall and within every 
individual group of studies analyzed. A positive duration-response 
relationship was observed in those studies presenting results according 
to employment duration. The weighted, pooled estimates of relative risk 
were identical for case-control and cohort studies and nearly identical 
for studies with or without smoking adjustments.
    The pooled relative risk from all 29 exposure effects (estimated 
from 23 studies) was RR = 1.33, with a 95-percent confidence interval 
(CI), adjusted for heterogeneity, extending from 1.24 to 1.44. For just 
the smoking-adjusted studies, it was 1.35 (CI: 1.20 to 1.52); and for 
cohort studies making internal comparisons, it was 1.43 (CI: 1.29 to 
1.58). Based on their evaluation of the all the analyses on various 
subgroups, Bhatia et al. (1998) concluded that the elevated risk of 
lung cancer observed among exposed workers was unlikely to be due to 
chance, that confounding from smoking was unlikely to explain all of 
the excess risk, and that ``this meta-analysis supports a causal 
association between increased risks for lung cancer and exposure to 
diesel exhaust.''
    The pooled relative risks estimated in both meta-analyses equal 
1.33 and exceed 1.4 for studies making internal comparisons, or 
comparisons to similar groups of workers. Both meta-analyses found 
these results to be statistically significant, meaning that they cannot 
be explained merely by random or unexplained variability in the risk of 
lung cancer that occurs among both exposed and unexposed workers. 
Although both meta-analyses relied, by necessity, on an overlapping 
selection of studies, the inclusion criteria were different and some 
studies included in one meta-analysis were excluded from the other. 
They used different statistical models for deriving a pooled estimate 
of relative risk, as well as different means of analyzing heterogeneity 
of effects. Nevertheless, they derived the same estimate of the overall 
exposure effect and found similar sources of heterogeneity in the 
results from individual studies.\68\ One commenter observed that--
---------------------------------------------------------------------------

    \68\ Several commenters suggested that because the two meta-
analyses both received direct or indirect funding from the same 
governmental agency, they were not independently conducted. These 
commenters speculated that Dr. Allan Smith, a co-author of Cal-EPA 
(1998) and Bhatia et al. (1998), contributed to both meta-analyses. 
Although an earlier version of Lipsett and Campleman (1999) appeared 
as an appendix to Cal-EPA (1998), commenters provided no evidence 
that Dr. Smith contributed anything to that appendix. Dr. Smith is 
not listed as a co-author of Lipsett and Campleman (1999).

    Lung cancer relative risks for occupational ``control groups'' 
vary over a range from 0.4 to 2.7 * * *. Therefore, the level of 
relative risks being reported in the dpm epidemiology fall within 
---------------------------------------------------------------------------
this level of natural variation. [IMC Global]

This argument is refuted by the statistical significance of the 
elevation in risk detected in both meta-analyses in combination with 
the analyses accounting for heterogeneity of exposure effects.
    The EMA objected that MSHA's focus on these two meta-analyses 
``presents an incomplete picture because the counter-arguments of 
Silverman (1998) were not discussed in the same detail.'' IMC global 
also faulted MSHA for dismissing Dr. Silverman's views without adequate 
explanation.
    In her review,\69\ Dr. Silverman characterized Bhatia et al. (1998) 
as a ``careful meta-analysis'' and acknowledged that it ``add[s] to the 
credibility that diesel exhaust is carcinogenic * * *.'' She also 
explicitly endorsed several of its most important conclusions. For 
example, Dr. Silverman stated that ``[t]he authors convincingly show 
that potential confounding by cigarette smoking is likely to have 
little impact on the estimated RRs for diesel exhaust and lung 
cancer.'' She suggested, however, that Bhatia et al. (1998) 
``ultimately do not resolve the question of causality.'' (Silverman, 
1998)
---------------------------------------------------------------------------

    \69\ Silverman (1998) reviewed Bhatia et al. (1998) but not 
Lipsett and Campleman (1999) or the earlier version of that meta-
analysis (Lipsett and Alexeeff, 1998) cited in MSHA's proposed 
preamble.
---------------------------------------------------------------------------

    Dr. Silverman imposed an extremely high standard for what is needed 
to ultimately resolve the question of causality. The precise question 
she posed, along with her answer, was as follows:

    Has science proven causality beyond any reasonable doubt? 
Probably not. [Silverman, 1998, emphasis added.]

Neither the Mine Act nor applicable case law requires MSHA to prove 
causality ``beyond any reasonable doubt.'' The burden of proof that Dr. 
Silverman would require to close the case and terminate research is not 
the same burden of proof that the Mine Act requires to warrant 
protection of miners subjected to far higher levels of a probable 
carcinogen than any other occupational group. In this risk assessment, 
MSHA is evaluating the collective weight of the best available 
evidence--not seeking proof ``beyond any reasonable doubt.'' \70\
---------------------------------------------------------------------------

    \70\ It is noteworthy that, in describing research underway that 
might resolve the issue of causality, Dr. Silverman stressed the 
need for studies with quantitative exposure measurements and stated 
that ``underground miners may, in fact, be the most attractive group 
for study because their exposure to diesel exhaust is at least five 
times greater than that of previously studied occupational groups.'' 
(Silverman, 1998) She then mentioned a study on underground miners 
in Germany that had recently been initiated. The study of German 
underground potash miners (Saverin et al., 1999), published after 
Dr. Silverman's article, utilizes quantitative exposure measurements 
and is included in MSHA's selection of best available epidemiologic 
evidence (see Section 3.a.iii(1)(a) of this risk assessment). MSHA 
also includes in that selection another underground miner study 
utilizing quantitative exposure measurements (Johnston et al., 
1997). The 1997 study was available prior to Dr. Silverman's article 
but is not listed among her references.
---------------------------------------------------------------------------

    The EMA objected to MSHA's reliance on the two meta-analyses 
because of ``* * * serious deficiencies in each'' but did not, in 
MSHA's opinion, identify any such deficiencies. The EMA pointed out 
that ``most of the original studies in each were the same, and the few 
that were not common to each were not of significance to the outcome of 
either meta-analysis.'' MSHA does not regard this as a deficiency. 
Since the object of both meta-analyses was to analyze the available 
epidemiologic evidence linking dpm exposure with lung cancer, using 
defensible inclusion criteria, it is quite understandable that they 
would rely on overlapping information. The principal differences were 
in the types and methods of statistical analysis used, rather than in 
the data subjected to analysis; and MSHA considers it informative that 
different approaches yielded very similar results and conclusions. It 
is noteworthy, moreover, that both of the meta-analyses explicitly 
addressed the EMA's concern by performing analyses on various different 
sub-groupings of the available studies. The sensitivity of results to 
the inclusion criteria was also explicitly investigated and considered. 
MSHA believes that the conclusions of these meta-analyses did not 
depend on unreasonable inclusion or exclusion criteria.
    The EMA also argued that--

    [a] meta-analysis cannot compensate for basic deficiencies in 
the studies used to create the meta-analysis, and this fact is not 
clearly stated by MSHA. Instead, MSHA follows the tack of the meta-
analysis authors, who claim that the meta-analysis somehow overcomes 
deficiencies of the individual studies selected and presents a 
stronger case. This is simply not true. [EMA]

MSHA agrees that a meta-analysis cannot correct for all deficiencies 
that may be present in individual studies. It

[[Page 5835]]

can, however, correct for certain types of deficiencies. For example, 
individual studies may lack statistical power because of small study 
populations. By pooling results from several such studies, a meta-
analysis may achieve a level of statistical significance not attainable 
by the individual studies. Furthermore, both of the meta-analyses used 
well-defined inclusion criteria to screen out those studies with the 
most severe deficiencies. In addition, they both found that it was the 
more rigorous and technically more valid studies that reported the 
strongest associations between excess lung cancer and dpm exposure. 
They also performed separate analyses that ruled out inflationary 
effects of such ``deficiencies'' as lack of a smoking adjustment. For 
example, Lipsett and Campleman (1999) reported a pooled RR = 1.43 for 
20 smoking-adjusted results, as compared to a pooled RR = 1.25 for 19 
results with no smoking adjustment.
    IMC Global and MARG submitted five specific criticisms of the meta-
analyses, to which MSHA will respond in turn.

(1) Publication Bias

    * * * both studies * * * rely only on published studies. * * * 
the authors rely on statistical analysis in an attempt to uncover 
possible publication bias. * * * the only safeguard to protect 
against possible publication bias is to seek out unpublished results 
* * *. [IMC Global]

    Both meta-analyses compared the results of published and 
unpublished studies and found them to be similar. Bhatia et al. (1998) 
found several unpublished studies of lung cancer among truck drivers 
that ``* * * were not included in our analysis; however the risk ratios 
of these studies are similar to the [sic] those in published studies 
among truck drivers.'' (Bhatia et al., p. 90) Lipsett and Campleman 
(1999) checked ``[s]moking-adjusted relative risks for several diesel-
exposed occupations'' in an unpublished report on U.S. veterans and 
found them ``* * * consistent with those reported here.'' They remarked 
that ``although publication bias cannot be completely ruled out, it is 
an unlikely explanation for our findings.'' (Lipsett and Campleman, p. 
1015) In addition to comparing results directly against unpublished 
studies, both meta-analyses used the statistical method of ``funnel 
plots'' as an indirect means of checking for the existence of 
significant publication bias. It should also be noted that MSHA did not 
exclude unpublished studies from this risk assessment.

(2) Selection Bias

    * * * [the] meta-analyses have to provide a much more convincing 
rationale as to why all miners were excluded even when the 
confounders that are mentioned are not likely or important, for 
example in studies conducted in potash and salt mines. * * * IMC 
Global sees no reason why the older studies of potash workers 
[Waxweiler et al., 1973] and more recent studies on New South Wales 
coal miners [Christie et al., 1995] should not be included * * *. 
[IMC Global]
    Studies were selectively included or excluded, without good or 
sufficient explanation. [MARG]

    Contrary to the commenters' characterization, both meta-analyses 
listed each study excluded from the analysis of pooled relative risk 
and gave a good reason for its exclusion. For example, both meta-
analyses excluded studies that failed to allow for a minimum 10-year 
latency period for lung cancer to develop after first exposure. With 
respect to the exclusion of all studies on miners, Bhatia et al. (1998) 
pointed out that ``[s]ince studies of miners often indicate higher 
relative risks for lung cancer than those considered in this meta-
analysis, this was a conservative exclusion.'' Even if studies on 
miners had been considered, Waxweiler et al. (1973) and Christie et al. 
(1995) would have been excluded from both meta-analyses because of 
their failure to meet the 10-year minimum latency requirement.

(3) Lack of Actual Exposure Data

    * * * [N]ondifferential exposure or disease misclassification 
can sometimes produce bias away from the null * * * Thus, tests for 
heterogeneity performed in both these meta-analyses won't detect or 
correct this problem. [IMC Global]

    Lipsett and Campleman acknowledged that ``[e]xposure 
misclassification is a problem common to all studies of cancer and 
diesel emissions. In no case were there direct measurements of 
historical diesel exhaust exposures of the subjects.'' However, as Dr. 
Silverman pointed out in her review, ``* * * this bias is most likely 
to be nondifferential, and the effect would probably have been to bias 
point estimates toward the null value. Thus the summary RR of 1.33 may 
be an underestimate of the true lung cancer effect associated with 
diesel exposure.'' (Silverman, 1998)

(4) Smoking as a Confounder

    * * * The use of data manipulation and modeling adjustments in 
both these meta-analyses cannot rectify the flaws in the initial 
studies. [IMC Global]
    * * * misclassification of this exposure [cigarette smoking] 
could result in residual confounding of individual studies and, 
consequently, meta-analyses, of those studies. [MARG]

    Contrary to the commenter's suggestion, neither of the meta-
analyses made any attempt to manipulate or adjust the data in order to 
rectify what the commenter regards as ``flaws'' in the way smoking or 
other potential confounders were treated in the initial studies. Both 
meta-analyses, however, compared the pooled RR for studies with a 
smoking adjustment to the pooled RR for studies without any such 
adjustment. Both meta-analysis calculated a pooled RR for the smoking-
adjusted studies greater than or equal to that for the unadjusted 
studies. In addition, Bhatia et al. (1998) analyzed the impact of the 
smoking adjustment for the subgroup of studies reporting results both 
with and without such an adjustment and found that the ``small 
reduction in the pooled RR estimates would not be consistent with a 
major effect from residual confounding.'' Dr. Silverman concluded that 
``[t]he authors convincingly show that potential confounding by 
cigarette smoking is likely to have little impact on the estimated RRs 
for diesel exhaust and lung cancer.'' (Silverman, 1998)

(5) Inadequate Control in the Underlying Studies for Diet

    As noted by Lipsett and Campleman, ``Diet may also confound the 
diesel-lung cancer association.'' The researchers also caution that 
this risk factor was not controlled for in the nearly 50 diesel 
studies they examined. [MARG]

    Since inhalation is the primary route of dpm exposure, and the lung 
is the primary target organ, MSHA considers potential dietary 
confounding to be of minor importance in the diesel-lung cancer 
association. Lipsett and Campleman acknowledged that diet might be a 
relevant consideration for long-haul truck drivers, but stated that 
``diet would probably not be an important confounder in studies of 
other occupations, particularly those using internal or other 
occupationally active reference populations.'' Studies making internal 
comparisons, or comparisons to similar groups of workers, are unlikely 
to be seriously confounded by dietary differences, because the groups 
of workers being compared are likely to have very similar dietary 
habits, on average. The pooled relative risk for cohort studies making 
comparisons internally or to other active workers was 1.48 (95% CI = 
1.28 to 1.70). (Lipsett and Campleman, 1999, Table 3) This was 
considerably higher than the pooled RRs for studies making comparisons 
against regional or national populations, where dietary differences

[[Page 5836]]

(and also differences with respect to other potential confounders) 
would be more important.
(3) Potential Systematic Biases
    Citing failure to account for dietary differences as an example, 
some commenters argued that the meta-analyses may simply propagate 
weaknesses shared by the individual studies. These commenters contended 
that many of the studies MSHA considered in this risk assessment share 
methodological similarities and that, therefore, a ``deficiency'' 
causing bias in one study would probably also bias many other studies 
in the same direction. According to these commenters, no matter how 
great a majority of studies report a 30- to 40-percent increase in the 
risk of lung cancer for exposed workers, the possibility of systematic 
bias prevents the collective evidence from being strong or sufficient.
    Although this point has some theoretical foundation, it has no 
basis in fact for the particular body of epidemiologic evidence 
relating lung cancer to diesel exposure. The studies considered were 
carried out by many different researchers, in different countries, 
using different methods, and involving a variety of different 
occupations. Elevated risk was found in cohort as well as case-control 
studies, and in studies explicitly adjusting for potential confounders 
as well as studies relying on internal comparisons within homogeneous 
populations. The possibility that systematic bias explains these 
results is also rendered less plausible by results from studies of a 
radically different type: the elevated risk of lung cancer associated 
with chronic environmental exposures to PM2.5 (Dockery et 
al. 1993; Pope et al., 1995).
    Furthermore, the commenters advancing this argument presented no 
evidence that the studies shared any deficiencies of a type that would 
systematically shift results in the direction of showing a spurious 
association. As explained in Subsection 2.c.i(2)(a), exposure 
misclassification, healthy worker effect, and low power due to 
insufficient latency generally have the opposite effect--systematically 
diluting and masking results. Although many studies may share a similar 
susceptibility to bias by dietary differences or residual smoking 
effects,\71\ there is no reason to expect that such effects will 
consistently bias results in the same direction, across all occupations 
and geographic regions.
---------------------------------------------------------------------------

    \71\ The term ``residual smoking effects'' refers to the 
potentially confounding effects of smoking that may remain after a 
smoking adjustment has been made.
---------------------------------------------------------------------------

    Associations between dpm exposure and excess lung cancer are 
evident in a wide variety of occupational and geographical contexts, 
and it is unlikely that all (or most) would be biased in the same 
direction by lifestyle effects. There is no reason to suppose that, in 
nearly all of these studies, exposed subjects were more likely than 
unexposed subjects to have lifestyles (apart from their occupations) 
that increased their risk of lung cancer. On the other hand, exposures 
to other occupational carcinogens, such as asbestos dust, radon 
progeny, and silica, could systematically cause studies in which they 
are not taken into account to exhibit spurious associations between 
lung cancer and occupational diesel exhaust exposures. Silica dust and 
radon progeny are frequently present in mining environments (though not 
usually in potash mines), and this was the reason that studies on 
miners were excluded from the two meta-analyses.
    IMC Global argued that because of the possibility of being misled 
by systematic biases, epidemiologic evidence can be used to identify 
only those hazards that, at a minimum, double the risk of disease 
(i.e., RR  2.0). IMC Global explained this viewpoint by 
quoting an epidemiologist as follows:

    * * * [E]pidemiologic methods can only yield valid documentation 
of large relative risks. Relative risks of low magnitude (say, less 
than 2) are virtually beyond the resolving power of the 
epidemiologic microscope. We can seldom demonstrably eliminate all 
sources of bias, and we can never exclude the possibility of 
unidentified and uncontrolled confounding. If many studies--
preferably based on different methods--are nevertheless congruent in 
producing markedly elevated relative risks, we can set our 
misgivings aside. If however, many studies produce only modest 
increases, those increases may well be due to the same biases in all 
the studies. [Dr. Samuel Shapiro, quoted by IMC Global]

    It is important to note that, unlike IMC Global, Dr. Shapiro did 
not suggest that results of RR  2.0 be counted as ``negative.'' He 
contended only that low RRs do not completely rule out the possibility 
of a spurious association due to unidentified or uncontrolled 
confounding. More importantly, however, this restriction would allow 
workers to be exposed to significant risks and is, therefore, 
unacceptable for regulatory purposes. For purposes of protecting miners 
from lung cancer, certainty is not required; and an increase in the 
relative risk of less than 100 percent can increase the absolute risk 
of lung cancer by a clearly unacceptable amount. For example, if the 
baseline risk of lung cancer is six per thousand, then increasing it by 
33 percent amounts to an increase of two per thousand for exposed 
workers.
    IMC Global went on to argue that--

    * * * only a few of these studies have relative risks that 
exceed 2.0, and some of the studies that do exceed 2.0 exhibit 
biases that make them unsuitable for rulemaking purposes in our 
opinion. * * * Thus, in IMC Global's opinion, the epidemiologic 
evidence demonstrates an artificial association that can be 
explained through common biases probably due to smoking habits and 
lifestyle factors. [IMC Global]

    This line of reasoning leaps from the possibility that systematic 
biases might account for observed results to a conclusion that they 
actually do so. Furthermore, after proposing to allow for possible 
biases by requiring that only relative risks in excess of 2.0 be 
counted as positive evidence, IMC global has ignored its own criterion 
and discounted results greater than 2.0 for the same reason. Contrary 
to IMC Global's claim that ``only a few of the studies have relative 
risks that exceed 2.0,'' Tables III-4 and III-5 show 23 separate 
results greater than 2.0, applying to independent categories of workers 
in 18 different studies.
    According to Stober and Abel (1996), the potential confounding 
effects of smoking are so strong that ``residual smoking effects'' 
could explain even statistically significant results observed in 
studies where smoking was explicitly taken into account. MSHA agrees 
that variable exposures to non-diesel lung carcinogens, including 
relatively small errors in smoking classification, could bias 
individual studies. However, the potential confounding effect of 
tobacco smoke and other carcinogens can cut in either direction. 
Spurious positive associations of dpm exposure with lung cancer would 
arise only if the group exposed to dpm had a greater exposure to these 
confounders than the unexposed control group used for comparison. If, 
on the contrary, the control group happened to be more exposed to 
confounders, then this would tend to make the association between dpm 
exposure and lung cancer appear negative. Therefore, although smoking 
effects could potentially distort the results of any single study, this 
effect could reasonably be expected to make only about half the studies 
that were explicitly adjusted for smoking come out positive. Smoking is 
unlikely to have been responsible for finding an excess prevalence of 
lung cancer in 17 out of 18 studies in which a smoking adjustment was 
applied. Based on a 2-tailed sign test, this possibility can be

[[Page 5837]]

rejected at a confidence level greater than 99.9 percent.
    Even in the 29 studies for which no smoking adjustment was made, 
tobacco smoke and other carcinogens were important confounders only to 
the extent that the populations exposed and unexposed to diesel exhaust 
differed systematically with respect to these other exposures. Twenty-
four of these studies, however, reported some degree of excess lung 
cancer risk for the diesel-exposed workers. This result could be 
attributed to other occupational carcinogens only in the unlikely event 
that, in nearly all of these studies, diesel-exposed workers happened 
to be more highly exposed to these other carcinogens than the control 
groups of workers unexposed to diesel.
    Like IMC Global, Stober and Abel (1996) do not, in MSHA's opinion, 
adequately distinguish between a possible bias and an actual one. 
Potential biases due to extraneous risk factors are unlikely to account 
for a significant part of the excess risk in all studies showing an 
association. Excess rates of lung cancer were associated with dpm 
exposure in all epidemiologic studies of sufficient size and scope to 
detect such an excess. Although it is possible, in any individual 
study, that the potentially confounding effects of differential 
exposure to tobacco smoke or other carcinogens could account for the 
observed elevation in risk otherwise attributable to diesel exposure, 
it is unlikely that such effects would give rise to positive 
associations in 41 out of 47 studies. As stated by Cohen and Higgins 
(1995):

    * * * elevations [of lung cancer] do not appear to be fully 
explicable by confounding due to cigarette smoking or other sources 
of bias. Therefore, at present, exposure to diesel exhaust provides 
the most reasonable explanation for these elevations. The 
association is most apparent in studies of occupational cohorts, in 
which assessment of exposure is better and more detailed analyses 
have been performed. The largest relative risks are often seen in 
the categories of most probable, most intense, or longest duration 
of exposure. In general population studies, in which exposure 
prevalence is low and misclassification of exposure poses a 
particularly serious potential bias in the direction of observing no 
effect of exposure, most studies indicate increased risk, albeit 
with considerable imprecision. [Cohen and Higgins (1995), p. 269].

    Several commenters identified publication bias as another possible 
explanation for the heavy preponderance of studies showing an elevated 
risk of lung cancer for exposed workers. As described earlier, both of 
the available meta-analyses investigated and rejected the hypothesis of 
significant publication bias affecting the overall results. This was 
based on both a statistical technique using ``funnel plots'' and a 
direct comparison between results of published and unpublished studies. 
Commenters presented no evidence that publication bias actually exists 
in this case. After the 1988 NIOSH and 1989 IARC determinations that 
diesel exhaust was a ``potential'' or ``probable'' human carcinogen, 
negative results would have been of considerable interest, and, in the 
absence of any evidence specifically applying to dpm studies, there is 
no reason to assume they would not have been published.

(4) Causality

    MSHA must draw its conclusions based on the weight of evidence. In 
the absence of any statistical evidence for differential confounding or 
significant publication bias, the weight of epidemiologic evidence 
strongly favors a causal connection. On the one side, it is evident 
that virtually all of the studies that adjusted for smoking and other 
known confounders, or controlled for them by comparing against similar 
groups of workers, showed positive associations (i.e., relative risk or 
odds ratio > 1.0). Also on this side of the balance are all eight of 
the studies MSHA identified as comprising the best available human 
evidence. These include three studies reporting positive exposure-
response relationships based on quantitative dpm exposure assessments: 
two recent studies specifically on underground miners (one coal and one 
potash) and one on trucking industry workers.\72\ On the other side of 
the balance is the possibility that publication bias or other 
systematic biases may have been responsible for some unknown portion of 
the overall 30- to 40-percent elevation in lung cancer risk observed--a 
possibility that, while conceivable, is based on speculation. After 
considering other viewpoints (addressed here and in the next 
subsection), MSHA has accepted what in its view is the far more likely 
alternative: that the vast majority of epidemiologic studies showed an 
elevated risk in association with occupational exposures to diesel 
exhaust because such exposures cause the risk of lung cancer to 
increase. The toxicity experiments discussed in Subsection 2.d.iv of 
this risk assessment support the causal interpretation that MSHA has 
placed on the associations observed in epidemiologic studies.
---------------------------------------------------------------------------

    \72\ These studies (respectively: Johnston et al., 1997; Saverin 
et al., 1999; Steenland et al., 1998) are discussed in detail in 
Subsection 2.c.i(2)(a) of this risk assessment.
---------------------------------------------------------------------------

    In this risk assessment, MSHA is basing its conclusions primarily 
on epidemiologic studies. However, the results obtained from animal 
studies confirm that diesel exhaust can increase the risk of lung 
cancer in some species and help show that dpm (rather than the gaseous 
fraction of diesel exhaust) is the causal agent. The fact that dpm has 
been proven to cause lung cancer in laboratory rats only under 
conditions of lung overload does not make the rat studies irrelevant to 
miners. The very high dpm concentrations currently observed in some 
mines could impair or even overwhelm lung clearance for miners already 
burdened by respirable mineral dusts, thereby inducing lung cancer by a 
mechanism similar to what occurs in rats (Nauss et al., 1995). It must 
also be noted, however, that most of the human studies show an 
increased risk of lung cancer at dpm levels lower than what might be 
expected to cause overload. Therefore, the human studies suggest that 
overload is not a necessary condition for dpm to induce or promote lung 
cancer among humans. Salvi et al. (1999) reported marked inflammatory 
responses in the airways of healthy human volunteers after just one 
hour of exposure to dpm at a concentration of 300 g/m\3\. 
Animal studies provide evidence that inhalation of dpm has related 
effects, such as induction of free oxygen radicals, that could promote 
the development of human lung cancers by mechanisms not requiring lung 
overload. (See Sec. III.2.d.iv(2).)
    Similarly, the weight of genotoxicity evidence helps support a 
causal interpretation of the associations observed in the epidemiologic 
studies. This evidence shows that dpm dispersed by alveolar surfactant 
can have mutagenic effects, thereby providing a genotoxic route to 
carcinogenesis that is independent of overloading the lung with 
particles. After a comprehensive review of the evidence, IPCS (1996) 
concluded that both the particle core and the associated organic 
materials have biological activity. The biological availability of 
carcinogens present in the organic portion of dpm may, however, differ 
significantly in different species. Chemical byproducts of 
phagocytosis, which occurs even when the lung is not overloaded, may 
provide another genotoxic route. Inhalation of diesel emissions has 
been shown to cause DNA adduct formation in peripheral lung cells of 
rats and monkeys, and increased levels of human DNA adducts have been 
found in association with occupational exposures. (See Sec. 
III.2.d.iv(1)) None of this evidence

[[Page 5838]]

suggests that a lung cancer threshold exists for humans exposed to dpm, 
despite its importance in the rat model. Nor does this evidence suggest 
that lung overload is necessary for dpm to induce lung cancer in 
humans. Indeed, lung overload may be only one of many mechanisms 
through which lung cancer is produced in humans.
    Results from the epidemiologic studies, the animal studies, and the 
genotoxicity studies are coherent and mutually supportive. After 
considering all these results, MSHA has concluded that the 
epidemiologic studies, supported by the experimental data establishing 
the plausibility of a causal connection, provide strong evidence that 
chronic occupational dpm exposure increases the risk of lung cancer in 
humans.
    In a review, submitted by MARG, of MSHA's proposed risk assessment, 
Dr. Jonathan Borak asserted that MSHA's determination that results from 
the epidemiologic and toxicity studies were ``coherent and mutually 
reinforcing'' involved circular reasoning. He supported this assertion 
by incorrectly attributing to MSHA the view that ``most of the 
individual [epidemiologic] studies are not very good'' and that their 
suggestion of an association between dpm and lung cancer is ``made 
credible in light of the animal data.'' To complete his argument that 
MSHA relied on circular reasoning, Dr. Borak then suggested that the 
epidemiologic data provided MSHA's sole basis for considering the 
animal data relevant to humans. In a similar vein, Kennecott Minerals 
claimed there was an ``absence of toxicological support for 
epidemiologic findings that are themselves inconclusive.''
    Contrary to Dr. Borak's assertion, MSHA has not characterized most 
of the epidemiologic studies as ``not very good.'' Nor has MSHA 
suggested that the epidemiologic evidence would not be credible or 
plausible in the absence of supporting animal data. As Dr. Borak 
correctly noted, MSHA acknowledged that ``none of the existing human 
studies is perfect'' and that ``no single one of the existing 
epidemiological studies, viewed in isolation, provides conclusive 
evidence of a causal connection * * *.'' That a study is not 
``perfect,'' however, does not imply that it is ``not very good.'' 
MSHA's position has consistently been that, as demonstrated by the two 
available meta-analyses, the collective epidemiologic evidence is not 
merely credible but statistically significant and indicative of a 
causal association. Although MSHA views the toxicity data as supporting 
and reinforcing the epidemiologic evidence, MSHA believes that the 
collective epidemiologic evidence is highly credible in its own right.
    Furthermore, MSHA does not consider the animal data relevant to 
humans simply because of the positive epidemiologic evidence. The 
animal evidence is also credible in its own right. As MSHA has 
repeatedly pointed out, dust concentrations in some mines have been 
measured at levels of the same order of magnitude as those found to 
have caused lung cancer in rats. Such high exposures, especially when 
combined with occupational exposures to respirable mineral dusts and 
exposures to particles in tobacco smoke, could overload the human lung 
and promote lung cancer by a mechanism similar to that hypothesized for 
rats. (Hattis and Silver, 1992, Figures 9, 10, 11). Also, many of the 
animal experiments have elucidated genotoxic effects that, while 
apparently not responsible for the excess lung cancers observed for 
rats, may be responsible for some or all of the excess risk reported 
for humans.
    MSHA has not relied on circular reasoning. If either the animal 
data or the toxicity data had failed to show any link between dpm and 
effects implicated in the induction or promotion of lung cancer, then 
MSHA's conclusion would have been weakened. The existence of 
experimental evidence confirming that there is such a link is not 
imaginary and is logically independent of the epidemiologic evidence. 
Therefore, contrary to Dr. Borak's characterization, the ``coherency 
and reinforcement'' arising from the epidemiologic, animal, and 
genotoxicity data are not the product of circular reasoning. A more apt 
description is that the three sources of evidence, like three legs of a 
tripod, support the same conclusion.
    Many commenters argued that a causal connection between dpm 
exposure and an increased human risk of lung cancer should not be 
inferred unless there is epidemiologic evidence showing a positive 
exposure-response relationship based on quantitative measures of 
cumulative dpm exposure. MSHA does not agree that a quantitative 
exposure-response relationship is essential in establishing causality. 
Such a relationship is only one of several factors, such as consistency 
and biological plausibility, that epidemiologists examine to provide 
evidence of causality. As mentioned earlier, however, there are three 
studies providing quantitative exposure-response relationships. One of 
these studies (Steenland et al., 1998) controlled for age, race, 
smoking, diet, and asbestos exposure, but relied on ``broad 
assumptions'' to estimate historical exposure levels from later 
measurements. Two of the studies, however, (Johnston et al., 1997, and 
Saverin et al., 1999) utilized measurements that were either 
contemporaneous with the exposures (Johnston) or that were made under 
conditions very similar to those under which the exposures took place 
(Saverin). Both of these studies were conducted on underground miners. 
The Saverin study used exposure measurements of total carbon (TC). All 
three of the studies combined exposure measurements for each job with 
detailed occupational histories to form estimates of cumulative dpm 
exposure; and all three reported evidence of increasing lung cancer 
risk with increasing cumulative exposure.
    Several commenters, expressing and endorsing the views of Dr. Peter 
Valberg, incorrectly asserted that the epidemiologic results obtained 
across different occupational categories were inconsistent with a 
biologically plausible exposure-response relationship. For example, 
MARG argued that--

    It is biologically implausible that, if dpm were (causally) 
increasing lung cancer risk by 50% for a low exposure (say, truck 
drivers), then the lung cancer risk produced by dpm exposure in more 
heavily exposed worker populations (railroad shop workers) would 
fall in this same range of added risk. The added lung-cancer risk 
for bus garage workers is half that of either railroad workers or 
truck drivers, but dpm concentrations are considerably higher. 
[MARG]

Earlier, MARG had argued to the contrary that, due to their lack of 
concurrent exposure measurements, these studies could not reliably be 
used for hazard identification. MARG then attempted to use them to 
perform the rather more difficult task of making quantitative 
comparisons of relative risk. If cumulative exposures are unknown, as 
MARG argued elsewhere, then there is little basis for comparing 
responses at different cumulative exposures.
    In an analysis submitted by the West Virginia Coal Association, Dr. 
Valberg extended this argument to miners as follows:

    * * * If dpm concentrations for truck drivers is in the range of 
5-50 g/m3, then we can assign the 0.49 excess 
risk (Bhatia's meta-analysis result) to the 5-50 g/
m3 exposure. Hence, dpm concentrations for miners in the 
range of 100-2,000 g/m3 should have yielded 
excess risks forty times larger, meaning that the RR for exposed 
miners would be expected to be about 21 (i.e., 1 + 19.6), whereas 
reported risk estimates are less than 3 (range from 0.74

[[Page 5839]]

2.67). Such an utter lack of concordance argues against a causal 
role for dpm in the reported epidemiologic associations.

    Based on a similar line of reasoning, IMC Global asserted that ``* 
* * the assumptions that MSHA used to develop [Figure III-4] * * * do 
not do make sense in the context of a dose-response relationship 
between lung cancer and dpm exposure.'' This was one of the reasons IMC 
Global gave for objecting to MSHA's comparison (in Section III.1.d) of 
exposure levels measured for miners to those reported for different 
occupations. IMC Global proposed that, as a consequence of this 
argument, MSHA should delete this comparison from its risk assessment.
    MSHA sees three major flaws in Dr. Valberg's argument and rejects 
it for the following reasons:
    (1) The argument glosses over the important distinction between 
exposure concentrations (intensity) and cumulative exposure (dose). 
Total cumulative exposure is the product of intensity and duration of 
exposure. Depending on duration, high intensity exposure may result in 
similar (or even lower) cumulative exposure than low intensity 
exposure. Furthermore, different industries, in different nations, 
introduced diesel equipment at different times. The studies being 
considered were carried out in a variety of different countries and 
covered a variety of different historical periods. Therefore, the same 
number of years in different studies can correspond to very different 
durations of occupational exposure.
    Many of the miners in the studies Dr. Valberg considered may have 
been occupationally exposed to dpm for relatively short periods of time 
or even not at all. Various forms of exposure misclassification would 
tend to obscure any exposure-response relationship across industries. 
Such obscuring would result from both exposure misclassification within 
individual studies and also variability in the degree of exposure 
misclassification in different industries.
    Furthermore, the exposure levels or intensities assigned to the 
various occupations would not necessarily be proportional to cumulative 
exposures, even if the average number of years of exposure were the 
same. Different job conditions, such as longer-than-average work hours, 
could have major, variable impacts on cumulative exposures. For 
example, lower dpm concentrations have been measured for truck drivers 
than for other occupationally-exposed workers. But as a group, the 
truck drivers who were studied, due to their work conditions, may have 
been in their trucks for longer than the standard 40-hour work week and 
therefore have larger cumulative dpm exposures. These truck drivers 
commonly congregated in parking areas and slept in their trucks with 
the engines idling, thereby disproportionately increasing their 
cumulative dpm exposures compared to miners and other types of workers.
    (2) The commenters advancing this argument assumed that an 
exposure-response relationship spanning occupations at different levels 
of exposure intensity would take the form of a straight line. This 
assumption is unwarranted, since carcinogens do not necessarily follow 
such a simple pattern across a broad range of exposure levels. There is 
little basis for assuming that the relationship between cumulative dpm 
exposures and the relative risk of lung cancer would appear as a 
straight line when plotted against exposure levels that may differ by a 
factor of 100. Steenland et al. (1998) reported a better statistical 
``fit'' to the data using a model based on the logarithm of cumulative 
exposure as compared to simple cumulative exposure. Even across the 
relatively limited range of exposures within the trucking industry, the 
logarithmic exposure model exhibits pronounced curvature towards the 
horizontal at the higher cumulative exposures (Steenland et al., 1998, 
Fig. 5). If this model is extrapolated out to the much higher exposures 
currently found in underground mining, then (as shown in Subsection 
3.b.ii(3)(b) of this risk assessment) it diverges even more from a 
straight-line model. Toxicological evidence of curvature in the dose-
response relationship has also been reported (Ichinose et al., 1997b, 
p. 190).
    Furthermore, the exposure-response pattern may depend on other 
aspects of exposure, besides how much is accumulated. For example, the 
National Research Council (NRC) has adopted a risk model for radon-
induced lung cancer in which the relative risk (RR) at any age depends 
on both accumulated exposure and the rate (reflecting the intensity of 
exposure) at which total exposure was accumulated. In this model, which 
was derived empirically from the epidemiologic data, exposures 
accumulated over long time periods at relatively low rates result in a 
greater risk of lung cancer than the same total exposures accumulated 
over shorter time periods at relatively higher rates (NRC, 1999). A 
similar effect for dpm could cause apparent anomalies in the pattern of 
relative risks observed for occupations ranked simply with respect to 
the intensity of their average exposures.
    (3) Mean exposures and relative risks reported for miners involved 
in the available studies were mischaracterized. Although dpm levels as 
high as 2000 g/m3 have been measured in some mines, 
the levels at most mines surveyed by MSHA were substantially lower (see 
Figures III-1 and III-2). The average levels MSHA measured at 
underground mines were 808 g/m3 and 644 g/
m3 for M/NM and coal mines using diesel equipment for face 
haulage, respectively (Table III-1). However, these were not 
necessarily the levels experienced by miners involved in the available 
studies. The mean TC exposure concentration reported by Saverin et al. 
(1999), for work locations having the highest mean concentration, was 
390 g/m3--corresponding to a mean dpm concentration 
of about 490 g/m3. In the only other study 
involving miners for which exposure measurements were available, 
Johnston et al. (1997) reported dpm concentrations for the most highly 
exposed category of workers (locomotive drivers), ranging from 44 
g/m3 to 370 g/m3. Therefore, 
the mean dpm concentration experienced by the most highly exposed 
miners involved in these two studies was not ``forty times larger'' 
than the level imputed to truck drivers, but closer to seven times 
larger.\73\ Applying Dr. Valberg's procedure, this yields an 
``expected'' relative risk of about 4.4 for the underground miners who 
happened to work at mines included in these particular studies (1 + 
7 x (0.49)). Miners exposed at higher levels would, of course, face a 
greater risk.
---------------------------------------------------------------------------

    \73\ The estimate of seven times larger dpm exposure in miners 
is the result of averaging data from Saverin et al. (1999) with data 
from Johnston et al. (1997) and comparing the combined average miner 
dpm exposure to the average truck driver dpm exposure.
---------------------------------------------------------------------------

    Dr. Valberg asserted that the highest relative risk reported for 
miners was 2.67 (from Boffetta et al., 1988). Dr. Valberg failed to 
note, however, that the upper 95-percent confidence limit for miners' 
relative risk in this study was 4.37, so that this result hardly 
qualifies as an ``utter lack of concordance'' with the 4.4 ``expected'' 
value for miners. Furthermore, even higher relative risks for miners 
have been reported in other studies. Burns and Swanson (1991) reported 
5.0 for operators of mining machinery, with an upper 95-percent 
confidence limit of 16.9. The relative risk estimated for the most 
highly exposed miners in the study by Johnston et al. (1997) was either 
5.5 or 11.0, depending on the statistical model used. These results 
appear to be quite consistent with the data for truck drivers.

[[Page 5840]]

(5) Other Interpretations of the Evidence

    After reviewing the same body of scientific evidence as MSHA, Dr. 
Peter Valberg came to a very different conclusion with respect to the 
likelihood of causality:

    Flawed methodology (lack of adequate control for smoking); 
values for relative risks (``RR'') that are low and often not 
statistically elevated above 1.0; inadequate treatment of sources of 
variability; reliance on multiple comparisons; and inadequate 
control over how authors choose to define dpm exposure surrogates 
(that is, job category within a profession, cumulative years of 
work, age at time of exposure, etc.), all undermine the assignment 
of causality to dpm exposure.

    On the other hand, many scientific organizations and governmental 
agencies have reviewed the available epidemiologic and toxicological 
evidence for carcinogenicity and, in accordance with MSHA's conclusion, 
identified dpm as a probable human carcinogen--at levels far lower than 
those measured in some mines--or placed it in a comparable category. 
These include:

YEAR

    2000  National Toxicology Program (NTP);
    1999  (tentative) U.S. Environmental Protection Agency (EPA)
    1998  (tentative) (American Conference of Governmental 
Industrial Hygienists (ACGIH); Currently on Y2K NIC list. Probable 
vote in 10/2000.
    1998  California Environmental Protection Agency (Cal-EPA);
    1998  Federal Republic of Germany;
    1996  International Programme on Chemical Safety (IPCS), a joint 
venture of the World Health Organization, the International Labour 
Organization, and the United Nations Environment Programme;
    1989  International Agency for Research on Cancer (IARC);
    1988  National Institute for Occupational Safety and Health 
(NIOSH).

    Nevertheless, several commenters strongly objected to MSHA's 
conclusion, claiming that the evidence was obviously inadequate and 
citing scientific authorities who, they claimed, rejected MSHA's 
inference of a causal connection. In some cases, views were 
inaccurately attributed to these authorities, and misleading quotations 
were presented out of context. For example, the Nevada Mining 
Association stated that its own review of the scientific literature led 
to--

    * * * the only reasonable conclusion possible: there is no 
scientific consensus that there is a causal link between dpm 
exposure and lung cancer. The HEI [1999 Expert Panel] report 
concludes that the causal link between diesel exhaust and lung 
cancer remains unproven, and that further study and analysis are 
clearly required. [Nevada Mining Assoc.]

Although HEI (1999) recommended further study and analysis for purposes 
of quantitative risk assessment, the report contains no findings or 
conclusions about the ``causal link.'' To the contrary, the report 
explicitly states that the panel ``* * * was not charged to evaluate 
either the broad toxicologic or epidemiologic literature concerning 
exposure to diesel exhaust and lung cancer for hazard identification 
purposes, which has been done by others.'' (HEI, 1999, p. 1) 
Furthermore, the HEI panel ``* * * recognize[d] that regulatory 
decisions need to be made in spite of the limitations and uncertainties 
of the few studies with quantitative data currently available.'' (HEI, 
1999, p. 20)
    MARG, along with the Nevada Mining Association and several other 
commenters, mischaracterized the Expert Panel's findings as extending 
beyond the subject matter of the report. This report was limited to 
evaluating the suitability of the data compiled by Garshick et al. 
(1987, 1988) and Steenland et al. (1990, 1992, 1998) for quantitative 
risk assessment. Contrary to the characterization by these commenters, 
HEI's Expert Panel explicitly stated:

    [The Panel] was not charged to evaluate the broad toxicologic or 
epidemiologic literature for hazard identification purposes, which 
has been done by others. State, national, and international agencies 
have all reviewed the broader animal and human evidence for 
carcinogenicity and, in either their draft or final reports, have 
all identified diesel exhaust as [a] probable human carcinogen or 
placed it in a comparable category.'' [HEI, 1999, p. 1]

The Panel then identified most of the organizations and governmental 
institutions listed above (HEI, 1999, p. 8).
    One commenter (MARG) also grossly misrepresented HEI (1999) as 
having stated that ``the available epidemiologic work has `study design 
flaws, including uncontrolled, confounding and lack of exposure 
measures, leading to a lack of convincing evidence.' '' (MARG post-
hearing comments) The opinion falsely attributed to HEI was taken from 
a sentence in which HEI's Diesel Epidemiology Expert Panel was 
describing opinions expressed in ``[s]ome reviews critical of these 
data.'' (HEI, 1999, p. 10) The Panel did not suggest that these 
opinions were shared by HEI or by any members of the Panel. In fact, 
the cited passage came at the end of a paragraph in which the Panel 
cited a larger number of other review articles that had ``discusse[d] 
this literature in depth'' and had expressed no such opinions. In the 
same paragraph, the Panel confirmed that ``[t]he epidemiologic studies 
generally show higher risks of lung cancer among persons occupationally 
exposed to diesel exhaust than among persons who have not been exposed, 
or who have been exposed to lower levels or for shorter periods of 
time.'' (HEI, 1999, p. 10)
    Several commenters noted that the U.S. EPA's Clean Air Scientific 
Advisory Committee (CASAC) issued a report (CASAC, 1998) critical of 
the EPA's 1998 draft Health Assessment Document for Diesel Emissions 
(EPA, 1998) and rejecting some of its conclusions. After the HEI (1999) 
Expert Panel report was published, the EPA distributed a revised draft 
of its Health Assessment Document (EPA, 1999). In the 1999 draft, the 
EPA characterized human exposures to diesel exhaust as ``highly 
likely'' to be carcinogenic to humans at ambient (i.e., environmental) 
exposure levels. After reviewing this draft, CASAC endorsed a 
conclusion that, at ambient levels, diesel exhaust is likely to be 
carcinogenic to humans. Although CASAC voted to recommend that the 
designation in the EPA document be changed from ``highly likely'' to 
``likely,'' this change was recommended specifically for ambient rather 
than occupational exposures. The CASAC report states that ``[a]lthough 
there was mixed opinion regarding the characterization of diesel 
emissions as `highly likely' to be a human carcinogen, the majority of 
the Panel did not agree that there was sufficient confidence (i.e., 
evidence) to use the descriptor `highly' in regard to environmental 
exposures.'' (CASAC, 2000, emphasis added)
    MSHA recognizes that not everyone who has reviewed the literature 
on lung cancer and diesel exposure agrees about the collective weight 
of the evidence it presents or about its implications for regulatory 
decisions. IMC Global, for example, stated:

    After independently reviewing most [of the] * * * epidemiologic 
studies, the literature reviews and the two meta-analyzes, IMC 
Global believes * * * MSHA has misrepresented the epidemiologic 
evidence in the Proposed Rule. The best conclusion that we can reach 
based on our review of this information is that different reputable 
studies reach conflicting conclusions * * *. [IMC Global]

IMC Global continued by expressing concern that MSHA had ``dismissed'' 
opposing arguments critical of the positive studies, especially 
``regarding lack of statistical significance; small magnitudes of 
relative risk * * *; and the impact of confounding factors, especially 
smoking * * *. [IMC Global]''

[[Page 5841]]

    MSHA has addressed these three issues, as they relate to the 
evaluation of individual studies, in Section 2.c.i(2)(a) of this 
preamble. The argument that confounding factors such as smoking may 
have been systematically responsible for the positive results was 
discussed above, under the heading of ``Potential Systematic Biases.'' 
Statistical significance of the collective evidence is not the same 
thing as statistical significance of individual studies. Application of 
the sign test, as described Subsection 3.a.iii(1) above, is one way 
that MSHA has addressed statistical significance of the collective 
evidence. Another approach was also described above, under the heading 
of ``Meta-Analyses.''
    IMC Global quoted Morgan et al. (1997) as concluding that 
``[a]lthough there have been a number of papers suggesting that diesel 
fumes may act as a carcinogen, the weight of the evidence is against 
this hypothesis.'' This conclusion was based largely on the authors' 
contention, shared by IMC Global, that the epidemiologic results were 
inconsistent and of insufficient strength (i.e., RR  2.0) to rule out 
spurious associations due to potential confounders. MSHA, on the other 
hand, interprets the epidemiologic studies as remarkably consistent, 
given their various limitations, and has argued that the strength of 
evidence from individual studies is less important than the strength of 
evidence from all studies combined. Dr. Debra Silverman has referred to 
the ``striking consistency'' of this evidence. (Silverman, 1998)
    Ironically, Morgan et al. point out many of the very limitations in 
individual studies that may actually explain why the studies do not 
yield entirely equivalent results. The 1997 Morgan article was written 
before the meta-analyses became available and resolved many, if not 
all, of the apparent inconsistencies in the epidemiologic results. 
Since none of the existing human studies is perfect and many contain 
important limitations, it is not surprising that reported results 
differ in magnitude and statistical significance. The meta-analyses 
described earlier showed that the more powerful and carefully designed 
studies tended to show greater degrees of association. MSHA has 
addressed the joint issues of consistency and strength of association 
above, under the heading of ``Consistency of Epidemiologic Evidence.''
    The Engine Manufacturers Association (EMA) quoted Cox (1997) as 
concluding: ``* * * there is no demonstrated biological basis for 
expecting increased risk at low to moderate levels of [diesel] 
exposure.'' (Cox, 1997, as quoted by EMA] The EMA, however, prematurely 
terminated this quotation. The quoted sentence continues: ``* * * low 
to moderate levels of exposure (those that do not lead to lasting soot 
deposits, chronic irritation, and perhaps GSH enzyme depletion in the 
lung).'' MSHA does not regard concentrations of dpm exceeding 200 
g/m\3\ as ``low to moderate,'' and the EMA presented no 
evidence that the effects Dr. Cox listed do not occur at the high 
exposure levels observed at some mines. Salvi et al. (1999) reported 
marked inflammatory responses in the airways of healthy human 
volunteers after just one hour of exposure to dpm at a concentration of 
300 g/m\3\. The deleted caveat ending the quotation is 
especially important in a mining context, since mine atmospheres 
generally contain respirable mineral dusts that may diminish clearance 
rates and contribute to meeting thresholds for chronic irritation and 
inflammation leading to oxidative damage. Based on miners' testimony at 
the public hearings and workshops, there is, in fact, reason to believe 
that exposed miners experience lasting soot deposits and chronic 
irritation as a result of their exposures.
    With respect to the epidemiologic evidence, the EMA quoted Dr. Cox 
as concluding: ``* * * among studies that demonstrate an increased 
relative risk, it appears plausible that uncontrolled biases in study 
design and data analysis methods can explain the statistical increases 
in relative risk without there being a true causal increase.'' (Cox, 
1997, quoted by EMA) Dr. Cox refers to non-causal explanations for 
positive epidemiologic results as ``threats to causal inference.'' In 
considering Dr. Cox's discussion of the evidence, it is important to 
bear in mind that his purpose was ``* * * not to establish that any (or 
all) of these threats do explain away the apparent positive 
associations between [dpm] and lung cancer risk * * * but only to point 
out that they plausibly could * * *.'' (Cox, 1997, p. 813) Dr. Cox's 
stated intent was to identify non-causal characteristics of positive 
studies that could potentially ``explain away'' the positive results. 
This is a relatively simple exercise that could misleadingly be applied 
to even the strongest of epidemiologic studies. As stated earlier, no 
epidemiologic study is perfect, and it is always possible that unknown 
or uncontrolled risk factors may have given rise to a spurious 
association. Neither the EMA nor Dr. Cox pointed out however, that 
there are characteristics common to the negative studies that plausibly 
explain why they came out negative: insufficient latency allowance, 
nondifferential exposure misclassification, inappropriate comparison 
groups (including healthy worker effect, negative confounding by 
smoking or other variables. A similar approach could also be used to 
explain why many of the positive studies did not exhibit stronger 
associations. As observed by Dr. Silverman, ``an unidentified negative 
confounder may have produced bias across studies, systematically 
diluting RRs.''

b. Significance of the Risk of Material Impairment to Miners

    The fact that there is substantial and persuasive evidence that dpm 
exposure can materially impair miner health in several ways does not 
imply that miners will necessarily suffer such impairments at a 
significant rate. This section will consider the significance of the 
risk faced by miners exposed to dpm.
i. Meaning of Significant Risk
(1) Legal Requirements
    The benzene case, cited earlier in this risk assessment, provides 
the starting point for MSHA's analysis of this issue. Soon after its 
enactment in 1970, OSHA adopted a ``consensus'' standard for exposure 
to benzene, as authorized by the OSH Act. The standard set an average 
exposure limit of 10 parts per million over an 8-hour workday. The 
consensus standard had been established over time to deal with concerns 
about poisoning from this substance (448 U.S. 607, 617). Several years 
later, NIOSH recommended that OSHA alter the standard to take into 
account evidence suggesting that benzene was also a carcinogen. (Id. at 
619 et seq.). Although the ``evidence in the administrative record of 
adverse effects of benzene exposure at 10 ppm is sketchy at best,'' 
OSHA was operating under a policy that there was no safe exposure level 
to a carcinogen. (Id., at 631). Once the evidence was adequate to reach 
a conclusion that a substance was a carcinogen, the policy required the 
agency to set the limit at the lowest level feasible for the industry. 
(Id. at 613). Accordingly, the Agency proposed lowering the permissible 
exposure limit to 1 ppm.
    The Supreme Court rejected this approach. Noting that the OSH Act 
requires ``safe or healthful employment,'' the court stated that--

    * * * `safe' is not the equivalent of `risk-free' * * * a 
workplace can hardly be considered ``unsafe'' unless it threatens 
the

[[Page 5842]]

workers with a significant risk of harm. Therefore, before he can 
promulgate any permanent health or safety standard, the Secretary is 
required to make a threshold finding that a place of employment is 
unsafe--in the sense that significant risks are present and can be 
eliminated or lessened by a change in practices. [Id., at 642, 
italics in original].

The court went on to explain that it is the Agency that determines how 
to make such a threshold finding:

    First, the requirement that a `significant' risk be identified 
is not a mathematical straitjacket. It is the Agency's 
responsibility to determine, in the first instance, what it 
considered to be a `significant' risk. Some risks are plainly 
acceptable and others are plainly unacceptable. If, for example, the 
odds are one in a billion that a person will die from cancer by 
taking a drink of chlorinated water, the risk clearly could not be 
considered significant. On the other hand, if the odds are one in a 
thousand that regular inhalation of gasoline vapors that are 2% 
benzene will be fatal, a reasonable person might well consider the 
risk significant and take appropriate steps to decrease or eliminate 
it. Although the Agency has no duty to calculate the exact 
probability of harm, it does have an obligation to find that a 
significant risk is present before it can characterize a place of 
employment as ``unsafe.'' [Id., at 655].

The court noted that the Agency's ``* * * determination that a 
particular level of risk is `significant' will be based largely on 
policy considerations.'' (Id., note 62).
    Some commenters contended that the concept of significant risk, as 
enunciated by the Supreme Court in the Benzene case, requires support 
by a quantitative dose-response relationship. For example, one 
commenter argued as follows:

    * * * OSHA had contended in * * * [the benzene] case that 
``because of the lack of data concerning the linkage between low-
level exposures and blood abnormalities, it was impossible to 
construct a dose-response curve at this time''. 448 U.S. at 632-633. 
The court rejected the Agency's attempt to support a standard based 
upon speculation that ``the benefits to be derived from lowering'' 
the permissible exposure level from 10 to 1 ppm were `likely' to be 
`appreciable'.'' 448 U.S. at 654.
    One year after the Benzene case, the Court in American Textile 
Mfr's Inst. v. Donovan, 452 U.S. 490 (1981), upheld OSHA's ``cotton 
dust'' standard for which a dose-response curve had been established 
by the Agency. The Court relied upon the existence of such data to 
find that OSHA had complied with the Benzene mandate, stating: ``In 
making its assessment of significant risk, OSHA relied on dose-
response curve data * * * It is difficult to imagine what else the 
agency could do to comply with this Court's decision in the Benzene 
case.'' Id. at 505, n. 25. See also Public Citizen Research Group v. 
Tyson, 796 F. 2d 1479, 1496, 1499 (D.C. Cir. 1986) (where a dose 
response curve was constructed for the ethylene oxide standard and 
the agency [had] gone to great lengths to calculate, within the 
bounds of available scientific data, the significance of the risk); 
United Steelworkers of America v. Marshall, 647 F. 2d 1189, 1248 
(D.C. Cir. 1980), cert. denied, 453 U.S. 913 (1981) (where in 
promulgating a new lead standard ``OSHA amassed voluminous evidence 
of the specific harmful effects of lead at particular blood levels 
and correlated these blood lead levels with air lead levels''). 
[NMA]

    A dose-response relationship has been established between exposure 
to PM2.5 (of which dpm is a major constituent) and the risk 
of death from cardiovascular, cardiopulmonary, or respiratory causes 
(Schwartz et al.,1996; EPA, 1996). Furthermore, three different 
epidemiologic studies, including two carried out specifically on mine 
workers, have reported evidence of a quantitative relationship between 
dpm exposure and the risk of lung cancer (Johnston et al., 1997, 
Steenland et al., 1998, Saverin et al., 1999). However, the Secretary 
has carefully reviewed the legal references provided by the commenters 
and finds there is no requirement in the law that the determination of 
significant risk be based on such a relationship. The cited court 
rulings appear to describe sufficient means of establishing a 
significant risk, rather than necessary ones. Indeed, as stated earlier 
in this section, the Benzene court explained that:

    * * * the requirement that a ``significant'' risk be identified 
is not a mathematical straitjacket. It is the Agency's 
responsibility to determine, in the first instance, what it 
considered to be a ``significant'' risk. * * * the Agency has no 
duty to calculate the exact probability of harm * * *.

    The Agency has set forth the evidence and rationale behind its 
decision to propose a rule restricting miner exposure to dpm, obtained 
an independent peer review of its assessment of that evidence, 
published the evidence and tentative conclusions for public comment, 
held hearings, kept the record open for further comments for months 
after the hearings, and re-opened the record so that stakeholders could 
comment on the most recent evidence available. Throughout these 
proceedings, the Agency has carefully considered all public comments 
concerning the evidence of adverse health effects resulting from 
occupational dpm exposures. Based on that extensive record, and the 
considerations noted in this section, the Agency is authorized under 
the statute and relevant precedents to act on this matter--despite the 
fact that a more conclusive or definitively established exposure-
response relationship might help address remaining doubts among some 
members of the mining community.
    As the Supreme Court pointed out in the benzene case, the 
appropriate definition of significance also depends on policy 
considerations of the Agency involved. In the case of MSHA, those 
policy considerations include special attention to the history of 
extraordinary occupational risks leading to the Mine Act. That history 
is intertwined with the toll to the mining community of silicosis and 
coal workers' pneumoconiosis (CWP or ``black lung''), along with 
billions of dollars in Federal expenditures.
(2) Standards and Guidelines for Risk Assessment
    Several commenters suggested that this risk assessment, as 
originally proposed, deviated from established risk assessment 
guidelines, because it did not provide a sufficiently quantitative 
basis for evaluating the significance of miners's risks due to their 
dpm exposures. One of these commenters (Dr. Jonathan Borak) maintained 
that a determination of significant risk based on a ``qualitative'' 
assessment ``has no statistical meaning.''
    MSHA recognizes that a risk assessment should strive to provide as 
high a degree of quantification and certainty as is possible, given the 
best available scientific evidence. However, in order to best protect 
miners' health, it is not prudent to insist on a ``perfect'' risk 
assessment. Nor is it prudent to delay assessing potentially grave 
risks simply because the available data may be insufficient for an 
ideal risk assessment. The need for regulatory agencies to act in the 
face of uncertainty was recognized by the HEI's Diesel Epidemiology 
Expert Panel as follows: ``The Panel recognizes that regulatory 
decisions need to be made in spite of the limitations and uncertainties 
of the few studies with quantitative data currently available.'' (HEI, 
1999) When there is good, qualitative evidence--such as the sight and 
smell of heavy smoke--that one's house is on fire, an inference of 
significant risk may be statistically meaningful even without 
quantitative measurements of the smoke's density and composition.
    Moreover, as will be demonstrated below, the question of whether a 
quantitative assessment is or is not essential is, in this case, moot: 
this risk assessment does, in fact, provide a quantitative evaluation 
of how significant the risk is for miners occupationally exposed to 
dpm.

[[Page 5843]]

ii. Significance of Risk for Underground Miners Exposed to dpm
    An important measure of the significance of a risk is the 
likelihood that an adverse effect actually will occur. A key factor in 
the significance of risks that dpm presents to miners is the very high 
dpm concentrations to which a number of those miners are currently 
exposed--compared to ambient atmospheric levels in even the most 
polluted urban environments, and to workers in diesel-related 
occupations for which positive epidemiologic results have been 
reported. Figure III-4 compared the range of median dpm exposure levels 
measured for mine workers at various mines to the range of medians 
estimated for other occupations, as well as to ambient environmental 
levels. Figure III-7 presents a similar comparison, based on the 
highest mean dpm level observed at any individual mine, the highest 
mean level reported for any occupational group other than mining, and 
the highest monthly mean concentration of dpm estimated for ambient air 
at any site in the Los Angeles basin.\74\ As shown in Figure III-7, 
underground miners are currently exposed at mean levels up to 10 times 
higher than the highest mean exposure reported for other occupations, 
and up to 100 times higher than the highest mean environmental level 
even after adjusting the environmental level upwards to reflect an 
equivalent occupational exposure.
---------------------------------------------------------------------------

    \74\ For comparability with occupational lifetime exposure 
levels, the environmental ambient air concentration has been 
multiplied by a factor of approximately 4.7. This factor reflects a 
45-year occupational lifetime with 240 working days per year, as 
opposed to a 70-year environmental lifetime with 365-days per year, 
and assumes that air inhaled during a work shift comprises half the 
total air inhaled during a 24-hour day.
---------------------------------------------------------------------------

    Given the significant increases in mortality and other acute health 
effects associated with increments of 25 g/m3 in 
fine particulate concentration (see Table III-3), the relative risk of 
acute effects for some miners (especially those already suffering 
respiratory problems) appears to be extremely high. Acute responses to 
dpm exposures have been detected in studies of stevedores, whose 
exposures were likely to have been less than one tenth the exposure of 
some miners on the job. Likewise, the risk of lung cancer due to dpm 
exposure would appear to be far greater for those underground miners 
who are exposed at such high levels than for other workers or general 
urban populations.

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[[Page 5845]]

    Several commenters asserted that current dpm exposures in 
underground mines are lower than they were when MSHA conducted its 
field surveys and that MSHA had not taken this into account when 
assessing the significance of dpm risk to miners. A related comment was 
that MSHA had not designed its sampling studies to provide a 
statistically representative cross section of the entire industry but 
had nevertheless used the results in concluding that the risk to 
underground miners was significant.
    In accordance with Sec. 101.(a)(6) of the Mine Act, MSHA is basing 
this risk assessment on the best available evidence. None of the 
commenters provided evidence that dpm levels in underground metal/
nonmetal mines had declined significantly since MSHA's field studies, 
or provided quantitative estimates of any purported decline in average 
dpm concentrations, or submitted data that would better represent the 
range of dpm concentrations to which underground miners are typically 
exposed at the present time. Although MSHA's field studies were not 
designed to be statistically representative in a way that can be 
readily quantified, they were performed at locations selected, 
according to MSHA's best engineering judgement, to be typical of the 
type of diesel equipment used. Furthermore, as will be shown below, 
MSHA's evaluation of the significance of risks presented to underground 
metal/nonmetal miners by their dpm exposures does not rely on the 
highest levels, or even the average levels, that MSHA has measured. As 
documented in Section 1.d of this risk assessment, some of the highest 
of MSHA's measurements were made as recently as 1996-1997. It is 
important to note, as is shown below, the cancer risks of dpm exposure 
are clearly significant even at a concentration of 300 g/
m3--less than half of the average level that MSHA observed 
in its field studies. Therefore, MSHA believes that a reduction in 
exposure of more than 50 percent in the last couple of years is highly 
implausible.

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[[Page 5846]]

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BILLING CODE 4510-43-C

[[Page 5847]]

    Earlier in this risk assessment, MSHA identified three types of 
material impairment that can result from occupational exposures to dpm. 
The next three subsections present the Agency's evaluation of how much 
of a risk there is that miners occupationally exposed to dpm will 
actually incur such consequences. Each part addresses the risk of 
incurring one of the three types of material impairment identified 
earlier.
(1) Sensory Irritations and Respiratory Symptoms (Including Allergenic 
Responses)
    It is evident from the direct testimony of numerous miners working 
near diesel equipment that their exposures pose a significant risk of 
severe sensory irritations and respiratory symptoms. This was 
underscored during the workshops and public hearings by several miners 
who noted that such effects occurred immediately and consistently after 
episodes of intense exposure (Section 2.b.i). There is also persuasive 
experimental evidence that exposure at levels found in underground 
mines frequently cause eye and nose irritation (Rudell et al., 1996) 
and pulmonary inflammation (Salvi et al., 1999). Section 2.a.ii and 
3.a.i of this risk assessment explain why these effects constitute 
``material impairments'' under the Mine Act and why they threaten 
miners' safety as well as health. Therefore, it is clear that even 
short-term exposures to excessive concentrations of dpm pose 
significant risks.
    MSHA's quantitative evaluation of how significant the risks of 
sensory irritations and respiratory symptoms are for miners is limited, 
by the quantitative evidence available, to acute respiratory symptoms 
linked to fine particulate exposures (PM2.5) in ambient air 
pollution studies. MSHA recognizes that, for miners exposed to dpm, 
this type of risk cannot be quantified with great confidence or 
precision based on the available evidence. This is because 
PM2.5 is not solely comprised of dpm and also because 
miners, as a group, have different demographic and health 
characteristics from the general populations involved in the relevant 
studies. However, MSHA believes that the quantitative evidence suffices 
to establish a lower bound on the significance of this type of risk to 
miners exposed to dpm. Even at this lower bound, which is likely to 
substantially underestimate the degree of risk, the probability that a 
miner's occupational exposure to dpm will cause adverse respiratory 
effects is clearly significant.
    As shown in Table III-3, the risk of acute lower respiratory tract 
symptoms has been reported to increase, at a 95-percent confidence 
level, by 15 to 82 percent (RR = 1.15 to 1.82) for each incremental 
increase of 20 g/m\3\ in the concentration of PM2.5 
in the ambient air. This means that the relative risk estimated for a 
given PM2.5 concentration ranges between (1.15)\k\ and 
(1.82)\k\, where k = the concentration of PM2.5 divided by 
20 g/m\3\. For example, for a PM2.5 concentration 
of 40 g/m\3\, the RR is estimated to be between (1.15)\2\ and 
(1.82)\2\, or 1.32 to 3.31. MSHA believes that part of the reason why 
the range is so wide is that the composition of PM2.5 varied 
in the data from which the estimates were derived.
    MSHA acknowledges that there are substantial uncertainties involved 
in converting 24-hour environmental exposures to 8-hour occupational 
exposures. However, since mining often involves vigorous physical 
activity (thereby increasing breathing depth and frequency) and sleep 
is characterized by reduced respiration, it is highly likely that 
miners would inhale at least one-third of their total 24-hour intake of 
air during a standard 8-hour work shift. If it is assumed that the 
acute respiratory effects of inhaling dpm at a concentration of 60 
g/m\3\ over an 8-hour workshift are at least as great as those 
at a concentration of 20 g/m\3\ over a 24-hour period, then it 
is possible to estimate a lower bound on the relative risk of such 
effects.
    Based solely on the fact that dpm consists almost entirely of 
particles much smaller than 2.5 micrometers in diameter, the dpm would 
be expected to penetrate the lower respiratory tract at least as 
effectively as PM2.5. Also, given the complex chemical 
composition of dpm, and its generation within a confined space, there 
is no reason to suspect that dpm in an underground mining environment 
is less potent than ambient PM2.5 in inducing respiratory 
symptoms. Under these assumptions, a short-term environmental exposure 
to PM2.5 at a concentration of 20 g/m\3\ would 
correspond to a short-term occupational exposure to dpm at a 
concentration of 60 g/m\3\. Consequently, the RR at an 
occupational exposure level of Y g/m\3\ would equal the RR 
calculated for an ambient exposure level of 20 x (Y/60) g/
m\3\. For example, the relative risk (RR) of acute lower respiratory 
symptoms at an occupational exposure level of 300 g/m\3\ dpm 
would, at a minimum, correspond to the RR at an ambient exposure level 
equal to 5 x 20 g/m\3\ PM2.5. (See Table III-3) A 
dpm concentration of 300 g/m\3\ happens to be the level at 
which Salvi et al. (1999) found a marked pulmonary inflammatory 
response in healthy human volunteers after just one hour of exposure.
    Under these assumptions, the risk of lower respiratory tract 
symptoms for a miner exposed to dpm for a full shift at a concentration 
of 300 g/m\3\ or more, would be at least twice the risk of 
ambient exposure (i.e., RR = (1.15)\5\ = 2.01). This would imply that 
for miners exposed to dpm at or above this level, the risk of acute 
lower respiratory symptoms would double, at a minimum. The Secretary 
considers such an increase in risk to be clearly significant.
(2) Premature Death From Cardiovascular, Cardiopulmonary, or 
Respiratory Causes
    As in the case of respiratory symptoms, the nature of the best 
available evidence limits MSHA's quantitative evaluation of how large 
an excess risk of premature death, due to causes other than lung 
cancer, there is for miners exposed to dpm. As before, this evidence 
consists of acute effects linked to fine particulate exposures 
(PM2.5) in ambient air pollution studies. Therefore, the 
analysis is subject to similar uncertainties. However, also as before, 
MSHA believes that the quantitative evidence suffices to place a lower 
bound on the increase in risk of premature mortality for miners 
occupationally exposed to dpm. As will be shown below, even this lower 
bound, which is likely to substantially underestimate the degree of 
increase, indicates that a miner's occupational exposure to dpm has a 
clearly significant impact on the likelihood of premature death.
    Schwartz et al. (1996) found an average increase of 1.5 percent in 
daily mortality associated with each increment of 10 g/m\3\ in 
the daily concentration of fine particulates. Higher increases were 
estimated specifically for ischemic heart disease (IHD: 2.1 percent), 
chronic obstructive pulmonary disease (COPD: 3.3 percent), and 
pneumonia (4.0 percent). The corresponding 95-percent confidence 
intervals for the three specific estimates were, respectively, 1.4% to 
2.8%, 1.0% to 5.7%, and 1.8% to 6.2%, per increment of 10 g/
m\3\ in daily PM2.5 exposure. Within the range of dust 
concentrations studied, the response appeared to be linear, with no 
threshold. The investigators checked for but did not find any 
consistent or statistically stable relationship between increased 
mortality and the atmospheric concentration of ``course'' respirable

[[Page 5848]]

particles--i.e., those with aerodynamic diameter greater than 2.5 
micrometers but less than 10 micrometers.
    As explained earlier, it is highly likely that miners would inhale 
at least one-third of their total 24-hour intake of air during a 
standard 8-hour work shift. Therefore, under the same assumptions made 
in the previous subsection, the 24-hour average concentrations of 
PM2.5 measured by Schwartz et al. are no more potent, in 
their impact on mortality risk, than eight-hour average concentrations 
that are three times as high. As discussed in Section 2.a.iii of this 
risk assessment, underground miners may be less, equally, or more 
susceptible than the general population to the acute mortality effects 
of fine particulates such as dpm. However, miners who smoke tobacco 
and/or suffer various respiratory ailments fall into groups identified 
as likely to be especially sensitive (EPA, 1996). Consequently, for 
such miners occupationally exposed to dpm, the relative risk of each 
type of premature mortality would be at least equal to the 
corresponding lower 95-percent confidence limit specified above.
    Therefore, MSHA estimates that, on average, each increment of 30 
g/m\3\ in the dpm concentration to which miners are exposed 
increases the risk of premature death due to IHD, COPD, and pneumonia 
by a factor of at least 1.4 percent, 1.0 percent, and 1.8 percent, 
respectively. As noted earlier, these estimates are based on the 
evidence of acute effects linked to fine particulate exposures 
(PM2.5) in ambient air pollution studies. A lower bound on 
the increased risk expected at an occupational dpm concentration 
greater than 30 g/m\3\, is obtained by raising the relative 
risks equivalent to these factors (i.e., 1.014, 1.01, and 1.018) to a 
power, k, equal to the ratio of the concentration to 30 g/
m\3\. For a concentration of 300 g/m\3\, k = 10; so MSHA 
estimates the lower bounds on relative risk to be: (1.014)\10\ = 1.149 
for IHD; (1.01)\10\ = 1.105 for COPD; and (1.018)\10\ = 1.195 for 
pneumonia. This means that for miners exposed to dpm at or above this 
level, MSHA expects the risks to increase by at least 14.9 percent for 
IHD, 10.5 percent for COPD, and 19.5 percent for pneumonia. The 
Secretary considers increases of this magnitude to be clearly 
significant, since the causes of death to which they apply are not rare 
among miners.
(3) Lung Cancer
    In contrast to the two types of risk discussed above, the available 
epidemiologic data can be used to relate the risk of lung cancer 
directly to dpm exposures. Therefore, the significance of the lung 
cancer risk can be evaluated without having to make assumptions about 
the relative potency of dpm compared to the remaining constituents of 
PM2.5. This removes an important source of uncertainty 
present in the other two evaluations.
    There are two different ways in which the significance of the lung 
cancer risk may be evaluated. The first way is based on the relative 
risk of lung cancer observed in the best available epidemiologic 
studies involving miners (identified as such in Subsections 3.a.iii(1) 
(b) and (d) of this risk assessment). As will be explained below, this 
approach leads to an estimated tripling of lung cancer risk for miners 
exposed to dpm, compared to a baseline risk for unexposed miners. The 
second way is to calculate the lung cancer risk expected at exposure 
levels MSHA has observed in underground mines, assuming a specified 
occupational lifetime and using the exposure-response relationships 
estimated for underground miners by Johnston et al. (1997) and Saverin 
et al. (1999). As will be explained further below, this second approach 
yields a wide range of estimates, depending on which exposure-response 
relationship and statistical model is used. All of the estimates, 
however, show at least a doubling of baseline lung cancer risk, 
assuming dpm exposure for a 45-year occupational lifetime at the 
average concentration MSHA has observed. Most of the estimates are much 
higher than this. If the exposure-response relationship estimated for 
workers in the trucking industry by Steenland et al. (1998) is 
extrapolated to the much higher exposure levels for miners, the 
resulting estimates fall within the range established by the two mine-
specific studies, thereby providing a degree of corroboration. Since 
lung cancer is not a rare disease, the Secretary considers even the 
very lowest estimate--a doubling of baseline risk--to represent a 
clearly significant risk.
    Both of these methods provide quantitative estimates of the degree 
by which miners' risk of lung cancer is increased by their occupational 
dpm exposures. The estimate based on exposure-response relationships is 
more refined, in that it ties the increased risk of lung cancer to 
specific levels of cumulative dpm exposure. However, this added 
refinement comes at the price of an additional source of uncertainty: 
the accuracy of the exposure-response relationship used to calculate 
the estimate. This additional uncertainty is reflected, in MSHA's 
evaluation, by a broad range of relative risk estimates, corresponding 
to the range of exposure-response relationships derived using different 
statistical models and epidemiologic data. The next two subsections 
present the details of MSHA's two approaches to analyzing lung cancer 
risk for miners exposed to dpm, along with MSHA's responses to the 
relevant public comments.
(a) Risk Assessment Based on Studies Involving Miners
    As one commenter pointed out, the epidemiologic evidence showing an 
elevated risk of lung cancer for exposed workers is mostly based on 
occupations estimated to experience far lower exposure levels, on 
average, than those observed in many underground mines:

* * *[U]nderground coal, metal and non-metal miners face a 
significant risk of lung cancer from occupational exposure to diesel 
particulate. Numerous epidemiologic studies of workers exposed to 
levels far below those experienced by coal, metal and non-metal 
miners have found the risk for exposed workers to be 30-50% greater 
than for unexposed workers. [Washington State Dept. of Labor and 
Industries]

    Indeed, although MSHA recognizes that results from animal studies 
should be extrapolated to humans with caution, it is noteworthy that 
dpm exposure levels recorded in some underground mines (see Figures 
III-1 and III-2) have been well within the exposure range that produced 
tumors in rats (Nauss et al., 1995).
    Both existing meta-analyses of the human studies relating dpm 
exposure and lung cancer excluded studies on miners but presented 
evidence showing that, averaged across all other occupations, dpm 
exposure is responsible for an increase of about 40 percent in lung 
cancer risk (See Section 3.a.iii(2) of this risk assessment). Even a 
40-percent increase in the risk of lung cancer would clearly be 
significant, since this would amount to more than two cases of lung 
cancer per year per thousand miners at risk, and to an even greater 
risk for smoking miners. The best available evidence, however, 
indicates (1) that exposure levels in underground mines generally 
exceed exposures for occupations included in the meta-analyses and (2) 
that lung cancer risks for exposed miners are elevated to a greater 
extent than for other occupations.
    As Dr. Valberg and other commenters pointed out, the epidemiologic 
studies used in the meta-analyses involved much lower exposure levels 
than those depicted for mines in Figures III-1 and III-2. The studies 
supporting a 40-percent excess risk of lung cancer were

[[Page 5849]]

conducted on populations whose average exposure is estimated to be less 
than 200 g/m\3\--less than one tenth the average concentration 
MSHA observed in some underground mines. More specifically, average 
exposure levels in the two most extensively studied industries--
trucking (including loading dock workers) and railroads--have been 
reported to be far below the levels observed in underground mining 
environments. For workers at docks employing diesel forklifts--the 
occupational group estimated to be most highly exposed within the 
trucking industry--the highest average dpm concentration reported was 
about 55 g/m\3\ EC at an individual dock (NIOSH, 1990). As 
explained in Subsection 1.d of this risk assessment, this corresponds 
to less than 150 g/m\3\ of dpm, on average. Published dpm 
measurements for railworkers have generally also been less than 150 
g/m\3\ (measured as respirable particulate matter other than 
cigarette smoke). The reported mean of 224 g/m\3\ for hostlers 
displayed in Figure III-7 represents only the worst-case occupational 
subgroup (Woskie et al., 1988). In contrast, in the study on 
underground potash miners by Saverin et al. (1999), the mean TC 
concentration measured for production areas was 390 g/m\3\--
corresponding to a mean dpm concentration of about 490 g/m\3\. 
As shown in Table III-1, the mean dpm exposure level MSHA observed in 
underground production areas and haulageways was 644 g/m\3\ 
for coal mines and 808 g/m\3\ for M/NM.
    In accordance with the higher exposure levels for underground 
miners, the five studies identified in Section III.3.a.iii(1)(d) as 
comprising the best available epidemiologic evidence on miners all show 
that the risk of lung cancer increased for occupationally exposed 
miners by substantially more than 40 percent. The following table 
presents the relative risk (RR) of lung cancer for miners in these 
studies, along with the geometric mean based on all five studies:

------------------------------------------------------------------------
                                                                Relative
                                                                risk of
                            Study                                 lung
                                                                 cancer
------------------------------------------------------------------------
Boffetta et al., 1988........................................       2.67
Burns & Swanson, 1991........................................       5.03
Johnston et al., 1997 (mine-adjusted model applied at highest       5.50
 cumulative exposure)........................................
Lerchen et al., 1987.........................................       2.1
Saverin et al., 1999 (highest vs least exposed)..............       2.17
geometric mean...............................................       3.2
------------------------------------------------------------------------

    As shown in this table, the estimated RR based on these five 
studies is 3.2 for miners exposed to dpm. In other words, the risk of 
lung cancer for the highly exposed miners is estimated to be 3.2 times 
that of a comparable group of occupationally unexposed workers. The 
geometric mean RR remains 3.2 if the two studies on which MSHA places 
less weight (by Burns & Swanson and by Lerchen) are excluded from the 
calculation. This represents a 220-percent increase in the risk of lung 
cancer for exposed miners, in contrast to the 40-percent increase 
estimated, on average, for other occupationally exposed workers. The 
Secretary believes that a 40-percent increase in the risk of lung 
cancer already exceeds, by a wide margin, the threshold for a clearly 
significant risk. However, a 220-percent increase to more than three 
times the baseline rate is obviously of even greater concern.
    Some commenters questioned whether increased lung cancer risks of 
this magnitude were plausible, since they were not aware of any 
unusually high lung cancer rates among workers at mines with which they 
were familiar and which used diesel equipment. There are several 
reasons why an elevated risk of lung cancer might not currently be 
conspicuous among U.S. miners exposed to dpm. Lung cancer not only may 
require a latency period of 30 or more years to develop, but it may 
also not develop until beyond the normal retirement age of 65 years. 
Cases of lung cancer developing after retirement may not all be known 
to members of the mining community. Also, in a population that includes 
many tobacco smokers, it may be difficult to discern cases of lung 
cancer specifically attributable to dpm exposure when they first begin 
to become prevalent. Two commenters expressed some of the relevant 
considerations as follows. Although they were referring to coal miners, 
the same points apply to M/NM miners.

    Because the latency period for lung cancer is so long, and 
diesel-powered equipment has only been used extensively in U.S. coal 
mines for about 25 years, the epidemic may well be progressing 
unnoticed. [UMWA]
    If dpm exposure will cause cancer, there is a huge population of 
miners here in the West that have already been exposed. Considering 
the latency periods indicated by MSHA, these miners should be 
beginning to develop cancers. [Canyon Fuels]
(b) Risk Assessment Based on Miners' Cumulative Exposure
    Although it is evident that underground miners currently face a 
significant risk of lung cancer due to their occupational exposure to 
dpm, there are certain advantages in utilizing an exposure-response 
relationship to quantify the degree of risk at specific levels of 
cumulative exposure. As some commenters pointed out, for example, dpm 
exposure levels may change over time due to changes in diesel fuel and 
engine design. The extent and patterns of diesel equipment usage within 
mines also has changed significantly during the past 25 years, and this 
has affected dpm exposure levels as well. Furthermore, exposure levels 
at the mines involved in epidemiologic studies were not necessarily 
typical or representative of exposure levels at mines in general. A 
quantitative exposure-response relationship provides an estimate of the 
risk at any specified level of cumulative exposure. Therefore, using 
such a relationship to assess risk under current or anticipated 
conditions factors in whatever differences in exposure levels may be 
relevant, including those due to historical changes.
(i) Exposure-Response Relationships from Studies Outside Mining
    Stayner et al. (1998) summarized quantitative risk assessments 
based on exposure-response relationships for dpm published through 
1998. These assessments were broadly divided into those based on human 
studies and those based on animal studies. Depending on the particular 
studies, assumptions, statistical models, and methods of assessment 
used, estimates of the exact degree of risk varied widely even within 
each broad category. However, as presented in Tables III and IV of 
Stayner et al. (1998), all of the very different approaches and methods 
published through 1998 produced results indicating that levels of dpm 
exposure measured at some underground mines present an unacceptably 
high risk of lung cancer for miners--a risk significantly greater than 
the risk they would experience without the dpm exposure.\75\
---------------------------------------------------------------------------

    \75\ In comments submitted by MARG, Dr. Jonathan Borak asserted 
that MSHA had ``misrepresented the findings of a critical study'' by 
stating that all methods showed an ``unacceptably high risk'' at 
exposure levels found at some mines. Dr. Borak claimed that Stayner 
et al. (1998) had described an analysis by Crump et al. ``that 
reached an opposite conclusion.'' Dr. Borak failed to distinguish 
between a finding of high risk and a finding of changes in that risk 
corresponding to changes in estimated exposures. The findings to 
which Dr. Borak referred pertained only to the exposure-response 
relationship within the group of exposed workers. Garshick (1981), 
Crump (1999), and HEI (1999) all noted that the risk of lung cancer 
was nevertheless elevated among the exposed workers, compared to 
unexposed workers in the same cohort, and they all identified 
reasons why the data used in this study might fail to detect a 
positive exposure-response relationship among the exposed workers.

---------------------------------------------------------------------------

[[Page 5850]]

    Quantitative risk estimates based on the human studies were 
generally higher than those based on analyses of the rat inhalation 
studies. As indicated by Tables 3 and 4 of Stayner et al. (1998), a 
working lifetime of exposure to dpm at 500 g/m\3\ yielded 
estimates of excess lung cancer risk ranging from about 1 to 200 excess 
cases of lung cancer per thousand workers based on the rat inhalation 
studies and from about 50 to 800 per thousand based on the 
epidemiologic assessments. Stayner et al. (1998) concluded their report 
---------------------------------------------------------------------------
by stating:

    The risk estimates derived from these different models vary by 
approximately three orders of magnitude, and there are substantial 
uncertainties surrounding each of these approaches. Nonetheless, the 
results from applying these methods are consistent in predicting 
relatively large risks of lung cancer for miners who have long-term 
exposures to high concentrations of DEP [i.e., dpm]. This is not 
surprising given the fact that miners may be exposed to DEP [dpm] 
concentrations that are similar to those that induced lung cancer in 
rats and mice, and substantially higher than the exposure 
concentrations in the positive epidemiologic studies of other worker 
populations.

    Restricting attention to the exposure-response relationships 
derived from human data, Table IV of Stayner et al. (1998) presented 
estimates of excess lung cancer risk based on exposure-response 
relationships derived from four different studies: Waller (1981) as 
analyzed by Harris (1983); Garshick et al. (1987) as analyzed by Smith 
and Stayner (1991); Garshick et al. (1988) as analyzed by California 
EPA (1998); and Steenland et al. (1998). Harris (1983) represented 
upper bounds on risk; and all of the other estimates represented the 
most likely value for risk, given the particular data and statistical 
modeling assumptions on which the estimate was based. Three different 
ranges of estimates were presented from the California EPA analysis, 
corresponding to various statistical models and assumptions about 
historical changes in dpm exposure among the railroad workers involved. 
As mentioned above and in the proposed version of this risk assessment, 
the low end of the range of estimates was 50 lung cancers per 1000 
workers occupationally exposed at 500 g/m\3\ for a 45-year 
working lifetime. This estimate was one of those based on railroad 
worker data from Garshick et al. (1988).
    Several commenters objected to MSHA's reliance on any of the 
exposure-response relationships derived from the data compiled by 
Garshick et al. (1987) or Garshick et al. (1988). These objections were 
based on re-analyses of these data by Crump (1999) and HEI (1999), 
using different statistical methods and assumptions from those used by 
Cal-EPA (1998). For example, the NMA quoted HEI (1999) as concluding:

    At present, the railroad worker cohort study * * * has very 
limited utility for QRA [quantitative risk assessment] of lifetime 
lung cancer risk from exposure to ambient levels of diesel exhaust * 
* * [NMA, quoting HEI (1999)]

    From this, the NMA argued as follows:

    What then is the relevance of this data to the proceedings at 
issue? Simply put, there is no relevance. The leading epidemiologist 
[sic], including Dr. Garshick himself, now agree that the data are 
inappropriate for conducting risk assessment. [NMA]

    MSHA notes that the HEI (1999) conclusion cited by the NMA referred 
to quantitative risk assessments at ambient, not occupational, exposure 
levels. Also, HEI (1999) did not apply its approach (i.e., 
investigating the correlation between exposure and relative risk within 
separate job categories) to the Armitage-Doll model employed by Cal-EPA 
in some of its analyses. (Results using this model were among those 
summarized in Table IV of Stayner et al., 1998). Therefore, the 
statistical findings on which HEI (1999) based its conclusion do not 
apply to exposure-response relationships estimated using the Armitage-
Doll model. Furthermore, although HEI concluded that the railroad 
worker data have ``very limited utility for QRA * * * at ambient 
levels'' [emphasis added], this does not mean, even if true, that these 
data have ``no relevance'' to this risk assessment, as the NMA 
asserted. Even if they do not reliably establish an exposure-response 
relationship suitable for use in a quantitative risk assessment, these 
data still show that the risk of lung cancer was significantly elevated 
among exposed workers. This is the only way in which MSHA is now using 
these data in this risk assessment.
    In the proposed risk assessment, MSHA did not rely directly on the 
railroad worker data but did refer to the lowest published quantitative 
estimate of risk, which happened, as of 1998, to be based on those 
data. MSHA's reasoning was that, even based on the lowest published 
estimate, the excess risk of lung cancer attributable to dpm exposure 
was clearly sufficient to warrant regulation. If risk assessments 
derived from the railroad worker data are eliminated from 
consideration, the lowest estimate remaining in Table IV of Stayner et 
al. (1998) is obviously even higher than the one that MSHA used to make 
this determination in the proposed risk assessment. This estimate 
(based on one of the analyses performed by Steenland et al., 1998) is 
89 excess cases of lung cancer per year per thousand workers exposed at 
500 g/m\3\ for a 45-year working lifetime.
    HEI (1999) also evaluated the use of the Steenland data for 
quantitative risk assessment, but did not perform any independent 
statistical analysis of the data compiled in that study. Some 
commenters pointed out HEI's reiteration of the cautionary remark by 
Steenland et al. (1998) that their exposure assessment depended on 
``broad assumptions.'' The HEI report did not rule out the use of these 
data for quantitative risk assessment but suggested that additional 
statistical analyses and evaluations were desirable, along with further 
development of exposure estimates using alternative assumptions. MSHA 
has addressed comments on various aspects of the analysis by Steenland 
et al., including the exposure assumptions, in Section 2.c.i(2)(a) of 
this risk assessment.
    One commenter noted that Steenland et al. (1998) had recognized the 
limitations of their analysis and had, therefore, advised that the 
results ``should be viewed as exploratory.'' The commenter then 
asserted that MSHA had nevertheless used these results as ``the basis 
for a major regulatory standard'' and that ``[t]his alone is sufficient 
to demonstrate that MSHA's proposal lacks the necessary scientific 
support.'' [Kennecott Minerals]
    The Secretary does not accept the premise that MSHA should exclude 
``exploratory'' results from its risk assessment, even if it is granted 
that those results depend on broad assumptions possibly requiring 
further research and validation before they are widely accepted by the 
scientific community. Steenland et al. (1998) estimated risks 
associated with specific cumulative exposures, based on estimates of 
historical exposure patterns combined with data originally described by 
Steenland et al., 1990 and 1992. Regardless of whether the cumulative 
exposure estimates used by Steenland et al. (1998) are sufficiently 
reliable to permit pinpointing the risk of lung cancer at any given 
exposure level, the quantitative analysis indicates that as cumulative 
exposure increases, so does the risk. Therefore, the 1998 analysis adds 
significantly to the weight of evidence supporting a causal

[[Page 5851]]

relationship. However, MSHA did not use or propose to use exposure-
response estimates derived by Steenland et al. (1998) as the sole basis 
for any regulatory standard.
    The exposure-response relationships presented by Steenland et al. 
were derived from exposures estimated to be far below those found in 
underground mines. As Stayner et al. (1998) point out, questions are 
introduced by extrapolating an exposure-response relationship beyond 
the exposures used to determine the relationship. The uncertainties 
implicit in such extrapolation are demonstrated by comparing results 
from two statistical models based on five-year lagged exposures--one 
using simple cumulative exposure and the other using the natural 
logarithm of cumulative exposure (Steenland et al., 1998, Table II).
    Assuming that, on average, EC comprises 40 percent of total 
dpm,\76\ the formula for calculating a relative risk (RR) using 
Steenland's simple cumulative exposure model is RR = 
exp(0.4 x 0.389 x CumExp), where CumExp is occupationally accumulated 
dpm exposure (expressed in mg-yr/m\3\), ignoring the most recent five 
years. Again assuming EC=0.4 x dpm, the corresponding formula using 
Steenland's Log(CumExp) model is: RR = 
exp(0.1803 x (Log(0.4 x 1000 x CumExp + BG)-Log(BG))), still ignoring 
occupational dpm exposure in the most recent five years.\77\
---------------------------------------------------------------------------

    \76\ The assumption is that, on average, EC = TC/2 and TC = 
0.8 x dpm.
    \77\ BG, expressed in g-yr/m\3\, accounts for an 
assumed background (i.e., non-occupational) EC exposure level of 1.0 
g/m\3\. At age 70, after a 45-year worklife and an 
additional 5-year lag after retirement, BG is assumed to equal 70 
g-yr/m\3\. ``Log'' refers to the natural logarithm, and 
``exp'' refers to the antilogarithm of the subsequent quantity.
---------------------------------------------------------------------------

    The risk estimates from these two models are similar at the 
cumulative exposure levels estimated for workers involved in the study, 
but the projected risks diverge markedly at the higher exposures 
projected for underground miners exposed to dpm for a 45-year 
occupational lifetime. For example, a cumulative dpm exposure of 2.5 
mg-yr/m\3\ (i.e., 45 years of occupational exposure at an average dpm 
concentration of about 55.6 g/m\3\) is within the range of 
cumulative exposures from which these exposure-response relationships 
were estimated. At this level of cumulative exposure, the models (both 
lagged five years) yield relative risk estimates of 1.48 (based on 
simple cumulative exposure) and 1.64 (based on the logarithm of 
cumulative exposure, with BG=70 g-yr/m\3\). On the other hand, 
45 years of occupational exposure at an average dpm concentration of 
808 g/m\3\ amounts to a cumulative dpm exposure of 36,360 
g-yr/m\3\, or about 36.4 mg-yr/m\3\. At this level, which lies 
well beyond the range of data used by Steenland et al. (1998), the 
simple and logarithmic exposure models produce relative risk estimates 
of about 300 and 2.6, respectively.
    Despite the divergence of these two models at high levels of 
cumulative exposure, they can provide a useful check of excess lung 
cancer risks estimated using exposure-response relationships developed 
from other studies. For highly exposed miners, the Steenland models 
both produce estimates of lung cancer risk within the range established 
by the two miner studies discussed below. This corroborates the upper 
and lower limits on such risk as estimated by the various statistical 
models used in those two studies.
(ii) Exposure-Response Relationships from Studies on Miners
    As described in Section 2.c.i(2)(a) of this risk assessment, two 
epidemiologic studies, both conducted on underground miners, provide 
exposure-response relationships based on fully quantitative dpm 
exposure assessments. Johnston et al. (1997) conducted their study on a 
cohort of 18,166 underground coal miners, and Saverin et al. (1999) 
conducted theirs on a cohort of 5,536 underground potash miners. Each 
of these studies developed a number of possible exposure-response 
relationships, depending on the statistical model used for analysis 
and, in the case of Saverin et al. (1999), inclusion criteria for the 
cohort analyzed. For purposes of this risk assessment, MSHA has 
converted the units of cumulative exposure in all of these exposure-
response relationships to mg-yr/m\3\.
    Two exposure-response relationships derived by Johnston et al. 
(1997) are used in this risk assessment, based on a ``mine-adjusted'' 
and a ``mine-unadjusted'' statistical model. In both of these models, 
cumulative dpm exposure is lagged by 15 years.\78\ This reflects the 
long latency period required for development of lung cancer and means 
that the most recent 15 years of exposure are ignored when the relative 
risk of lung cancer is estimated. The exposure-response relationships, 
as reported by the investigators, were expressed in terms of g-hr/m\3\ 
of cumulative dpm exposure. MSHA has converted the exposure units to 
mg-yr/m\3\ by assuming 1920 work hours per year.
---------------------------------------------------------------------------

    \78\ The 15-year lagged mine-unadjusted and mine-adjusted models 
are respectively denoted by M/03 and M/06 in Table 11.2 of Johnston 
et al. (1997). As explained earlier, the individual mines considered 
in this study differed significantly with respect to both dpm 
exposures and lung cancer experience. The investigators could not 
determine exactly how much, if any, of the increased lung cancer 
risk associated with dpm exposure depends on other, unknown factors 
differentiating the individual mines. The mine-adjusted model 
allocates a significant number of the lung cancers otherwise 
attributable to dpm exposure to the ``norm'' for specific mines. 
Therefore, if the differences in lung cancer prevalence between 
mines is actually due to corresponding differences in mean dpm 
exposure, then this model will mask a significant portion of the 
risk due to dpm exposure. After adjusting for miners' age and 
smoking habits, the mine-unadjusted model attributes differences in 
the prevalence of lung cancer between mines to corresponding 
differences in mean dpm exposure. However, the mine-adjusted model 
has the advantage of taking into account differences between mines 
with respect to potentially confounding factors, such as radon 
progeny and silica levels.
---------------------------------------------------------------------------

    Two different methods of statistical analysis were applied by 
Sauverin et al. (1999) to both the full cohort and to a subcohort of 
3,258 miners who had worked underground, in relatively stable jobs, for 
at least ten years. Thus, the investigators developed a total of four 
possible exposure-response relationships from this study. Since they 
were based on measurements of total carbon (TC), these exposure-
response relationships were expressed in terms mg-yr/m\3\ of cumulative 
TC exposure. MSHA has converted the exposure units to mg-yr/m\3\ of 
cumulative dpm exposure by assuming that, on average, TC comprises 80 
percent of total dpm.
    The following table summarizes the exposure-response relationships 
obtained from these two studies. Each of the quantitative relationships 
is specified by the unit relative risk (RR) per mg-yr/m\3\ of 
cumulative dpm exposure. To calculate the relative risk estimated for a 
given cumulative dpm exposure (CE), it is necessary to raise the unit 
RR to a power equal to CE. For example, if the unit RR is 1.11 and CE = 
20, then the estimated relative risk is (1.11)\20\ = 8.1. Therefore, 
the estimated relative risk of lung cancer increases as CE increases. 
For the two Johnston models, CE does not include exposure accumulated 
during the 15 years immediately prior to the time in a miner's life at 
which the relative risk is calculated.

[[Page 5852]]



Exposure-Response Relationships Obtained From Two Studies on Underground
                                 Miners.
------------------------------------------------------------------------
                                                             Unit RR per
                Study and statistical model                   mg-yr/m\3\
                                                                 dpm
------------------------------------------------------------------------
Saverin et al. (1999)\1\:
  Poisson, full cohort.....................................        1.024
  Cox, full cohort.........................................        1.089
  Poisson, subcohort.......................................        1.110
  Cox, subcohort...........................................        1.176
Johnston et al. (1997)\2\ :
  15-year lag, mine-adjusted...............................        1.321
  15-year lag, mine-unadjusted.............................       1.479
------------------------------------------------------------------------
\1\ Unit RR calculated from Tables III and IV, assuming TC = 0.8 x dpm.
\2\ Unit RR calculated from Table 11.2, assuming 1920 work hours per
  year.

    For example, suppose a miner is occupationally exposed to dpm at an 
average level of 500 g/m3. Then each year of 
occupational exposure would contribute 0.5 mg-yr/m3 to the 
miner's cumulative dpm exposure. Suppose also that this miner's 
occupational exposure begins at age 45 and continues for 20 years until 
retirement at age 65. Consequently, at or above age 65, this 
hypothetical miner would have accumulated a total of 10 mg-yr/
m3 of occupational dpm exposure. According to the Saverin-
Cox-subcohort model, the relative risk estimated for this miner after 
retirement is RR = (1.176)10 = 5.1. This means that, at or 
above age 65, the retired miner's risk of lung cancer is estimated (by 
this model) to be about five times that of another retired miner having 
the same age and smoking history but no occupational dpm exposure.
    Since the two Johnston models exclude exposure within the last 15 
years, it is instructive to calculate the relative risk using these 
models for the same hypothetical retiree at age 75. Since this miner 
retired at age 65, immediately after 20 years of occupational exposure, 
the cumulative exposure used in applying the Johnston models must be 
reduced by the 2.5 mg-yr/m3 accumulated from age 60 to age 
65. Therefore, according to the Johnston mine-adjusted model, the 
relative risk estimated for this retired miner at age 75 is RR = 
(1.321)7.5 = 8.1. At age 80 or above, however, this model 
predicts that the relative risk would increase to RR = 
(1.321)10 = 16.2.
    The six exposure-response relationships obtained from these two 
studies establish a range of quantitative risk estimates corresponding 
to a given level of cumulative dpm exposure. This range provides lower 
and upper limits on the risk of lung cancer for workers exposed at the 
given level, relative to similar workers who were not occupationally 
exposed. The lower limit of this range is established by Saverin's full 
cohort Poisson model. Therefore, the lowest estimate of relative risk 
after 45 years of occupational dpm exposure is RR = 
(1.024)45 x 0.644 = 2.0 at a mean concentration of 644 
g/m3 or RR = (1.024)45 x 0.808 = 2.4 at 
mean concentration of 808 g/m3. These exposure 
levels correspond to the averages presented in Table III-1 for 
underground coal and underground M/NM mines, respectively.
    A relative risk of 2.0 amounts to a doubling of the baseline lung 
cancer risk, and all of the models project relative risks of at least 
2.0 after 45 years of exposure at these levels. Therefore, MSHA expects 
that underground miners exposed to dpm at these levels for a full 45-
year occupational lifetime would, at a minimum, experience lung cancer 
at a rate twice that of unexposed but otherwise similar miners. Five of 
the six statistical models, however, predict a relative risk much 
greater than 2.0 after 45 years at a mean dpm concentration of 644 
g/m3. The second-lowest estimate of relative risk, 
for example, is RR = (1.089)45 x 0.644 = 11.8, predicted by 
Saverin's full cohort Cox model.\79\
---------------------------------------------------------------------------

    \79\ Some commenters contended that MSHA cannot establish a 
reliable exposure-response relationship because of potential 
interferences in MSHA's dpm concentration measurements. More 
specifically, some of these commenters claimed that MSHA's dpm 
measurements in underground coal mines were significantly inflated 
by submicrometer coal dust.
    As explained in Subsection 1.a of this risk assessment, the 
sampling device MSHA used to measure dpm in underground coal mines 
was designed specifically to allow for the submicrometer fraction of 
coal dust. Both the size-selective and RCD methods are reasonably 
accurate when dpm concentrations exceed 300 g/
m3. Moreover, neither of these methods was used to 
establish the exposure-response relationships presented by Saverin 
et al. (1999) or Johnston et al. (1997).
---------------------------------------------------------------------------

    In the next subsection of this risk assessment, relative risks will 
be combined with baseline lung cancer and mortality data to estimate 
the lifetime probability of dying from lung cancer due to occupational 
dpm exposure.
    (iii) Excess Risk at Specific dpm Exposure Levels. The ``excess 
risk'' discussed in this subsection refers to the lifetime probability 
of dying from lung cancer resulting from occupational exposure to dpm 
for 45 years. This probability is expressed as the expected excess 
number of lung cancer deaths per thousand miners occupationally exposed 
to dpm at a specified level. The excess is calculated relative to 
baseline, age-specific lung cancer mortality rates taken from standard 
mortality tables. In order to properly estimate this excess, it is 
necessary to calculate, at each year of life after occupational 
exposure begins, the expected number of persons surviving to that age 
with and without dpm exposure at the specified level. At each age, 
standard actuarial adjustments must be made in the number of survivors 
to account for the risk of dying from causes other than lung cancer.
    Table III-7 shows the excess risk of death from lung cancer 
estimated across the range of exposure-response relationships obtained 
from Saverin et al. (1999) and Johnston et al. (1997). Estimates based 
on the 5-year lagged models from Steenland et al. (1998) fall within 
this range and are included for comparison. Based on each of the eight 
statistical models, the excess risk was estimated at four levels of dpm 
exposure: 200 g/m3, 500 g/m3, 
644 g/m3 (the mean dpm concentration observed by 
MSHA at underground coal mines, as shown in Table III-1), and 808 
g/m3 (the mean dpm concentration observed by MSHA 
at underground M/NM mines, as shown in Table III-1).

BILLING CODE 4510-43-P

[[Page 5853]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.074


BILLING CODE 4510-43-C

[[Page 5854]]

    All of the estimates in Table III-7 assume that occupational 
exposure begins at age 20 and continues until retirement at age 65. 
Excess risks were calculated through age 85 as in Table IV of Stayner 
et al. (1998). Table III-7 differs from Table IV of Stayner et al. in 
that results from Johnston et al. and Saverin et al. are substituted 
for results based on the two studies by Garshick et al. Nevertheless, 
at 500 g/m3, the range of excess risks shown in 
Table III-7 is nearly identical to the range (50 to 810 g/
m3) presented in Table IV of Stayner et al. (1998).
    MSHA considers the exposure levels shown in Table III-1 to be 
typical of current conditions in underground coal mines using diesel 
face equipment. At the mean dpm concentration observed by MSHA at 
underground M/NM mines (808 g/m3), the eight 
estimates range from 83 to 830 excess lung cancer deaths per 1000 
affected miners. At the mean dpm concentration observed by MSHA at 
underground coal mines (644 g/m3), the estimates 
range from 61 to 811 excess lung cancer deaths per 1000 affected 
miners. MSHA recognizes that these risk estimates involved 
extrapolation beyond the exposure experience of the miner cohorts in 
Saverin et al. (1999) and Johnston et al. (1997). However, the degree 
of extrapolation was less for those two studies than the extrapolation 
that was necessary for the diesel-exposed truck drivers in Steenland et 
al. The lowest excess lung cancer risk in dpm exposed miners found in 
Table III-7 is 61/1000 per 45-year working lifetime. Based on the 
quantitative rule of thumb established in the benzene case, this 
estimate indicates a clearly significant risk of lung cancer 
attributable to dpm exposure at current levels. [Industrial Union vs. 
American Petroleum; 448 U.S. 607, 100 S.Ct. 2844 (1980)].
c. The Rule's Expected Impact on Risk
    MSHA strongly disagrees with the views of some commenters who 
asserted that the proposed rules would provide no known or quantifiable 
health benefit to mine workers. On the contrary, MSHA's assessment of 
the best available evidence indicates that reducing the very high 
exposures currently existing in underground mines will significantly 
reduce the risk of three different kinds of material impairment to 
miners: (1) Acute sensory irritations and respiratory symptoms 
(including allergenic responses); (2) premature death from 
cardiovascular, cardiopulmonary, or respiratory causes; and (3) lung 
cancer. Furthermore, as will be shown below, the reduction in lung 
cancer risk expected as a result of the rule can readily be quantified 
based on the estimates of excess risk at exposure levels given in Table 
III-7.
    Using exposure-response relationships and assumptions described in 
Subsections 3.b.ii(1) and 3.b.ii(2) of this risk assessment, MSHA 
estimated lower bounds on the significance of risks faced by miners 
occupationally exposed to dpm with respect to (1) acute sensory 
irritations and respiratory symptoms or (2) premature death from 
cardiovascular, cardiopulmonary, or respiratory causes. MSHA expects 
the rules to significantly and substantially reduce all three kinds of 
risk. However, MSHA is unable, based on currently available data, to 
quantify with confidence the reductions expected for the first two 
kinds. A 24-hour exposure at 20 g/m\3\ may not have the same 
short-term effects as an 8-hour exposure at 60 g/m\3\. 
Furthermore, this concentration is only 30 percent of the maximum dpm 
concentration that MSHA expects once the rules are fully implemented 
and represents an even smaller fraction of average dpm concentrations 
many underground miners currently experience. It is unclear whether the 
same incremental effects on acute respiratory symptoms and premature 
mortality would apply at the much higher exposure levels found in 
underground mines. Additionally, as MSHA suggested in the proposed 
preamble and several commenters repeated, the toxicity of dpm and 
PM2.5 may differ because of differences in composition. 
Finally, underground miners as a group may differ significantly from 
the populations for which the PM2.5 exposure-response 
relationships were derived.
    Therefore, MSHA's quantitative assessment of the rule's impact on 
risk is restricted to its expected impact on the third kind of risk--
the risk of lung cancer. The rule will limit dpm concentrations to 
which miners in underground M/NM mines are exposed. The rule will limit 
these dpm concentrations to approximately 200 g/m\3\ by 
limiting the measured concentration of total carbon to 160 g/
m\3\. Assuming that, in the absence of this rule, underground M/NM 
miners would be occupationally exposed to dpm for 45 years at a mean 
level of 808 g/m\3\, the following table contains the 
estimated reductions in lifetime risk expected to result from full 
implementation of the rule, based on the various exposure-response 
relationships obtained from Saverin et al. (1999) and Johnston et al. 
(1997). These estimates were obtained by calculating the difference 
between the corresponding estimates of excess lung cancer mortality, at 
808 g/m\3\ and 200 g/m\3\, shown in Table III-7. The 
Regulatory Impact Analysis (RIA), presented later in this preamble, 
contains further quantitative discussion of the benefits anticipated 
from this rule.

 Reduction in Lifetime Risk of Lung Cancer Mortality Expected as Result
 of Reducing Exposure Level From 808 g/m\3\ to 200 g/
                                  m\3\.
------------------------------------------------------------------------
                                                             Expected
                                                           reduction in
                                                            lung cancer
               Study and statistical model                  deaths per
                                                           1000 affected
                                                             miners\1\
------------------------------------------------------------------------
Saverin et al. (1999):
Poisson, full cohort....................................              68
Cox, full cohort........................................             507
Poisson, subcohort......................................             600
Cox, subcohort..........................................             620
Johnston et al. (1997):
15-year lag, mine-adjusted..............................             487
15-year lag, mine-unadjusted............................            317
------------------------------------------------------------------------
\1\ Calculated from Table III-7.

    Although the Agency expects that health risks will be substantially 
reduced by this rule, the best available evidence indicates that a 
significant risk of adverse health effects due to dpm exposures will 
remain even after the rule is fully implemented. As explained in Part V 
of this preamble, however, MSHA has concluded that, due to monetary 
costs and technological limitations, the underground M/NM mining sector 
as a whole cannot feasibly reduce dpm concentrations further at this 
time.

4. Conclusions

    MSHA has carefully considered all of the evidence and public 
comment submitted during these proceedings to determine whether dpm 
exposures, at levels observed in some mines, present miners with 
significant health risks. This information was evaluated in light of 
the legal requirements governing regulatory action under the Mine Act. 
Particular attention was paid to issues and questions raised by the 
mining community in response to the Agency's ANPRM and NPRM and during 
workshops on dpm held in 1995. Based on its review of the record as a 
whole, the agency has determined that the best available evidence 
warrants the following conclusions:
    1. Exposure to dpm can materially impair miner health or functional 
capacity. These material impairments include acute sensory irritations 
and respiratory symptoms (including allergenic responses); premature 
death

[[Page 5855]]

from cardiovascular, cardiopulmonary, or respiratory causes; and lung 
cancer.
    2. At dpm levels currently observed in underground mines, many 
miners are presently at significant risk of incurring these material 
impairments due to their occupational exposures to dpm over a working 
lifetime.
    3. By reducing dpm concentrations in underground mines, the rule 
will substantially reduce the risks of material impairment faced by 
underground miners exposed to dpm at current levels.
    In its response to MSHA's proposals, the NMA endorsed these 
conclusions to a certain extent, as follows:

    The members of NMA have come to recognize that it would be 
prudent to limit miners' exposure to the constituents of diesel 
exhaust in the underground environment. [NMA]

    A number of commenters, however, urged MSHA to defer rulemaking for 
either the coal or M/NM sector, or both, until results were available 
from the NCI/NIOSH study currently underway. For example, referring to 
the M/NM proposal, one commenter stated:

    Vulcan agrees with MSHA that underground miner dpm exposure 
needs to be addressed by mine operators. Vulcan agrees with MSHA 
that a permissible exposure level (PEL) should be established, but 
disagrees that adequate information is currently available to set a 
PEL. [Vulcan Materials]

MSHA believes that expeditious rulemaking, in both underground mining 
sectors, is necessary for the following reasons:
    (1) The NCI/NIOSH study currently in progress will eventually 
provide additional information on lung cancer mortality. Non-cancer 
health effects, such as sensory irritations, respiratory symptoms, or 
premature death from cardiovascular, cardiopulmonary, or respiratory 
causes will not be addressed. MSHA believes that these non-cancer 
effects constitute material impairments.
    (2) NIOSH itself has recommended that, ``* * * given the length of 
time to complete this study and the current state of knowledge 
regarding dpm exposures and health effects in miners,'' MSHA should 
``proceed with rulemaking based on the evidence currently available as 
presented in this FR notice.'' [NIOSH testimony by Paul Schulte, dated 
5/27/99]
    (3) Given the very high exposure levels measured at some 
underground mines, miners should not be required to serve as human 
guinea pigs in order to remove all doubts about the excess risks of dpm 
exposures in underground mines. While additional studies are in 
progress, miners should be protected by reducing dpm concentrations to 
a level more nearly commensurate with exposures in other industries.
    Referring to some commenters' position that further scientific 
study was necessary before regulatory action could be justified, a 
miner at one of the dpm workshops held in 1995 said:

    * * * if I understand the Mine Act, it requires MSHA to set the 
rules based on the best set of available evidence, not possible 
evidence * * * Is it going to take us 10 more years before we kill 
out, or are we going to do something now * * *? (dpm Workshop; 
Beckley, WV, 1995).

Similar concern with the risk of waiting for additional scientific 
evidence was expressed by another miner, who testified:

    * * * I got the indication that the diesel studies in rats could 
no way be compared to humans because their lungs are not the same * 
* * But * * * if we don't set the limits, if you remember probably 
last year when these reports come out how the government used human 
guinea pigs for radiation, shots, and all this, and aren't we doing 
the same thing by using coal miners as guinea pigs to set the value? 
(dpm Workshop; Beckley, WV, 1995).

    MSHA shares these sentiments. That is why MSHA considers it 
imperative to protect miners based on the weight of existing evidence, 
rather than to wait for the results of additional studies.

IV. Section by Section Discussion of Final Rule

    This part of the preamble describes the provisions of the final 
rule on a section-by-section basis. As appropriate, this part 
references discussions in other parts of this preamble: in particular, 
the background discussions on measurement methods and controls in part 
II, and the feasibility discussions in part V.
    The final rule would add nine new sections to 30 CFR Part 57 
immediately following Sec. 57.5015. It would not amend any existing 
sections of that part.
    Many provisions of the final rule are identical to the proposed 
rule, but some provisions have been changed. The following table 
provides a quick overview of the key changes:

------------------------------------------------------------------------
             Section                Final rule (changes from proposal)
------------------------------------------------------------------------
57.5060.........................  When specified conditions have been
                                   met and various precautions have been
                                   taken (including use of proper PPE),
                                   miners performing certain inspection,
                                   maintenance and repair activities may
                                   be granted permission from MSHA to
                                   work in certain areas where miners
                                   normally work and travel, but where
                                   the dpm concentration limit is
                                   exceeded (not authorized in proposed
                                   rule)
57.5061.........................  Compliance sampling must always be
                                   done with submicrometer impactor
                                   (unspecified in proposed rule)
57.5067.........................  Engines meeting the applicable EPA
                                   requirements as per a table provided
                                   in the rule may be introduced
                                   underground after rule's effective
                                   date (under proposal, only MSHA
                                   approved engines were so allowed)
------------------------------------------------------------------------

Section 57.5060  Limit on Concentration of Diesel Particulate Matter

    Summary. This section of the final rule limits the concentration of 
dpm in underground metal and nonmetal mines. It has six subsections.
    Subsection (a) provides that 18 months after the date of 
promulgation, dpm concentrations would be limited by restricting total 
carbon to 400 micrograms per cubic meter of air 
(400TCg/m\3\). The reason why the concentration 
limit for dpm is expressed in terms of total carbon is explained below. 
A total carbon limit of 400TCg/m\3\ is the 
equivalent of about 500 micrograms per cubic meter of air of dpm 
(500DPMg/m\3\). This limit would apply only for a 
period of 42 months; accordingly, it is sometimes referred to in this 
preamble as the ``interim'' concentration limit. The final rule is the 
same as the proposed rule in this regard.
    Subsection (b) provides that five years after the date of 
promulgation, the concentration limit would be reduced, restricting 
total carbon to 160 micrograms per cubic meter of air 
(160TCg/m\3\, or about 200DPMg/
m\3\). This is sometimes referred to in this preamble as the ``final'' 
concentration limit. The final rule is the same as the proposed rule in 
this regard.
    Subsection (c) provides for a special extension of up to two 
additional years in order for a mine to comply with the final 
concentration limit. This special extension is only available when the 
mine operator can establish that the final concentration limit cannot 
be met

[[Page 5856]]

within the five years allotted due to technological constraints. The 
final rule establishes the information that must be contained in the 
application for an extension, the procedure to follow to make 
application, and the conditions that must be observed during the 
special extension period. Subsection (c) of the final rule refers to 
this extension as ``special'' because the final rule provides all mines 
in this sector with an extension of time (five years) to meet the final 
concentration limit. The final rule is the same as the proposed rule in 
this regard.
    Subsection (d) provides that under certain conditions, a miner 
engaged in inspection, repair or maintenance activities in certain 
areas of a mine may work in concentrations of dpm in excess of the 
applicable concentration limit. Among the conditions that must be met 
in order for such work to be permitted is the use of proper personal 
protective equipment. This exception was not included in the proposed 
rule.
    Subsection (e) provides that apart from the extraordinary 
circumstances where the use of such controls may be authorized under 
subsections (c) and (d), an operator must not utilize personal 
protective equipment to comply with either the interim or final 
concentration limit. The wording in the final rule clarifies the intent 
of the proposed rule, and accommodates new subsection (d).
    Subsection (f) provides that an operator must not utilize 
administrative controls to comply with either the interim or final 
concentration limit. The proposed rule included the same requirement, 
but in the final rule this has been separated into a separate 
paragraph.
    General Comments. Some commenters questioned MSHA's rationale for 
establishing concentration limits at this time. They pointed out that a 
large scale study by NIOSH of the health risks of dpm exposure is still 
on-going. Accordingly, they accused MSHA of acting prematurely, and 
urged delaying implementation of any limits until the health risks of 
dpm exposure are fully quantified. MSHA was also challenged to justify 
the specific numerical values chosen for the limits; several commenters 
suggested that these limits are based on unsubstantiated and 
unquantified health risks, and that therefore, the levels chosen cannot 
be justified. But another commenter suggested that the health risks are 
sufficiently documented to justify even lower limits than were 
contained in the proposed rule. This commenter suggested 100 g 
and 50 g for the interim and final limits, respectively. As 
these comments involve questions about the risk to underground metal 
and nonmetal miners, they are addressed in Part III of this preamble.
    Some commenters also objected to the proposed concentration limits 
because they argued that MSHA lacked evidence that the limits were 
technologically feasible and economically feasible, and some objected 
to the use of unvalidated simulations to demonstrate the feasibility of 
compliance. An alternative to concentration limits was proposed wherein 
mine operators would ``Examine and adopt technically and economically 
feasible methods of preventing potentially hazardous or irritating 
exposure to diesel exhaust.'' But another commenter argued that the 
metal and nonmetal industry could feasibly meet even lower 
concentration limits than those proposed. And another suggested that a 
concentration limit alone will not adequately protect miner health 
because, given the freedom to choose control options, mine operators 
may elect to boost ventilation rather than cut emissions. As these 
comments concern feasibility, they are generally discussed in part V of 
this preamble.
    A number of commenters argued that MSHA should allow operators 
considerable additional flexibility dealing with dpm. Some felt 
operators should be left complete flexibility on controls, and that a 
concentration limit at all was inappropriate. Others argued that the 
range of operator choice of controls should include personal protective 
equipment as well as administrative controls. These comments are 
discussed below in connection with this section (Sec. 57.5060).
    Still other commenters argued that concentration limits should not 
be proposed, or should be much higher, because they argue MSHA lacks a 
method to measure dpm concentrations in underground metal and nonmetal 
mines that provides the accuracy, consistency, and reliability that are 
needed for compliance determinations. These comments are discussed in 
this part in connection with Sec. 57.5061.
    Another commenter expressed concern about the interplay between 
this rule and those already in effect for diesel gases. This commenter 
expressed concern that, in addition to complying with the interim and 
final dpm concentration limits, mine operators would be required to 
comply with a concentration limit that considers the additive effect of 
diesel particulate matter and the principal gaseous emissions from a 
diesel engine (carbon monoxide, carbon dioxide, nitric oxide, and 
nitrogen dioxide).
    MSHA's risk assessment in part III does not specifically evaluate 
the possible additive effects of diesel particulate matter and diesel 
gases. Accordingly, the agency does not at this time have a basis upon 
which to enforce either the interim or final dpm concentration limit in 
combination with any other substance or substances, including diesel 
exhaust gases. MSHA will, of course, continue to enforce the limits 
applicable to diesel gases, but this enforcement will be separate from 
the enforcement of the dpm concentration limits under the final dpm 
rule. The Agency understands that Canada does consider the additive 
effect of diesel exhaust gases and particulate, and will notify the 
mining community if it decides to look into this matter further based 
upon additional information.
    Finally, the Agency notes it received only two comments on a 
related matter on which it specifically sought comment--whether to 
establish an ``Action Level'' for dpm (63FR 58119). An ``Action Level'' 
is a defined contaminant level (usually one-half of the compliance 
limit) which, if exceeded, triggers actions that must be taken to 
effectuate control of the contaminant. In the preamble to the proposed 
rule, MSHA noted it had considered the possibility of establishing an 
Action Level because the dpm concentration at which exposure does not 
result in adverse health effects is not known at this time. If an 
Action Level were in place and compliance sampling results exceeded 
this level, certain remedial steps, or ``best practices,'' would have 
to be initiated by management to reduce exposures, such as limits on 
fuel type, idling, and engine maintenance--whatever steps MSHA 
determined would be feasible at the Action Level for this sector as a 
whole. One comment that addressed this approach recommended against 
establishing an Action Level because the commenter was of the view that 
no limits at all could be justified at this time based on available 
health risk data. The other commenter suggested that an Action Level 
should be adopted in lieu of a rule incorporating a concentration limit 
requiring mandatory compliance.
    After further consideration, MSHA determined it does not have 
enough information to proceed with an Action Level at this time, 
although it notes that the concept of an Action Level is well 
recognized in occupational health protection and included in many other 
standards. Furthermore, MSHA determined that these ``best practices'' 
are technologically and economically feasible for all mines, so there 
is no reason to withhold their

[[Page 5857]]

implementation until an Action Level is reached. The rationale for 
requiring these ``best practices'' is discussed in more detail later in 
this section under ``Meeting the concentration limit: operator choice 
of controls.''
    Concentration limit expressed as an ``average eight-hour equivalent 
full shift airborne concentration.'' MSHA recognizes that work shifts 
longer than eight hours are common in the underground metal and 
nonmetal mining industry. It is for this reason that MSHA expressed its 
concentration limit as an ``average eight-hour equivalent full shift 
airborne concentration.'' Health-related standards for airborne 
contaminants are typically established on the basis of an eight-hour 
work shift. Standard industrial hygiene practice, and MSHA's past 
practice for metal and nonmetal health sampling, involve adjusting the 
actual measured concentration of an airborne contaminant to an eight-
hour equivalent concentration when work shifts are longer than eight 
hours. This adjusts an exposure occurring over an extended workshift 
(e.g., 10 or 12 hours) to enable a valid comparison to an established 
exposure limit that is based on an 8-hr workshift.
    The mathematical formula for making this adjustment is thoroughly 
described in the MSHA Metal and Nonmetal Health Inspection Procedures 
Handbook. This formula is as follows:
[GRAPHIC] [TIFF OMITTED] TR19JA01.075

    When the sampling pump flow rate is expressed in units of liters 
per minute, the formula results in a contaminant concentration 
expressed in units of mg or g per cubic meter. The factor of 
480 minutes is used regardless of actual shift duration so as to adjust 
the actual concentration to an eight-hour equivalent concentration that 
can be appropriately compared to a standard limit.
    MSHA specifically asked for comment on whether a more explicit 
definition is required in this regard (63 FR 58183). The agency did not 
receive any such suggestions. However, it is apparent that the term may 
be confusing to some. For example, one commenter observed that ``miners 
working overtime hours would be exposed to more dpm than miners on a 
normal eight-hour shift,'' and that a formula to determine eight-hour 
equivalency should be included. Another commenter expressed concern 
that the final rule would place a restriction on the number of hours or 
overtime hours miners could work.
    MSHA disagrees with these interpretations of the rule. The only 
impact of the rule relative to work hours is the aforementioned 
determination of ``average eight-hour equivalent full shift airborne 
concentration'' for dpm-exposed miners whose work shifts exceed eight 
hours. Although the Agency has no suggestions for a more clear 
formulation, it will endeavor to clarify this matter further for 
operators in its compliance guide.
    Dpm concentration limits expressed in terms of total carbon. The 
purpose of the interim and final concentration limits is to limit the 
amount of diesel particulate matter; but the limit is being expressed 
in terms of a restriction on the amount of total carbon. The reason for 
this involves the measurement method that MSHA intends to utilize to 
determine the concentration of dpm. As discussed in connection with 
Sec. 57.5061(a), the final rule specifies that MSHA will use a sampling 
and analytical method developed by NIOSH (NIOSH Method 5040) to measure 
dpm concentrations for compliance purposes. Using NIOSH's analytical 
method, the amount of total carbon (TC) contained in a dpm sample from 
any underground metal and nonmetal mine can be determined; the method 
does not directly yield the amount of dpm in a particular sample. 
However, as explained in detail in Part II of this preamble, TC 
represents approximately 80-85 percent of the total mass of dpm emitted 
in the exhaust of a diesel engine. The remaining 15-20 percent consists 
of sulfates and the various elements bound up with the organic carbon 
to form the adsorbed hydrocarbons. Using the lower boundary of this 
range, limiting the concentration of total carbon to 400 micrograms per 
cubic meter (400TC g/m\3\) effectively limits the 
concentration of whole diesel particulate to about 500 DPM 
g/m\3\. Similarly, limiting the concentration of total carbon 
to 160TC g/m\3\ effectively limits the 
concentration of whole diesel particulate to about 200DPM 
g/m\3\. Expressing the concentration limit in terms of total 
carbon enables miners, mine operators and inspectors to directly 
compare a measurement result with the applicable limit.
    Where the concentration limit applies. The concentration limits--
both interim and final--would apply only in areas where miners normally 
work or travel. The purpose of this restriction is to ensure that mine 
operators do not have to monitor and control dpm concentrations in 
areas where miners do not normally work or travel--e.g., abandoned 
areas of a mine where, for example, the roof may not be monitored for 
safety or ventilation may not be provided. At the same time, it should 
be noted that the interim and final concentration limits apply in any 
and all areas of a mine where miners normally work or travel--not just 
where miners might be present at any particular time.
    MSHA generally intends for inspectors to determine which portions 
of a given mine are subject to the concentration limit based on whether 
normal work or travel activities routinely do, or could occur there, 
whether areas are designated as ``abandoned'' on mine maps, whether 
areas are made ``off limits'' through the use of signs or barricades, 
etc.
    MSHA has, however, in the final rule (Sec. 57.5060(d)), explicitly 
authorized the Secretary, upon making certain findings and ensuring 
that certain protections are in place for miners, to allow miners 
engaged in certain inspection, maintenance or repair activities to work 
in areas of a mine which are considered areas in which miners normally 
work or travel but that exceed the concentration limits. These 
situations are discussed immediately below.
    Exception: Specific mining activities which may be conducted in 
areas which exceed the concentration limit. Although feasible 
engineering and work practice controls were found to exist for most 
underground metal and nonmetal mining situations, MSHA did determine 
that certain maintenance and repair activities might have to be 
performed in areas where feasible engineering and work practice 
controls may not be capable of maintaining the dpm concentration at or 
below the applicable concentration limit. Therefore, in the final rule, 
Sec. 57.5060(d) under certain conditions permits miners to work in 
areas where the concentration limit is exceeded, and only when 
specified precautions have been implemented to protect affected miners. 
As explained in

[[Page 5858]]

detail below, principal among these precautions is the use by all 
affected miners, of proper personal protective equipment (i.e., 
respiratory protection devices) within the context of a comprehensive 
respiratory protection program.
    More specifically, Sec. 57.5060(d)(1) permits, with the pre-
approval of the Secretary, employees engaged in inspection, 
maintenance, or repair activities to work in concentrations of dpm 
exceeding the applicable limit if they are protected by appropriate 
respiratory protective equipment. This provision applies only to miners 
performing the identified activities, and only when certain mandatory 
protections are implemented. If respiratory protective equipment is 
used, the final rule requires implementation of a respiratory 
protection program consistent with the minimum requirements established 
in Sec. 56/57.5005 (a) and (b), which address such factors as 
selection, maintenance, training, fitting, supervision, and cleaning. 
These requirements include by reference, the elements of a minimally 
acceptable respiratory protection program as delineated in the American 
National Standard on ``Practices For Respiratory Protection'' (ANSI 
Z88.2-1969).
    The rule specifies that areas for which a request to allow 
employees to work in areas that exceed the concentration limit are 
limited to--areas where miners work or travel infrequently or for brief 
periods of time for equipment or mine inspection; areas where miners 
otherwise work exclusively inside of enclosed and environmentally 
controlled cabs, booths and similar structures with filtered breathing 
air; and in shafts, inclines, slopes, adits, tunnels and similar 
workings that are designated as return or exhaust air courses and that 
are also used for access into, or egress from an underground mine.
    The standard applies in areas of the mine where miners ``normally'' 
work or travel. Normally does not equate to frequency, but rather to 
the nature of the area. Areas where miners work or travel infrequently 
are treated by the rule no differently than areas where miners work or 
travel frequently. For example, if a remote pump is checked on a weekly 
basis, the area in which that pump is located would be considered an 
area where miners normally work or travel, even though the area is 
visited infrequently.
    Approval to allow miners to work in areas that exceed the 
concentration limit would be contingent on the Secretary determining 
that engineering controls are not feasible, and that adequate 
safeguards would be employed by the mine operator to prevent hazardous 
exposure to dpm. The final rule requires mine operators to submit a 
plan to the Secretary to justify the infeasibility of engineering 
controls, and to explain the circumstances of the job, the location 
where work will be performed, resulting dpm exposures, and controls to 
be used, including, but not necessarily limited to personal protective 
equipment.
    In order for MSHA to determine the reasonableness of a mine 
operator's request for approval under 5060(d), certain details 
regarding the work need to be provided. These include the types of 
inspection, maintenance or repair activities planned, the locations of 
such activities, the dpm concentrations at these locations, the reasons 
why engineering controls would not be feasible, the anticipated 
frequency of these activities, the anticipated number of miners 
involved, and the safeguards the mine operator will employ to minimize 
dpm exposures. These factors will tend to change over time as the mine 
develops, as new equipment or procedures are introduced, as ventilation 
system parameters change, etc. MSHA believes that an annual updating of 
these factors is necessary to insure that approval is granted only 
where justified by the actual circumstances.
    In essence, this exemption allows the use of personal protective 
equipment as a substitute for engineering controls under a limited 
number of circumstances. Many commenters suggested MSHA permit the use 
of PPE much more broadly in lieu of engineering controls; MSHA's review 
and reaction to these comments is discussed below.
    One commenter, a mine operator, agreed with MSHA's approach that 
stresses engineering controls first and foremost. The commenter stated 
that, ``engineering controls, as close to the source of the diesel 
emission as possible, must be the first line of DPM exposure control.'' 
The commenter further suggested that, ``The proposed rule should allow 
personal protective equipment to be used as a last resort. The proposed 
rule should require written documentation explaining how the mine 
determined the appropriate exposure controls. This written 
documentation should clearly explain why engineering controls, commonly 
used in industry to control diesel emissions, are not technically or 
economically feasible.''
    Although MSHA has embraced the commenter's basic idea of requiring 
written documentation when personal protective equipment is proposed as 
an alternative to engineering controls, the final rule includes other 
necessary safeguards to insure that this option is used only when 
absolutely necessary and that appropriate steps are taken to insure 
that respirator wearers are adequately protected. The final rule 
requires such plans to identify, at a minimum, the types of anticipated 
inspection, maintenance, and repair activities that must be performed 
for which there are no feasible engineering controls sufficient to 
comply with the concentration limit, the locations where such 
activities could take place, the concentration of dpm in these 
locations, the reasons why engineering controls are not feasible, the 
anticipated frequency of such activities, the anticipated duration of 
such activities, the anticipated number of miners involved in such 
activities, and the safeguards that will be employed to limit miner 
exposure to dpm, including, but not limited to the use of respiratory 
protective equipment.
    The final rule requires mine operators to utilize all feasible 
engineering and work practice controls, however, the exception under 
subsection (d) permits such controls to be supplemented with respirator 
use in certain limited situations where reliance solely on feasible 
engineering and work practice controls would be inadequate to control 
exposures below the applicable concentration limit. The proposal's 
prohibition on administrative controls under any and all circumstances 
is retained in the final rule in subsection (e).
    Examples of situations where MSHA believes engineering controls 
might not be feasible include cleaning up a roof fall in an exhaust air 
course, replacing a conveyor belt idler in a conveyor tunnel that is 
carrying exhaust air, or shaft inspection in an exhaust air shaft. The 
provisions of subsection (d) are not intended to suggest that MSHA 
believes these and similar activities should automatically be 
considered exempt from the requirement to utilize engineering and work 
practice controls to comply with the concentration limit. Rather, MSHA 
recognizes that under certain site specific circumstances, feasible 
engineering and work practice controls alone may not be capable of 
achieving compliance with the concentration limit. Therefore, MSHA 
agrees that respirator use should be permitted if the applications are 
sufficiently justified and approved in advance.
    MSHA does not intend that plans submitted for advance approval need 
to identify specifically and individually

[[Page 5859]]

every activity for which advance approval is sought. The intent is that 
plans must identify, in a generic sense, the types of activities and 
related circumstances as can reasonably be anticipated, sufficient to 
enable the Secretary to determine whether advance approval is 
warranted.
    Meeting the concentration limit: operator choice of engineering 
controls. The final rule contemplates that an operator of an 
underground metal or nonmetal mine have considerable discretion over 
the controls utilized to bring down dpm concentrations to the interim 
and final concentration limits. For example, an operator could filter 
the emissions from diesel-powered equipment, install cleaner-burning 
engines, increase ventilation, improve fleet management, use traffic 
controls, or use a variety of other readily available controls. A 
combination of several control measures, including both engineering 
controls and work practices, may be necessary, depending on site 
specific conditions.
    MSHA intends for engineering controls to refer to controls that 
remove the dpm hazard by applying such methods as substitution, 
isolation, enclosure, and ventilation. MSHA intends for work practice 
controls to refer to specified changes in the way work tasks are 
performed that reduce or eliminate a hazard, such as traffic controls 
(speed limits, one-way travel, etc.), prohibiting unnecessary engine 
idling, or designating areas that are off-limits for diesel equipment 
operations. As discussed below, the final rule does not permit 
utilization of administrative controls as a means of complying with the 
dpm concentration limit. In the context of this rule, MSHA intends for 
administrative controls to refer to controls that limit a miner's 
exposure to dpm by distributing the exposure among other miners through 
various work scheduling and worker rotation practices.
    Some commenters asserted that implementation of certain dpm control 
measures may create other, unrelated health or safety problems. One 
example given concerned the complications and safety trade-offs of 
increasing ventilation to control dpm concentrations. The increased 
ventilation would tend to dry out roadways, causing increased problems 
with respirable silica bearing dust exposure. This problem, would, in 
turn, require application of greater amounts of water on the roadways 
for dust control, which, in turn, would create traction problems for 
vehicles. Increased ventilation might also accelerate the drying out of 
certain roof strata, creating roof control problems. Another commenter 
worried that enclosed cabs can reduce an equipment operator's field-of-
view, and dirt or glare on windows can obscure visibility, possibly 
creating safety problems.
    MSHA acknowledges that dpm control measures need to be selected and 
implemented carefully, both to insure they achieve the desired effect 
on dpm concentrations, and to minimize or avoid undesirable effects on 
other aspects of the mine's health and safety environment. In most 
cases, implementation of a given control will not have any undesirable 
effects. In other isolated cases, the undesirable effects of a given 
control can most likely be negated through additional work practice 
controls or other measures. For example, the increased application of 
water on roadways to reduce dust control problems caused by higher 
ventilation rates may require that equipment be operated at slower 
speeds. Roof control problems resulting from the accelerated drying out 
of strata may require a reassessment of the mine's roof control plan, 
such as its roof bolting practices. Vehicle operator field-of-view and 
visibility problems could be addressed by instituting new traffic 
controls, requiring slower speeds, and use of window washers. For these 
reasons, MSHA does not wish to explicitly deny operators a particular 
type of engineering control because in some circumstances an adjustment 
to customary mining practices may have to be made.
    Because information on available controls has been described in 
other parts of this preamble (part II and part V), further discussion 
is not provided here. Mine operators are also directed to the MSHA 
``estimator'' model to help them determine which control or combination 
of controls would be best able to produce the reduction in dpm 
concentrations necessary to comply with the appropriate concentration 
limit. The ``estimator'' mathematically calculates the effect of any 
combination of engineering and ventilation controls on existing dpm 
concentrations in a given production area of a mine. This model is in 
the form of a spreadsheet template permitting instant display of 
outcomes as inputs are altered. The model and some examples 
illustrating its potential utility are described in Part V of this 
preamble.
    Several commenters expressed disappointment that the proposal did 
not embrace what they sometimes referred to as ``MSHA's toolbox 
approach.'' In some cases, this appears to mean the commenters want 
operators to have the flexibility to use personal protective equipment 
and administrative controls, as well as engineering and work practice 
controls, to meet the required concentration limits. In other cases, 
however, it appears the commenters meant that MSHA should allow them 
the discretion not only to choose the controls they wish, but to choose 
whether or not to use controls at all. In other words, to these 
commenters, the ``toolbox approach'' means voluntary implementation of 
controls without enforcement of a concentration limit.
    By way of background, in 1997, MSHA published a pocket-sized 
handbook called, ``Practical Ways to Reduce Exposure to Diesel Exhaust 
in Mining---A Toolbox.'' This handbook describes and discusses a 
variety of emission control equipment, methods, and strategies, both in 
terms of laboratory emissions testing and in-mine experience. The 
rationale for a ``toolbox approach'' to controlling diesel emissions is 
explained in the handbook. ``A toolbox offers a choice of tools, each 
with a specific purpose. One tool after another may be used to find a 
solution to a problem, or several tools may be tried at the same time.* 
* * Reducing exposure to diesel emissions lends itself to a toolbox 
approach because no single method or approach to reducing exposure may 
be suitable for every situation.'' Since its publication, this 
handbook, which is referred to simply as the ``MSHA toolbox'' or 
``toolbox'' has become quite well known and is widely used in the 
mining industry.
    Commenters who urged MSHA to adopt a ``toolbox approach'' in its 
rulemaking praised the approach taken in MSHA's publication, and 
indicated that they had successfully implemented some of the control 
strategies discussed. They urged MSHA to maintain this flexibility. One 
commenter suggested that, ``The toolbox is just simply best practices, 
if you would. If we're doing this, this, and this, then we're doing all 
we can without enforcement.* * * That's what a toolbox is. A toolbox is 
not an enforcement tool.''
    The MSHA Toolbox was issued before this rulemaking, in which, after 
considering all the evidence, MSHA has concluded that miners are at 
significant risk of material impairment at the concentration levels 
still found in underground metal and nonmetal mines. When MSHA makes 
such a finding, it is required to act to protect miners to the extent 
feasible. MSHA has concluded that requiring operators to comply with a 
concentration limit using engineering controls is necessary to protect 
miners and feasible for the mining industry as a whole, while still

[[Page 5860]]

providing underground metal and nonmetal mine operators with maximum 
flexibility to address this problem. Thus, MSHA believes the final rule 
does incorporate the ``toolbox approach'' by allowing mine operators to 
choose, from among numerous alternatives, the mix of control measures 
most suitable for the site specific conditions at a given mine--
provided that the controls bring exposures down to the required limit.
    MSHA has determined that certain types of controls discussed in the 
toolbox--PPE and administrative controls--are not considered acceptable 
ways to meet a concentration limit. PPE does not reduce the 
concentrations of a contaminant in the environment, though such 
equipment does offer limited protection to miners who must work in 
areas where the applicable concentration limit cannot be achieved using 
feasible engineering or work practice controls. The rule permits PPE to 
be used to protect miners in those limited situations where it permits 
work to take place despite dpm concentrations in excess of the 
concentration limit (special extension of time to meet final 
concentration limit under paragraph (c), discussed below, and special 
permission to perform inspection, maintenance and repair activities in 
areas that exceed the concentration limit under paragraph (d), 
discussed above.) Administrative controls (e.g., limiting the hours 
worked by a particular miner in a high concentration area) simply 
spread risk among miners. The reasons for MSHA's position in this 
regard are discussed in detail below.
    MSHA has also determined that certain other types of dpm control 
measures discussed in the toolbox must be implemented at all 
underground metal and nonmetal mines that use diesel equipment, 
regardless of the dpm concentration level, to minimize miner risks. 
These ``best practices'' include such requirements as low sulfur 
content diesel fuel, limits on unnecessary idling of diesel engines, 
maintenance standards, and a requirement for newly introduced engines 
to be MSHA approved or meet certain EPA standards. MSHA's rationale for 
why it is mandating such ``best practices'' is summarized below. 
Further detail is provided in the preamble to the proposal (63FR 
58119), and in the sections of this Part which discuss the individual 
practices themselves (diesel fuel (Sec. 57.5065(a)), maintenance 
(Sec. 57.5066), and engines that are MSHA approved or meet EPA 
standards (Sec. 57.5067).
    In the proposal, MSHA explained that it had considered implementing 
an ``Action Level'' for dpm, possibly at a level one-half of the final 
concentration limit, or 80TC g/m\3\ because the dpm 
concentration at which exposure does not result in adverse health 
effects is not known at this time. Under this approach, when dpm levels 
exceeded the Action Level, implementation of certain ``best practice'' 
controls, such as limits on fuel types, idling, and engine maintenance 
would have been required. However, this approach was not incorporated 
into the proposal, nor has it been incorporated into the final rule. 
MSHA determined it does not have enough information to proceed with an 
Action Level at this time, although it notes that the concept of an 
Action Level is well recognized in occupational health protection and 
included in many other standards. Instead, MSHA determined that these 
``best practices'' would be required for all mines at all times.
    MSHA followed this course for several reasons, including: (1) 
Sampling by both mine operators and MSHA would have been much more 
frequent under an approach incorporating an Action Level; (2) tracking 
equipment maintenance requirements would have been much more 
complicated, as diesel equipment could move from an area of the mine 
where the dpm concentration was less that the Action Level, to another 
area where the Action Level had been exceeded; (3) these ``best 
practices'' are already in place, and have proven to be workable and 
practical in coal mines; (4) given the history of lung problems 
associated with the mining industry, and considering that these 
practices were determined to be economically and technologically 
feasible for the industry as a whole, a more protective course seemed 
prudent; and (5) a number of the work practices appear to have 
significant benefits, such as improving the efficiency of maintenance 
operations.
    One commenter suggested that other ``best practices'' related to 
mine ventilation should be mandated in the final rule. This commenter 
recommended requiring mine operators to provide details on the design 
and operating parameters of auxiliary ventilation systems, that they be 
required to utilize an appropriate air measurement and recording 
program, and that they properly attend to uncontrolled recirculations 
and leakages. MSHA believes that existing ventilation regulations 
adequately address these concerns, and that mine operators, in 
utilizing a ``toolbox approach'' to implement dpm control measures, 
have the option of incorporating ventilation system improvements if 
they are judged to be feasible, practical, desirable, and appropriate 
to the site specific conditions at a given mine. Thus, MSHA did not 
include a mandate to use such ventilation ``best practices'' in the 
final rule.
    Concentration limit: time to meet. As noted, the dpm limitation 
requires metal and nonmetal mines to reduce total carbon concentrations 
in areas where miners normally work or travel to 160 micrograms per 
cubic meter of air (equating to about 200 micrograms of dpm per cubic 
meter of air.) Sec. 57.5060 provides for an extension of time for 
underground metal and nonmetal mines to meet the concentration limit. 
Mines do not have to meet any limit for the first 18 months after the 
final rule is promulgated. Instead, this period will be used to provide 
compliance assistance to the metal and nonmetal mining community to 
ensure it understands how to measure and control diesel particulate 
matter concentrations in individual operations. Moreover, the rule 
provides all mines in this sector an extension of three and a half 
additional years to meet the final concentration limit established by 
Sec. 57.5060(b). During this extension, however, all mines will have to 
bring total carbon concentrations down to 400 micrograms per cubic 
meter, equating to a limit of 500 micrograms per cubic meter in dpm.
    Comments on the implementation schedule for the concentration 
limits focused on the technological and economic feasibility of 
complying within the time frames established. Commenters expressed the 
view that the rule is technology forcing, and that the mining sector of 
the economy is too small to justify the expense by manufacturers 
(mining equipment, diesel engines, aftertreatment devices, etc.) to 
develop the necessary products to enable mine operators to fully comply 
by the deadlines contained in the final rule.
    MSHA provided these phase-in times for meeting the interim and 
final concentration limits after carefully reviewing comments on the 
economic and technological feasibility of requiring all mines in this 
sector to meet the applicable limits using available controls. This 
review is presented in Part V of this preamble. MSHA has studied a 
number of metal and nonmetal mines in which it believed dpm might be 
particularly difficult to control. The Agency has concluded that in 
combination with the ``best practices'' required under other provisions 
of the

[[Page 5861]]

final rule (Secs. 57.5065, 57.5066 and 57.5067), engineering and work 
practice controls are available that can bring dpm concentrations in 
all underground metal and nonmetal mines down to or below 
400TC g/m\3\ within 18 months. Moreover, the Agency 
has concluded that controls are available to bring dpm concentrations 
in all underground metal and nonmetal mines down to or below 
160TC g/m\3\ within 5 years. The Agency has 
concluded that it is not feasible to require this sector, as a whole, 
to lower dpm concentrations further, or to implement the required 
controls more swiftly.
    Despite its conclusions on the feasibility of these timeframes for 
the underground metal and nonmetal industry as a whole, MSHA has 
included a provision in the final rule to allow an additional two years 
for mines experiencing difficulty in complying due to technological 
problems. A discussion of this special extension follows.
    Special extension. Pursuant to Sec. 5060(c), an operator may 
request more than five years to comply with the final concentration 
limit only in the case of technological problems. In light of the risks 
to miners posed by dpm, however, the Agency has concluded that the 
economic constraints of a particular operator are not an adequate basis 
for a further extension of time for that operator, and the final rule 
does not provide for any extension grounded in economic concerns. 
Moreover, if it is technologically feasible for an operator to reduce 
dpm concentrations to the final limit within the established five year 
compliance period, no extension would be permitted even if a more cost 
effective solution might be available in the future for that operator.
    However, the Agency has determined that if an operator can actually 
demonstrate that there is no technological solution that could reduce 
the concentration of dpm to 160TC g/m\3\ within 
five years, a special extension would be warranted.
    Extension application. Sec. 57.5060(c)(1) provides that if an 
operator of an underground metal or nonmetal mine can demonstrate that 
there is no combination of controls that can, due to technological 
constraints, be implemented within five years to reduce the 
concentration of dpm to the limit, MSHA may approve an application for 
an extension of time to comply.
    Such a special extension is available only once, and is limited to 
2 years. In this regard, MSHA does not anticipate that an extension 
will automatically last 2 years, and the agency will closely scrutinize 
applications to determine how much time is really required to implement 
a technological solution. To obtain a special extension, an operator 
must show that diesel powered equipment was used in the mine prior to 
publication of the rule, demonstrate that there is no off-the-shelf 
technology available to reduce dpm to the limit specified in 
Sec. 57.5060, and establish the lowest concentration of dpm attainable. 
In this regard, the Agency reiterates that cost is not a consideration; 
thus, simply because a more cost-effective solution will become 
available in the future is not an acceptable reason for an extension.
    One commenter questioned whether it is reasonable to limit mine 
operators to one special extension when the necessary technology to 
comply with the concentration limits does not exist today. This 
commenter suggests a five to ten year compliance schedule is more 
realistic to allow time to develop the technology and to phase in the 
replacement of equipment. MSHA believes that very few, if any, 
underground metal and nonmetal mining operations should need a special 
extension, based on the feasibility information discussed in part V of 
this preamble. Despite this information, the final rule makes specific 
provision for a special extension for the very few mines that might 
experience technical problems that cannot be foreseen at this time. In 
the unlikely event any mines experience such technical problems, MSHA 
believes that a two year extension, in addition to the five years 
granted in the final rule for all mines, will be sufficient for them to 
achieve compliance.
    The final rule further requires that to establish the lowest 
achievable concentration, the operator must provide sampling data 
obtained using NIOSH Method 5040 (the method MSHA will use when 
determining concentrations for compliance purposes; this sampling 
method is further discussed in connection with Sec. 57.5061(a)).
    The application would also require the mine operator to specify the 
actions that are to be taken to ``maintain the lowest concentration of 
diesel particulate achievable'' (such as ensuring strict adherence to 
an established control plan) and to minimize miner exposure to dpm 
(e.g., such as providing and requiring the use of suitable respirators 
at mines or areas of mines under extension). MSHA's intent is to ensure 
that personal protective equipment is permitted only as a last and 
temporary resort to bridge the gap between what can be accomplished 
with engineering and work practice controls and the concentration 
limit. It is not the Agency's intent that personal protective equipment 
be permitted during the extension period as a substitute for 
engineering and work practice controls that can be implemented 
immediately.
    Filing, posting and approval of extension application. The final 
rule requires that an application for an extension be filed no later 
than 6 months (180 days) in advance of the date of the final 
concentration limit (160TC g/m\3\), and a copy of 
the extension be posted at the mine site for the duration of the 
extension period. In addition, a copy of the application would also 
have to be provided to the designated representative of the miners.
    The application must be approved by MSHA before it becomes 
effective. While pre-approval of plans is not the norm in this sector, 
an exception to the final concentration limit cannot be provided 
without careful scrutiny. Moreover in some cases, the examination of 
the application may enable MSHA to point out to the operator the 
availability of solutions not considered to date. MSHA notes that it 
received no comments on this requirement for pre-approval.
    While the final rule is not explicit on the point, it is MSHA's 
intent (as set forth in the preamble to the proposed rule, 63 FR 58184) 
that primary responsibility for processing of the operator's 
application for an extension will rest with MSHA's District Managers. 
This ensures familiarity with the mine conditions, and provides an 
opportunity to consult with miners as well. At the same time, MSHA 
recognizes that District Managers may not have the expertise required 
to keep fully abreast of the latest technologies and of solutions being 
used in similar mines elsewhere in the country. Accordingly, and again 
consistent with the preamble to the proposed rule, the Agency intends 
to establish, within its Technical Support Directorate a special panel 
to consult on these issues and to provide assistance and guidance to 
its District Managers. In the preamble to the proposed rule (63 FR 
58184) the Agency requested comment on whether further specifics 
regarding this approach to approving applications for special 
extensions should be incorporated into the final rule, however, no such 
comments were received.
    The rule specifies that a mine operator shall comply with the terms 
of any approved application for a special extension, and provides that 
a copy of the approved application be posted at the mine site for the 
duration of the application.

[[Page 5862]]

    Personal protective equipment and administrative controls. In the 
proposal, mine operators were expressly forbidden to use personal 
protective equipment (e.g., respirators) or administrative controls 
(e.g., job rotation) to comply with either the interim or final dpm 
concentration limit. MSHA's rationale for these provisions was that 
limiting individual miner exposure through the use of respirators or 
job rotation would not reduce the airborne concentrations of dpm in the 
mine. Rather, in the proposal, MSHA chose to incorporate the widely 
accepted industrial hygiene concept of ``hierarchy of controls'' which 
places the highest priority on eliminating or minimizing hazards at the 
source through implementation of engineering and work practice 
controls.
    The ``hierarchy of controls'' paradigm regards administrative 
controls and the use of personal protective equipment to be inherently 
inferior methods of controlling contaminant exposures in the workplace. 
Support for this position is virtually universal in the field of 
industrial hygiene. Patty's Industrial Hygiene and Toxicology (Vol I, 
General Principles) states, ``Evidence of the importance of engineering 
control of the work environment among the various alternative solutions 
to industrial hygiene problems is found in every current industrial 
hygiene text: all list the possible solutions in priority fashion as 
engineering controls, administrative controls, and as a last resort, 
use of personal protective equipment.'' The National Safety Council's 
Fundamentals of Industrial Hygiene states, ``Engineering controls 
should be used as the first line of defense against workplace hazards 
whenever feasible. Such built-in protection, inherent in the design of 
a process, is preferable to a method that depends on continual human 
implementation or intervention.''
    This text goes on to describe administrative controls as, ``not as 
satisfactory as engineering controls,'' and notes that such controls 
``have been criticized by some as a means of spreading exposures 
instead of reducing or eliminating the exposure.'' This latter 
statement is particularly relevant to dpm, and to carcinogens in 
general, because administrative controls, such as job rotation, result 
in placing more workers at risk. Among the reasons Patty's Industrial 
Hygiene and Toxicology recommends that a given chemical should not be 
controlled by administrative reduction of exposure time is that it may 
be a carcinogen.
    In the proposed rule, MSHA prohibited administrative controls as an 
acceptable dpm control method because they fail to eliminate the 
exposure hazard and result in placing more miners at risk. Since MSHA 
determined that compliance with the interim and final dpm concentration 
limits was feasible for the underground metal and nonmetal mining 
industry as a whole using exclusively engineering and work practice 
controls, the Agency logically chose to prohibit personal protective 
equipment as a compliance option as well.
    In the Preamble to the proposed rule, MSHA stated that it intended 
that the normal meaning be given to the terms personal protective 
equipment and administrative controls, and asked for comment as to 
whether more specificity would be useful. MSHA noted that it assumed 
the mining community understands, for example, that an environmentally 
controlled cab for a piece of equipment is an engineering control and 
not a piece of personal protective equipment.
    Numerous commenters took issue with the proposal's prohibition on 
administrative controls and personal protective equipment as compliance 
options. They noted that both administrative controls and personal 
protective equipment are accepted industrial hygiene exposure control 
methods that should be permitted under the rule. Most commenters agreed 
that engineering controls would be the preferred option for reducing an 
occupational health exposure, but that engineering controls sufficient 
to reduce dpm concentrations below the applicable concentration limit 
might not be the most cost-effective approach, and more importantly, 
that engineering controls may not be feasible in all situations. They 
argued that prohibiting administrative controls and personal protective 
equipment would, as a result, place mine operators in an impossible 
compliance dilemma.
    It is significant to note that the commenters did not disagree with 
MSHA's fundamental reasoning for using the ``hierarchy of controls'' 
concept as the basis for prohibiting administrative controls and 
personal protective equipment. Likewise, there was no direct 
disagreement with MSHA's endorsement of the widely accepted industrial 
hygiene principle that administrative controls are inappropriate in the 
case of exposure to carcinogens because job rotation will expose more 
miners to the hazard.
    Rather, commenters argued that administrative controls and personal 
protective equipment should be permitted simply to give mine operators 
greater flexibility in dealing cost effectively with a workplace 
contaminant, and because certain situations exist where no feasible 
engineering control would be available to enable compliance with the 
concentration limit.
    Regarding the question of affording greater operator flexibility, a 
typical commenter observed that, ``If MSHA's goal is protection of 
miners, in the context of a viable and profitable industry, it should 
encourage flexible control approaches to the control of dpm exposure, 
and not penalize operators for using all effective means available--
including administrative controls and PPE.'' Another commenter asked 
MSHA to, ``reconsider the use of personal protective equipment as a 
cost effective solution when appropriate.'' MSHA responds to these 
comments by noting that it did incorporate compliance flexibility into 
the requirements for this rule. As noted earlier under the discussion 
on ``Meeting the concentration limit: operator choice of engineering 
controls,'' mine operators do have considerable freedom to choose the 
control, or combination of controls necessary to achieve and maintain 
compliance with the applicable concentration limit in their mines. 
However, this freedom is not total, particularly with respect to 
administrative controls and personal protective equipment. Operator 
flexibility, convenience, or cost effectiveness are not acceptable 
bases for permitting dpm control methods that are widely acknowledged 
to be inherently inferior to engineering and work practice controls.
    Regarding the question of the feasibility of controls, several 
commenters argued that there are situations where engineering controls 
are either economically infeasible, technologically infeasible, or 
both. Some typical examples of these comments include a mining company 
that objected to, ``the Agency's continued downgrading of 
administrative controls and the use of personal protective equipment in 
favor of considerably more expensive, presently infeasible, engineering 
controls.'' Another commenter complained that, ``the standard must be 
attained with engineering controls alone,'' and that, ``personal 
protective equipment and other means cannot be used even where 
compliance with engineering controls is not feasible.'' Still another 
commenter observed that, ``The proposal is not [economically or 
technologically] feasible for metal mines * * * which are designed 
specifically for use of diesel equipment. In these

[[Page 5863]]

mining scenarios, use of electric equipment is not cost-effective, and 
elimination of diesel equipment would eliminate the process for which 
the mines were designed.''
    The question of economic feasibility will be discussed separately 
from the question of technological feasibility. MSHA acknowledges that 
administrative controls or the use of personal protective equipment may 
be less costly than engineering or work practice controls in certain 
situations. However, a difference in cost between two approaches is 
simply that--a difference in cost. MSHA does not regard a cost 
difference per se as prima facia proof that an approach is economically 
infeasible simply because a less expensive alternative exists.
    Commenters also questioned MSHA's compliance cost estimates, 
asserting that compliance costs will actually be much higher. MSHA's 
compliance cost estimates are discussed in the REA. However, in answer 
to this comment, MSHA determined that exclusive reliance on engineering 
and work practice controls are economically feasible for the 
underground metal and nonmetal mining industry as a whole (with the 
exception of the situations addressed in Sec. 57.5050(d)). Thus, MSHA 
rejects the argument that administrative controls and the use of 
personal protective equipment should be permitted based on 
consideration for economic feasibility.
    Regarding the question of the technological feasibility of 
engineering and work practice controls, the high number of comments 
addressing this issue suggested that the underground metal and nonmetal 
mining industry considered it to be of vital importance. Despite their 
number, however, none of these comments identified specific equipment 
or mining situations where exclusive reliance on engineering or work 
practice controls to achieve and maintain compliance with the 
applicable dpm concentration limit would be impossible due to 
technological infeasibility.
    In the preamble to the proposed rule, MSHA provided extensive 
information on how mine operators might use a computer program known as 
the ``Estimator'' to conduct assessments of controls that might be 
necessary to deal with problems in individual mines, and requested 
comments based on such specific information. The comments that were 
received were critical of the ``Estimator'' because it produces an 
estimate of average dpm concentration in a given area, not the specific 
concentration that might exist at a specified sampling location; and 
because its accuracy depends on the quality of the input data, which is 
suspect due to the perceived inherent inaccuracy of the dpm sampling 
methods which must be used to obtain the input data.
    Regarding the first criticism, MSHA notes that the average dpm 
concentration in a given area, which is the output obtained from the 
``Estimator,'' is a more accurate indicator of the potential dpm hazard 
than a specific concentration that might exist at a specified sampling 
location. Since compliance is based on a shift weighted average 
concentration produced by diesel equipment that is normally in constant 
motion throughout the shift, the average dpm concentration in a given 
area is a better predictor of compliance or noncompliance than a 
determination of specific concentration that might exist at a specified 
sampling location. It might also be advisable to consider relocating a 
miner who, by virtue of their specific work location, is thought to be 
at risk of being exposed to a concentration of dpm that is greater than 
the average for that area (for example, move the miner from being in 
the direct line of the exhaust stream). Finally, MSHA notes that the 
``Estimator'' is just that, a means of estimating dpm concentration. It 
was never claimed that this model could predict dpm concentrations with 
pinpoint accuracy. However, in verification testing of the model, MSHA 
has observed good agreement between predicted and measured dpm 
concentrations (as discussed in part II, section 3 of this preamble).
    Regarding the second criticism, MSHA notes that users have the 
option of inputting actual dpm data, or estimating such values. If 
users desire to input in-mine measurements of dpm concentrations, MSHA 
is confident that dpm sampling and analysis using the NIOSH Method 
5040, as described elsewhere in this preamble, will accurately 
represent actual dpm concentrations.
    Nonetheless, MSHA reevaluated the feasibility of engineering and 
work practice controls as the exclusive means of complying with the 
applicable dpm concentration limits. This reevaluation identified 
potential compliance problems related to performing certain inspection, 
repair, and maintenance work if only engineering and work practice 
controls were permitted as means of achieving compliance. Therefore, 
the Agency has adjusted the final rule to allow such work, when 
sufficiently justified and preapproved by the Secretary, to be 
performed using personal protective equipment as a supplement to 
engineering and work practice controls. But apart from these very 
limited situations, the Agency has concluded that the use of 
engineering controls to meet the concentration limit is both 
economically and technologically feasible for the underground mining 
industry as a whole, and in light of the health risks to miners, and 
the superiority of engineering controls, the Agency has concluded that 
they (and not PPE or administrative controls) must be utilized to meet 
the concentration limit.

57.5061 Compliance Determinations

    Summary. This section of the final rule establishes the criteria 
for determining compliance with the concentration limits. It has three 
subsections.
    Subsection (a) provides for compliance sampling to be performed by 
MSHA directly, requires that such compliance sampling be done in 
accordance with the other requirements of this section, and further 
provides that a single such sample will be adequate to establish a 
violation. This is consistent with the proposed rule.
    Subsection (b) provides that MSHA will collect dpm samples using a 
respirable dust sampler equipped with a submicrometer impactor, and 
analyze such samples for the amount of total carbon (TC) using NIOSH 
Method 5040 (or by using any method of collection and analysis 
subsequently determined by NIOSH to provide equal or improved accuracy 
for the measurement of dpm in underground metal and nonmetal mines). 
This is like the proposed rule except that the final rule explicitly 
requires a submicrometer impactor to be used in collecting all dpm 
compliance samples in underground metal and nonmetal mines.
    Subsection (c) provides for MSHA inspectors to determine the 
appropriate sampling strategy for compliance determinations--personal 
sampling, occupational sampling, or area sampling--based on the 
circumstances of the particular exposure or exposures to be evaluated. 
This provision was not explicitly stated in the proposed rule; it was, 
however, stated in the preamble to the proposed rule as MSHA's intent. 
The final rule makes explicit MSHA's discretion in this regard.
    As discussed in more detail in Part II, section 3, an important 
factor in the agency's decision as to which sampling practice to 
utilize in a particular situation, and how the sampling should be 
conducted (e.g., how far away from a smoker or source of oil mist), is 
a careful review of other sources of total carbon in the environment to 
be

[[Page 5864]]

sampled which could cast doubt on whether the sample result was based 
solely on the amount of dpm present. MSHA will provide guidance in this 
regard to metal and nonmetal inspectors and the mining community--based 
on the information noted already in Part II, section 3 of this 
preamble, such new information as may be developed, and continued 
experience in this regard--so as to avoid wasting the limited resources 
of the Agency and its counsel, the Mine Safety and Health Review 
Commission, and the underground metal and nonmetal mining community by 
taking compliance samples whose validity is questionable.
    Numerous comments were received on this section--addressing the 
validity of single samples for determining compliance with an 
occupational health standard; the accuracy, precision, appropriateness, 
and practicality of using the NIOSH Method 5040 for determining dpm 
concentrations for enforcement purposes; and the legitimacy of using 
area sampling to determine compliance with a health standard. These 
comments, and MSHA's response to them, are discussed below.
    Single sample compliance determination. Pursuant to 
Sec. 57.5061(a), a single dpm sample showing that the applicable TC 
concentration limit has been exceeded on any individual shift will 
constitute a citable violation. Such a violation will also trigger 
further action pursuant to Sec. 57.5062, as discussed below in 
connection with that section.
    As is standard practice with other health compliance measurements, 
MSHA intends to account for normal variability in the sampling and 
analytical process by allowing a margin of error in the sampling result 
before issuing a citation. This margin of error will be based on the 
accuracy of the sampling and analytical method (Method 5040) used to 
measure the total carbon (TC) concentration in the mine environment, 
after correcting for potential interferences.
    The variability associated with Method 5040, as expressed by the 
relative standard deviation (RSD), decreases with increased load on the 
filter. Based on a laboratory experiment, NIOSH has determined that, at 
a TC concentration as low as 23 g/m\3\, the variability 
associated with an 8-hour sample using Method 5040 and a pump flow rate 
of 2.0 L/min is approximately 8.5 percent. (NIOSH Manual of Analytical 
Methods, Method 5040, Issue 2, 1998)
    MSHA will issue a citation for exceeding the applicable 
concentration limit only when such a citation can be issued at a 
confidence level of at least 95 percent. Each measurement made for 
purposes of compliance determination may be adjusted, if necessary, to 
compensate for any expected biases due to interferences such as tobacco 
smoke and oil mist. To account for sampling and analytical variability 
associated with Method 5040, the adjusted measurement will then be 
compared to the appropriate level established in Sec. 57.5060 
multiplied by an ``error factor.'' The error factor will be calculated 
so as to achieve the required 95-percent confidence that a violation 
has actually occurred. Based on the standard normal distribution for 
measurement errors, this will be 1 + 1.645 times the variability of the 
sampling and analytical method, as expressed by its RSD.
    For example, assuming the 8.5-percent limit on the RSD established 
by NIOSH under laboratory conditions, the error factor would be 1 + 
1.645 x .085 = 1.14. Suppose MSHA takes a sample during the interim 
period when the limit is 400TC g/m\3\. Then, if 
expected interferences are negligible, MSHA would cite noncompliance 
only if the TC measurement exceeded 1.14 x 400 = 456 g/m\3\.
    MSHA recognizes that measurement uncertainty may be higher for 
samples collected under mining conditions than under laboratory 
conditions. Therefore, MSHA intends to base the margin of error 
required to achieve a 95-percent confidence level for all noncompliance 
determinations on samples collected under field conditions. The Agency 
anticipates that the sampling and analytical error factor will be 
somewhere between 1.1 and 1.2. The Agency will, however, be governed by 
the actual data obtained to establish an appropriate margin of error.
    Several comments were received regarding the value of the error 
factor for dpm sampling using NIOSH Method 5040. One commenter asserted 
that it will be impossible to establish a meaningful error factor, 
stating, ``* * * there is insufficient information available to 
quantify the margin of error with any level of certainty.'' Another 
commenter expressed confusion with respect to the various ways in which 
measurement uncertainty was quantified in the proposal. This commenter 
argued as follows:

    MSHA states on page 58116 that the 5040 Method meets NIOSH's 
accuracy criteria that measurements come within 25% of the 
concentration at least 95% of the time. This standard is for a known 
particle size distribution in a laboratory setting, not a mine 
environment. Then on page 58184 states that, ``the variability 
associated with the Method 5040 to be approximately 6% (one relative 
standard deviation)''! These do not compare! Then it states MSHA 
will issue a citation if the measured value was 10% over the 
established level! There is a contradiction somewhere in the MSHA 
proposal--how can MSHA take 25% NIOSH laboratory criteria and shrink 
it to 6% in a mining environment?

    This commenter has apparently misunderstood the NIOSH Accuracy 
Criterion. Any unbiased method for which the RSD is known to be less 
than 12.75 percent meets the criterion, because any RSD less than 12.75 
percent implies (assuming no measurement bias) that measurements will 
come within 25 percent of the true value at least 95 percent of the 
time. An RSD of 6 percent meets the NIOSH accuracy criterion, simply 
because 6 percent is less than 12.75 percent. In order to achieve 95-
percent confidence that a specific measurement demonstrates 
noncompliance, a 6-percent RSD would, nevertheless, have to be 
multiplied by a 1-tailed 95-percent confidence coefficient of 1.645, 
yielding the 10-percent adjustment to which the commenter was 
referring. Therefore, these quantities are internally consistent. As 
stated earlier, however, MSHA intends to base its estimate of the RSD 
on data appropriate for field conditions in underground mining 
environments.
    Another commenter suggested that the NIOSH Method 5040 is prone to 
excessive errors because it is ``complex and requires highly skilled 
technicians.'' The inherent capacity of the method to produce accurate 
results was criticized by one commenter who stated, ``* * * it is not 
possible to evaluate the accuracy of the method. In fact, the method 
has been shown to produce massive errors when side-by-side samples and 
control filters are analyzed. Even blank filters produce high and 
widely-varying readings for TC.''
    Based on MSHA's extensive experience using NIOSH Method 5040 and 
related sampling practices, the Agency is confident that such sampling 
and analysis will meet or exceed MSHA's accuracy criteria. This is 
discussed in detail in Part II, section 3, and later in this section 
under ``Using NIOSH Method 5040 for compliance determinations.''
    Regarding the issue of uncertainty in the sampling and analytical 
process for field measurements, MSHA has not yet completed its 
determination of an appropriate error factor for this method. As noted 
above, MSHA will determine an appropriate factor and apply it when 
enforcing the applicable compliance

[[Page 5865]]

limit. As a matter of general practice, however, the Agency does not 
include error factors in occupational health rules, since the accuracy 
of measurement methods may change over time. When this determination is 
made, the error factor, along with its derivation, will be promptly 
communicated to the underground metal and nonmetal mining industry 
through the appropriate channels.
    MSHA recognizes that in recent years courts have closely 
scrutinized Agency actions to ensure they are consistent with the 
requirements of the Administrative Procedures Act and, in MSHA's case, 
with the requirements of the Mine Safety and Health Act as well. Courts 
have held that certain actions, traditionally regarded as enforcement 
policies issued at an agency's discretion, require notice and comment 
and even the development of feasibility analyses. MSHA has carefully 
considered its obligations in light of these precedents and has 
concluded that the determination of a margin of error to be allowed 
before issuing a citation remains among the type of actions left to 
Agency discretion. To require the Agency to go through rulemaking each 
time such an error factor is established or updated based upon improved 
sampling or analytical methods would not serve the best interests of 
the mining community. Therefore, MSHA wishes to emphasize that the 
Agency does not regard the determination of an appropriate margin of 
error as a necessary part of this rulemaking, but rather as strictly a 
matter of enforcement policy. As noted explicitly in the rule, the 
Agency is retaining discretion to switch to better techniques should 
NIOSH certify that they provide ``equal or improved accuracy for the 
measurement of diesel particulate matter in'' underground metal and 
nonmetal mines. (Sec. 57.5061(b))
    Notwithstanding its decision not to be explicit in this standard 
about the error factor to be used, MSHA recognizes the strong interest 
the underground metal and nonmetal mining community has in this issue 
and will ensure the matter is fully discussed with that community 
before the concentration limits are scheduled to go into effect. In 
working with this community on diesel particulate matter controls (see 
the history of this rulemaking in Part II of this preamble), the Agency 
has repeatedly demonstrated its commitment to good communications in 
this regard--e.g., the workshops, the advance and final circulation of 
the diesel toolbox, the use of the Agency's web site and direct 
notification in appropriate cases.
    As explained elsewhere in this preamble, MSHA has determined that 
it is feasible for underground M/NM mines to maintain dpm 
concentrations at or below the limits specified in Sec. 57.5060 on each 
and every shift, everywhere that miners normally work or travel, with 
the exception of the circumstances defined in Sec. 57.5060(d). 
Therefore, MSHA will protect miners' health to the maximum extent 
feasible by citing a violation whenever a single sample demonstrates 
that the limit has been exceeded on a full shift at any appropriate 
sampling location. This single-sample enforcement strategy is 
consistent with all other occupational health enforcement practices in 
the metal and nonmetal sector. As per long-standing policy in this 
sector, single out-of-compliance samples for dust (e.g., silica-bearing 
respirable dust, total nuisance particulate, etc.), gas (e.g., CO, 
NO2, solvent vapors, etc.), mist (e.g., cutting oil mist, 
spray paint, etc.), fume (e.g., welding fumes, fumes from melting 
furnaces, etc.), and noise are all considered citable violations of the 
respective standards. Nevertheless, the Agency decided it would be 
best, in this rulemaking, to avoid any possible ambiguity in this 
regard by explicitly stating in the rule itself that a single sample by 
the Agency would provide the basis for a citation. MSHA highlighted 
this matter in the preamble of its proposed rule (63 FR 58117, part of 
Question and Answer 12).
    Some commenters suggested that MSHA should collect numerous samples 
and base noncompliance determinations on the average value of all 
samples collected. These commenters argued that a single sample is not 
a statistically valid representation of the subject's ``typical'' or 
``normal'' exposure to the contaminant. The commenters noted that a 
single sample, if taken on a randomly selected work day, could result 
in an unusually high measurement (unusual with respect to a ``typical'' 
or ``normal'' day). Therefore, a single sample could give rise to a 
noncompliance determination, even if the environment being sampled is 
in compliance on most shifts. These commenters contended that such a 
sample was ``unrepresentative'' of typical exposure concentrations and 
should not, therefore, be used as a basis for a noncompliance 
determination.
    MSHA recognizes that the day-to-day exposure of a miner will not be 
constant and that on some days the sample collected over a single shift 
may be lower than the miner's long term average and on other days 
higher. However, MSHA has several compelling reasons for considering 
noncompliance on any individual shift to be a citable violation of the 
dpm concentration limit.
    First, MSHA has identified significant risks associated with short-
term dpm exposures (i.e., exposures over a 24-hour period). As 
documented in Part III of this preamble, adverse health effects 
associated with short-term exposures include (1) acute sensory 
irritations and respiratory symptoms (including allergenic responses) 
and (2) premature death from cardiovascular, cardiopulmonary, or 
respiratory causes. These risks alone would fully justify enforcing the 
concentration limits established in Sec. 57.5060 on each and every 
shift.
    Second, the concentration limits that MSHA has established are not 
expected to fully protect miners from these risks or from the excess 
risk of lung cancer associated with chronic dpm exposure. Instead, they 
are based on what can be feasibly achieved at this time to control dpm. 
By requiring compliance with the concentration limit on each shift 
measurement, it is MSHA's intent to protect miners to the maximum 
extent feasible.
    Third, it is not MSHA's objective, when sampling for compliance 
determination purposes, to estimate average dpm concentrations for any 
period greater than the shift sampled or for any mine location other 
than the location sampled. Some commenters confused the objective of 
estimating cumulative exposures for purposes of risk assessment with 
the objective of limiting cumulative exposures for purposes of risk 
management. MSHA's objective is to limit exposures to protect miners 
against both short- and long-term effects. It is not practical for MSHA 
to track miners' cumulative exposures over an occupational lifetime. 
Therefore, as a practical matter of enforcement policy, MSHA can best 
protect miners from both the health risks associated with acute 
exposures and from the excess lung cancer risk due to chronic dpm 
exposure by limiting exposure on each shift wherever miners normally 
work or travel.
    In addition, MSHA wants to emphasize that compliance limits in the 
metal and nonmetal sector, whether personal exposure limits or 
concentration limits, apply to every individual work shift. Every full-
shift exposure, not just the typical, or ``average'' exposure, must be 
in compliance with the limit. Basing compliance on the typical, or 
``average'' shift would permit frequent or sustained exposures to the 
contaminant at concentrations significantly higher than the compliance 
limit.

[[Page 5866]]

    Although MSHA's dpm compliance limit was not derived from any 
corresponding ACGIH TLV, the explanation of the proper interpretation 
and application of TLV's provided in the 1999 TLV's and BEI's booklet 
(American Conference of Governmental Industrial Hygienists, 1999), is 
relevant to this discussion. Compliance limits are specifically 
intended to be applied over a conventional eight-hour work day and 
forty-hour workweek, and not to the average exposure received during a 
series of consecutive work shifts or workweek. Although an allowance is 
made in some instances for calculating exposures on the basis of a 
workweek average concentration, MSHA believes such an exception should 
not apply to dpm because of (1) the seriousness of associated health 
risks (such as lung cancer and premature death from cardiovascular, 
cardiopulmonary, or respiratory causes) and (2) the significant risk of 
adverse health effects associated with short-term exposures).
    The only circumstance in which a single, out-of-compliance sample 
would not be used as the basis for a non-compliance determination is if 
the sample itself were considered invalid; for example, an inspector 
following an improper sampling procedure. MSHA is of course concerned 
primarily with the health and safety of miners so the magnitude of any 
citation for a single out-of compliance sample will take into account 
the actual risk posed to miners.
    MSHA's policy on health inspections requires inspectors to 
rigorously follow established sampling procedures to ensure the 
validity of samples collected. As a practical matter, MSHA will not 
sample for diesel particulate at the tailpipe of any diesel powered 
equipment in metal and nonmetal underground mines. As discussed below, 
MSHA's sampling strategy for determining operator compliance is 
established in paragraph (c) of Section 57.5062. That section 
specifically states that MSHA will conduct personal sampling, 
occupational sampling, and/or area sampling, depending upon the 
circumstances of the particular exposure. Because MSHA has an 
environmental exposure limit, MSHA is interested in obtaining the level 
of diesel particulate in the environment where miners normally work or 
travel. In the alternative, MSHA may conduct personal sampling where 
circumstances necessitate it. For example, if a mine operator has a 
miner working inside a cab and there are no other workers in that area 
working outside the cab, MSHA will conduct personal sampling of the cab 
operator and not conduct environmental sampling outside the cab in the 
same area of the mine. Moreover, MSHA's sampling would be conducted 
inside the cab rather than outside the cab. On the other hand, if there 
are miners working outside the enclosed cab, MSHA will sample the 
environment to determine the level of exposure to dpm for these miners. 
Also, if an operator has a miner who is operating a shuttle car, and 
that miner is replaced by another miner during that shift, MSHA intends 
to place the sampler on the shuttle car in the vicinity of the miner 
and not at the tailpipe. However, in no case will area sampling be 
performed closer than five feet to a piece of operating diesel 
equipment, and no tailpipe sampling will be performed to determine 
compliance with any concentration limit.
    Among other precautions, sampling equipment is maintained and 
operated in strict accordance with manufacturer recommendations, and 
pumps are calibrated before and after samples are collected. Sampling 
media are blank-corrected, and all laboratory handling and analytic 
procedures are in accordance with AIHA laboratory certification. Sample 
integrity is ensured through chain-of-custody seals. If any breach in 
procedure occurs, all affected samples are invalidated.
    In order to assure compliance with the limit, mine operators need 
to implement controls sufficient to ensure that the entire range of 
concentration values is always safely below the compliance limit. The 
purpose of both MSHA sampling and mine operator monitoring is to 
verify, on an on-going basis, that this limit is always met on every 
shift.
    When mine operators implement effective engineering controls, the 
range of the concentration values becomes narrower so that once control 
of dpm is demonstrated, it is unlikely that the concentration limit 
will be exceeded.
    MSHA believes the same justification for determining noncompliance 
based on a single sample applies to dpm as to other contaminants and 
noise. Therefore, MSHA has retained the provision permitting a 
noncompliance determination to be based on a single sample.
    Using NIOSH Method 5040 for compliance determinations. Pursuant to 
paragraph (b) of section 5061 of the final rule, MSHA will collect dpm 
samples for compliance using a respirable dust sampler equipped with a 
submicrometer impactor, and analyze such samples for the amount of 
total carbon using NIOSH Method 5040 (or by using any method of 
collection and analysis subsequently determined by NIOSH to provide 
equal or improved accuracy) for the measurement of dpm in underground 
metal and nonmetal mines. As noted above, this is like the proposed 
rule except that the final rule explicitly requires that a 
submicrometer impactor be used in collecting all dpm compliance samples 
in underground metal and nonmetal mines.
    Section 3 of part II of this preamble discusses alternative methods 
for measuring dpm concentrations, and reviews the many comments MSHA 
received on this topic. As noted in that discussion, methods other than 
NIOSH Method 5040 do not at this time provide the accuracy required to 
support compliance determinations at the concentration levels required 
to be achieved under this rule. Moreover, after a careful review of the 
comments and hearing record, the available technical information 
submitted in response to MSHA's proposed rule, and the results of 
studies performed by agency experts to ascertain the veracity of those 
comments and submissions, MSHA has determined that NIOSH method 5040 
provides an accurate method of determining the total carbon content of 
a sample collected in any underground metal or nonmetal mine when a 
submicron impactor is used with the otherwise prescribed sampling 
procedure, and when sampling strategies avoid sampling under 
circumstances that could compromise the integrity of the analytical 
process. Accordingly, MSHA will use this method for determining TC 
concentrations for compliance purposes, and the rule has been 
specifically amended to require that such samples be taken with a 
submicron impactor.
    As indicated in the discussion of the proposed rule (p. 58129), 
utilizing the submicron impactor--a device that limits particles 
entering the sampler to those less than 0.9 micron in size when 
operated at a flow rate of 1.7 LPM--does cause a reduction in the 
amount of dpm that can enter the sampler, since some dpm is larger than 
0.9 microns. Thus, in making this amendment, MSHA recognizes that 
underground metal and nonmetal miners will be exposed to more dpm than 
will be ascertained by these compliance measurements. However, for the 
reasons noted in section 3 of Part II, MSHA has determined that 
requiring use of the impactor is the only way to ensure that certain 
potential interferences (sources of total carbon other than dpm) are 
avoided at this time. Thus, to ensure the integrity of the sampling 
method, the agency has determined that it must use such an impactor.

[[Page 5867]]

    One commenter suggested that, in addition to basing concentration 
limit compliance determinations on samples collected pursuant to 
Sec. 57.5061, samples collected and analyzed in accordance with 
Sec. 7.89 should also be used as a basis for compliance determinations. 
Section 57.5061 is the compliance determination for the ambient 
concentrations in the mine. Based on the ventilation being supplied, 
the number of engines being used, the condition of the engines, the 
duty cycle of the machines, the sample will show if the mine is in 
compliance with the dpm standard. Section 7.89 is the laboratory test 
for the diesel in engine in the lab to measure the raw dpm from the 
engine. The Sec. 7.89 test data is used to calculate the particulate 
index for a single engine. Section 7.89 data can give the mine operator 
an idea of the dpm being emitted from the single engine and can use 
this data in the ``Estimator'' to calculate an estimated dpm ambient 
concentration. However, as explained elsewhere in the preamble, this is 
an estimate to set up proper ventilation when adding other pieces of 
equipment or deciding on which engine to buy. The section 7.89 dpm 
concentration does not take into account the duty cycle of the engine. 
Section 7.89 tests all engines on a specific test cycle. Section 7.89 
test data can only be used to estimate dpm, cannot be used to know 
exactly what the concentration is in a mine at any given time. The test 
in 57.5061 is used for that determination. MSHA believes this procedure 
is inappropriate for determining compliance with the concentration 
limits and provision for doing so has not been included in the final 
rule.
    Sampling strategy--personal, occupational, and area sampling. 
Subsection (c) of section 5061 provides for MSHA inspectors to 
determine the appropriate sampling strategy for compliance 
determinations: personal sampling (attaching a sampler to an individual 
miner within the miner's breathing zone), area sampling (sampling at a 
fixed location where miners normally work or travel), or occupational 
sampling (locating the sampler on a piece of equipment where a miner 
may work).
    Personal sampling is well understood in the metal and nonmetal 
sector because it is commonly used by MSHA to determine compliance with 
TLV's for silica-bearing respirable dust, welding fumes, and 
other airborne contaminants. Area sampling is less well known in this 
sector, but it is used by MSHA for compliance determinations in some 
situations, such as where miners are exposed to a contaminant having a 
ceiling limit. Occupational sampling is not well known in the metal and 
nonmetal sector because it is not currently used by MSHA for compliance 
determinations in this sector. However, MSHA does employ occupational 
sampling in the coal sector for compliance determinations.
    Occupational sampling is a method which measures the exposure of an 
occupation to a given contaminant, as opposed to personal sampling, 
which measures the exposure of an individual, or area sampling, which 
measures the contaminant concentration at a fixed location throughout 
the working shift. All three methods determine contaminant 
concentration on a shift weighted average basis (see previous 
discussion of ``Concentration limit expressed as an average eight hour 
equivalent full shift airborne concentration'' under Sec. 57.5060). In 
occupational sampling, a full-shift sample is collected from the 
working environment of the occupation. The sampling apparatus (sample 
pump, size selection devices, sample filter, etc.) remains in the 
environment of the work position being sampled rather than with the 
individual miner, even when miners change positions or alternate duties 
during the shift.
    A very common example of where occupational sampling would be the 
appropriate sampling method is where the sampling objective is to 
determine the full shift exposure of the operator of a particular piece 
of equipment, but where two or more individuals alternate operating the 
equipment. Personal sampling would capture both the exposure received 
while the equipment is being operated, as well as the exposure received 
while performing other duties. Area sampling would be limited to 
measuring the contaminant concentration in the general area where the 
equipment is operated, but would not capture the operator's exposure. 
In this example, occupational sampling, with the sample apparatus 
remaining in the cab or operator's compartment of the equipment 
throughout the shift, would be the only sampling method that could 
satisfy the sampling objective.
    As noted above, the provision for utilizing either personal 
sampling, area sampling, or occupational sampling was not explicitly 
stated in the proposed rule. It was, however, clearly stated in the 
preamble to the proposed rule as MSHA's intent; indeed, a specific 
Question and Answer was devoted to the topic. (63 FR 58117, Question 
and Answer 14; the topic is further explored at 63 FR 58185). Moreover, 
in explaining its adoption of a ``concentration limit'', MSHA noted 
that its intention was to emulate the approach taken with coal mine 
dust, where inspectors have similar discretion (63 FR 58184) in the 
preamble to the proposal). Accordingly, the mining community was fully 
informed in this regard. The topic was the subject of considerable 
discussion at the hearings and received considerable comment.
    After evaluating the comments, and reviewing the verification data 
on possible interferences discussed in Part II of this preamble, MSHA 
determined that its proposed position in this regard should be 
explicitly incorporated into the final rule. At the same time, as a 
result of the comments, the Agency has refined its thinking as to when 
various types of sampling would be appropriate. The Agency will provide 
further information in this regard in its compliance guide, but is 
using this opportunity to inform the underground metal and nonmetal 
mining community of its current views on how it will initially approach 
this matter.
    Numerous commenters expressed concern about the proposed rule's 
provision for using either personal sampling or area sampling for 
determining compliance with the concentration limit for dpm. They 
pointed out that area sampling was a departure from previous 
enforcement practice in metal and nonmetal mines. They also questioned 
whether it was appropriate to use area sampling to determine compliance 
when there may be no one exposed (or very limited miner exposure) to 
dpm at the time and in the location where the area sample is taken, as 
well as in situations where miners work in enclosed cabs with filtered 
breathing air, and in other areas where engineering controls are not 
feasible. One commenter also argued that sampling at a fixed location 
(area sampling) and then equating the results with a personal exposure 
was invalid.
    Commenters also asserted that the superiority of personal sampling 
for quantifying worker exposures is a commonly accepted industrial 
hygiene principle. Some commenters noted that in underground mines 
which use mobile diesel equipment, the positions of diesel-powered 
vehicles with respect to intake and return air streams vary from hour 
to hour. Therefore, they asserted, it is virtually impossible to obtain 
meaningful information from stationary instruments. One commenter 
stated that area sampling was appropriate as a screening tool to 
determine whether personal sampling would be warranted, or to evaluate 
the effectiveness of controls, but that it

[[Page 5868]]

should not be used to determine compliance with a mandatory limit.
    In responding to these comments, MSHA would like to emphasize to 
the metal and nonmetal mining community, as it did in the preamble to 
the proposed rule, that while the concept of a concentration limit is 
new for this sector, it is a well established concept in the mining 
industry, and has been implemented for many years with respect to coal 
dust. Questions about whether a particular sampling method are 
appropriate in a given situation have been raised and resolved many 
times.
    Moreover, the courts have upheld MSHA's use of area sampling for 
enforcing compliance. In a 1982 decision (American Mining Congress v. 
Secretary of Labor, Nos. 80-1581 and 80-2166), the U.S. Court of 
Appeals, Tenth Circuit ruled that the decision to employ area sampling 
for respirable dust compliance determinations was a reasonable exercise 
of MSHA's discretion and authority. The court stated:

    ``Nothing in the record supports the conclusion that either type 
of sampling provides a perfect measure of exposure to respirable 
dust. Since there is no perfect sampling method, the Secretary has 
discretion to adopt any sampling method that approximates exposure 
with reasonable accuracy. The Secretary is not required to impose an 
arguably superior sampling method as long as the one he imposes is 
reasonably calculated to prevent excessive exposure to respirable 
dust. On this record, the difference between area and personal 
sampling is not shown to be so great as to make Secretary's choice 
of an area sampling program irrational. Keeping in mind that our 
task is not to determine which method is better, we hold that the 
Secretary's choice of area sampling over personal sampling is not 
legally arbitrary and capricious.''
    ``We are not unmindful that area sampling may effectively 
require lower dust levels than might be required under a personal 
sampling program.''
    ``The fact that in theory the regulation may require operators 
to maintain a dust level below [the limit] in its person-by-person 
impact does not render the regulation arbitrary and capricious. We 
repeat that all proposed sampling methods are less than perfect and 
are designed to provide only estimates of actual exposure. Since 
measurement error is inherent in all sampling, the very fact that 
Congress authorized a sampling program indicates that it intended 
some error to be tolerated in enforcement of the dust standard. The 
method selected by the Secretary, while perhaps more burdensome in 
its impact on mine operators than other methods, is not beyond the 
scope of his discretion.''

In addition to affirming MSHA's discretion to employ area sampling on 
the basis that it can be ``reasonably calculated to prevent excessive 
exposure,'' the court also observed that area sampling can be 
considered superior to personal sampling for enforcement purposes:

    ``The area sampling program has several advantages over a 
personal sampling program. The most important advantage is that area 
sampling not only measures the concentration of respirable dust, it 
allows identification and thus control of dust generation sources. 
Control of dust at the source will obviously contribute to reducing 
the level of personal exposure. By contrast, the results of personal 
samples do not allow identification of dust sources due to the 
movement of miners through various areas of the mine during the 
course of a working shift. Thus, while a personal sampling system 
makes possible the identification of discrete individuals who have 
been overexposed, it does nothing to ensure reduction of dust 
generation because the source of the dust cannot be determined. 
Therefore, it clearly appears that area sampling can rationally be 
found to be superior to personal sampling as a means of enforcing 
(as opposed to merely measuring) compliance with [the standard].''

Although this decision relates specifically to respirable dust, it is 
clear that the Court of Appeals did not find that area sampling is 
inherently unreliable. Moreover, the logic expressed by the Court in 
describing the application of area sampling to respirable coal mine 
dust applies equally to dpm. Both are solid particulates that are 
produced from discrete sources during mining and are transported via 
the mine's ventilation system and inhaled by miners.
    Accordingly, the fact that some in the metal and nonmetal sector, 
or some not engaged in mining at all, may not be familiar with this 
approach does not make it invalid or inappropriate.
    Implementation by MSHA of its discretion. For the reasons noted 
above, MSHA has determined that personal sampling, occupational 
sampling, and area sampling are all viable sampling methods, and that 
inspectors should have the discretion to utilize whichever sampling 
strategy is appropriate in a given situation to determine compliance 
with the concentration limit for dpm. Accordingly, all three approaches 
are permitted in the final rule.
    The Agency will provide further information about how these 
approaches should be used for dpm sampling in its compliance guide; 
however, it is using this opportunity to inform the underground metal 
and nonmetal mining community of its current views on some common 
situations.
    For example, one commenter noted that an area sample could be taken 
adjacent to where a piece of diesel equipment was accelerating at low 
RPM, which is the time that an engine is working at its lowest 
efficiency. This commenter expressed concern that such a sample could 
indicate that the applicable dpm concentration was exceeded, even 
though the duty cycle as a whole for that equipment might be in 
compliance. MSHA believes this situation shouldn't result in a 
violation, because such an area sample would be taken for an entire 
shift, not just for the short time period when the piece of diesel 
equipment passes by the sampler.
    Moreover, MSHA recognizes that it would not provide an accurate 
measure of the concentration of dpm to place a sampler in the area 
immediately around a machine's tailpipe when no workers would be in 
that location for any great length of time. An area sample would not be 
taken in that manner. But if a worker were assigned to work in a 
location on or immediately adjacent to diesel equipment, a personal or 
occupational sample might well be appropriate to determine if the limit 
is being exceeded for that worker or for such occupation.
    Similarly, the agency would not consider it appropriate to conduct 
area sampling for compliance determinations in areas where dpm 
exposures, if any, would be infrequent and brief; in areas where miners 
work exclusively inside enclosed cabs; and in shafts, inclines, slopes, 
adits, tunnels and similar workings that are designated as return or 
exhaust air courses and that are also used for access into, or egress 
from an underground mine.
    Examples of the first situation would be work areas that are 
visited infrequently and briefly, such as a remote pump that needs to 
be checked weekly, or a remote area where roof conditions need to be 
inspected at periodic intervals. These areas would clearly be subject 
to the concentration limit because miners ``normally work or travel'' 
there. Area sampling in such areas would be inconsistent with the 
regulation's intent to, `` * * * limit the concentration of [dpm] to 
which miners are exposed * * *,'' because exposure would occur for only 
a few minutes per week, or possibly less.
    Examples of the second situation would be production areas or 
haulageways where the only miners present work inside of enclosed and 
isolated cabs with appropriate filtration of breathing air, and 
underground crushing stations where crusher operator booths or similar 
fixed structures are provided with appropriately filtered breathing 
air. Area sampling outside such cabs or structures, which would have 
been permitted under the proposed rule,

[[Page 5869]]

would be inconsistent with the regulation's intent to, `` * * *limit 
the concentration of [dpm] to which miners are exposed * * *,'' because 
miners in these areas are not exposed; they are already protected by an 
accepted engineering control. This approach is consistent with MSHA's 
intent as stated in the preamble to the proposed rule (63 FR 58184). It 
also reflects MSHA's awareness that enclosed cabs may provide many 
other important health and safety benefits, such as reducing noise 
exposure and reducing exposure to silica bearing respirable dust.
    However, as a result of the comments concerning whether NIOSH 
method 5040 can effectively be used to determine compliance when miners 
are smoking, the agency recognizes that it faces a particular 
difficulty in sampling miners when they smoke inside an enclosed cab or 
booth, whether such sampling is area, occupational, or personal. As 
noted in Part II, section 3, MSHA has verified that sampling using 
NIOSH method 5040 immediately adjacent to smokers can undermine the 
validity of the sample result--since some of the total carbon detected 
may be from the smoke). While MSHA can generally avoid this problem by 
not sampling immediately near smokers, as discussed in that section of 
this preamble, it does face a problem when the area to be sampled is an 
enclosed cab or booth: it can neither sample inside nor outside an 
enclosed cab or booth if the subject miner smokes. The Agency intends 
to address this problem by obtaining the concurrence of the miner not 
to smoke while sampling the environment of the cab.
    MSHA is troubled that, under certain circumstances, it will need to 
rely on miners voluntarily refraining from smoking in order to perform 
compliance sampling for dpm. Since miners are usually free to choose to 
smoke if they wish, this need to rely on the voluntarily cooperation of 
miners could seriously limit the agency's ability to sample when and 
where it desires. Though MSHA has determined that sampling of 
nonsmokers would usually be unaffected by the presence of smokers 
elsewhere in the mine, there will be situations where sampling of a 
specifically targeted area, occupation, or person would be prevented 
due to the presence of a smoker at that immediate location. Therefore, 
MSHA intends to continue to search for a means to reliably measure dpm 
concentrations despite the presence of cigarette, cigar, and pipe smoke 
in close proximity to the sampling equipment.
    As noted in Part II, section 3, MSHA has determined that samples 
analyzed only for elemental carbon are unaffected by the presence of 
cigarette smoke. At this time, however, MSHA cannot limit its analysis 
to elemental carbon, because no consistent quantitative relationship 
has been established between elemental carbon concentration and the 
concentration of whole dpm.
    MSHA intends to implement any newly developed sampling procedure 
and/or analytical method that is capable of directly or indirectly 
measuring the concentration of whole dpm in the presence of cigarette, 
cigar, and pipe smoke, provided such procedure and/or method is 
determined by NIOSH to provide equal or improved accuracy compared to 
the NIOSH Method 5040. If MSHA decides that such a change in sampling 
procedure and/or analytic method should be adopted, the agency will 
utilize standard communication channels to provide specific 
notification of its intention in this regard to the underground metal 
and nonmetal mining industry. However, MSHA wishes to be clear that, in 
accordance with Sec. 57.5061(b), implementing such a change does not 
require new rulemaking.
    Examples of the third situation include return or exhaust air 
courses that are shafts, inclines, slopes, adits, tunnels, etc. which 
terminate on the surface, but which are also used for mine access or 
egress by mine personnel.
    Since the purpose of a return or exhaust air course is to collect 
and remove contaminated air from the mine, one would expect such an air 
course could contain high dpm levels. However, being a major travelway, 
one would naturally consider them to be areas ``where miners normally 
work or travel.'' As miners travel into the mine at the beginning of 
the shift and out of the mine at the end of the shift through these 
mine openings, relatively brief exposures to potentially high dpm 
levels could be expected. Full shift area sampling in such a location 
would likely indicate dpm levels in excess of the concentration limit. 
Should area sampling in such an air course result in a determination of 
noncompliance (which would be highly likely), the mine operator would 
be required to implement a change of some kind to bring the area into 
compliance, such as requiring that miners use a different access to the 
mine that is an intake or neutral air course, or that the ventilation 
system would need to be changed so that the access in question is no 
longer a return or exhaust air course. Since neither of these options 
may be feasible, the operator would be placed in an impossible 
compliance situation.
    In such situations, MSHA believes that it would not be appropriate 
to use area sampling; rather, personal sampling would be more 
appropriate. Personal sampling would capture the exposure as miners 
travel into the mine at the beginning of the shift and depart at the 
end of the shift. Since the exposure time is brief, overexposure on a 
full-shift basis would be unlikely (assuming dpm levels in the working 
places are in compliance). Also, since exposure time is brief, the 
health risk associated with the exposure would be minimal.
    It should be noted, however, that miners whose jobs require them to 
spend significant periods of time in these areas would continue to be 
at risk of overexposure if the dpm levels are high. For example, a 
haulage truck driver that spends much of the shift driving in and out 
of the mine through exhaust air hauling material to a surface dump 
point or crusher may need to be protected with an enclosed cab that is 
provided with filtered breathing air. Personal sampling on miners who 
engage in such activities would reveal the problem.
    Another situation requiring clarification as to MSHA's intended 
compliance sampling procedures concerns miners who perform multiple 
work tasks during a shift. If a miner's work on a given shift includes 
a task or tasks for which the sampling procedures would not provide an 
accurate measurement of the dpm, MSHA would not use that measurement 
for the basis of a compliance determination. An example would be a 
miner who begins the shift operating a diesel-powered loader, and who 
finishes the shift operating a jack leg drill equipped with an in-line 
oil bowl. While operating the loader, MSHA would consider a personal or 
occupational sampling procedure to be acceptable for obtaining an 
accurate measurement for compliance purposes. However, as noted in 
Section II, MSHA would not consider personal or occupational sampling 
to be acceptable for sampling a miner who is operating a jack leg drill 
equipped with an in-line oil bowl, because there is the potential that 
oil mist emitted from the drill may be collected on the sample filter 
causing an inaccurate measurement of dpm to be made.
    In this case, full shift area sampling would be performed at a 
location where the oil mist would not interfere with the measurement of 
dpm. If the drilling operation takes place in a different location from 
the loading operation (a different stope, for example), MSHA would 
consider full shift area sampling in both locations, if appropriate.

[[Page 5870]]

However, if no source of dpm is present at the drilling location, the 
inspector would probably choose to sample only the location where the 
loader is operating.
    The agency considered whether it would be appropriate to deal with 
these situations through an amendment of the rule, and decided this 
would not be appropriate. The specific facts in a specific situation 
should determine the appropriateness of the sampling approach; trying 
to lock down this situation or that in the rule would prove very 
complex and restrict the flexibility to react to developments in the 
industry. The rule reserves to MSHA the flexibility to adjust the use 
of sampling approaches for any situation where use of one or another 
method might not be appropriate.
    At the same time, the Agency wishes to make it clear that in 
putting explicitly into the rule that the Agency can use any of the 
three methods specified, it intends by that action to ensure that any 
policy that would broadly restrict the use of one or another of these 
methods would have to be the subject of new rulemaking. Thus, for 
example, any policy to significantly restrict the use of area sampling 
to enforce compliance with this rule would have to be the subject of 
new rulemaking action, as the availability of that method was a key 
consideration in MSHA's decision that it could implement a 
concentration limit.

Section 57.5062  Diesel Particulate Matter Control Plan

    Under the final rule, a determination of noncompliance with either 
the interim or final concentration limit prescribed by Sec. 57.5060 
would trigger two requirements: first, the operator must establish a 
diesel particulate matter control plan (dpm control plan) meeting 
certain basic requirements--or modify the plan if one is already in 
effect; and second, the operator must demonstrate that the new or 
modified plan will be effective in controlling the concentration of dpm 
to the applicable concentration limit. The final rule also sets forth a 
number of other specific details about such plans, and states that 
failure of an operator to comply with the provisions of a plan or to 
conduct required verification sampling will be a violation of Part 57 
without regard for the concentration of dpm that may be present. In all 
respects, this section of the final rule is essentially the same as in 
the proposed rule.
    Only a few comments were directed specifically at Sec. 57.5062. 
Some of those were supportive of the concept, such as the remark by one 
mine operator that, ``Generally, the Diesel Particulate Matter Control 
Plan (DPMCP) contained in Sec. 57.5062 is well conceived.'' One 
commenter noted that once a plan is in place, failure to abide by its 
provisions is a citable violation, even if dpm levels are below the 
applicable concentration limit. Another commenter recommended that 
rather than a single out-of-compliance sample triggering the 
requirement to implement a plan, the provisions of Sec. 57.5062 should 
not be triggered unless there is a significant history of non-
compliance with the limit. Another commenter questioned why a 
determination of non-compliance requires MSHA to obtain only one non-
compliant sample, whereas proof of operator compliance (both with 
respect to Sec. 57.5062 and Sec. 57.5071) requires multiple operator 
samples. A commenter also observed that a single sample is not 
``statistically significant or representative and cannot determine if 
the mine is out of compliance.'' The same commenter argued that the 
requirements for documenting dpm control plan effectiveness were 
unnecessary, burdensome, and duplicated other MSHA requirements.
    Triggering plan. Under the final rule, a single out-of-compliance 
dpm sample constitutes a citable violation of the applicable 
concentration limit and triggers the requirement to implement a diesel 
particulate matter control plan. As noted above, one commenter 
recommended that a diesel particulate matter control plan should not be 
required unless a mine has a significant history of non-compliance with 
the applicable dpm concentration limit. MSHA disagrees with the 
commenter's position because MSHA does consider a single sample to be a 
valid means of determining compliance (see discussion under 
Sec. 57.5060 on single sample), and because a ``significant history of 
non-compliance'' at a given mine, would almost certainly be accompanied 
by significant, prolonged, and repeated exposure of miners to dpm 
levels in excess of the applicable concentration limit. Such exposures 
cannot be tolerated. When sampling indicates non-compliance, remedial 
action consisting of the implementation of a dpm control plan, or 
modification of an existing plan, must be initiated without delay. This 
will insure a timely reduction in dpm levels, and will help prevent dpm 
levels from rising above the applicable concentration limit in the 
future.
    No advance approval of plans required. Sec. 57.5062 will maintain 
the Agency's metal and nonmetal mine plan tradition by not invoking a 
formal plan approval process. That is, the plan would not require 
advance approval of the MSHA District Manager. As noted in the 
discussion of Sec. 57.5060(c) and (d), MSHA is requiring advance 
approval for an operator to obtain a special extension of up to 2 years 
to meet the final concentration limit, and/or to allow miners 
performing inspection, maintenance or repair work to conduct such 
activities in areas that exceed the concentration limit. But a plan 
required because the limit has been exceeded need not obtain such 
advance approval.
    In the preamble to the proposal for this Part, MSHA requested 
comment from the mining industry as to whether dpm control plans should 
require pre-approval by the Agency (p. 58119). The only comment 
received was in support of the Agency's proposal that such plans not 
require pre-approval.
    A dpm control plan would, however, have to meet certain 
requirements set forth in the final rule, and as noted in the preamble 
to the proposed rule, it would be a violation of Sec. 57.5062 if MSHA 
determines that the operator has failed to adequately address each of 
the plan's required elements.
    Moreover, as discussed subsequently in connection with paragraph 
(f) of this section, once in place, a dpm control plan becomes law for 
that mine, and an operator must comply with it.
    Elements of plan. Under Sec. 57.5062(b), a dpm control plan must 
describe the controls the operator will utilize to maintain the 
concentration of diesel particulate matter to the applicable limit 
specified by Sec. 57.5060. The plan must also include a list of diesel-
powered units maintained by the mine operator, together with 
information about any unit's emission control device and the parameters 
of any other methods used to control the concentration of diesel 
particulate matter.
    Relationship to ventilation plan. At the discretion of the 
operator, the dpm control plan may be consolidated with the ventilation 
plan required by Sec. 57.8520.
    Demonstration of plan effectiveness. The final rule would require 
monitoring to verify that the dpm control plans are actually effective 
in reducing dpm concentrations in the mine to the applicable 
concentration limit. Because the dpm control plan was initiated as a 
result of a compliance action, the final rule would require the use of 
the same measurement method used by MSHA in compliance determinations--
total carbon using NIOSH method 5040--to conduct verification sampling. 
As a result, mine operators who are required to establish a dpm control 
plan would need to acquire the necessary sampling equipment to conduct 
the verification sampling, or arrange for such sampling

[[Page 5871]]

to be conducted for them. As noted in Part II, the necessary sampling 
equipment is commercially available.
    MSHA recognizes concerns about the commercial availability of the 
sampling equipment for NIOSH Method 5040. It is important that 
operators know whether they are in compliance with the standard. MSHA 
understands that the equipment will be available before this standard 
is in effect. MSHA will not use any equipment for sampling for 
compliance with this standard that is not commercially available. If 
the equipment is not commercially available by the effective date of 
the standard it is MSHA's intention not to enforce the dpm levels in 
the standard until the sampling equipment is available.
    Effectiveness must be demonstrated by ``sufficient'' monitoring to 
confirm that the plan or amended plan will control the concentration of 
diesel particulate to the applicable limit under conditions that can be 
``reasonably anticipated'' in the mine.
    The final rule, like the proposed rule, does not specify that any 
defined number of samples must be taken--the intent is that the 
sampling provide a fair picture of whether the plan or amended plan is 
working. Instead, as indicated in the preamble to the proposed rule, 
MSHA will determine compliance with this obligation based on a review 
of the situation involved. While an MSHA compliance sample may be an 
indicator that the operator has not fulfilled the obligation under this 
section to undertake monitoring ``sufficient'' to verify plan 
effectiveness, it would not be conclusive on that point.
    One commenter questioned the fairness of holding operators 
responsible for verifying plan effectiveness, the need for 
documentation to verify that plans will control dpm to the applicable 
limit, and for the requirement that such documentation must be provided 
upon request by MSHA. This commenter suggested that mine operators are 
already required to show compliance with air quality standards under 
Sec. 57.5002, and that further documentation relating to the diesel 
particulate matter control plan therefore duplicates existing 
requirements.
    While it is true that Sec. 57.5002 requires mine operators to 
conduct ``dust, gas, mist, and fume surveys'' as frequently as 
necessary to determine the adequacy of control measures, this 
regulation does not specifically address diesel particulate matter, nor 
does it specify that dpm concentrations must be determined using the 
NIOSH Method 5040 (as is required in Sec. 57.5062(c)). Thus, compliance 
with Sec. 57.5002 will not insure compliance with the intent of 
Sec. 57.5062. Section 57.5062(c) also requires that mine operators 
demonstrate that dpm concentrations will be controlled to applicable 
limits, not only under current conditions (i.e., that a compliant 
sample be obtained), but also under reasonably anticipated conditions 
in the future.
    MSHA disagrees with the commenter's suggestion that ``rigorous 
enforcement of existing TLV's and air quality rules, and * * 
* utilization of recommendations in the `Diesel Toolbox''' will result 
in ``adequate safety levels.'' The 1973 Threshold Limit 
Values or TLV's (the TLV©'s incorporated by 
reference in Sec. 57.5001, and therefore currently enforceable in 
underground metal and nonmetal mines) do not include a limit of any 
kind for dpm. It is interesting to note that, as indicated in Table II-
2 of Part II, section 5, the TLV's enforced by MSHA are 
derived from recommendations of the American Conference of Governmental 
Industrial Hygienists (ACGIH). That organization has recently proposed 
a limit for dpm (ACGIH Notice of Intended Changes for 1999) of 
50DPMg/m3, well below what is being 
established by this rule. As noted in Part V of this preamble, MSHA has 
concluded that 50DPMg/m3 is an 
unreasonably low limit for dpm concentration in underground metal and 
nonmetal mines because MSHA's technological and economic feasibility 
assessment indicate that this level cannot be achieved using feasible 
control measures.
    If a diesel particulate matter control plan is in effect, the final 
rule specifies that monitoring must be ``sufficient to verify that the 
plan will control the concentration of diesel particulate matter to the 
applicable limit under conditions that can be reasonably anticipated in 
the mine.'' Again, as conditions and circumstances in the mine change, 
the mine operator must demonstrate, on a continuing basis, through 
sampling results using NIOSH Method 5040, that compliance with the 
applicable concentration limit is consistently achieved.
    MSHA believes that dpm control requires a holistic approach. A 
piecemeal solution to a dpm problem may result in shifting an 
overexposure from one area to another, but not eliminating the problem 
entirely. If an overexposure in one part of the mine is addressed by 
re-routing more ventilation air to that area, it means another part of 
the mine will have to give up some air, possibly causing an 
overexposure there. If an overexposure in one part of the mine is 
addressed by exchanging a dirty machine for a clean machine, it means 
the dirty machine is still polluting somewhere else. In these examples, 
the actions taken may simply move an overexposure to a different 
location, or they may result in overall compliance. The only way of 
knowing for sure whether the problem has actually been solved, is to 
consider the effects of a given action on the mine as a whole. That is 
what the regulation requires. MSHA does expect operators will focus 
their control plans on the areas of the mine in which dpm presents a 
hazard to miners.
    The reason that MSHA can determine non-compliance based on a single 
sample whereas mine operators need multiple samples to demonstrate 
compliance is due to the fundamental difference between proving non-
compliance versus proving compliance. For example, proving that at 
least one non-compliance condition exists somewhere in a mine requires 
only one non-compliant sample result. Proving conditions are fully 
compliant everywhere in a mine all the time requires more than one 
compliant sample result. The actual number of compliant samples 
necessary to prove that every location in the mine is fully compliant 
all the time would have to be determined, but it would rarely, if ever, 
be only one.
    The differences between determining non-compliance versus 
determining compliance are incorporated into standard industrial 
hygiene practice. For example, regarding the evaluation of the exposure 
of a worker over a single day by means of a full-period measurement 
(which is MSHA's compliance sampling approach), Patty's Industrial 
Hygiene and Toxicology (3rd Edition, 1994) states, ``In that case, the 
error variance is determined by only the sampling and analytical error, 
and confidence limits tend to be quite narrow.'' By appropriately 
accounting for sampling and analytic errors, MSHA will assure, at the 
95% confidence level, that an out-of-compliance sample accurately 
reflects an out-of-compliance condition in the mine.
    This contrasts with the mine operator's need to verify compliance. 
Patty's states, ``Usually, however, our concern is with the totality of 
a workers exposure, and we wish to use the data collected to make 
inferences about other times not sampled. There is little choice; 
unless the universe of all exposure occasions is measured, we must 
``sample,'' that is, make statements about, the whole based on 
measurement of some parts.''
    ``The American Industrial Hygiene Association has addressed the 
issue of

[[Page 5872]]

appropriate sample size (Hawkins et al., 1991) and recommends in the 
range of 6-10 random samples per homogeneous exposure group. Fewer than 
6 leaves a lot of uncertainty and more than 10 results in only marginal 
improvement in accuracy. Also, it is usually possible to make a 
reasonable approximation of the exposure distribution with 10 samples 
although a rigorous goodness-of-fit test often requires 30 or more.'' 
Although a single sample is not adequate to demonstrate compliance, 
MSHA does not specify in the final rule, a minimum number of samples 
that will constitute adequate verification of compliance in all cases. 
It is the mine operator's responsibility to determine the appropriate 
level of sampling effort and explain the rationale in the diesel 
particulate matter control plan.
    Like the final rule, the proposed rule provided that verification 
sampling would be conducted under conditions that can be ``reasonably 
anticipated'' in the mine. The Agency very specifically solicited 
comment on ``whether, and how, it should define the term `reasonably 
anticipated.' '' (63 FR 58185) The agency noted that with respect to 
coal dust, the Dust Advisory Committee recommended that ``MSHA should 
define the range of production values which must be maintained during 
sampling to verify the plan. This value should be sufficiently close to 
maximum anticipated production.'' (MSHA, 1996) For dpm, the Agency 
suggested, the equivalent approach might be based on worst-case 
operating conditions of the diesel equipment--e.g., all equipment is 
being operated simultaneously with the least ventilation. No comments 
were received on this point.
    Recordkeeping retention and access. Pursuant to section 5062(b), a 
copy of the current dpm control plan is to be maintained at the mine 
site during the duration of the plan and for one year thereafter. 
Section 5062(c) requires that verification sample results be retained 
for 5 years. And, section 5062(d) provides that both the control plan 
and sampling records verifying effectiveness be made available for 
review, upon request, by the authorized representative of the 
Secretary, the Secretary of Health and Human Services, and/or the 
authorized representative of miners. Upon request of the District 
Manager or the authorized representative of miners, a copy of these 
records is to be provided by the operator.
    Duration. The final rule requires the dpm control plan to remain in 
effect for three years from the date of the violation resulting in the 
establishment/modification of the plan. Section 57.5062(e)(1) and 
(e)(2). MSHA has concluded that operators have sufficient time under 
the final rule to come into compliance with the concentration limits; 
if a problem exists, maintaining a plan in effect long enough to ensure 
that daily mine practices really change is an important safeguard. MSHA 
noted its view in this regard in the preamble to the proposed rule; no 
comments were received on this point.
    Modification during plan lifetime. If a diesel particulate matter 
control plan is already in effect at a mine, section 57.5062(a) 
requires the mine operator to modify the current plan upon a subsequent 
violation of section 57.5060, and to demonstrate the effectiveness of 
the modified plan.
    Section 57.5062(e)(3) would require the mine operator to 
independently initiate the modification of an existing dpm control plan 
to reflect changes in mining equipment and/or the mine environment, and 
requires the operator to demonstrate the effectiveness of the modified 
plan.
    It should also be noted that a mine operator, based on dpm sampling 
data or other information or analysis, may at any time, modify the 
provisions of a dpm control plan to make it less restrictive, provided 
sufficient sampling data confirm the plan's continuing effectiveness in 
controlling dpm to compliant levels. A modification made in this manner 
does not affect the 3-year duration of the plan (end date unaffected). 
These plans made by the operator do not require advance approval by 
MSHA.
    Compliance with plan requirements. Section 57.5062(f) states that 
failure by a mine operator to comply with the provisions of a diesel 
particulate matter control plan is a violation of the rule, regardless 
of the concentration of dpm that may be present at any time. Once an 
underground metal or nonmetal mine operator adopts a dpm control plan, 
it is considered law for the mine. Section 57.5062(f) specifically 
provides that MSHA would not need to establish (by sampling) that an 
operator is currently in violation of the applicable concentration 
limit under Sec. 57.5060 in order to determine (by observation) that an 
operator has failed to comply with any requirement of the mine's dpm 
control plan.
    One commenter observed that, ``It does seem odd * * * that 
Sec. 57.5062(f) contemplates that the mere failure to adhere to the 
[dpm control plan] itself is deemed a violation of the regulation--
irrespective of the fact that the exposure to dpm may indeed be less 
than the [concentration limit].''
    MSHA's rationale for making a mine's dpm control plan law for that 
mine derives from the rule's approach to setting control requirements. 
MSHA recognizes that every mine faces a unique set of conditions and 
circumstances relating to equipment, engines, emission controls, 
ventilation, etc. that would make uniform dpm control requirements 
across the entire underground metal and nonmetal mining industry 
unworkable, impractical, and ineffective. Hence, the final rule, with 
just a few exceptions, permits mine operators considerable freedom to 
select the mix of dpm control options they believe are necessary to 
comply with the applicable concentration limit. An operator can filter 
the emissions from diesel-powered equipment, install cleaner-burning 
engines, increase ventilation, improve fleet management, or use a 
variety of other readily available controls, all without consulting 
with, or seeking approval from MSHA.
    However, if MSHA sampling indicates non-compliance with the 
applicable concentration limit, the rule requires the operator reduce 
to writing his or her specific plans for controlling dpm to the 
concentration limit and to adhere to that plan. MSHA considers miner 
exposure to dpm, a probable carcinogen, as a very serious matter, and 
has not established that exposures, even at the concentration limit, 
are safe. That is why a single non-compliant sample triggers the 
requirement for a compliance plan. The plan lays out the minimum steps 
the operator has determined must be followed in that mine to insure 
compliance. Failure to adhere to the requirements of the operator-
developed plan must thus be viewed as a failure to take actions that 
are necessary for compliance with the concentration limit.
    Because of the importance of adhering strictly to an effective dpm 
control plan, a means of enforcing such adherence is necessary. The 
plan is made law for that mine so that its provisions can be enforced 
by MSHA. The plan need not be approved by the MSHA District Manager, 
but it is, nonetheless, law for that mine, and any violation of the 
plan is therefore a violation of the regulation. As discussed above, an 
operator is free to modify a dpm control plan to make it less 
restrictive at any time during its life, and as often as desired, as 
long as sufficient sampling data confirm the plan's continuing 
effectiveness in controlling dpm to compliant levels. MSHA is of course 
concerned primarily with the health and safety of miners so the 
magnitude of any citation for a

[[Page 5873]]

violation of the plan will take into account the actual risk posed to 
miners.
    With respect to the required diesel particulate matter control 
plan, the mine operator is essentially telling MSHA what steps are 
necessary for that mine to comply with the applicable concentration 
limit. If MSHA observes a violation of the plan, it is only reasonable 
and proper for MSHA to conclude that full compliance is therefore not 
possible. If enforcement of the provisions of the dpm control plan 
depended upon obtaining an out-of-compliance dpm sample, plan 
enforcement would be greatly diminished, both in terms of timeliness 
and effectiveness. If such a sample were taken, and found to be out of 
compliance, implementation of needed corrective measures would be 
delayed because MSHA could not require the mine operator to take 
remedial actions until the sample results were obtained from the 
analytic laboratory, which could involve several weeks of time. If such 
a sample were taken, and found to be in compliance, that fact would not 
constitute conclusive evidence that the plan as a whole was fully 
effective (see earlier discussion on the need for multiple samples to 
establish continuing compliance). Thus, while providing inconclusive 
information at best, such a sampling outcome would prevent MSHA from 
enforcing a provision of the plan. Regardless of sampling outcome, it 
is important to remember that a violation of the plan means the mine 
operator did not adhere to the very requirements that were represented 
to MSHA by the operator as being necessary for compliance.
    It should also be noted that MSHA already has similar enforcement 
authority relative to various other plans that are required in the 
underground metal and nonmetal sector. Mine operators are required to 
prepare plans for such purposes as escape and evacuation, rock bursts, 
ventilation, and training. MSHA has the authority to enforce the 
provisions of these plans without first verifying that the observed 
violation has caused an immediate outcome which itself, is prohibited 
by regulation. There is also ample precedent for citing health-related 
violations without sampling, such as Sec. 58.620 on drill dust control, 
and Sec. 57.5005 on respiratory protection.
    The mine operator is required to modify dpm control plans to 
reflect changes in mining equipment or circumstances. The mine operator 
is also required to modify dpm control plans if the plan proves to be 
inadequate, as evidenced by a subsequent non-compliance determination 
during the three year period that the plan is in effect. In either 
case, the modifications to the original plan become law for that mine, 
and violations are subject to enforcement action by MSHA regardless of 
dpm concentration.
    It is also important to remember that dpm levels are determined by 
the complex interaction of numerous factors, such as equipment type, 
engine size, type, and horsepower, duty cycles, engine maintenance, 
equipment operator training and work practices, fuel and fuel 
additives, the characteristics and performance of exhaust filtering 
systems, mine ventilation flows, and many others. Effectively 
controlling dpm levels throughout a mine requires a systematic approach 
that acknowledges the interrelationships and interactions between these 
factors to produce the desired end result, which is compliance with the 
applicable concentration limit. A determination of non-compliance 
indicates that the system of controls has failed. Thus, an effective 
permanent solution requires a comprehensive approach which not only 
corrects the immediate cause of the non-compliance (an out-of-tune 
engine, for example), but also addresses the underlying system failure 
(deficient maintenance management, inadequate dpm monitoring, 
ineffective equipment operator training, failure to tag equipment 
believed to require maintenance, etc.).
    The implementation of a dpm control plan avoids piecemeal solutions 
that result in a repetitive pattern of mines being in and out of 
compliance without ever coming to grips with underlying problems. The 
required elements of a dpm control plan force a comprehensive approach, 
and facilitate effective, permanent solutions to systemic failures. The 
three year duration of such plans insures that the necessary system 
changes become institutionalized and integrated into daily mine 
practices. This, in turn, will increase the chances that mines will be 
in compliance with the applicable concentration limit on a continuous, 
on-going basis.
    MSHA recognizes that some operators may want to supplement the 
compliance plans required by the regulation with additional internal 
instructions that provide supplementary protection--i.e., to achieve 
concentration levels below those required. MSHA does not want to 
discourage such supplemental plans; indeed, it would like to encourage 
them. Accordingly, MSHA will, upon request, work closely with mine 
operators to help avoid confusion by mine and Agency personnel between 
required compliance plans that contain the minimum elements considered 
essential to achieve compliance (and whose provisions are therefore 
enforceable by MSHA) and non-required supplemental plans that contain 
elements the mine operator wishes to implement as a matter of company 
policy (but whose provisions are not enforceable by MSHA).

Section 57.5065 Fueling Practices

    Summary. This section of the final rule establishes the 
requirements for fueling practices in underground metal and nonmetal 
mines. Unlike the proposed rule, the final rule has two subsections.
    Subsection (a) limits the amount of sulfur that may be contained in 
diesel fuel used to power equipment in underground areas, and requires 
mine operators to maintain purchase records that verify the sulfur 
content of the fuel they use.
    Subsection (b) requires that fuel additives used in underground 
diesel-powered equipment be restricted to those registered by the U.S. 
Environmental Protection Agency.
    These subsections of the final rule have not been changed from the 
proposed rule.
    The practices being required by these two subsections are accepted 
industry practices to reduce dpm emissions. They are among the methods 
for reducing dpm explicitly included in MSHA's toolbox publication, and 
were made requirements for underground coal mines as part of MSHA's 
diesel equipment rulemaking. They are among the ``best practices'' for 
reducing dpm emissions that MSHA has determined are technologically and 
economically feasible for all underground metal and nonmetal mines. 
Part II of this preamble contains some background information on these 
practices together with information about the rules currently 
applicable in underground coal mines.
    Low-sulfur fuel. In the final rule, Sec. 57.5065(a) would require 
underground metal and nonmetal mine operators to use only low-sulfur 
fuel having a sulfur content of no greater than 0.05 percent. This 
requirement is identical to that currently required for diesel 
equipment used in underground coal mines [30 CFR 75.1901(a)]. Both 
number 1 and number 2 diesel fuel meeting the sulfur content 
requirement of this rule are commercially available.
    Sulfur content can have a significant effect on diesel emissions. 
Use of low-sulfur diesel fuel reduces the sulfate fraction of dpm 
matter emissions, and

[[Page 5874]]

reduces objectionable odors associated with diesel exhaust.
    Another major benefit of using low-sulfur fuel is that the 
reduction of sulfur allows oxidation catalysts to perform properly. 
Some diesel emission aftertreatment devices, such as catalytic 
converters and catalyzed particulate traps, are ``poisoned'' with fuels 
having high-sulfur content (greater than 0.05 percent sulfur). MSHA 
believes the use of these aftertreatment devices is important to the 
mining industry because they will be necessary for many mines to meet 
the specified concentration limits. The requirement to use low-sulfur 
fuel will allow these devices to be used without additional adverse 
effects caused by the high-sulfur fuel.
    Several commenters questioned why low-sulfur fuel was mandated, 
even for operators who could meet the applicable concentration limit 
using other means. MSHA responds by noting that the use of low-sulfur 
fuel is one of the ``best practices'' that MSHA requires all mines to 
follow, regardless of current dpm levels. Further elaboration on the 
rationale for mandating these ``best practices'' was included in the 
preamble to the proposed rule (63 FR 58119), and a summary was provided 
in this Part under the portion of Sec. 57.5060 that discussed ``Meeting 
the concentration limit, operator choice of engineering controls.'' As 
noted in those discussions, MSHA is required by statute to reduce a 
significant risk to the extent feasible; the use of low-sulfur fuel is 
feasible, has not created any problems in the underground coal sector 
where it is required as a result of the diesel equipment rule, and its 
use will reduce dpm emissions from underground engines.
    In the preamble to the proposal (63 FR 58186), MSHA indicated it 
did not believe a requirement mandating the use of low-sulfur fuel will 
add additional compliance costs. Several commenters contradicted this 
conclusion, arguing that the provision requiring low-sulfur fuel would 
have an adverse cost impact. One commenter supplied actual cost figures 
that showed their fuel costs increased over $18,000 per year after they 
switched to low-sulfur fuel. However, it is significant to note that 
this increase is quite small on both a cost per gallon of fuel basis 
(less than $0.03 per gallon), and a cost per ton basis (about $0.008 
per ton), and that this mine had already made the switch to low-sulfur 
fuel, apparently because they perceived that the benefits justified the 
small additional expense.
    As discussed in the Section IV of the PRIA, MSHA determined that 
the cost difference between high-sulfur and low-sulfur diesel fuel was 
less than $0.02 per gallon in many parts of the country, and in some 
areas, there was no difference at all, or a slight cost advantage to 
using low-sulfur fuel. Fuel used in over-the-road diesel engines is 
currently required by EPA regulations to meet the same 0.05% sulfur 
content limit that is being implemented for underground metal and 
nonmetal mines. Because over-the-road diesel engines represent the bulk 
of the diesel fuel market, such low-sulfur fuel is already readily 
available throughout the country. EPA has proposed regulations that 
would further reduce allowable fuel sulfur content to 0.0015% for over-
the-road diesel engines. Current MSHA regulations limit the sulfur 
content of diesel fuel used in underground coal mines to 0.05%, and the 
availability of this fuel in remote coal mining areas has not been a 
problem for coal mine operators. As discussed above, MSHA has 
determined, based on extensive study of the metal and nonmetal mining 
industry, that compliance with the rule is economically feasible for 
the industry as a whole. Thus, although the provision requiring use of 
only low-sulfur fuel may, in some instances, result in a small cost 
increase for some operators, MSHA estimates that on average, the 
overall measurable impact is negligible. When they are measurable, it 
is because the mine is located in an area where heating fuel has 
relatively large market share compared to diesel fuel used for 
vehicles. This circumstance is unrelated to mine size. Most mines are 
not located in these regions and there is no evidence that small mines 
are disproportionately concentrated in these regions.
    Fuel additives. Paragraph (b) of this section requires mine 
operators to use only diesel fuel additives that have been registered 
by the Environmental Protection Agency (40 CFR Part 79). Again, this 
rule is consistent with current requirements for diesel equipment used 
in underground coal mines [30 CFR 75.1901(c)], and is another of the 
``best practices'' that MSHA considers to be feasible for all 
underground metal and nonmetal mines. The restricted use of additives 
would ensure that diesel particulate concentrations would not be 
inadvertently increased, while also protecting miners against the 
emission of other toxic contaminants. MSHA has published Program 
Information Bulletin No. P97-10, issued on May 5, 1997, that discusses 
the fuel additives list. The requirements of this paragraph do not 
place an undue burden on mine operators because operators need only 
verify with their fuel suppliers or distributors that the additive 
purchased is included on the EPA registration list. To assist mine 
operators in this regard, EPA's Internet site contains a current 
listing of additives registered with EPA. This site can be accessed at 
the following address: http://www.epa.gov/oms/regs/fuels/additive/web-dies.txt. No commenters objected to this requirement.
    Idling practices. Proposed paragraph (c) of Sec. 57.5021 would have 
prohibited idling of mobile diesel-powered equipment, except as 
required for normal mining operations. After further consideration of 
all comments received during the comment period, as well as testimony 
presented at the public hearings, MSHA has decided to delete this 
requirement from the final rule. Therefore, the final rule does not 
contain a restriction for operators on idling diesel-powered equipment. 
MSHA does, however, recommend as a best practice that mine operators do 
not allow miners to idle diesel-powered equipment unnecessarily.
    Although commenters generally agreed with MSHA's statement in the 
proposal that this requirement would aid in the reduction of dpm 
concentrations at the mine, they pointed out that the total amount of 
diesel particulate matter emitted from this single source might have 
little effect on the levels of dpm in the overall mining environment. 
Also, several commenters questioned the need for an idling restriction 
in light of the proposed concentration limits established in the 
regulation. Additionally, another commenter indicated that the 
provision was not necessary because mine operators, in an effort to 
comply with the applicable concentration limits, would be forced to 
institute work rules to this effect anyway. Moreover, as pointed out by 
commenters, nothing in the regulatory language prohibits operators from 
voluntarily restricting idling at the mine, eliminating the need to 
include this provision. Accordingly, we have deleted proposed paragraph 
(c) from the final rule.

Section 57.5066 Maintenance standards.

    Summary. This section of the final rule establishes maintenance 
standards for diesel-powered equipment operated in underground areas of 
metal and nonmetal mines. It has three subsections.
    Subsection (a) addresses maintenance of diesel engines, emission 
related components, and emission or particulate control devices.

[[Page 5875]]

    Subsection (b) institutes a mandatory procedure by which diesel 
equipment operators must be authorized and required to tag equipment 
they believe requires maintenance in order to comply with subsection 
(a) above, for mine operators to insure that equipment so tagged is 
promptly examined, and for mine operators to retain a log of tagged 
equipment and the corresponding equipment examinations.
    Subsection (c) requires that persons maintaining diesel equipment 
in underground metal and nonmetal mines be appropriately qualified by 
virtue of training or experience, and that mine operators must retain 
evidence of the competence of such persons.
    The provisions of this section in the final rule are unchanged from 
the proposal.
    Maintain Approved engines in approved condition. Sec. 57.5066(a)(1) 
requires that mine operators maintain any approved diesel engine in 
``approved'' condition. Under MSHA's approval requirements, engine 
approval is tied to the use of certain parts and engine specifications. 
When these parts or specifications are changed (i.e., an incorrect part 
is used, or the engine timing is incorrectly set), the engine is no 
longer considered by MSHA to be in approved condition.
    Often, engine exhaust emissions will deteriorate when this occurs. 
Maintaining approved engines in their approved condition will ensure 
near-original performance of an engine, and maximize vehicle 
productivity and engine life, while keeping exhaust emissions at 
approved levels. The maintenance requirements for approved engines in 
this rule are already applicable to underground coal mines. 30 CFR 
75.1914.
    Thus in practice, with respect to approved engines, mine 
maintenance personnel will have to maintain the following engine 
systems in near original condition: air intake, cooling, lubrication, 
fuel injection and exhaust. These systems shall be maintained on a 
regularly scheduled basis to keep the system in its ``approved'' 
condition and thus operating at its expected efficiency.
    One of the best ways to ensure these standards are observed is to 
implement a proper maintenance program in the mine--but the final rule 
would not require operators to do this. A good program should include 
compliance with manufacturers' recommended maintenance schedules, 
maintenance of accurate records and the use of proper maintenance 
procedures. MSHA's diesel toolbox provides more information about the 
practices that should be followed in maintaining diesel engines in 
mines.
    Maintain emissions related components of non-approved engines to 
manufacturer specifications. For any non-approved diesel engine, 
paragraph (a)(2) requires mine operators to maintain the emissions 
related components to manufacturer specifications.
    The term ``emission related components,'' refers to the parts of 
the engine that directly affect the emission characteristics of the raw 
exhaust. These are basically the same components which MSHA examines 
for ``approved'' engines. They are the piston, intake and exhaust 
valves, cylinder head, injector, fuel injection pump, governor, turbo 
charger, after cooler, injection timing and fuel pump calibration.
    Engine manufacturers are required to build engines in a manner that 
ensures continued compliance with EPA emissions levels and to establish 
specifications for adjusting and maintaining these engines to the 
engine manufacturer's specifications to ensure that the engines 
continue to perform properly and emit acceptable levels of emissions.
    As it indicated in the preamble to the proposed rule, the Agency 
does not intend that this requirement could be misconstrued as 
establishing the basis for ``picky'' citations. It is not MSHA's intent 
that engines be torn down and the engine components be compared against 
the specifications in manufacturer maintenance manuals (63 FR 58187). 
Primarily, the Agency is interested in ensuring that engines are 
maintained in accordance with the schedule recommended by the 
manufacturer. However, if it becomes evident that the engines are not 
being maintained to the correct specifications or are being rebuilt in 
a configuration not in line with manufacturers' specifications or 
approval requirements, an inspector may ask to see the manuals to 
confirm that the right manuals are being used, or call in MSHA experts 
to examine an engine to confirm whether basic specifications are being 
properly observed.
    This explanation of MSHA's intent relative to its enforcement of 
this provision was included in the Preamble to the proposed rule, 
accompanied by an invitation for comment from the mining industry to 
suggest alternative ways to rephrase this requirement so the Agency has 
a basis for ensuring compliance while minimizing the opportunity for 
overprescriptiveness (63 FR 58187). However, no such suggestions were 
received.
    Maintain emission or Particulate Control Devices in effective 
operating condition. Paragraph (a)(3) requires that any emission or 
particulate control device installed on diesel-powered equipment be 
maintained in effective operating condition. Depending on the type of 
devices installed on an engine, this would involve having trained 
personnel perform such basic tasks as regularly cleaning aftertreatment 
filters, using methods recommended by the manufacturer for that 
purpose, or inserting appropriate replacement filters when required, 
checking for and repairing any exhaust system leaks, and other 
appropriate actions. This explanation of MSHA's intent relative to 
subsection (a)(3) was contained in the preamble to the proposed rule 
(63 FR 58187). One comment was received on this subsection from a 
commenter who submitted a complete regulatory alternative to MSHA's 
proposed dpm rule. The section of this regulatory alternative that 
corresponds to subsection (a)(3) of both the proposed and final rules 
reads as follows: ``Emission related components of diesel powered 
equipment shall be maintained in effective operating condition.'' This 
alternative language is functionally identical to both the proposed and 
final rules. It incorporates the phrase ``Emission related components 
of diesel powered equipment * * *,'' whereas the rules incorporate the 
phrase, ``Any emission or particulate control device installed on the 
equipment * * *,'' however, the requirement that such equipment, 
``shall be maintained in effective operating condition,'' is identical. 
Therefore, MSHA concluded that no change from the proposal was 
necessary.
    Ensuring equipment that may be out of compliance with maintenance 
standards is attended to--Tagging. Section 57.5066(b)(1) of the final 
rule requires underground metal and nonmetal mine operators to 
authorize and require miners operating diesel powered equipment to 
affix a visible and dated tag to the equipment at any time the 
equipment operator ``notes any evidence that the equipment may require 
maintenance in order to comply with the maintenance standards of 
paragraph (a) of this section.'' Moreover, Sec. 57.5066 (b)(2) requires 
that the equipment be ``promptly'' examined by a person authorized by 
the mine operator to maintain diesel equipment, and prohibits removal 
of the tag until such examination has been completed. Section 57.5066 
(b)(3) requires a log to be retained of all equipment tagged.
    In proposing this approach, MSHA noted its view that tagging would

[[Page 5876]]

provide an effective and efficient method of alerting all mine 
personnel that a piece of equipment needs to be checked by qualified 
service personnel for possible emission problems, and that such a check 
is performed in a timely way (63 FR 58187).
    The agency noted that the presence of a tag serves as a caution 
sign to miners working on or near the equipment, as well as a reminder 
to mine management, as the equipment moves from task to task throughout 
the mine. While the equipment is not barred from service, operators 
would be expected to use common sense and not use it in locations in 
which diesel particulate concentrations are known to be high.
    The agency noted it was not requiring that equipment tagged for 
potential emission problems be automatically taken out of service. The 
rule is not, therefore, directly comparable to a ``tag-out'' 
requirement such as OSHA's requirement for automatic powered machinery, 
nor is it as stringent as MSHA's requirement to remove from service 
certain equipment ``when defects make continued operation hazardous to 
persons'' (see 30 CFR 57.14100). In the Preamble to the proposed rule, 
MSHA indicated that it did not think there was a need for something as 
stringent as these requirements because, although exposure to dpm 
emissions does pose a serious health hazard for miners, the existence 
or scope of an equipment problem cannot be determined until the 
equipment is examined or tested by a person competent to assess the 
situation. Moreover, the danger is not as immediate as, for example, an 
explosive hazard.
    In the preamble to the proposed rule, MSHA also provided additional 
insights into how this approach would be implemented. It noted, for 
example, that the tag may be affixed because the equipment operator 
detects a problem through a visual exam conducted before the equipment 
is started, or because of a problem that comes to the attention of the 
equipment operator during mining operations, (i.e., black smoke while 
the equipment is under normal load, rough idling, unusual noises, 
backfiring, etc.) MSHA also noted it had not defined the term 
``promptly'' with respect to how quickly tagged equipment must be 
examined by a qualified person, and sought comment on whether it should 
define this term--for example, by limiting the number of shifts it 
could operate before the required examination is performed (63 FR 
58187).
    The equipment tagging requirement was the subject of numerous 
comments. Most commenters were concerned that equipment operators would 
be authorized and required to make judgements about equipment function 
(and malfunction) for which they are unqualified, namely, to tag 
equipment they believe requires maintenance due to a problem related to 
dpm emissions. The commenters argued that, although equipment operators 
may be highly skilled in operating equipment, they are not necessarily 
qualified to make judgements concerning equipment maintenance 
requirements. Even though the regulation would not require tagged 
equipment to be removed from service, the commenters were concerned 
that such tags would cause unnecessary ``scurrying about of mechanics'' 
whose time could be more productively spent performing actual needed 
maintenance, rather than reacting to tags affixed for reasons that 
might be dubious, at best.
    Commenters noted that, in addition to unnecessary maintenance 
inspections and the possibility of unnecessarily removing equipment 
from service, this requirement could result in a safety hazard if a tag 
affixed under Sec. 57.14100(c) is mistaken for a tag affixed under 
Sec. 57.5066(b)(1). The former addresses safety defects that ``make 
continued operation hazardous to persons,'' and it requires the 
equipment to be immediately removed from service. The latter relates to 
dpm emissions, and does not require the piece of equipment to be 
removed from service. If a tag under Sec. 57.14100(c) is mistaken for a 
tag under Sec. 57.5066(b)(1), the affected equipment would be allowed 
to remain in service, exposing the operator, and possibly others, to 
potentially dangerous conditions.
    Some commenters suggested that the tagging requirement in the final 
rule was completely unnecessary because its intent is already satisfied 
by existing Sec. 57.14100, and that for the sake of simplicity, 
Sec. 57.5066(b)(1) should be eliminated. Another commenter noted that 
Sec. 57.5066(b)(1) was unnecessary because mine operators already have 
effective mechanisms in place to identify and correct maintenance 
problems on diesel equipment, including emissions-related problems. 
Another commenter worried that a citation could be issued if an 
inspector believes an operator failed to tag a piece of diesel 
equipment with a ``smoky'' exhaust, even if the operator believes the 
exhaust is within the normal range. Several commenters speculated that 
disgruntled employees would deliberately shut down equipment by tagging 
it for an emissions check.
    Several commenters suggested alternative requirements, including 
incorporating emissions checks into the pre-shift equipment inspection 
required under Sec. 57.14100(a), requiring equipment operators to 
either inform their supervisors of any suspected emissions-related 
problems or note any suspected emissions-related problems in a log book 
provided in every piece of equipment for that purpose, and requiring 
the mine operator to insure that a qualified person examines any piece 
of equipment for which an emissions-related problem has been 
identified.
    MSHA has considered these comments, and determined that the 
requirements contained in the proposal are both necessary, and more 
protective than the alternatives suggested by the commenters. For these 
reasons, the requirements contained in the proposal have been retained 
without change in the final rule.
    MSHA believes that, since equipment operators spend more time 
running the equipment than other employees (such as mechanics), and are 
present when the equipment functions under the widest range of 
operating conditions, they are often better able to detect emissions-
related problems than are mechanics. For this reason, the final rule 
requires that equipment operators be authorized and required to affix a 
visible and dated tag if they note any evidence that the equipment may 
need maintenance in order to comply with the rule's maintenance 
requirements. Even though equipment operators may not be trained or 
qualified as diesel mechanics, they often know the difference between 
normal and abnormal equipment performance, especially as it relates to 
diesel particulate matter generation, which is often plainly visible or 
apparent (i.e., black smoke while the equipment is under normal load, 
rough idling, unusual noises, backfiring, etc.).
    MSHA acknowledges that an equipment operator's judgement should not 
necessarily be relied upon to remove a piece of diesel equipment from 
service, precisely because equipment operators are not specifically 
trained or qualified to make such a judgement. Accordingly, the final 
rule does not require equipment operators to be granted this authority; 
only that they be granted authority to visibly identify a potential 
problem machine by affixing a tag. It is then the responsibility of the 
mine operator to appropriately respond to the presence of a tag. Note 
that the response by the mine operator need not be immediate, nor does 
it necessarily require the affected equipment to be removed from 
service, as some commenters feared. Mine operators have the authority 
to establish work rules and procedures to prevent equipment from

[[Page 5877]]

being removed from service unnecessarily. Equipment operators and 
mechanics simply need to be trained as to their respective authority 
and responsibility under this section; namely, that equipment operators 
need to tag equipment suspected of requiring maintenance attention, and 
that qualified mechanics need to follow up to determine if a problem 
actually exists, and if so, what corrective maintenance work is needed.
    It is highly unlikely that a tag intended to indicate a suspected 
emissions-related problems, if properly designed, would be confused 
with a tag intended to indicate a safety problem as per 
Sec. 57.14100(c). Such tags could be differentiated by size, color, or 
other obvious visual characteristics so that mistaking one for the 
other would be virtually impossible. As noted below, the final rule 
allows mine operators the freedom to develop a design that suits their 
circumstances. In contrast, a design mandated by MSHA might be too 
similar to a given mine's existing Sec. 57.14100(c) safety tag.
    MSHA believes that the equipment tagging requirements of 
Sec. 57.14100(c) and Sec. 57.5066(b)(1) are inherently and 
significantly different, to the extent that the Sec. 57.14100(c) 
requirement, even if modified to include health hazards, could not 
achieve the desired effect of Sec. 57.5066(b)(1). The purpose of 
Sec. 57.14100(c) is to immediately remove equipment from service if it 
poses a safety hazard, whereas the purpose of Sec. 57.5066(b)(1) is to 
identify a potential emissions-related problem that might require 
maintenance, but does not justify immediate removal from service. 
Another important difference is that examinations under 
Sec. 57.14100(c) occur before a piece of equipment is placed in 
operation on that shift, whereas Sec. 57.5066(b)(1) applies throughout 
a work shift. These fundamental differences would make any attempt to 
combine the rules overly complicated, which would defeat the 
commenter's purpose of simplifying the rule.
    As discussed above, MSHA believes that equipment operators should 
be authorized and required to note emissions-related deficiencies at 
all times during a work shift, and not be limited to making such 
observations during a pre-shift equipment inspection or before the 
equipment is placed into operation. Some emissions-related problems may 
not become apparent until after the equipment has been fully engaged 
for some time in heavy duty cycle activities. If the only time 
emissions-related deficiencies could be identified is before the 
equipment is placed into operation, the mine operator might never learn 
about such problems, or the corresponding notification might be 
unnecessarily delayed.
    MSHA acknowledges that many underground metal and nonmetal mine 
operators utilize effective maintenance programs to identify and 
correct emissions-related problems in a timely manner. However, MSHA 
believes that Secs. 57.5066(b)(1) and (2) are ``best practices'' that 
should be implemented at all mines. At mines that already have an 
effective program, this provision would serve as a complementary 
element. At mines that have no effective program, this provision would 
create an important safeguard. Further elaboration on the rationale for 
mandating these ``best practices'' was included in the preamble to the 
proposal (p. 58119), and a summary was provided in this Part under the 
portion of Sec. 57.5060 that discussed ``Meeting the concentration 
limit, operator choice of engineering controls.''
    The tagging provision of Sec. 57.5066(b) requires judgement on the 
parts of both the equipment operator and the MSHA inspector. There is 
no absolute standard which precisely defines the physical proof that 
constitutes, ``evidence that the equipment may require maintenance in 
order to comply with the maintenance standards of paragraph (a) of this 
section.'' Thus, MSHA inspectors will be guided by a standard of 
reasonableness, based on an equipment operator's ability to 
differentiate normal emissions from grossly abnormal emissions. MSHA 
does not expect operators to tag equipment whenever there is a minor 
aberration or excursion from an optimum or perfect emissions condition, 
or that an inspector should make a fine distinction between emissions 
that are ``slightly too smoky'' versus ``barely acceptable.'' However, 
MSHA inspectors will not ignore an operator's failure to tag a piece of 
equipment suffering from a serious emissions-related problem that is so 
obvious as to suggest the mine operator is indifferent to, or even 
discourages such tagging.
    MSHA believes that disgruntled employees' attempts to shut down 
equipment by affixing tags indicating possible emissions-related 
problems can be effectively controlled and prevented by mine operators 
through work rules and procedures, and employee discipline policies. 
Mine operators should treat the inappropriate exercise of this 
provision by a disgruntled employee no differently than any other 
disruptive or malicious behavior. In addition to being preventable, 
MSHA believes the inappropriate tagging of equipment would have minimal 
impact on mining operations because tagged equipment need not be 
immediately removed from service. The maintenance examination that is 
triggered by a tag might not take place until the next shift or the 
shift after, and if there is truly nothing wrong with the equipment, it 
would be obvious to the mechanic performing the examination, and would 
therefore only require a few minutes of a mechanic's time.
    MSHA considers the provision for tagging equipment to be preferable 
to a system which permits equipment operators to simply notify their 
supervisor of a suspected emissions-related problem, because the 
presence of a tag serves as a caution sign to other miners working on 
or near the equipment, as well as a reminder to mine management that 
this piece of equipment needs to be examined. Simply informing the 
supervisor does not provide this ongoing visual indicator or reminder, 
and as miners and equipment are reassigned to different jobs in 
different parts of a mine, information that is communicated verbally 
can be easily forgotten. A major advantage of tagging is that the tag 
goes with the equipment throughout the mine, alerting all who come in 
contact with it of the potential dpm emissions problem. In this sense, 
tagging requirements are particularly valuable for mobile equipment 
that travels from place to place throughout the shift, and may have 
multiple operators over the course of several shifts.
    Design of the tag. MSHA proposed that the design of the tag be left 
to the discretion of the mine operator, with the exception that the tag 
must be able to be marked with a date. MSHA sought comment on ``whether 
some or all elements of the tag should be standardized to ensure its 
purpose is met''.
    Several commenters suggested that MSHA should design the tag to be 
used for indicating equipment suspected of needing emissions-related 
maintenance.
    As noted above, the final rule leaves this decision to the 
discretion of the mine operator. Since the design of tags required 
under Sec. 57.14100(c) is left to the discretion of the operator, it 
would be impossible for MSHA to insure that any mandated design for a 
tag under Sec. 57.5066(b)(1) would be easily distinguishable from an 
existing Sec. 57.14100(c) tag. However, MSHA strongly urges mine 
operators to adopt a design for their Sec. 57.5066(b)(1) tags that is 
easily distinguishable from the design of their Sec. 57.14100(c) tags, 
using, for example, different sizes, colors, or other obvious visual 
characteristics.

[[Page 5878]]

    Time to inspect equipment. As noted above, MSHA sought specific 
comment on whether to define the term ``promptly.'' One commenter 
referred to ``promptly examined'' as, ``whatever that is,'' indicating 
they believed the term ``promptly examined'' is too vague. Another 
commenter suggested that a definite time period for examining equipment 
should be specified; namely, ``by the end of the next shift.'' However, 
another commenter agreed with MSHA that equipment tagged by an operator 
should be, ``promptly examined'' by an authorized diesel maintenance 
person. Another commenter proposed that, ``the required examination be 
conducted during normally scheduled maintenance cycles.''
    The final rule, like the proposal, does not define the term 
``promptly''. Operating and maintenance practices vary from mine to 
mine to such an extent that a proscriptive requirement mandating a 
specific time period within which an examination must be completed may 
be infeasibly short for some operators and unnecessarily long for other 
operators. However, MSHA's intent is that mine operators will insure 
such examinations are performed without undue delay. If a tag is 
affixed during a given shift, it would not be unreasonable to complete 
that shift before the maintenance examination. If no qualified mechanic 
is scheduled to work on the following shift, the equipment could be 
operated during that shift as well. However, if a qualified mechanic 
was scheduled to work on the next shift, the examination would be 
required before the equipment was used.
    Tagged Equipment Log. Section 57.5066(b)(3) requires a log to be 
retained of all equipment tagged. Moreover, the log must include the 
date the equipment is tagged, the date the tagged equipment is 
examined, the name of the person making the examination, and the action 
taken as a result of the examination. Records in the log about a 
particular incident must be retained for at least one year after the 
equipment is tagged.
    MSHA does not expect the log to be burdensome to the mine operator 
or mechanic examining or testing the engine. Based on MSHA's 
experience, it is common practice to maintain a log when equipment is 
serviced or repaired, consistent with any good maintenance program. The 
records of the tagging and servicing, although basic, provide mine 
operators, miners and MSHA with a history that will help in determining 
whether a maintenance program is being effectively implemented, and 
whether emissions-related components on the equipment are being 
maintained in a proper and timely fashion.
    Several comments addressing the equipment log were received. 
Proposed revisions generally retained the requirement for an equipment 
log, but varied as to who would maintain the log (equipment operators, 
mechanics or supervisors), and how long they should be kept (one year 
versus until the condition is examined and remedied). It was also 
suggested that all record keeping could be accomplished under 
``existing mobile equipment examination standards and maintenance work 
order systems,'' and that additional standards were therefore not 
needed.
    MSHA has concluded that the requirements in the proposal relative 
to tagged equipment logs are essential to effectively controlling dpm, 
and have therefore been retained in the final rule without change. They 
enable both the mine operator and MSHA to track emissions-related 
problems on equipment, and the actions taken by the mine operator to 
resolve the problems that occur. The logs are also important because 
they provide a written record documenting when equipment was tagged, 
and how the mine operator responded.
    The log creates an accountability chain that clearly indicates the 
date the equipment was tagged, the date the tagged equipment was 
examined, the name of the person making the examination, and the action 
taken as a result of the examination. Without the written record, MSHA 
would be unable to ascertain the extent to which mine operators respond 
in a timely and appropriate manner to emissions-related problems on 
diesel equipment. The one-year record retention requirement is 
necessary so that MSHA can review the emissions-related maintenance 
history on a given piece of equipment over a meaningful time period. 
This will enable MSHA to judge the mine operator's on-going commitment 
to proper and timely maintenance of these components. If the log were 
kept only until a given maintenance operation was completed, MSHA's 
opportunity to assess the mine operator's on-going responsiveness to 
emissions-related problems would be limited to the few chance occasions 
where a piece of equipment is tagged during an MSHA inspection of the 
mine.
    These requirements are protective to miners because they force mine 
operators to address dpm emissions problems through a systematic and 
effective program. The combination of equipment tagging and logging 
helps insure problems will be identified and resolved quickly. If 
either or both requirements were eliminated, mine operators would be 
less likely to receive timely notice of a potential problem, and once 
notified, would be less motivated to promptly initiate the required 
examination and corrective measures.
    Persons qualified to perform maintenance. Section 57.5066(c) 
requires that persons who maintain diesel equipment in underground 
metal and nonmetal mines be ``qualified,'' by virtue of training or 
experience, to ensure the maintenance standards of Sec. 57.5066(a) are 
observed. Paragraph (c) also requires that an operator retain 
appropriate evidence of ``the competence of any person to perform 
specific maintenance tasks'' in compliance with the requirement's 
maintenance standards for one year.
    The requirements being established in this regard are not as 
stringent as those in effect for the maintenance of diesel powered 
equipment in underground coal mines. Operators of underground coal 
mines where diesel-powered equipment is used are required, as of 
November 25, 1997, to establish programs to ensure that persons who 
perform maintenance, tests, examinations and repairs on diesel-powered 
equipment are qualified (30 CFR 75.1915). The unique conditions in 
underground coal mines require the use of specialized equipment. 
Accordingly, the persons who maintain this equipment generally must be 
appropriately qualified.
    If repairs and adjustments to diesel engines used in underground 
metal and nonmetal mines are to be done properly, personnel performing 
such tasks must be properly trained. MSHA does not believe, however, 
that the qualifications required to perform this work in underground 
metal and nonmetal mines necessarily require the same level of training 
as is required for similar work in underground coal mines. Under the 
final rule, the training required would be that which is commensurate 
with the maintenance task involved. If examining and, if necessary, 
changing a filter or air cleaner is all that is required, a miner who 
has been shown how to do these tasks would be qualified by virtue of 
training or experience to do those tasks. For more detailed work, 
specialized training or additional experience would be required. 
Training by a manufacturer's representative, completion of a general 
diesel engine maintenance course, or practical experience performing 
such repairs could also serve as evidence of having the qualifications 
to perform the service.

[[Page 5879]]

    In practice, the appropriateness of the training or experience of 
the maintenance personnel will be revealed by the performance of the 
equipment, both the diesel engine itself and any emission 
aftertreatment devices. If MSHA finds a situation where maintenance 
appears to be shoddy, where the log indicates an engine has been in for 
repair with more frequency than should be required, or where repairs 
have damaged engine approval status or emission control effectiveness, 
MSHA would ask the operator to provide evidence that the person(s) who 
worked on the equipment was properly qualified by virtue of training or 
experience.
    It is MSHA's intent that equipment sent off-site for maintenance 
and repair is also subject to the requirement that the personnel 
performing the work be qualified by virtue of training or experience 
for the task involved. It is not MSHA's intent that a mine operator 
have to examine the training and experience record of off-site 
mechanics, but a mine operator will be expected to observe the same 
kind of caution as one would observe with a personal vehicle--e.g., 
selecting the proper kind of shop for the nature of the work involved, 
and considering prior direct experience with the quality of the shop's 
work.
    One commenter objected to the requirement that mine operators must 
retain evidence of the competence of such workers for one year after 
any applicable maintenance task is completed. MSHA believes the 
provision is important because the evidence retained by the mine 
operator is the only means by which MSHA can judge compliance with the 
competency requirement.
    Another commenter recommended this provision be dropped from the 
final rule because it is unnecessary. This commenter argued that it is 
in a mine operator's self interest to employ only qualified diesel 
mechanics to perform maintenance on equipment that is critical to the 
productive capacity of the mine. Another commenter stated that the rule 
is unnecessary because they already keep a file on mechanic training. 
MSHA believes this provision is important because not all mine 
operators are as careful in employing only qualified persons to 
maintain the emissions-related components of their diesel equipment. 
For mine operators that do, this requirement should not be burdensome. 
For mine operators that don't, this requirement will prevent 
unqualified persons from performing improper maintenance procedures on 
this equipment, thereby preventing this equipment from generating 
potentially excessive diesel emissions.
    Another commenter recommended that the final rule should include 
minimum qualifications for persons responsible for ventilation at 
underground metal and nonmetal mines. The recommendation applied to 
mines employing greater than 20 miners, and suggested that the minimum 
qualification should be a mining engineering degree from an accredited 
university having a program that includes training in the theory and 
practice of underground metal and nonmetal mine ventilation, and that 
qualified persons should also have some minimum level of operating 
experience in this field. MSHA believes that its existing ventilation 
regulations and this final dpm rule are appropriately performance 
oriented regarding the use of mine ventilation as a dpm control 
measure. Mine operators who rely on ventilation will be judged by MSHA 
according to their success in complying with the final concentration 
limit. Therefore, the final rule has not been changed to require 
persons who are responsible for ventilation at mines employing more 
than 20 miners to meet any minimum qualifications.

Section 57.5067 Engines

    The final rule requires that, with the exception of diesel engines 
used in ambulances and fire-fighting equipment, any diesel engines 
added to the fleet of an underground metal or nonmetal mine in the 
future have to either be engines approved by MSHA under part 7 or part 
36 or engines that meet or exceed the applicable dpm emission 
requirements of the EPA explicitly incorporated into a table in the 
rule. This requirement takes effect 60 days after the date this rule is 
promulgated. Only engines approved by MSHA as permissible can be used 
in areas of the mine where permissible diesel equipment is required. 
The composition of the existing fleet in an underground metal and 
nonmetal mine is not impacted by this part of the final rule. However, 
after the rule's effective date, any engine introduced into the 
underground areas of the mine must be either MSHA approved or meet the 
applicable EPA requirements. The term ``introduced'' is explicitly 
defined in the final rule to eliminate uncertainty regarding MSHA's 
intent. Engines that are introduced means engines in newly purchased 
equipment, engines in used equipment brought into the mine, or 
replacement engines that have a different serial number than the engine 
it is replacing. The term introduced does not include engines that were 
previously part of the mine inventory and rebuilt.
    The final rule reflects a change from the proposed rule. The 
proposed rule would have required that, with the exception of diesel 
engines used in ambulances and fire-fighting equipment, any diesel 
engines added to the fleet of an underground metal or nonmetal mine in 
the future would have to have been approved by MSHA under Part 7 or 
Part 36. As discussed below, after reviewing the comments on this 
topic, MSHA concluded that it could accomplish the same goal, while 
providing operators with considerable extra flexibility, by permitting 
engines compliant with applicable EPA standards as an alternative to 
MSHA approved engines.
    Table Sec. 57.5067-1 in the final rule lists the applicable EPA dpm 
standards for diesel engines. The EPA standards represent the dpm 
emission limits set by EPA for light duty vehicles, light duty trucks, 
heavy duty highway engines, and nonroad engines. MSHA believes that all 
engines used in underground M/NM mines would come from these 
categories. MSHA chose the current on-highway dpm standards that have 
been in effect since 1994 for any commercially available on-highway 
vehicle. For nonroad, MSHA mainly used the EPA tier 1 standards that 
have been in effect starting in 1996 through 2000.
    MSHA did notice one gap in the EPA nonroad standards. For engines 
in the 50 to 175 horsepower range, EPA did not list a dpm standard for 
tier 1. A tier 2 standard is listed in the final rule table for this 
reason. Full EPA implementation of the tier 2 standard for this 
horsepower range will become effective in 2003 for engines from 50-100 
horsepower and in 2004 for engines 100 to 175 horsepower. However, MSHA 
believes that engines in this horsepower range are available now to 
meet the standard. MSHA has approved many engines under part 7 in this 
horsepower range that would meet the standard, and engine manufacturers 
are also producing other engine models in this horsepower range that 
meet the standard. The dpm requirement is the same for this engine 
horsepower range as was specified for engines in light duty vehicles in 
the coal final rule. Therefore, MSHA does not believe that mine 
operators will have problems introducing engines that meet any of the 
requirements of this section.
    Several commenters questioned the need for engine restrictions at 
all if the applicable concentration limit could be achieved through 
other means. The rationale for this requirement is to promote the 
gradual turnover of the

[[Page 5880]]

existing fleet to better, less-polluting engines, thereby reducing dpm 
concentrations and attendant health risks. Without this requirement, 
there would be no constraint on the introduction of engines that are 
inherently higher polluting into underground metal and nonmetal mines. 
Such engines, regardless of the level of maintenance they receive, 
produce significantly higher dpm emissions than the low polluting 
engines mandated in the final rule. MSHA acknowledges that older, high 
polluting engines will eventually be replaced with low polluting 
engines through the normal equipment turnover process, because EPA 
emission requirements (and similar requirements imposed by foreign 
regulatory bodies) will make high polluting engines increasingly 
difficult for manufacturers to sell for any application. Even if a mine 
operator wanted to continue using high polluting engines, such engines 
will become more and more scarce over time. But in light of the risks 
of dpm exposure to miners, and the history of the underground mining 
industry to bring old engines underground and keep them operating for a 
long period of time, MSHA has concluded that a rule is required to 
bring about the transition to newer engines more quickly than would 
otherwise be the case. MSHA considers the gradual introduction of 
cleaner engines to be one of the ``best practices'' that is feasible 
for all underground metal and nonmetal mines. Further elaboration on 
the rationale for mandating these ``best practices'' was included in 
the preamble to the proposal (63 FR 58119), and a summary was provided 
in this Part under the portion of Sec. 57.5060 that discussed ``Meeting 
the concentration limit, operator choice of engineering controls.''
    Other commenters recommended that EPA certification be an 
acceptable alternative to MSHA approval. As noted above, after 
considering the matter, MSHA agrees that engines certified as meeting 
applicable EPA standards would provide an acceptable level of 
protection to miner health comparable to that which can be achieved by 
requiring MSHA approved engines. (For detailed information about the 
various ``tiers'' of EPA engine requirements, and the various types of 
engine categories, please see Part II, section 5). Therefore, under the 
final rule, engines meeting or exceeding applicable particulate 
emission requirements of the Environmental Protection Agency (as listed 
in the table in Sec. 57.5067(b)) are an acceptable alternative to 
engines approved by MSHA as nonpermissible under subpart E of Part 7 of 
this title. This change in the final rule will provide mine operators 
with a wider choice of acceptable engines, and may reduce compliance 
costs.
    MSHA is developing a program that will streamline the procedures by 
which manufacturers of diesel engines intended for use in outby areas 
of underground coal mines can gain Agency approval. The program will 
draw on the EPA approval programs for engines used in off-road 
applications. MSHA will continue to issue approvals for mining engines, 
but the application process will be abbreviated. Many of the provisions 
of part 7 are intended to ensure that engines continue to be 
manufactured in the same configuration and with the same emissions as 
the engine tested by MSHA. Procedures within the EPA approval programs 
reach the same end. Additionally, EPA has the resources and the 
regulatory authority to conduct an extensive quality assurance program 
to monitor emissions from production engines. In addition to 
streamlining the application process, MSHA will establish a program 
under which the engine emission tests conducted for EPA approval will 
satisfy the part 7 testing requirements. The test cycles under which 
emissions are tested for both MSHA and EPA are identical, and the 
gaseous emission results from the EPA tests can be used to establish 
the ventilating air quantity that appears on the engine approval plate 
and is referenced in mine ventilation regulations. MSHA will announce 
the specifics of the program when it is finalized. A listing of MSHA 
approved nonpermissible engines has been provided on MSHA's Internet 
web site. This listing can be accessed at the following address: http://www.msha.gov/S&HINFO/DESLREG/1909a.HTM.
    Many underground metal and nonmetal mines are accustomed to 
employing front end loaders, haulage trucks, and other production 
equipment that is developed for, and primarily marketed to the surface 
mining and construction industries. Likewise, where conditions permit, 
underground metal and nonmetal mines often employ support vehicles such 
as pickup trucks, sport utility vehicles, and other small to medium 
sized trucks that are developed for, and primarily marketed to the 
surface over-the-road market. Mine operators employ this equipment 
because it is significantly less costly than purpose-built underground 
mining equipment, which has special mine-duty features and is produced 
in relatively low volume.
    The engines in newly manufactured surface off-road equipment and 
over-the-road vehicles are already required to comply with EPA dpm 
emission regulations. EPA regulations are fashioned in a Tier structure 
whereby engines in designated horsepower ranges are required to meet 
increasingly stringent emissions levels. By changing the final rule as 
indicated above to accept engines meeting or exceeding applicable 
particulate emission requirements of the EPA, MSHA is, in essence, 
allowing mine operators to continue the long-standing and cost-
effective practice of employing standard off-road equipment and over-
the-road vehicles underground (if they are equipped with engines 
meeting the appropriate EPA requirements), without requiring 
potentially costly retrofits of approved engines. This change will 
enable mine operators and mine workers to gain the added benefits of 
engines that incorporate the most recent emission reducing technology.
    Laboratory testing to certify that an engine meets the applicable 
EPA particulate matter limit or MSHA approval requirements is not the 
responsibility of the mine operator. MSHA approved engines carry an 
approval plate so they are easy to distinguish. Engines produced after 
the date indicated in the Table incorporated into 5067(b) will meet the 
EPA requirements for the listed category of engines.
    Engines in diesel-powered ambulances and fire-fighting equipment 
are exempted from these requirements. This exemption is identical with 
that in the rule for diesel-powered equipment in underground coal 
mines. The rationale for this exemption is that the usage of these 
vehicles and equipment is so limited that their contribution to overall 
dpm levels in a mine is negligible. MSHA wishes to caution mine 
operators, however, that this exemption is intended to apply only to 
equipment that is used exclusively as an ambulance or fire fighting 
equipment. This exemption does not apply to vehicles and equipment that 
are normally used for other purposes, but serve as an ambulance or fire 
fighting equipment in the event of an accident or mine emergency.

Section 57.5070 Miner Training

    Section 57.5070 requires any miner ``who can reasonably be expected 
to be exposed to diesel emissions'' be trained annually in: (a) The 
health risks associated with dpm exposure; (b) the methods used in the 
mine to control dpm concentrations; (c) identification of the personnel 
responsible for

[[Page 5881]]

maintaining those controls; and (d) actions miners must take to ensure 
the controls operate as intended. The final rule is the same as that 
proposed, and is identical to the rule being established for 
underground coal miners through MSHA's rulemaking limiting dpm 
concentrations in underground coal mines.
    The purpose of these requirements is to promote miner awareness. 
Exposure to diesel particulate is associated with a number of harmful 
effects as discussed in Part III of this preamble, and the safe level 
is unknown. Miners who work in mines where they are exposed to this 
risk ought to be reminded of the hazard often enough to make them 
active and committed partners in implementing actions that will reduce 
that risk.
    The training need only be provided to miners who can reasonably be 
expected to be exposed at the mine. The training is to be provided by 
operators; hence, it is to be without fee to the miner.
    The rule places no constraints on the operator as to how to 
accomplish this training. MSHA believes that the required training can 
be provided at minimal cost and minimal disruption. The proposal would 
not require any special qualifications for instructors, nor would it 
specify the hours of instruction.
    Instruction could take place at safety meetings before the shift 
begins. Devoting one of those meetings to the topic of dpm would be a 
very easy way to convey the necessary information. Simply providing 
miners with a copy of MSHA's ``Toolbox'' and, a copy of the plan, if a 
control plan is in effect for the mine, and reviewing these documents, 
can cover several of the training requirements. One-on-one discussions 
that cover the required topics are another approach that can be used.
    Operators could also choose to include a discussion on diesel 
particulate matter emissions in their Part 48 training, provided the 
plan is approved by MSHA. There is no existing requirement that Part 48 
training include a discussion of the hazards and control of diesel 
emissions. While mine operators are free to cover additional topics 
during the Part 48 training sessions, the topics that must be covered 
during the required time frame may make it impracticable to cover the 
additional material on dpm. Where adequate time is available at mines 
using diesel-powered equipment, operators would be free to include the 
dpm instruction in their Part 48 training plans. Since inclusion of 
dpm-related training in Part 48 training plans is not explicitly 
prohibited in the final rule, MSHA does not believe special language is 
required to permit this practice.
    The final rule does not require the mine operator to separately 
certify the completion of the dpm training, but some evidence that the 
training took place would have to be produced upon request. A serial 
log with the employee's signature is an acceptable practice. To assist 
mine operators with this training requirement, it is MSHA's intent to 
develop an instructor's guide and corresponding training materials.
    A few comments were received on Sec. 57.5070, including the 
suggestion that such training be included under Part 48, and the 
opposing view that such training be independent of Part 48. Arguments 
in favor of including the training under Part 48 focused on the need to 
simplify the rule by not requiring separate diesel particulate 
emissions training and training recordkeeping. Arguments opposed 
focused on the difficulty of including more subject matter into a Part 
48 training plan that is already overfilled. It was also noted that 
Part 48 training requires MSHA-certified instructors. By separating 
Part 48 training from the training required under Sec. 57.5070, mine 
operators would have greater flexibility in choosing instructors.
    MSHA believes the final rule satisfies both positions because 
inclusion of the specified diesel particulate emissions training topics 
under Part 48 training is neither required nor prohibited. Mine 
operators wishing to incorporate diesel emissions training in their 
Part 48 training plan are free to do so, whereas those wishing to 
conduct diesel emissions training separate from Part 48 training are 
equally free to choose that option. MSHA believes it is significant 
that none of the commenters discounted the importance of providing dpm-
exposed miners with such training; their comments only addressed the 
mechanics of how such training should be delivered.
    In its preamble to the proposed rule, MSHA specifically invited 
comment as to whether special language should be included in the final 
rule that would expressly permit required dpm training to be 
incorporated into Part 48 training. Only one commenter responded, 
expressing the view that special language was not necessary. Therefore, 
MSHA did not change this provision in the final rule.
    Another commenter suggested that training required under 
Sec. 57.5070 incorporate mandatory coverage of underground metal and 
nonmetal mine ventilation, that such training address auxiliary 
ventilation and the use of elementary ventilation measurement 
instruments, and that similar training be mandatory for first and 
second line supervisors.
    MSHA agrees that ventilation is an important topic and that 
ventilation can have a significant effect on dpm concentrations 
underground. However, MSHA believes it would be inappropriate to 
specify the content of dpm-related miner training to the level of 
detail suggested by the commenter. Since MSHA allows mine operators 
considerable freedom to choose dpm control measures, MSHA expects 
significant variability from mine to mine in the mix of controls 
selected. For example, some mines may rely heavily on ventilation to 
comply with the applicable concentration limit, but other mines may 
rely more on enclosed cabs or diesel particulate filters. As a result, 
the most important training subject or subjects at one mine could be 
quite different at another mine.
    By requiring training in the health risks associated with dpm 
exposure, the methods used in the mine to control dpm concentrations, 
identification of the personnel responsible for maintaining those 
controls, and the actions miners must take to ensure the controls 
operate as intended, MSHA believes it has established performance-based 
training requirements that are applicable to all mines.
    As with the proposed rule, the final rule does not require the mine 
operator to separately certify the completion of dpm training, but some 
evidence that the training took place will have to be produced upon 
MSHA request. In this regard, as noted in the preamble to the proposed 
rule, a serial log with the employee's signature is an acceptable 
practice. Nevertheless, some commenters complained that the 
recordkeeping requirements in the training provisions are burdensome, 
and don't reduce diesel emissions. MSHA believes that dpm training is 
an essential element of a comprehensive dpm control program because 
miners who are fully informed are more apt to become active and 
committed partners in implementing an effective dpm control strategy. 
In this way, training can have an indirect, yet substantive and 
positive influence on reducing dpm exposure. The corresponding 
recordkeeping requirements are important, because the records are the 
means by which MSHA can insure that the mine operator is complying with 
the training requirements.
    As noted in the preamble to the proposed rule, to assist mine 
operators with this training requirement, it is MSHA's intent to 
develop an instruction outline that mine operators can use as

[[Page 5882]]

a guide for training personnel. Instruction materials will be provided 
with the outline.

Section 57.5071 Environmental Monitoring

    The final rule requires mine operators to monitor as often as 
necessary to effectively evaluate, under conditions that can be 
reasonably anticipated in the mine--(1) whether the concentration of 
dpm in an area where miners normally work or travel exceeds the 
applicable concentration limit; and (2) the average full shift airborne 
concentration at any position or on any person designated by the 
Secretary. This section also requires operators to provide affected 
miners and their representatives with notice and an opportunity to 
observe monitoring, to initiate corrective action by the next work 
shift should monitoring reveal a violation and to promptly complete 
such action, and requires certain posting and recordkeeping. The final 
rule is the same as the proposed rule.
    Operator's Monitoring Responsibility. Section 57.5071(a) requires 
mine operators to monitor the underground mine environment to insure 
dpm concentrations are within compliance limits wherever the limits 
apply. Sampling, which could be area sampling, personal sampling, or 
occupational sampling, is required as often as necessary to 
``effectively determine''--under conditions that can be reasonably 
anticipated in the mine--(1) whether the dpm concentration in any area 
of the mine where miners normally work or travel exceeds the applicable 
limit; and (2) the average full shift airborne concentration at any 
position or on any person designated by the Secretary.
    This requirement is similar to existing Sec. 57.5002 which requires 
mine operators to conduct dust, gas, mist, and fume surveys as 
frequently as necessary to determine the adequacy of control measures, 
and to existing Sec. 62.110(a) and (b) which requires mine operators to 
measure each miner's noise dose sufficient to determine continuing 
compliance with the established noise limits. Under Sec. 57.5071(a), 
mine operators are required to monitor dpm concentrations in much the 
same way they are already required to monitor dust, gas, mist, fume, 
and noise.
    There are three important aspects of this operator monitoring 
requirement.
    First, the responsibility for dpm monitoring rests with the mine 
operator, not with MSHA. Mine operators cannot rely on MSHA inspectors 
to conduct dpm monitoring whenever and wherever necessary to ensure 
compliance with the applicable dpm concentration limit. The purpose of 
operator monitoring is to determine continuing compliance, whereas the 
purpose of MSHA sampling is to identify non-compliance. MSHA sampling 
is neither intended for, nor capable of determining continued 
compliance.
    Second, the information gathered through operator monitoring is to 
be used by the operator to determine whether action is necessary to 
maintain compliance anywhere the applicable concentration limits apply 
in the mine. Gathering dpm concentration data, though necessary, is not 
the final goal in itself. The reason for gathering this information is 
so it can be used by the mine operator to assess the effectiveness of 
dpm control measures. Sampling results which indicate non-compliance 
should prompt the mine operator to initiate whatever actions are 
required (i.e., implementation of appropriate engineering controls and 
work practices) to achieve compliance wherever the applicable 
concentration limits apply.
    Third, this requirement ensures special attention will be focused 
on locations or persons known to MSHA to have a significant potential 
for overexposure to dpm.
    The obligation of operators to ``effectively determine'' dpm 
concentrations in a mine is a separate obligation from that to keep dpm 
levels below the established limit, and can be the basis of a separate 
citation from MSHA. The final rule is performance-oriented in that the 
regularity and methodology used to make this evaluation are not 
specified. However, MSHA expects mine operators to sample with such 
frequency that they and the miners working at the mine site are aware 
of dpm levels in their work environment. In this regard, MSHA's own 
measurements will assist the Agency in verifying the effectiveness of 
an operator's monitoring program. If an operator is ``effectively 
determining'' the concentration of dpm at designated positions, for 
example, MSHA would not expect to regularly record concentrations above 
the limit when it samples at that location. If MSHA does find such a 
problem, it will investigate to determine how frequently an operator is 
sampling, where the operator is sampling, and what methodology is being 
used, so as to determine whether the obligation in this section is 
being fulfilled. (See previous discussion in this Part in the portion 
of Sec. 57.5062 that addressed ``Demonstration of plan effectiveness'' 
for further information on the number of samples required to 
demonstrate continuing compliance.)
    Operator Monitoring Methods. The final rule requires that full-
shift diesel particulate concentrations be determined during periods of 
normal production or normal work activity in areas where miners work or 
travel. The rule does not specify a particular monitoring method or 
frequency; rather, the rule is performance-oriented. Operators may, at 
their discretion, conduct their monitoring using the same sampling and 
analytical method as MSHA, or they may use any other method that 
enables that mine to ``effectively determine'' the concentrations of 
dpm.
    As required by Sec. 57.5061, MSHA will collect samples using a 
respirable dust sampler equipped with a submicrometer impactor, and use 
NIOSH Method 5040, the sampling and analytical method that NIOSH has 
developed for accurately determining the concentration of total carbon, 
to determine compliance. Operators who must comply with the terms of a 
diesel particulate control plan pursuant to Sec. 57.5062 must, as noted 
in the requirements of that section, use the same sampling and 
analytical method as MSHA to verify plan effectiveness; monitoring 
performed for that purpose would probably meet the obligation under 
Sec. 5071 if it is done with enough sufficiency to meet the obligation 
under Sec. 57.5062(c). But the method may not be necessary to 
effectively determine dpm in some mines for purposes of 
Sec. 57.5071(a). For example, dpm measurements in limestone, potash and 
salt mines could be determined using the RCD method, since there are no 
large carbonaceous particles present that would interfere with the 
analysis. For hydrated minerals such as gypsum and trona, a two-step 
RCD method would be necessary, wherein the first step would elevate the 
temperature of the sample sufficient to cause dehydration (105  deg.C). 
The sample is then reweighed, and the conventional RCD analysis 
procedure is followed. Such estimates can be useful in determining the 
effectiveness of controls and where more refined measurements may be 
required.
    Of course, mine operators using the RCD or size-selective methods 
to monitor their diesel particulate concentrations would have to 
convert the results to a TC equivalent to ascertain their compliance 
status. At the present time, MSHA has no conversion tables for this 
purpose, however a simple conversion approach would be to adjust the 
sampling result to the corresponding estimated whole dpm concentration, 
then multiply that value by 0.8. In most cases, the other methods will 
provide a good indication of

[[Page 5883]]

whether controls are working and whether further action is required.
    Part II of this preamble provides information on monitoring methods 
and their constraints, and on laboratory and sampler availability.
    One commenter observed that area sampling outside of an enclosed 
cab would defeat the purpose of installing the cab, and would diminish 
the status of such a cab, which is a recognized engineering control, to 
that of personal protective equipment, which is prohibited under the 
rule. MSHA agrees that area sampling is inappropriate where miners are 
protected by enclosed cabs with filtered breathing air and no other 
miners are required to work in the area outside of the cab. As 
discussed under section 5061(c)(3), area sampling by MSHA for 
compliance purposes would not be conducted outside of an enclosed cab 
unless miners are working in the area outside of such cabs, and MSHA 
would urge operators to follow the same approach. Also, as noted in 
discussing that section, personal sampling within cabs operated by 
smokers should only be conducted if the equipment operator agrees not 
to smoke during the sampling period.
    Observation of Monitoring. Section 103(c) of the Mine Act requires 
that:

    The Secretary, in cooperation with the Secretary of Health, 
Education, and Welfare, shall issue regulations requiring operators 
to maintain accurate records of employee exposures to potentially 
toxic materials or harmful physical agents which are required to be 
monitored or measured under any applicable mandatory health or 
safety standard promulgated under this Act. Such regulations shall 
provide miners or their representatives with an opportunity to 
observe such monitoring or measuring, and to have access to the 
records thereof.

    In accordance with this legal requirement, Sec. 57.5071(b) of the 
final rule requires a mine operator to provide affected miners and 
their representatives with an opportunity to observe exposure 
monitoring required by this section. Mine operators must give prior 
notice of the date and time of intended monitoring so that affected 
miners and their representatives can exercise their right to observe 
the monitoring if they so choose.
    Comments addressing Sec. 57.5071(b) questioned the meaning of the 
terms ``miner's representative'' and ``affected miners,'' and objected 
to paying miners to observe dpm monitoring.
    MSHA intends for miner's representative to mean any authorized 
representative of the miners. A representative of the miners could, but 
does not necessarily have to be, a representative of a certified union. 
Limiting representatives of miners to certified unions is a violation 
of the Mine Act and departs from previous MSHA practice.
    MSHA intends for affected miners to mean the miners that are 
potentially exposed to the diesel particulate matter being monitored. 
The commenter suggested that this provision ``* * * leaves too much for 
interpretation. How many employees may observe? For how long?'' 
Consistent with the Mine Act, MSHA does not intend to limit the number 
of miners who may observe dpm monitoring, however, such miners need not 
be paid if, as a result of observing the monitoring, they are not 
performing their jobs.
    Corrective Action if Concentration Is Exceeded. Section 57.5071(c) 
provides that if any monitoring performed under this section indicates 
that the applicable dpm concentration limit has been exceeded, an 
operator shall initiate corrective action by the next work shift, 
promptly post a notice of the corrective action being taken and 
promptly complete such corrective action.
    The Agency wishes to emphasize that operator monitoring of dpm 
concentrations would not take the place of MSHA sampling for compliance 
purposes; rather, this requirement is designed to ensure the operator 
checks dpm concentrations on a more regular basis than is possible for 
MSHA to do. Paragraph (c) provides that if sampling results indicate 
the concentration limit has been exceeded in an area of a mine, an 
operator would initiate corrective action by the next work shift and 
promptly complete such action. Paragraph (c) does not require an 
operator to establish a dpm control plan. The establishment of a dpm 
control plan is triggered by a non-compliance determination based on 
sampling conducted by the Secretary.
    In certain types of cases (e.g., 30 CFR 75.323), MSHA has required 
that when monitoring detects a hazardous level of a substance, miners 
must be immediately withdrawn from an area until abatement action has 
been completed. Although MSHA did not include such a requirement in the 
final rule, MSHA in its proposal did solicit comment from the mining 
industry concerning this practice, especially in light of the evidence 
presented on the various risks posed by exposure to diesel particulate, 
including material presented in the preamble to the proposal that acute 
short-term increases in exposure can pose significant risks to miner 
health. The comments that were received in response to this 
solicitation were opposed to a provision requiring immediate 
withdrawal.
    The agency also specifically asked for comments on three other 
points (63 FR 58189, 58190). First, the agency noted that it welcomed 
comments as to what guidance to provide with respect to corrective 
actions required where an operator is not using the total carbon 
analytical method. Second, the agency noted it welcomed comment as to 
whether personal notice of corrective action would be more appropriate 
than posting, given the health risks involved. Third, the agency 
solicited comment on whether clarification of the proposed requirement 
was needed in light of the fact that operators using more complex 
analytical procedures (e.g., the total carbon method) may not receive 
the results for some time period after the posting has taken place.
    No comments addressing these points were received.
    Posting of Sample Results. Section 57.5071(d)(1) requires that 
monitoring results be posted on the mine bulletin board within 15 days 
of receipt, and remain posted for 30 days. A copy of the results must 
also be provided to the authorized miners' representative. Posting of 
the results will ensure that miners are kept aware of the hazard so 
they can actively participate in efforts to control dpm.
    Comments that addressed this paragraph recommended that sampling 
results should not be given to the representative of the miners because 
this information is private, and recommended that mine operators should 
not be cited for posting sampling results that exceed the applicable 
concentration limit.
    MSHA disagrees with the assertion that dpm sampling results are 
private, and therefore, such results should not be given the 
representative of the miners. The Mine Act clearly states that miners 
or their representatives have a legal right to access to exposure 
monitoring information.
    Regarding the question of MSHA issuing a citation based on a mine 
operator posting sampling results that exceed the applicable 
concentration limit, it is not MSHA's intent to issue a citation under 
these circumstances. If such sampling indicates that dpm levels exceed 
the applicable concentration limit, a citation may be issued if the 
mine operator fails to initiate corrective action by the next work 
shift, as required under Sec. 57.5071(c). However, mine operator 
sampling results that exceed the applicable limit is not, by itself, a 
violation.
    MSHA recognizes that this is an important point, and reiterates 
that, as indicated in Sec. 57.5061, MSHA itself is to conduct 
compliance sampling.

[[Page 5884]]

    Retention of Sample Results. Section 57.5071(d)(2) requires that 
records of the sampling method and the sample results themselves be 
retained by mine operators for five years. This is because the results 
from a monitoring program can provide insight as to the effectiveness 
of controls over time, and provide a history of occupational exposures 
at the mine.
    In the preamble to the proposed rule, MSHA welcomed comments on the 
sample retention period appropriate for the risks involved. None were 
received.
    In the preamble to the proposed rule, MSHA also asked for comments 
regarding the advisability of instituting a system of medical 
surveillance of miners exposed to dpm to identify miners suffering ill 
effects of dpm exposure, and the subsequent medical removal of miners 
who are determined to be suffering such ill effects. The comments 
received in response to this request suggested that medical 
surveillance for excessive dpm exposure is not feasible at this time 
because the appropriate biological tests or markers do not exist. One 
commenter observed that they were, ``* * * unaware of any recognized or 
generally accepted examinations or tests for detecting whether miners 
are suffering from ill effects as a result of diesel particulate or 
exhaust exposure. This view is supported by EPA's Health Assessment 
Document for Diesel Emissions which states, `There is no single medical 
test to determine if DP exposure has occurred. Many symptoms of 
episodic DP exposure are similar to symptoms caused by other agents or, 
in some cases, onset of a common cold. Invasive sampling of particle 
deposits in the upper respiratory tract or lung could be done, yet such 
particles may not be readily distinguishable from particulate matter 
from other sources' [EPA, 1998].'' MSHA agrees with these commenters 
that appropriate medical testing protocols are not currently available. 
Therefore, provision for neither medical surveillance nor medical 
removal protections have been incorporated into the final rule.

Section 57.5075 Diesel Particulate Records

    Various recordkeeping requirements are set forth in the provisions 
of the final rule. For the convenience of the mining community, these 
requirements are also listed in a table entitled ``Diesel Particulate 
Recordkeeping Requirements,'' which can be found in Sec. 57.5075(a). 
Each row involves a record that must be kept. The section requiring the 
record be kept is noted, along with the retention time.
    This approach--having a summary table of recordkeeping requirements 
included in various sections of the rule--is identical to that taken in 
the proposed rule. MSHA indicated in the preamble to the proposed rule 
that it would welcome input from the mining community as to whether it 
liked this approach or found it duplicative or confusing, however, no 
comments were received.
    Location of Records. Section 57.5075(b)(1) provides that any record 
which is required to be retained at the mine site may be retained 
elsewhere if it is immediately accessible from the mine site by 
electronic transmission. Compliance records need to be accessible to an 
inspector so they can be viewed during the course of an inspection, as 
the information in the records may determine how the inspection 
proceeds. If the mine site has a fax machine or computer terminal, 
there is no reason why the records cannot be maintained elsewhere. 
MSHA's approach in this regard is consistent with Office of Management 
and Budget Circular A-130.
    One commenter, though supporting the concept of off-site electronic 
records storage, questioned MSHA's intent relative to the term 
``immediately accessible.'' As noted above, MSHA intends that records 
maintained off-site be made available to an MSHA inspector so the 
information can be used to guide inspection decisions. Thus, undue 
delay in retrieving this information from off site electronic storage 
would impede an inspection, and would not be permitted. If the records 
are maintained in hardcopy form at an off-site location, and 
considering the time required to contact off-site personnel to request 
the records, for those personnel to locate and remove the records from 
the files, and to fax the records to the mine site, a delay of one or 
two hours would not be unreasonable. If records are maintained in an 
off-site electronic database, it is reasonable to assume they could be 
electronically transmitted to the mine site even faster; perhaps one 
hour or less.
    These time frames are in contrast to the requirement in MSHA's new 
noise regulation for noise records to be accessible to the MSHA 
inspector, but not ``immediately accessible.'' The guideline 
established in the Preamble to the final noise rule states that records 
must be provided to the MSHA inspector within one business day or less 
(p. 49625).
    The commenter notes further that, ``Even with Y2K compliant 
systems, computer and electronic transmission equipment is not 100% 
reliable, especially in remote mining environments.'' MSHA agrees that 
an insistence on 100% reliability of computer and electronic 
transmission equipment is unreasonable. However, MSHA will not accept 
chronic computer or electronic transmission problems as a justification 
for the repeated denial of timely access to the required records. If 
chronic computer or electronic transmission problems make ``immediate'' 
access to records problematic, such records would have to be kept at 
the mine site.
    Records Access. Section 57.5075(b) also covers records access. 
Consistent with the statute, upon request from an authorized 
representative of the Secretary of Labor, the Secretary of Health and 
Human Services, or from the authorized representative of miners, mine 
operators are to promptly provide access to any record listed in the 
table in this section. A miner, former miner, or, with the miner's or 
former miner's written consent, a personal representative of a miner, 
is to have access to any exposure record required to be maintained 
pursuant to Sec. 57.5071 to the extent the information pertains to the 
miner or former miner. Upon request, the operator must provide the 
first copy of such record at no cost. Whenever an operator ceases to do 
business, that operator would be required to transfer all records 
required to be maintained by this part to any successor operator.
    General Effective Date of Part 57. The rule provides that unless 
otherwise specified, its provisions take effect 60 days after the date 
of promulgation of the final rule. Thus, for example, the requirements 
to implement certain work practice controls (e.g., fuel type) go into 
effect 60 days after the final rule is published.
    A number of provisions of the final rule contain separate effective 
dates that provide more time for technical support. For example, the 
initial concentration limit for underground metal and nonmetal mines 
would be delayed for 18 months.
    A general outline of effective dates is summarized in Part I of 
this preamble.
    Additionally, the paperwork provisions will not become effective 
until approved by the Office of Management and Budget.

V. Adequacy of Protection and Feasibility of Final Rule; 
Alternatives Considered

    The Mine Act requires that in promulgating a standard, the 
Secretary, based on the best available evidence, shall attain the 
highest degree of health

[[Page 5885]]

and safety protection for the miner with feasibility a consideration.
    Overview. This part begins with a summary of the pertinent legal 
requirements, followed by a general profile of the economic health and 
prospects of the metal and nonmetal mining industry.
    The final rule establishes a concentration limit for dpm, 
supplemented by monitoring and training requirements. An operator in 
the metal and nonmetal sector would have the flexibility to choose any 
type or combination of engineering controls to keep dpm levels at or 
below the concentration limit. This part evaluates the final rule to 
ascertain if, as required by the statute, it achieves the highest 
degree of protection for underground metal and nonmetal miners that is 
feasible, both technologically and economically, for underground metal 
and nonmetal mine operators to provide.
    Several regulatory alternatives to the final rule were also 
reviewed by MSHA in light of the record. The Agency has concluded that 
compliance with these alternatives either provide less protection than 
the feasible approach being adopted, or are not technologically or 
economically feasible for the underground metal and nonmetal industry 
as a whole at this time.
    Pertinent Legal Requirements. Section 101(a)(6)(A) of the Federal 
Mine Safety and Health Act of 1977 (Mine Act) states that MSHA's 
promulgation of health standards must:

    * * * [A]dequately assure, on the basis of the best available 
evidence, that no miner will suffer material impairment of health or 
functional capacity even if such miner has regular exposure to the 
hazards dealt with by such standard for the period of his working 
life.

    The Mine Act also specifies that the Secretary of Labor 
(Secretary), in promulgating mandatory standards pertaining to toxic 
materials or harmful physical agents, base such standards upon:

    * * * [R]esearch, demonstrations, experiments, and such other 
information as may be appropriate. In addition to the attainment of 
the highest degree of health and safety protection for the miner, 
other considerations shall be the latest available scientific data 
in the field, the feasibility of the standards, and experience 
gained under this and other health and safety laws. Whenever 
practicable, the mandatory health or safety standard promulgated 
shall be expressed in terms of objective criteria and of the 
performance desired. [Section 101(a)(6)(A)].

    Thus, the Mine Act requires that the Secretary, in promulgating a 
standard, based on the best available evidence, attain the highest 
degree of health and safety protection for the miner with feasibility a 
consideration.
    In relation to feasibility, the legislative history of the Mine Act 
states that:

    * * *Section further provides that ``other considerations'' in 
the setting of health standards are ``the latest available 
scientific data in the field, the feasibility of the standards, and 
experience gained under this and other health and safety laws.'' 
While feasibility of the standard may be taken into consideration 
with respect to engineering controls, this factor should have a 
substantially less significant role. Thus, the Secretary may 
appropriately consider the state of the engineering art in industry 
at the time the standard is promulgated. However, as the circuit 
courts of appeal have recognized, occupational safety and health 
statutes should be viewed as ``technology-forcing'' legislation, and 
a proposed health standard should not be rejected as infeasible when 
the necessary technology looms in today's horizon. AFL-CIO v. 
Brennan, 530 F.2d 109 (1975); Society of the Plastics Industry v. 
OSHA, 509 F.2d 1301, cert. denied, 427 U.S. 992 (1975).

    Similarly, information on the economic impact of a health standard 
which is provided to the Secretary of Labor at a hearing or during the 
public comment period, may be given weight by the Secretary. In 
adopting the language of [this section], the Committee wishes to 
emphasize that it rejects the view that cost benefit ratios alone may 
be the basis for depriving miners of the health protection which the 
law was intended to insure. S. Rep. No. 95-181, 95th Cong., 1st Sess. 
21 (1977).
    Court decisions have clarified the meaning of feasibility. The 
Supreme Court, in American Textile Manufacturers' Institute v. Donovan 
(OSHA Cotton Dust), 452 U.S. 490, 101 S.Ct. 2478 (1981), defined the 
word ``feasible'' as ``capable of being done, executed, or effected.'' 
The Court stated that a standard would not be considered economically 
feasible if an entire industry's competitive structure was threatened. 
According to the Court, the appropriate inquiry into a standard's 
economic feasibility is whether the standard is capable of being 
achieved.
    Courts do not expect hard and precise predictions from agencies 
regarding feasibility. Congress intended for the ``arbitrary and 
capricious standard'' to be applied in judicial review of MSHA 
rulemaking (S.Rep. No. 95-181, at 21.) Under this standard, MSHA need 
only base its predictions on reasonable inferences drawn from the 
existing facts. MSHA is required to produce reasonable assessment of 
the likely range of costs that a new standard will have on an industry. 
The agency must also show that a reasonable probability exists that the 
typical firm in an industry will be able to develop and install 
controls that will meet the standard. See, Citizens to Preserve Overton 
Park v. Volpe, 401 U.S. 402, 91 S.Ct. 814 (1971); Baltimore Gas & 
Electric Co. v. NRDC, 462 U.S. 87 103 S.Ct. 2246, (1983); Motor Vehicle 
Manufacturers Assn. v. State Farm Mutual Automobile Insurance Co., 463 
U.S. 29, 103 S.Ct. 2856 (1983); International Ladies' Garment Workers' 
Union v. Donovan, 722 F.2d 795, 232 U.S. App. D.C. 309 (1983), cert. 
denied, 469 U.S. 820 (1984); Bowen v. American Hospital Assn., 476 U.S. 
610, 106 S.Ct. 2101 (1986).
    In developing a health standard, MSHA must also show that modern 
technology has at least conceived some industrial strategies or devices 
that are likely to be capable of meeting the standard, and which 
industry is generally capable of adopting. United Steelworkers of 
America v. Marshall, 647 F.2d 1189, 1272 (1980). If only the most 
technologically advanced companies in an industry are capable of 
meeting the standard, then that would be sufficient demonstration of 
feasibility (this would be true even if only some of the operations met 
the standard for some of the time). American Iron and Steel Institute 
v. OSHA, 577 F.2d 825, (3d Cir. 1978); see also, Industrial Union 
Department, AFL-CIO v. Hodgson, 499 F.2d 467 (1974).
    Industry Profile. This industry profile provides background 
information about the structure and economic characteristics of the 
mining industry. It provides data on the number of mines, their size, 
the number of employees, and the diesel powered equipment used.
    The Structure of the Metal/Nonmetal Mining Industry. MSHA divides 
the mining industry into two major segments based on commodity: (1) 
Coal mines and (2) metal and nonmetal (M/NM) mines. These segments are 
further divided based on type of operation (e.g., underground mines or 
surface mines). MSHA maintains its own data on mine type, size, and 
employment, and the Agency also collects data on the number of 
independent contractors and contractor employees by major industry 
segment.
    MSHA categorizes mines by size based on employment. For the past 20 
years, for rulemaking purposes, MSHA has consistently defined a small 
mine to be one that employs fewer than 20 workers and a large mine to 
be one that employs 20 or more workers. To comply with the requirements 
of the Small

[[Page 5886]]

Business Regulatory Enforcement Fairness Act (SBREFA) amendments to the 
Regulatory Flexibility Act (RFA), however, an agency must use the Small 
Business Administration's (SBA's) criteria for a small entity-\3/4\ for 
mining, 500 or fewer employees \3/4\ when determining a rule's economic 
impact.
    Table V-1 presents the total number of small and large mines and 
the corresponding number of miners, excluding contractors, for the M/NM 
mining segment. The M/NM mining segment consists of metal mines 
(copper, iron ore, gold, silver, etc.) and nonmetal mines (stone 
including granite, limestone, dolomite, sandstone, slate, and marble; 
sand and gravel; and others such as clays, potash, soda ash, salt, 
talc, and pyrophyllite.) As Table II-1 indicates, 98 percent of all M/
NM mines are surface mines, and these mines employ some 90 percent of 
all M/NM miners, excluding office workers. Table V-2 presents 
corresponding data on the number of independent contractors and their 
employees working in the M/NM mining segment.

  Table V-1.--Distribution of M/NM Mine Operations and Employment (Excluding Contractors) by Mine Type and Size
                                                       \a\
----------------------------------------------------------------------------------------------------------------
                                                                                   Mine type
                                                             ---------------------------------------------------
                    Size of M/NM mine \b\                                                  Office
                                                              Underground    Surface      workers     Total M/NM
----------------------------------------------------------------------------------------------------------------
Fewer than 20 employees:
    Mines...................................................          134        9,635  ...........        9,769
    Employees...............................................        1,054       54,356        9,160       64,570
20 to 500 employees:
    Mines...................................................          124        1,419  ...........        1,543
    Employees...............................................       11,299       79,675       15,040      106,014
Over 500 employees:
    Mines...................................................            7           18  ...........           25
    Employees...............................................        4,594       16,836        3,543       24,973
All M/NM mines:
    Mines...................................................          265       11,072  ...........       11,337
    Employees...............................................       16,947      150,867       27,743     195,557
----------------------------------------------------------------------------------------------------------------
\a\ Source: U.S. Department of Labor, Mine Safety and Health Administration, Office of Standards, Regulations,
  and Variances based on 1998 MS data, CM441/CM935LA cycle 1998/198. Data for Total Office workers from Mine
  Injury and Worktime Quarterly (1997 Closeout Edition) Table 2, p. 6.
\b\ Based on MSHA's traditional definition, large mines include all mines with 20 or more employees. Based on
  SBA's definition, as required by SBREFA, large mines include only mines with over 500 employees.


         Table V-2.--Distribution of M/NM Contractors and Contractor Employment by Size of Operation \a\
----------------------------------------------------------------------------------------------------------------
                                                                                  Contractors
                                                             ---------------------------------------------------
                   Size of contractors \b\                                                 Office
                                                              Underground    Surface      workers       Total
----------------------------------------------------------------------------------------------------------------
Fewer than 20 employees:
    Mines...................................................          399        2,783  ...........        3,182
    Employees...............................................        1,717       14,155          649       16,521
20 to 500 employees:
    Mines...................................................           36          349  ...........          384
    Employees...............................................        1,639       17,979          802       20,420
Over 500 employees:
    Mines...................................................  ...........            3  ...........            3
    Employees...............................................  ...........        2,560          105        2,665
      Total contractors:
        Mines...............................................          434        3,135  ...........        3,569
        Employees...........................................        3,356       34,694        1,556      39,606
----------------------------------------------------------------------------------------------------------------
\a\ Source: U.S. Department of Labor, Mine Safety and Health Administration, Office of Standards, Regulations,
  and Variances based on 1998 MS data, CT441/CT935LA cycle 1998/198. Data for total office workers from Mine
  Injury and Worktime Quarterly (1998 Closeout Edition) Table 6, p. 21.
\b\ Based on MSHA's traditional definition, large mines include all mines with 20 or more employees. Based on
  SBA's definition, as required by SBREFA, large mines include only mines with over 500 employees.

    The M/NM mining sector consists of about 80 different commodities 
including industrial minerals. There were 11,337 M/NM mines in the U.S. 
in 1998, of which 9,769 (86%) were small mines and 1,568 (14%) were 
large mines, using MSHA's traditional definition of small and large 
mines. Based on SBA's definition, however, only 25 M/NM mines (0.2%) 
were large mines.\1\
---------------------------------------------------------------------------

    \1\ U.S. Department of Labor, MSHA, 1998 Final MIS data CM441 
cycle 1998/198.
---------------------------------------------------------------------------

    The data in Table V-1 indicate that employment at M/NM mines in 
1998 was 195,557, of which 64,570 workers (33%) were employed by small 
mines and 130,987 miners (67%) were employed by large mines, using 
MSHA's definition. Based on SBA's definition, however, 170,584 workers 
(87%) were employed by small mines and 24,973 workers (13%) were 
employed by large mines. Using MSHA's definition, the average 
employment is 7 workers at a small M/NM mine and 84 workers at a

[[Page 5887]]

large M/NM mine.\2\ Using SBA's definition, there are an average of 15 
workers in each small M/NM mine and 888 workers in each large M/NM 
mine.
---------------------------------------------------------------------------

    \2\ U.S. Department of Labor, MSHA, 1998 final MIS data CM441 
cycle 1998/198.
---------------------------------------------------------------------------

    Metal Mining. There are about 24 metal commodities mined in the 
U.S. Underground metal mines use a few basic mining methods, such as 
room and pillar and block caving. The larger mines rely more heavily on 
hydraulic drills and track-mounted haulage, and the smaller underground 
metal mines rely more heavily on hand-held pneumatic drills
    Surface metal mines normally include drilling, blasting, and 
hauling; such processes are typical in all surface mines, irrespective 
of commodity types. Surface metal mines in the U.S. rank among some of 
the largest mines in the world.
    Metal mines constitute 3 percent of all M/NM mines and employ 23 
percent of all M/NM miners. Under MSHA's traditional definition of a 
small mine, 45 percent of metal mines are small, and these mines employ 
2 percent of all miners working in metal mines. Using SBA's definition, 
94 percent of metal mines are small, and they employ 53 percent of all 
miners working in metal mines.\3\
---------------------------------------------------------------------------

    \3\ U.S. Department of Labor, Mine Safety and Health 
Administration, Office of Program Policy Evaluation, Mine Employment 
Size-Average Employment 1998.
---------------------------------------------------------------------------

    Stone Mining. In the stone mining subsector, there are eight 
different stone commodities, of which seven are further classified as 
either dimension stone or crushed and broken stone. Stone mining in the 
U.S. is predominantly by quarrying, with only a few slight variations. 
Crushed stone mines typically drill and blast, while dimension stone 
mines generally use channel burners, drills, or wire saws. Diesel 
powered-haulage is used to transfer the broken rock from the quarry to 
the mill where crushing and sizing are done.
    Stone mines constitute 33 percent of all M/NM mines, and they 
employ 41 percent of all M/NM miners. Using MSHA's definition of a 
small mine, 71 percent of stone mines are small, and these mines employ 
29 percent of all miners working in stone mines. Using SBA's 
definition, 99.9 percent of stone mines are small, and they employ 99 
percent of all miners working in stone mines.\4\
---------------------------------------------------------------------------

    \4\ U.S. Department of Labor, Mine Safety and Health 
Administration, Office of Program Policy Evaluation, Mine Employment 
Size-Average Employment 1998.
---------------------------------------------------------------------------

    Sand & Gravel Mining. Sand and gravel, for construction, is 
generally extracted from surface deposits using dredges or draglines. 
Further preparation involves washing and screening. As in other surface 
mining operations, sand and gravel uses diesel-driven machines, such as 
front-end loaders, trucks, and bulldozers, for haulage. The preparation 
of industrial sand and silica flour involves the use of crushers, ball 
mills, vibrating screens, and classifiers.
    The sand and gravel subsector represents the single largest 
commodity group in the U.S. mining industry when the number of mining 
operations is being considered. Sand and gravel mines comprise 57 
percent of all M/NM mines, and they employ 22 percent of all M/NM 
miners. Using MSHA's definition of a small mine, 95 percent of sand and 
gravel mines are small, and these mines employ 76 percent of all miners 
working in sand and gravel mines. Using SBA's definition, almost 100 
percent of sand and gravel mines are small, and they employ 
approximately 42,800 miners.\5\
---------------------------------------------------------------------------

    \5\ U.S. Department of Labor, Mine Safety and Health 
Administration, Office of Program Policy Evaluation, Mine Employment 
Size-Average Employment 1998.
---------------------------------------------------------------------------

    Other Nonmetal Mining. For enforcement and statistical purposes, 
MSHA separates stone and sand and gravel mining from other nonmetal 
mining. There are about 35 other nonmetal commodities, not including 
stone, and sand and gravel. Nonmetal mining uses a wide variety of 
underground mining methods such as continuous mining (similar to coal 
mining), in-situ retorting, block caving, and room and pillar. The 
mining method is dependent on the geologic characteristics of the ore 
and host rock. Some nonmetal operations use kilns and dryers in ore 
processing. Ore crushing and milling are processes common to both 
nonmetal and metal mining.
    As with underground mining, there is a wide range of mining methods 
utilized in extracting minerals by surface mining. In addition to 
drilling and blasting, other mining methods, such as evaporation and 
dredging, are also utilized, depending on the ore formation.
    ``Other'' nonmetal mines comprise 7 percent of all M/NM mines, and 
they employ 14 percent of all M/NM miners. Using MSHA's definition of a 
small mine, 66 percent of other nonmetal mines are small, and they 
employ 12 percent of all miners working in these nonmetal mines. Using 
SBA's definition, 99 percent of other nonmetal mines are small, and 
they employ 92 percent of all miners working in these nonmetal 
mines.\6\
---------------------------------------------------------------------------

    \6\ U.S. Department of Labor, Mine Safety and Health 
Administration, Office of Program Policy Evaluation, Mine Employment 
Size-Average Employment 1998.
---------------------------------------------------------------------------

    Economic Characteristics of the Metal/nonmetal Mining Industry. The 
value of all M/NM mining output in 1998 was estimated at $40 
billion.\7\ Metal mines, which include copper, gold, iron, lead, 
silver, tin, and zinc mines, contributed $17.8 billion. Nonmetal 
production was valued at $22.2 billion: $9.0 billion from stone mining, 
$5.2 billion from sand and gravel, and $8 billion from other nonmetals 
such as potash, clay, and salt.
---------------------------------------------------------------------------

    \7\ U.S. Department of Energy, Energy Information 
Administration, Annual Energy Review 1998, July 1999, pp. 3, 6, 142, 
158, and 160.
---------------------------------------------------------------------------

    The end uses of M/NM mining output are diverse. For example, iron 
and aluminum are used to produce vehicles and other heavy duty 
equipment, as well as consumer goods such as household equipment and 
soft drink cans. Other metals, such as uranium and titanium, have more 
limited uses. Nonmetals, like cement, are used in construction while 
salt is used as a food additive and for road deicing in the winter. 
Soda ash, phosphate rock, and potash also have a wide variety of 
commercial uses. Stone and sand and gravel are used in numerous 
industries and extensively in the construction industry.
    A detailed economic picture of the M/NM mining industry is 
difficult to develop because most mines are either privately held 
corporations or sole proprietorships, or subsidiaries of publicly owned 
companies. Privately held corporations and sole proprietorships are not 
required to make their financial data available to the public. Parent 
companies are not required to separate financial data for subsidiaries 
in their reports to the Securities and Exchange Commission. As a 
result, financial data are available for only a few M/NM companies, and 
these data are not representative of the entire industry.
    Adequacy of Miner Protection Provided by the Final Rule in 
Underground Metal and Nonmetal Mines. In evaluating the rule for this 
purpose, it should be remembered that MSHA has measured dpm 
concentrations in this sector as high as 5,570DPM 
g/m\3\--a mean of 808DPM g/m\3\. See Table 
III-1 and Figure III-2 in part III of the preamble. As discussed in 
detail in part III of the preamble, these concentrations place 
underground metal and nonmetal miners at significant risk of material 
impairment of their health,

[[Page 5888]]

and it does not appear there is any lower boundary to the risk. 
Accordingly, in accordance with the statute, the Agency has to set a 
standard which reduces these concentrations as much as is both 
technologically and economically feasible for this sector as a whole.
    Specifically, the standard establishes a concentration limit on 
dpm. The concentration limit is the equivalent of about 
200DPM g/m\3\ (as explained in Part IV, in the rule 
the concentration limit is expressed in terms of a restriction on the 
amount of total carbon because of the measurement system which MSHA 
will utilize for compliance sampling).
    Alternatives considered. In order to ensure that the maximum 
protection that is feasible for the underground mining industry as a 
whole is being provided, the Agency has considered three alternatives 
that would provide greater protection: a lower concentration limit, a 
significantly shorter implementation period, and requiring certain 
categories of metal and nonmetal equipment to be filtered in addition 
to observing a concentration limit. In addition, the agency has 
considered whether the approach it is taking in underground coal mines 
would be feasible in this sector. Specific alternatives and approaches 
suggested by industry and labor are discussed in detail in part IV.
    (1) Establish a lower concentration limit for underground metal/
nonmetal mines. Based on the Agency's risk assessment, a lower 
concentration limit would provide more miner protection. The Agency has 
concluded, however, that at this time it would not be feasible for the 
underground metal and nonmetal sector to reach a lower concentration 
limit. The problem is not technological feasibility, but rather 
economic feasibility.
    Technological feasibility of lower limit. In evaluating whether a 
lower concentration limit is technologically feasible for this sector, 
MSHA considered several examples of real-world situations. These 
examples, and a detailed description of the methodology by which they 
were developed, were published in the preamble to the proposed rule (65 
FR 58198 et seq.). The examples were based on data about equipment and 
ventilation from several actual underground metal and nonmetal mines: a 
salt mine; an underground limestone mine that operates two completely 
different shifts, one for production, and one for support; and a multi-
level underground gold mine. The data was placed into a computer model 
to estimate the ambient dpm that would remain in a mine section after 
the application of a particular combination of control technologies. 
The details of this computer model, referred to as ``The Estimator'', 
has subsequently been published in the literature (Haney and Saseen, 
Mining Engineering, April 2000). The results for the salt and limestone 
mines were written up in detail and placed into MSHA's record, with 
actual mine identifiers removed; the study of the underground gold mine 
is based on information supplied by inspectors, and all available data 
was presented in the preamble to the proposed rule.
    MSHA had picked these mines because the Agency originally thought 
the conditions there were such that these mines would have great 
difficulty in controlling dpm concentrations. As the results indicated, 
however, even in these apparently difficult situations the 
concentration of dpm could be lowered to well below 200DPM 
g/m\3\ with readily available control techniques. Moreover as 
noted above, MSHA can adopt a rule which is not feasible for every 
mine; the standard is that the rule be feasible for the industry as a 
whole.
    MSHA did receive comments on the Estimator. However, no specific 
examples of its application were received nor comments taking issue 
with the examples discussed above. Specific comments received on the 
Estimator are addressed in part IV.
    Economic feasibility of lower concentration limit. MSHA estimates 
that it will cost the underground metal and nonmetal industry about 
$25.1 million a year to comply with a concentration limit of 
160TC g/m\3\ (200DPM g/m\3\). 
For an average underground metal and nonmetal dieselized mine that uses 
diesel powered equipment, this amounts to about $128,000 per year.
    The assumptions used in preparing the cost estimates for the final 
review are discussed in detail in the Agency's REA. They are based on a 
careful review of the evidence on the capabilities of various controls, 
and a careful review of an economic analysis submitted on behalf of 
several industry associations. That analysis estimated costs to be 
three times as high as MSHA's initial estimate. MSHA's analysis and the 
industry analysis agree on many of their assumptions; however, MSHA 
believes the industry analysis to be an overestimation primarily 
because it failed to properly optimize.
    In general, MSHA has concluded that:
     The interim standard of 400TC g/m\3\ 
(500DPM g/m\3\) will be met primarily through the 
use of filters, but with cabs and ventilation in certain instances; and
     The final standard of 160TC g/m\3\ 
(200DPM g/m\3\) will be met through the use of more 
filters, ventilation changes, and the turnover in equipment and engines 
to less polluting models that will have occurred by the time the final 
standard goes into effect.
    Based on its cost estimates, the Agency has concluded that this 
sector would not find it economically feasible to reduce dpm 
concentrations to a lower limit at this time. The incremental cost of 
additional controls would rise sharply if the industry were required to 
reach a substantially lower concentration level. It would begin to be 
necessary to retrofit cabs on equipment that was not designed with cabs 
and/or did not have off-the-shelf parts--at a cost per unit nearly 
three times as great as the costs for more limited retrofitting of 
suitably designed equipment. Additional ventilation improvements (e.g., 
new shafts) could easily run into the millions of dollars--compared 
with the $300,000 estimate for more limited ``major system 
improvements'' used in the cost analysis. Additional replacement of 
engines beyond the natural turnover included in the baseline could run 
as high as $27,500 for the engine itself, with additional costs 
possibly as high as $65,000 for equipment modifications and 
installation.
    (2) Significantly shorten the phase-in time to reach the final 
concentration limit in underground metal/nonmetal mines. Under the 
rule, there is a phase-in period for a dpm concentration limit. 
Operators have 18 months to reduce dpm concentrations in areas of the 
mine where miners work or travel to 400TC g/m\3\ 
(500DPM g/m\3\), and up to 60 months in all to 
reduce dpm concentrations in those areas to 160TC 
g/m\3\ (200DPM g/m\3\).
    MSHA has established this phase-in period because it has concluded 
that it is economically infeasible for the underground metal and 
nonmetal mining industry as a whole to implement the requirements 
sooner. The costs of the rule would increase significantly were the 
final concentration limit to become effective significantly sooner. For 
example, the turnover of the fleet to less polluting engines would not 
be as complete by the time the final limit goes into effect; hence, 
operators would be required to purchase new engines ahead of schedule. 
Moreover, a substantial portion of the costs to implement these 
provisions were calculated using a 5-year discounting process to 
reflect the phase-in schedule.

[[Page 5889]]

    Technological feasibility problems might also be more frequent with 
a quicker implementation schedule. The rule includes a provision for a 
special time extension to deal with unique situations; shortening the 
normal time frame available to this sector would tend to increase the 
frequency upon which operators would have to apply for such extensions.
    Accordingly, MSHA has concluded that, for the underground metal and 
nonmetal sector as a whole, a significantly accelerated approach would 
not be feasible.
    (3) In addition to a concentration limit, require certain types of 
equipment to utilize an 80% efficiency filter. This approach would help 
reduce dpm concentrations in localized areas of a mine, and ensure that 
problems with ventilation controls will have less of an impact on miner 
exposures. Most filters can meet the 80% requirement. The requirement 
could be applied: (a) just to loading and hauling equipment (e.g., 
trucks and loaders); (b) to the equipment in (a) plus equipment used in 
the production process (e.g., drills, powered trucks); (c) to the 
equipment in (a) and (b) and also direct support equipment (e.g., 
scalers, lube trucks, generators, compressors and pumps); or (d) to all 
equipment except personnel carriers and supply trucks.
    Such an approach would limit operator flexibility on controls--the 
broader the requirement, the less the flexibility. And it would 
increase expense, since the most efficient way to achieve compliance 
with the concentration limit might well be another type of control 
(e.g., new engine, cab, ventilation, etc.). Accordingly, MSHA has 
determined that this approach would be infeasible for this sector at 
this time.
    (4) In lieu of a concentration limit, require certain types of 
equipment to reach tailpipe limits. In the underground coal sector, 
MSHA is requiring various categories of equipment to meet specific 
tailpipe limits. Compliance with these limits is determined through 
laboratory tests of engines and control devices. This approach avoids 
questions about MSHA in-mine compliance sampling which have been the 
focus of much discussion in coal mining. Accordingly, MSHA considered 
requiring a similar approach in underground metal and nonmetal mines. 
However, the agency determined that this would not be practical, 
because the engines in the current fleet are not approved; hence, the 
agency lacks information on their emission rates, a key piece of 
information needed to implement a tailpipe standard. Moreover, in many 
cases a cab or ventilation change might be a more effective solution to 
a localized dpm concentration in an underground metal and nonmetal mine 
than a change in the engine or emission control device--and perhaps 
less expensive for equipment of this size. One of the advantages of a 
concentration limit is the flexibility of controls that the operator 
can apply to meet the limit.
    Feasibility of the final rule for underground metal and nonmetal 
mining sector. The Agency has carefully considered both the 
technological and economic feasibility of the rule being promulgated 
for the underground metal and nonmetal mining sector as a whole.
    Technological feasibility of final rule. There are arguably two 
separate issues with respect to technological feasibility--(a) the 
existence of technology that can accurately and reliably measure dpm 
concentration levels in all types of underground metal and nonmetal 
mines; and (b) the existence of control mechanisms that can bring dpm 
concentrations down to the proposed limit in all types of underground 
metal and nonmetal mines. Both have been addressed elsewhere in this 
preamble.
    The first of these questions, concerning measurement, is reviewed 
in considerable detail in section 3 of Part II and in the discussion of 
section 57.5061 of the rule in Part IV. For the reasons set forth in 
those discussions, MSHA has concluded that with the use of a 
submicrometer sampler as required by the final rule, and with a 
sampling strategy that avoids the inteferences which can compromise 
individual samples in certain situations, it does have a 
technologically feasible measurement method that operators and the 
agency can use to determine if the limits established by the standard 
are in fact being met.
    The second of these questions, concerning controls, is discussed 
earlier in this part [See ``(1) Establish a lower concentration limit 
for underground metal/nonmetal mines'']. MSHA has performed various 
studies which suggest that even in the most difficult situations, it is 
technologically feasible for operators to meet the rule's final 
concentration limit. In fact, these studies suggest it is 
technologically feasible for operators in this sector to reduce their 
dpm concentrations to an even lower concentration limit. In addition, 
as discussed in section 6 of Part II of this preamble, considerable 
progress has been made in recent years on the effectiveness of filters 
and cabs. MSHA very carefully reviewed this information with reference 
to the kinds of engines and equipment found in underground metal and 
nonmetal mines, and their ventilation, and is confident that the final 
rule is technologically feasible.
    Although the agency has reached this conclusion, and moreover knows 
of no mine that cannot accomplish the required reductions in the 
permitted time, it has nevertheless retained in the final rule a 
provision that any underground metal or nonmetal mine may have up to an 
additional two years to install the required controls should it find 
that there are unforseen technological barriers to timely completion. A 
detailed discussion of the requirements for obtaining approval for such 
an extension of time to comply is provided in part IV of the preamble.
    Economic Feasibility. MSHA estimates that the rule would cost the 
underground metal and nonmetal sector about $25.1 million a year even 
with the extended phase-in time. The costs per underground dieselized 
metal or nonmetal mine are estimated to be about $128,000 annually. The 
yearly cost of the final rule represents about 0.67 percent of yearly 
industry revenue. MSHA uses a one-percent ``screen'' of costs relative 
to revenues as a presumptive benchmark of economic feasibility. 
Therefore, since the cost of the rule is less than one percent of 
revenues, MSHA anticipates that (subject to contrary evidence) the rule 
is economically feasible for the dieselized underground M/NM mining 
sector as a whole. Note, however, that the costs are sufficiently close 
to one percent of revenues that the rule could threaten the economic 
viability of affected mines on the economic margin and that more costly 
regulatory alternative could conceivably threaten the economic 
viability of a substantial fraction of this mining sector.
    As explained in the REA, nearly all ($24.1 million) of the 
anticipated yearly costs would be investments in equipment to meet the 
interim and final concentration limits. While operators have complete 
flexibility as to what controls to use to meet the concentration 
limits, the Agency based its cost estimates on the assumption that 
operators will ultimately need the following to get to the final 
concentration limit: (a) Fifty percent of the fleet will have new 
engines (these new engines do not impact cost of the rule). It is 
expected that the new engines will be more expensive and 
technologically superior to the ones that they replace. One aspect of 
this technological superiority will be substantially lower DPM 
emissions. It does not follow, however, that the

[[Page 5890]]

greater expense of these engines is an impact of this rule. Mine 
operators will not replace existing engines with the same type or model 
of engine. New engine technology makes engines much more efficient and 
productive than existing older engines. Particularly on larger 
equipment, greater productivity makes new engines an attractive 
investment that will pay back the greater costs. Moreover, due to EPA 
regulations which will limit DPM emissions from engines used in surface 
construction, surface mining, and over-the-road trucks (the major 
markets for heavy duty diesel engines), the market for low tech, 
``dirtier'' engines will dry up. Underground mine operators will thus 
purchase high tech, cleaner engines because they will be the only 
engines available for purchase.
    (b) One hundred percent of the production equipment and about fifty 
percent of the support equipment will be equipped with filters; (c) 
about thirty percent of all equipment will need to be equipped with 
environmentally controlled cabs; (d) twenty three percent of the mines 
will need new ventilation systems (fans and motors): (e) forty percent 
of the mines will need new motors on these fans; and (f) thirty two 
percent of the mines will need major ventilation upgrades.
    The Agency is taking a number of steps to mitigate the impact of 
the rule for the underground metal and nonmetal sector, particularly on 
the smallest mines in this sector. These are described in detail in the 
Agency's Regulatory Flexibility Analysis, which the Agency is required 
to prepare under the Regulatory Flexibility Act in connection with the 
impact of the rule on small entities. (The regulatory flexibility 
analysis can be found in part VI of this preamble, or packaged with the 
Agency's REA.)
    Based on its cost estimates, the Agency has concluded that this 
sector would not find it economically feasible to reduce dpm 
concentrations to a lower limit at this time. These assumptions and the 
rationale behind them are discussed in greater detail in the beginning 
of Chapter IV of the Regulatory Economic Analysis.
    After a careful review of the information about this sector 
available from the industry economic profile, and the other obligations 
of this sector under the Mine Act, MSHA has concluded that a reasonable 
probability exists that the typical firm in this sector will be able at 
this time to afford the controls that will be necessary to meet the 
proposed standard.
    Conclusion: metal and nonmetal mining sector. Based on the best 
evidence available at this time, the Agency has concluded that the 
final rule for the underground metal and nonmetal sector meets the 
statutory requirement that the Secretary attain the highest degree of 
health and safety protection for the miners in that sector, with 
feasibility a consideration.

VI. Regulatory Impact Analyses

    This part of the preamble reviews several impact analyses which the 
Agency is required to provide in connection with its final rulemaking. 
The full text of these analyses can be found in the Agency's Regulatory 
Economic Analysis (REA).

(A) Costs and Benefits: Executive Order 12866

    In accordance with Executive Order 12866, MSHA has prepared a 
Regulatory Economic Analysis (REA) of the estimated costs and benefits 
associated with the final rule for the underground metal and nonmetal 
mining sector.
    The key conclusions of the REA are summarized, together with cost 
tables, in part I of this preamble (see Item number 7). The complete 
REA is part of the record of this rulemaking, and is available from 
MSHA.
    The Agency considers this rulemaking ``significant'' under section 
3(f) of Executive Order 12866, and has so designated the rule in its 
semiannual regulatory agenda (RIN 1219-AA74). However, based upon the 
REA, MSHA has determined that the final rule does not constitute an 
``economically significant'' regulatory action pursuant to section 
3(f)(1) of Executive Order 12866.

(B) Regulatory Flexibility Act (RFA)

Introduction

    In accordance with section 605 of the Regulatory Flexibility Act of 
1980 as amended, MSHA has analyzed the impact of the final rule on 
small businesses. Further, MSHA has made a determination with respect 
to whether or not it can certify that this final rule will not have a 
significant economic impact on a substantial number of small entities 
that are affected by this rulemaking. Under the Small Business 
Regulatory Enforcement Fairness Act (SBREFA) amendments to the 
Regulatory Flexibility Act (RFA), MSHA must include a factual basis for 
this certification. If the final rule does have a significant economic 
impact on a substantial number of small entities, then the Agency must 
develop a final regulatory flexibility analysis.
    The Agency has, as required by law (5 U.S.C. 605), developed a 
final regulatory flexibility analysis which is set forth Chapter V of 
the REA. In addition to a succinct statement of the objectives of the 
final rule and other information required by the Regulatory Flexibility 
Act, the analysis reviews alternatives considered by the Agency with an 
eye toward minimizing the economic impact on small business entities.

Definition of a Small Mine

    Under the RFA, in analyzing the impact of a rule on small entities, 
MSHA must use the Small Business Administration (SBA) definition for a 
small entity or, after consultation with the SBA Office of Advocacy, 
establish an alternative definition for the mining industry by 
publishing that definition in the Federal Register for notice and 
comment. MSHA has not taken such an action, and hence is required to 
use the SBA definition.
    The SBA defines a small entity in the mining industry as an 
establishment with 500 or fewer employees (13 CFR 121.201). Of the 196 
underground M/NM mines that use diesel powered equipment and are 
therefore affected by this rulemaking, 189 (or all but 7) fall into 
this category and hence can be viewed as sharing the special regulatory 
concerns that the RFA was designed to address.
    Traditionally, the Agency has also looked at the impacts of its 
rules on a subset of mines with 500 or fewer employees \3/4\ those with 
fewer than 20 employees, which the mining community refers to as 
``small mines.'' The way these small mines perform mining operations is 
generally recognized as being different from the way larger mines 
operate. These small mines differ from larger mines not only in the 
number of employees, but also, among other things, in economies of 
scale in material produced, in the type and amount of production 
equipment, and in supply inventory. Therefore, their costs of complying 
with MSHA rules and the impact of MSHA rules on them will also tend to 
be different. It is for this reason that ``small mines,'' as 
traditionally defined by the mining community, are of special concern 
to MSHA.
    This analysis complies with the legal requirements of the RFA for 
an analysis of the impacts on ``small entities'' while continuing 
MSHA's traditional look at ``small mines.'' MSHA concludes that the 
final rule would not have a significant economic impact on small 
entities, as defined by SBA, when considered as a group. However, MSHA 
has determined that the final rule arguably would have a significant 
economic impact on a subset of small entities that are covered by this

[[Page 5891]]

rulemaking. That subset is small underground M/NM mines as 
traditionally defined by MSHA, those mines with fewer than 20 
employees. This subset of affected mines constitutes a substantial 
number of small entities.

Screening Analysis

    General Approach. The Agency's analysis of impacts on ``small 
entities'' begins with a ``screening'' analysis. The screening compares 
the estimated compliance costs of a rule for small entities in the 
sector affected by the rule to the estimated revenues for those small 
entities. When estimated compliance costs are less than 1 percent of 
the estimated revenues (for the size categories considered), the Agency 
believes it is generally appropriate to conclude that there is no 
significant economic impact on a substantial number of small entities. 
When estimated compliance costs exceed 1 percent of revenues, it tends 
to indicate that further analysis may be warranted.
    Derivation of Costs and Revenues. The compliance costs presented 
here were previously introduced in Chapter IV of the REA along with an 
explanation of how they were derived. Table VI-1 summarizes the total 
yearly cost of the final rule by mine size.

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[[Page 5893]]

    Data on underground M/NM mines published by the U.S. Geological 
Survey \1\ were used for tonnage and value of underground M/NM mines. 
These data, however, are not disaggregated by mine size class. MSHA 
collects data, by mine size, on both average employees and employee 
hours.\2\ MSHA has used these data to estimate revenues by mine size 
class.
---------------------------------------------------------------------------

    \1\ U.S. Geological Survey, ``Mineral Industry Surveys: Mining 
and Quarrying Trends, 1998 Annual Review, April 2000.
    \2\ U.S. Department of Labor, MSHA, 1998 Final MIS data CM441 
cycle 1998/198.
---------------------------------------------------------------------------

    MSHA has assumed that tonnage is proportional to employee hours. 
This assumption (rather than proportionality with employees) implicitly 
adjusts for different shift lengths associated with different sizes of 
mines. MSHA has also assumed that all underground M/NM mines use diesel 
powered equipment.\3\ Using these assumptions, MSHA has computed the 
percentages of employee hours of all underground M/NM mines that are 
accounted for by each size class. MSHA estimates that these percentages 
of total revenues are accounted for by the different mine size classes.
---------------------------------------------------------------------------

    \3\ This assumption ignores the fact that some very small mines 
do not use diesel powered equipment. MSHA believes, however, that 
these mines are generally very small (even among the mines with 
fewer than 20 employees) and that many of them operate only 
intermittently. Thus they account for employee hours proportionately 
far less than their numbers. Accordingly, MSHA believes that the 
most accurate way to interpret the data is to disregard the fact 
that these mines do not use diesel powered equipment.
---------------------------------------------------------------------------

    Results of the Screening Analysis. The final rule applies to 
underground M/NM mines that use diesel-powered equipment. Table VI-1 
shows that the estimated yearly cost of the final rule as a percentage 
of yearly revenues is about 0.8 percent for the affected underground M/
NM mines with 500 or fewer employees.
    However, for a subset of affected underground M/NM mines, those 
with fewer than 20 employees, estimated yearly costs are equal to about 
2.16 percent of yearly revenues for this subset of mines. The economic 
impact on these small mines, which constitute a substantial number of 
small entities affected by the final rule, is larger than one percent 
of their revenues. MSHA therefore cannot certify that the final rule 
would not have a significant impact on a substantial number of small 
entities.
    The Agency has prepared a final regulatory flexibility analysis, as 
required by law, which explains the steps MSHA has taken to minimize 
the burden on these small entities and justifies the costs placed on 
them.

  Table VI-2.--Estimated Yearly Costs of Final Rule Relative to Yearly Revenues for Underground Coal Mines That
                                          Use Diesel-Powered Equipment
----------------------------------------------------------------------------------------------------------------
                                                                    Final rule                       Costs as
                            Mine size                              yearly costs   Revenuesa  (In  Percentage  of
                                                                  (In thousands)    thousands)       revenues
----------------------------------------------------------------------------------------------------------------
20 emp..........................................................          $4,093        $189,305            2.16
500 emp..............................................          21,837       2,745,137           0.80
----------------------------------------------------------------------------------------------------------------
a Source: Mine Safety and Health Administration, Office of Injury and Employment Information, Denver, Colorado.
  1999, and U.S. Department of Energy, Energy Information Agency, Annual Energy Review 1998, DOE/EIA0384(98),
  July 1999, p.203.

Final Regulatory Flexibility Analysis

    As indicated above, the estimated yearly cost of the final rule on 
a subset of small entities, those with fewer than 20 employees, is 2.16 
percent of yearly revenue. This percentage is just over twice the value 
(1.0 percent) below which MSHA could say with reasonable confidence 
that the final rule does not have a significant economic impact on a 
substantial number of small entities. Accordingly, MSHA has prepared a 
final regulatory flexibility analysis.

Need for, and Objectives of, the Rule

    Need. The rule is needed because underground miners in mines that 
use diesel powered equipment are currently exposed to extremely high 
concentrations of diesel particulate matter (DPM). Based on MSHA field 
studies, median DPM concentrations to which underground miners are 
exposed range up to 200 times as high as average environmental 
exposures in the most heavily polluted urban areas and up to 10 times 
as high as median exposures estimated for the most heavily exposed 
workers in any occupational group other than underground miners.
    The available scientific information indicates that miners exposed 
to the extremely high DPM concentrations found in underground mines are 
at significant excess risk of experiencing three kinds of material 
impairment to their health:
     Increased risk of lung cancer has been linked to chronic 
occupational DPM exposure.
     Increased acute risk of death from cardiovascular, 
cardiopulmonary, or respiratory causes has been linked to short or long 
term DPM exposures.
     Sensory irritations and respiratory symptoms can result 
from even short term DPM exposures. Besides being potentially 
debilitating, such effects can distract miners from their 
responsibilities in ways that could pose safety hazards for everyone in 
the mine.
    Although definitive dose-response relationships have not yet been 
established (especially for the acute effects), the best available 
evidence indicates that the risks are substantial.
    Objective. The objective of the rule is to lower DPM exposures in 
underground M/NM mines to concentrations similar to the worst levels to 
which other occupational groups are exposed. By doing so, the rule is 
designed substantially to lower the health risks associated with DPM. 
Expected benefits include an estimated minimum of 8.5 lung cancer 
deaths avoided per year.

Significant Issues Raised in Response to the Initial RFA

    Comments. The principal issue raised in comments on the PREA was 
that, for a variety of reasons, MSHA had substantially understated the 
costs of controlling DPM. The implication of these comments was that 
the rule was economically infeasible. The most comprehensive comments 
along these lines were by Head,\4\ who argued (among other things) that 
MSHA had made the following errors and omissions in its analysis:
---------------------------------------------------------------------------

    \4\ H. John Head, Principal Mining Engineer, Harding Lawson 
Associates, ``Review of Economic and Technical Feasibility of 
Compliance Issues Related to: Department of Labor--MSHA, 30 CFR Part 
57--Proposed Rule for Diesel Particulate Matter Exposure of 
Underground Metal and Nonmetal Miners,'' Report prepared under 
contract with the National Mining Association, July 21, 1999.
---------------------------------------------------------------------------

     MSHA had (according to Head) understated the numbers of 
machines and mines affected, including:

[[Page 5894]]

     Understatement of the number of diesel units in 
underground M/NM mines by more than 50 percent, and
     Understatement of the number of ventilation upgrades 
needed by 20 percent to 40 percent
     MSHA had understated a number of costs, including:
     Understatement of the cost of replacement engines by up to 
one third,
     Understatement of the costs of filters on larger engines 
by 20 percent, and
     Understatement of the costs of vehicle cabs by about 60 
percent.
     MSHA had omitted some costs entirely, including:
     Installation costs of retrofitting new engines in old 
equipment, which ran as high as three times the costs of the engines 
themselves, and
     Major ventilation improvements needed by about one third 
of the mines.
    Based on his own numbers, Head estimated compliance costs to be 
three times as high as MSHA's estimate of the cost of the proposed rule 
of $19.2 million.
    Analytical Assessment of Issues. MSHA considered the comments and 
reviewed its assessment of costs very carefully. The assessment focused 
on Head's comments, since his exposition was detailed enough for 
analysis of the basis of his estimates. MSHA responded in a variety of 
ways, which are summarized below.
    The key to the issue of the number of diesel units affected by the 
rule was how one interpreted the number. MSHA resolved this issue by 
recognizing that not all diesel powered equipment would be affected in 
the same manner. In fact, the machines in Head's total count should be 
grouped into three categories: active, spares, and disused. Active 
diesel powered equipment (essentially MSHA's original count) needs to 
be fitted for everyday use. Spare equipment needs to be controlled for 
occasional use as back-up. Disused equipment is essentially not 
affected by the rule. A shift in the principal control strategy from 
engine replacement to ceramic filters (discussed further below) made 
these distinctions operational. With ceramic filters, both active and 
spare equipment can be fitted with filters (a relatively inexpensive 
operation), but filters need to be regenerated and changed (which 
encompasses most of the costs) only to the extent that the equipment is 
actually used.
    MSHA believes that Head was simply wrong about the number of mines 
needing upgrades to their ventilation systems. Head appeared to believe 
that MSHA's count was arbitrary, and the basis for his proposed number 
was obscure. In fact, MSHA has based its count on mine-specific data on 
the existence and rate of air flow of ventilation systems. Thus, MSHA 
retained its original count.
    MSHA's review of comments on costs produced different conclusions 
for different specific costs:
     MSHA accepted and used Head's estimate of costs of ceramic 
filters.
     MSHA does not entirely agree with Head's estimates of 
costs of new engines. Moreover, expensive new engines are 
technologically advanced and tend to produce substantial gains in 
productivity and savings in operating costs, which Head did not 
consider. The issue of engine costs became irrelevant, however, under a 
strategy of filters as the first-used control device.
     MSHA's re-examination of the costs of cabs indicated that 
MSHA's cost estimate is appropriate for equipment for which equipment 
manufacturers can provide off-the-shelf kits for retrofitting 
equipment, and Head's cost estimate is appropriate for equipment for 
which cabs have to be custom designed and retrofitted. Since the rule 
does not mandate cabs and MSHA expects cabs to be used on a relatively 
small proportion of equipment, however, MSHA believes that mine 
operators will not retrofit equipment for which cabs would need to be 
custom designed. Accordingly, MSHA has retained its original cost 
estimate.
     Head concurred with MSHA on the costs of ventilation 
improvements. While these costs appear to be an appropriate average 
estimate for M/NM mines as a whole, there is a distinct possibility 
that they may be too high for very small M/NM mines.\5\ In the context 
of regulatory flexibility analysis, MSHA considers these cost estimates 
to be fairly conservative.
---------------------------------------------------------------------------

    \5\ The issue is further complicated by the fact that mines that 
are ``small'' in terms of employment vary considerably among 
commodities and mining techniques in their physical size and 
ventilation requirements. Accordingly, MSHA has not attempted to 
make a separate cost estimate of ventilation improvement costs for 
``small'' M/NM minas as a group.
---------------------------------------------------------------------------

    MSHA agrees that certain costs were omitted, but the conclusions of 
MSHA's reconsideration of these costs also vary with the cost:
     MSHA has accepted Head's estimates for major ventilation 
improvements and has included them in the analysis of costs.
     Head's comment that MSHA had omitted the costs of 
retrofitting new engines in old equipment is correct, although MSHA 
does not agree with the size of Head's cost estimates. The key issue, 
however, is that the strategy of relying primarily on filters does not 
entail retrofitting engines. Thus Head's comment is not germane.
    Concentration Limits and the Toolbox. This standard for underground 
M/NM mines is a performance standard, with an interim DPM concentration 
limit of 500 micrograms/m\3\, followed by a final DPM concentration 
limit of 200 micrograms/m\3\. The rule encourages mine operators to use 
any combination of a ``toolbox'' of measures to meet these 
concentration limits. For cost estimation purposes, however, it is 
necessary to assume a specific set and sequence of control measures. 
Specifically, in the PREA MSHA assumed that:
     The interim standard would be met by replacing engines, 
installing oxidation catalytic converters, and improving ventilation; 
and
     The final standard would be met by adding cabs and 
filters.
    Both the general strategy and the specific proportions of diesel 
powered equipment to be controlled by each measure were based on an 
optimizing approach, in which the most cost-effective additional 
measures were selected for additional DPM reductions at each stage.
    In his comments, Head exactly replicated MSHA's assumptions about 
how many pieces of each kind of diesel equipment would be controlled, 
how they would be controlled, and the sequence in which controls would 
be used. Although his cost estimates differed substantially from 
MSHA's, Head made no attempt to optimize the use of DPM control 
``tools'' from the toolbox.
    Substantially the most important of Head's changes is to make 
filters much cheaper, relative to engine replacement. At the same time, 
data collected by MSHA since publication of the PREA indicate that 
filters are more effective than was previously understood. This finding 
has further enhanced the cost-effectiveness of filters, relative to 
engine replacement. These changes in information have caused MSHA to go 
back to the toolbox and rethink the optimized compliance strategy. The 
revised compliance strategy, upon which MSHA bases the revised 
estimates of compliance costs, reverses the two most widely used 
measures from the toolbox. MSHA now anticipates that:
     The interim DPM standard of 500 micrograms/m\3\ will be 
met with filters, cabs, and ventilation; and
     The final DPM standard of 200 micrograms/m\3\ will be met 
with more filters, ventilation, and such turnover in

[[Page 5895]]

equipment and engines as will have occurred in the baseline.
    This new approach uses the same toolbox and optimization strategy 
that was used in the PREA. Since relative costs are different, however, 
the tools used and costs estimated are quite different. The effects on 
costs is substantial. Most of the difference between Head's cost 
estimate and the cost estimate in the REA is attributable to this 
change in strategy.
    Changes in the Rule. Because the rule is a performance standard 
that uses a tool-box approach, most modifications that MSHA made in 
response to comments involved changes in the mix of tools within the 
framework of the rule, rather than changes in the rule per se. MSHA did 
make one significant change in the rule itself, however, by allowing 
compliance with listed EPA standards as a substitute for MSHA approval 
of new engines. Because most engines used in underground M/NM mining 
equipment are essentially the same engines used on the surface, which 
fall under EPA regulations, MSHA believes that virtually all new 
engines used in mining equipment will meet EPA standards. Therefore, 
this change resulted in eliminating a cost of approval that was 
estimated in the PREA to average $2,500 per new engine.

Small Entities to Which the Rule Will Apply

    For the purposes of this regulatory flexibility analysis, the 
working definition of ``small'' is MSHA's definition of fewer than 20 
employees. (Although SBREFA requires use of the SBA's definition, the 
impacts on mines with 500 or fewer employees as a whole are not 
economically significant.) Correspondingly, one element of a regulatory 
flexibility analysis involves developing a more focused definition of 
``small.''
    There are 77 M/NM mines that are ``small'' by this definition. 
These mines fall in four commodity groups:
     Stone is the largest group, accounting for 54 small 
underground M/NM mines that use diesel equipment (70 percent). These 
mines include limestone (46 mines), marble (5 mines), lime (2 mines), 
and granite (1 mine).
     Precious metals account for 10 small underground M/NM 
mines that use diesel equipment (13 percent). Most of these (9 mines) 
are gold mines; one mines both gold and silver.
     Other metals account for 4 small underground M/NM mines 
that use diesel equipment (5 percent). These mines include zinc (2 
mines), copper (1 mine), and a combination of copper and zinc (1 mine).
     The other 9 small underground M/NM mines that use diesel 
equipment (12 percent) are a miscellany that includes shale (3 mines) 
as well as calcite, clay, gemstone, perlite, sand (industrial), and 
talc (1 mine each).
    Collectively, these 77 mines have estimated revenues of $189.3 
million, or an average of $2.46 million per mine. The estimated total 
costs of the rule are $4.1 million, or an average of $53,160 per mine. 
Estimated costs of the rule are 2.16 percent of estimated revenues.
    Costs by Commodity Group and Mine Size. Table VI-3 shows the 
estimated yearly cost by size class for each commodity group in M/NM 
mines. Costs for Section 57.5060(a) and Section 57.5060(b) were 
recalculated for each commodity group, based on the diesel powered 
equipment and air flow of the mines in each commodity group. All other 
costs were very small, probabilistically distributed among mines, and/
or essentially constant for all mines or for all mines in a size class. 
For these costs, the average cost per mine in each size class (from 
Table VI-1) was used, as very little precision was lost through this 
simpler estimation procedure. Table VI-3 shows a fair degree of 
variation among commodity groups.
     For mines with fewer than 20 employees, the average cost 
per mine is estimated to be $53,158, and estimated costs per mine for 
commodity groups range from $31,500 to $60,500, with:
     Costs above average for stone mines ($60,500) and base 
metal ($54,400), and
     Costs below average for other M/NM mines ($31,500) and 
gold mines ($34,600).
     For mines with 20 to 500 employees, the average cost per 
mine is estimated to be $158,437, and estimated costs per mine for 
commodity groups range from $102,100 to $201,700, with:
     Costs above average for base metal mines ($201,700) and 
gold mines ($171,900),
     Costs roughly average for stone mines ($150,900) and 
evaporates mines ($149,100), and
     Costs below average for other M/NM mines ($102,100).
     For mines with over 500 employees, the average cost per 
mine is estimated to be $473,078, and estimated costs per mine for 
commodity groups range from $291,800 to $660,300, with:
     Costs above average for gold mines ($660,300) and base 
metal mines ($592,300), and
     Costs below average for evaporates mines ($291,800) and 
stone mines ($298,000).

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BILLING CODE 4510-43-C

[[Page 5897]]

    Thus by overall commodity group:
     Compliance costs are relatively high in gold mines (except 
for small mines) and base metal mines,
     Compliance costs are relatively low in evaporates mines 
and other M/NM mines, and
     Compliance costs of stone mines show no consistent pattern 
relative to average costs for all M/NM mines.
    The differences in cost per mine appear to be attributable to the 
interaction of three characteristics of the mines, which are included 
in Table VI-4:
     The percentage of mines that need new ventilation systems;
     The number of diesel powered machines per mine; and
     The proportion of diesel powered equipment that is large 
production equipment.
[GRAPHIC] [TIFF OMITTED] TR19JA01.078


[[Page 5898]]


    These three characteristics interact in somewhat different ways in 
the different mine size classes:
     For mines with fewer than 20 employees, the cost per mine 
is:
     Relatively high (or just above average) in commodity 
groups where two or all three of these factors have relatively high 
values, and
     Relatively low when two of these factors have relatively 
low values.
     For mines with 20 to 500 employees, the cost per mine is:
     Relatively high in commodity groups where the number of 
machines per mine and the proportion of machines that are large 
production equipment are both relatively large,
     Average when one of these two factors is relatively high 
and the other is relatively small, and
     Relatively low when all three of the factors have 
relatively low values.
     For mines with over 500 employees (none of which need new 
ventilation systems), the cost per mine is:
     Relatively high in commodity groups where the number of 
machines per mine is relatively large, and
     Relatively low when the number of machines per mine or the 
proportion of machines that are large production equipment is 
relatively small.
    Impacts on Small Mines by Commodity Group. The available data are 
not adequate to support a realistic estimate of impacts on small 
underground M/NM mines by commodity group, since revenues of individual 
commodities cannot be allocated to different size classes of mine. The 
analysis of costs per mine suggests, however, that stone is the only 
commodity group with impacts much above average. The costs per small 
stone mine are 13.6 percent higher than the average for all small 
underground M/NM mines. Impacts on small underground mines in other M/
NM commodity groups appear to be about average or less.

Projected Reporting, Recordkeeping, and Other Requirements of the 
Rule

    The rule requires several types of records and reports. Plans are 
required in conjunction with respirator use and DPM control if the 
concentration levels are violated, and these must be posted and 
provided to various parties. An extension may be applied for. 
Maintenance training, miner health training, and respirator training 
must be logged. Environmental monitoring results must be recorded and 
provided to miners upon request. While there are a number of reporting 
and recordkeeping requirements, however, each one is straightforward, 
and most are no more than the simplest form of documentation. Thus the 
total cost of recordkeeping is only about 0.35 percent of the 
compliance costs for small mines.
    The principal source of costs of the rule is controls to reduce the 
DPM concentrations in underground mines. MSHA has adopted a flexible 
``toolbox'' approach that allows mine operators to select the controls 
that will be most cost-effective for their mines. MSHA has based its 
cost estimates on extensive use of ceramic filters, less widespread use 
of cabs on equipment, and ventilation upgrades. MSHA also assumes that 
new diesel engines introduced into the mines as part of the baseline 
turnover of the fleet and its engines will be relatively clean and will 
contribute to reduced DPM levels. These control costs account for an 
estimated 95.6 percent of the yearly compliance costs of small mines. 
Of these costs, ventilation costs (47.1 percent) and filter costs (46.3 
percent) account for nearly half each, while the cost of cabs (6.6 
percent) is relatively minor.
    Only two other requirements impose costs of any size. Environmental 
monitoring accounts for about 2.6 percent of the estimated compliance 
costs of small mines. Occasional use of respirators (equipment, 
training, inspection, etc.) accounts for about 1.6 percent of estimated 
compliance costs. Maintenance training and miner health training 
account for less than 0.2 percent of compliance costs. The non-control 
requirements of the rule are quite modest.

Steps Taken to Minimize Impacts on Small Entities

    Constraints of the Mine Safety and Health Act. The Federal Mine 
Safety and Health Act of 1977 was enacted to protect miners. MSHA has 
always read the Act to prohibit discriminating among miners by 
providing different degrees of protection that varied systematically 
with the size of the mine in which they worked. Accordingly, the Mine 
Safety and Health Act rules out certain classes of regulatory 
flexibility alternatives, particularly exemption of small mines, but 
also any alternative that would result in systematically higher 
allowable DPM concentration levels in small mines. Because over 95 
percent of the yearly costs to be incurred by small mines are directly 
related to protection, there is little scope for distinct provisions 
for small mines.
    Built-In Flexibility. To minimize impacts on small entities, MSHA 
has taken steps to build as much flexibility into the rule itself as 
possible. The rule itself is a performance standard that allows mine 
operators to meet the DPM concentration limits with their own choice of 
``tools.'' While MSHA has selected a specific set of tools for the cost 
analysis, MSHA expects that operators of specific mines probably will 
often be able to come into compliance at lower costs by using a mix of 
techniques tailored to that specific mine.
    Other parts of the rule provide similar flexibility. Training and 
recordkeeping requirements indicate the information to be imparted or 
retained, for example, but they do not spell out how this is to be 
done. Much of the reporting is required only upon request, rather than 
routinely. Where a requirement (e.g., MSHA approval of new engines) 
appeared to be relatively expensive, MSHA added an alternative 
(compliance with listed EPA standards).
    Phasing in over five years is another element that MSHA has 
incorporated to minimize impacts (albeit for all mines, not just for 
small ones). This not only defers costs, it allows impacts to be 
reduced in a number of ways. Mine operators can spread major expenses 
out to avoid a capital crunch. To a great degree, mine operators will 
be able to take advantage of the natural turnover of their fleets, 
rather than doing extensive (and more expensive) retrofitting. In 
extreme cases, if a mine is quite marginal and/or is likely to shut 
down in a few years anyway, the five-year phase-in allows an orderly 
closure that minimizes impacts.
    Low Risk of Short-Term Closures. Ultimately, the issue of concern 
related to impacts whether mines may be forced to close. When costs are 
a significant but relatively small fraction of revenues (or profits), 
however, it is especially difficult to determine whether closure is an 
impact resulting from the rule or a baseline event that would have 
happened anyway. Given the fact that profits fluctuate widely over 
time, even the presence of losses is not necessarily a good indicator 
of whether businesses will recover or fail. In many cases where a 
business does fail, the true impact of a regulation is not causing its 
failure but rather hastening its failure. Because of the phasing of 
this rule, it affords an opportunity to consider the potential for 
hastening the failure of a small mine.
    If a mine is likely to close within five to seven years without the 
regulation, the impacts of the rule are different from the above 
analysis. In order to stay open for five years, a mine need only comply 
with the interim DPM concentration level. To this end, it needs to 
incur the costs of:

[[Page 5899]]

     Control costs necessary for Section 57.5060(a); \6\
     Respirator protection costs of Section 57.5060(d); \7\
     DPM control plan costs of Section 57.5062; \8\
---------------------------------------------------------------------------

    \6\ These controls include ceramic filters and cabs, but not 
ventilation (which MSHA did not estimate to be necessary for the 
interim DPM level. These costs, amortized over 5 years at an annual 
discount rate of 7.0 percent, are $1,119,800 for filters and 
$150,437 for cabs.
    \7\ These costs, amortized over 5 years at an annual discount 
rate of 7.0 percent, are $164,845.
    \8\ Annual costs are $1,408.
---------------------------------------------------------------------------

     Maintenance training, tagging, and examination costs of 
Section 57.5066(b) and Section 57.5066(c);\9\
---------------------------------------------------------------------------

    \9\ These costs, amortized over 5 years at an annual discount 
rate of 7.0 percent, are $5,681.
---------------------------------------------------------------------------

     Miner Health Training costs of Section 57.5071; \10\
---------------------------------------------------------------------------

    \10\ Annual costs are $5,226.
---------------------------------------------------------------------------

     Environmental monitoring costs of Section 57.5071; \11\ 
and
---------------------------------------------------------------------------

    \11\ Annual costs are $106,425.
---------------------------------------------------------------------------

     DPM record costs of Section 57.5075. \12\
---------------------------------------------------------------------------

    \12\ Annual costs are $204.
---------------------------------------------------------------------------

    Thus the yearly costs for small mines, amortized over 5 years at an 
annual discount rate of 7.0 percent, would be $1,554,086, or an average 
of $20,183 per mine. This is 0.82 percent of annual revenue, which is 
below the threshold for a significant economic impact. This is not the 
type of impact that would force a mine to close sooner rather than 
later. The conclusion is that any closure impacts would be mild and 
would occur foreseeably over time, rather than abruptly.

Compliance Assistance

    The Agency plans to provide extensive compliance assistance to the 
mining community. MSHA intends to focus these efforts on smaller metal 
and nonmetal operators, including training them to measure DPM 
concentrations, providing technical assistance on available controls, 
and establishing a system for addressing compliance inquiries from 
small businesses. The Agency will also issue a compliance guide, 
continue its current efforts to disseminate educational materials and 
software, and hold workshops to inform the mining community.
    In conclusion, MSHA believes that it has taken all of the steps 
consistent with the Mine Safety and Health Act that could substantially 
reduce the impacts of this rule on small entities.

(C) Alternatives Considered

    MSHA did explore a variety of alternatives in its Initial 
Regulatory Flexibility Analysis. See 63 FR 58212. For example, it 
looked at a regulatory approach that would have focused on limiting 
workers exposure rather than limiting particulate concentration. Under 
such an approach, operators would have been able to use administrative 
controls and respiratory protection equipment to reduce diesel 
particulate exposure. For the reasons explained in that Initial 
Analysis, the Agency declined to take such an approach. For MSHA's 
response to comments on the specific topics of administrative controls 
and respiratory protection equipment, see Part IV's discussion of 
57.5060(e) and 57.5060(f).

(D) Unfunded Mandates Reform Act of 1995

    For purposes of the Unfunded Mandates Reform Act of 1995, the final 
rule does not include any Federal mandate that may result in increased 
expenditures by State, local, or tribal governments, or increased 
expenditures by the private sector of more than $100 million.

(E) Paperwork Reduction Act of 1995

    The final rule contains information collections which are subject 
to review by the Office of Management and Budget (OMB) under the 
Paperwork Reduction Act of 1995 (PRA95). The final rule will impose two 
types of paperwork burden hours on underground M/NM mine operators that 
use diesel powered equipment. First, there are burden hours that will 
occur only in the first year the rule is in effect (hereafter known as 
first year burden hours). Second, there are burden hours that will 
occur every year that the rule is in effect, starting with the first 
year (hereafter known as ``annual'' burden hours).
    In the first year, mine operators will incur 3,571 burden hours and 
associated burden costs of about $171,926. After the first year, mine 
operators will incur 526 burden hours annually and associated costs of 
about $21,871.
    We have submitted a copy of this final rule to OMB for its review 
and approval of these information collections. Interested persons are 
requested to send comments regarding this information collection, 
including suggestions for reducing this burden, to the Office of 
Information and Regulatory Affairs, OMB New Executive Office Building, 
725 17th St., NW, Rm. 10235, Washington, DC 20503, Attn: Desk Officer 
for MSHA. Submit written comments on the information collection not 
later than 60 days after date of publication in the Federal Register.
    Our paperwork submission summarized above is explained in detail in 
the REA. The REA includes the estimated costs and assumptions for each 
final paperwork requirement related to this final rule. A copy of the 
REA is available from us. These paperwork requirements have been 
submitted to the Office of Management and Budget for review under 
section 3504(h) of the Paperwork Reduction Act of 1995. Respondents are 
not required to respond to any collection of information unless it 
displays a current valid OMB control number.

(F) National Environmental Protection Act

    The National Environmental Policy Act (NEPA) of 1969 requires each 
Federal agency to consider the environmental effects of final actions 
and to prepare an Environmental Impact Statement on major actions 
significantly affecting the quality of the environment. MSHA has 
reviewed the final rule in accordance with NEPA requirements (42 U.S.C. 
4321 et. seq.), the regulations of the Council of Environmental Quality 
(40 CFR Part 1500), and the Department of Labor's NEPA procedures (29 
CFR Part 11). As a result of this review, MSHA has determined that this 
rule will have no significant environmental impact.

(G) Executive Order 12360 Governmental Actions and Interference With 
Constitutionally Protected Property Rights

    This final rule is not subject to Executive Order 12360, 
Governmental Actions and Interference with Constitutionally Protected 
Property Rights, because it does not involve implementation of a policy 
with takings implications.

(H) Executive Order 13045 Protection of Children From Environmental 
Health Risks and Safety Risks

    In accordance with Executive Order 13045, MSHA has evaluated the 
environmental health and safety effects of the final rule on children. 
The Agency has determined that the rule will not have an adverse impact 
on children.

(I) Executive Order 12988 (Civil Justice)

    The Agency has reviewed Executive Order 12988, Civil Justice 
Reform, and determined that the final rule will not unduly burden the 
Federal court system. The rule has been written so as to provide a 
clear legal standard for affected conduct, and has been reviewed 
carefully to eliminate drafting errors and ambiguities.

[[Page 5900]]

(J) Executive Order 13084 Consultation and Coordination With Indian 
Tribal Governments

    MSHA certifies that the final rule will not impose substantial 
direct compliance costs on Indian tribal governments.

(K) Executive Order 13132 (Federalism)

    MSHA has reviewed the final rule in accordance with Executive Order 
13132 regarding federalism and has determined that it does not have 
``federalism implications.'' The final rule does not ``have substantial 
direct effects on the States, on the relationship between the national 
government and the States, or on the distribution of power and 
responsibilities among the various levels of government.''

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American Journal of Industrial Medicine, 34:220-228, 1998.
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Men,'' Cancer Epidemiology, Biomarkers & Prevention, 2:313-320, 
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Antigen-Induced Airway Inflammation and Local Cytokine Expression in 
Mice,'' American Journal of Respiratory and Critical Care Medicine, 
156:36-42, 1997.
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Results from Exposure to the Aromatic Hydrocarbons from Diesel 
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Immunol, 95-103-115, 1995.
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Studies to Assess Diesel Particulate Exposures and Control 
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Particles and Phenanthrene, a Major Polyaromatic Hydrocarbon 
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Pharmacology, 142:256-263, 1997.
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1990.
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``Evaluation of Catalyzed

[[Page 5906]]

Diesel Particulate Filters Used in an Underground Metal Mine'', 
Report of Investigations No. 9478, 1993.
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Service Performance of Catalyzed Ceramic Wall-Flow Diesel 
Particulate Filters,'' in Diesels in Underground Coal Mines: 
Measurement and Control of Particulate Emissions, Information 
Circular No. 9324, 1992.
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``Diesel in Underground Mines: Measurement and Control of 
Particulate Emissions,'' Information Circular No. 9324, 1992.
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public comment submitted in response to MSHA's January 1992 ANPRM, 
87-OFED-1, July 7, 1992.
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``Fuel Additive and Engine Operation Effects on Diesel Soot 
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Occupational Medicine, 35(2):149-154, February 1993.
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Particles and Oil Shale Particles Dispersed in Lecithin 
Surfactant,'' Journal of Toxicology and Environmental Health, 
21:163-171, 1987.
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Institute, Cambridge, MA., 1995.
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Underground Coal Mines,'' U.S. Bureau of Mines, Information Circular 
No. 9324, pp. 31-39, 1992.
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Matter in Underground Coal Mines,'' United States Department of 
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Role of Phagocyte-Generated Oxidants in Carcinogenesis,'' Blood, 
76(4):655-663, August 15, 1990.

West Virginia House Bill No. 2890, May 5, 1997.

    White House Press Release, Office of the Vice President, ``Vice 
President Gore Announces Joint Industry-Government Research Plan to 
Produce the World's Cleanest Diesels,'' July 23, 1997.
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in Matthew, O.P. and G. Sant' Ambrogio, eds., Respiratory Function 
of the Upper Airway, pp. 193-231, 1988.
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Typewith Occupation and Industry From the Third National Cancer 
Survey Interview,'' Journal of the National Cancer Institute, Vol. 
59, No. 4, October 1977.
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in rats inhaling diesel exhaust or carbon black. Inhalation 
Toxicology, 2:241-254, 1990.
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Equipment Operators Union with Potential Exposure to Diesel Exhaust 
Emissions,'' British Journal of Industrial Medicine, 42:435-448, 
1985.
    Woskie, Susan R., et al., ``Estimation of the Diesel Exhaust 
Exposures of Railroad Workers: I. Current Exposures,'' American 
Journal of Industrial Medicine, 13:381-394, 1988.
    Woskie, Susan R., et al., ``Estimation of the Diesel Exhaust 
Exposures of Railroad Workers: II. National and Historical 
Exposures,'' American Journal of Industrial Medicine, 13:395-404, 
1988.
    Zaebst, D.D., et al., ``Quantitative Determination of Trucking 
Industry Workers' Exposures to Diesel Exhaust Particles,'' American 
Industrial Hygiene Association Journal, (52), December 1991.

Supplementary References

    Below is a list of supplemental references that MSHA reviewed 
and considered in the development of the proposed rule. These 
documents are not specifically cited in the preamble discussion, but 
are applicable to MSHA's findings:
    Bice, D.E., et al., ``Effects of Inhaled Diesel Exhaust on 
Immune Responses after Lung Immunization,'' Fundamental and Applied 
Toxicology, 5:1075-1086, 1985.
    California Environmental Protection Agency, Air Resources Board, 
News Release, ``ARB Identifies Diesel Particulate Emissions as a 
Toxic Air Contaminant,'' August 27, 1998.
    Fischer, Torkel, and Bolli Bjarnason, ``Sensitizing and Irritant 
Properties of 3 Environmental Classes of Diesel Oil and Their 
Indicator Dyes,'' Contact Dermatitis, 34:309-315, 1996.
    Frew, A.J., and S.S. Salvi, ``Diesel Exhaust Particles and 
Respiratory Allergy,'' Clinical and Experimental Allergy, 27:237-
239, 1997.
    Fujimaki, Hidekazu, et al., ``Intranasal Instillation of Diesel 
Exhaust Particles and Antigen in Mice Modulated Cytokine Productions 
in Cervical Lymph Node Cells,'' International Archives of Allergy 
and Immunology, 108:268-273, 1995.
    Fujimaki, Hidekazu, et al., ``IL-4 Production in Mediastinal 
Lymph Node Cells in Mice Intratracheally Instilled with Diesel 
Exhaust Particles and Antigen,'' Toxicology, 92:261-268, 1994.
    Fujimaki, Hidekazu, et al., ``Inhalation of Diesel Exhaust 
Enhances Antigen-Specific IgE Antibody Production in Mice,'' 
Toxicology, 116:227-233, 1997.
    Ikeda, Masahiko, et al., ``Impairment of Endothelium-Dependent 
Relaxation by Diesel Exhaust Particles in Rat Thoracic Aorta,'' 
Japanese Journal of Pharmacology, 68:183-189, 1995.
    Muranaka, Masaharu, et al., ``Adjuvant Activity of Diesel-
Exhaust Particles for the Production of IgE Antibody in Mice,'' J 
Allergy Clin Immunology, 77:616-623, 1986.
    Northridge, Mary, ``Diesel Exhaust Exposure Among Adolescents in 
Harlem: A Community-Driven Study,'' American Journal of Public 
Health, (89) 998-1002, July 1999.
    Scientific Review Panel, Findings on the Report on Diesel 
Exhaust as a Toxic air Contaminant, as adopted at the Panel's April 
22, 1998 meeting.
    Stayner, Leslie, ``Protecting Public Health in the Face of 
Uncertain Risks: The Example of Diesel Exhaust,'' American Journal 
of Public Health, (89) 991-993, July 1999.
    Takafuji, Shigeru, et al., ``Diesel-Exhaust Particulates 
Inoculated by the Intranasal Route Have an Adjuvant Activity for IgE 
Production in Mice,'' J Allergy Clin Immunol, 79:639-645, 1987.
    Terada, Nobushisa, et al., ``Diesel Exhaust Particulates Enhance 
Eosinophil Adhesion to Nasal Epithelial Cells and Cause 
Degranulation,'' International Archives of Allergy and Immunology, 
114:167-174, 1997.
    Yang, Hui-Min, et al., ``Effects of Diesel Exhaust Particles on 
the Release of Interleukin-1 and Tumor Necrosis Factor-Alpha from 
Rat Alveolar Macrophages,'' Experimental Lung Research, 23:269-284, 
1997.

List of Subjects in 30 CFR Part 57

    Metal and nonmetal, Mine safety and health, Underground mines, 
Diesel particulate matter.

    Dated: January 8, 2001.
Robert A. Elam,
Acting Assistant Secretary for Mine Safety and Health.

    Chapter I of Title 30 of the Code of Federal Regulations is hereby 
amended as follows:

[[Page 5907]]

PART 57--[AMENDED]

    1. The authority citation for Part 57 continues to read as follows:

    Authority: 30 U.S.C. 811, 957, 961.

    2. The heading of Subpart D of Part 57 is revised to read as 
follows:

Subpart D--Air Quality, Radiation, Physical Agents, and Diesel 
Particulate Matter

    3. A new undesignated center heading and Secs. 57.5060 through 
56.5075 are added to subpart D.

DIESEL PARTICULATE MATTER--UNDERGROUND ONLY

Sec.
57.5060   Limit on concentration of diesel particulate matter.
57.5061   Compliance determinations.
57.5062   Diesel particulate matter control plan.
57.5065   Fueling and idling practices.
57.5066   Maintenance standards.
57.5067   Engines.
57.5070   Miner training.
57.5071   Environmental monitoring.
57.5075   Diesel particulate records.

DIESEL PARTICULATE MATTER--UNDERGROUND ONLY


Sec. 57.5060  Limit on concentration of diesel particulate matter.

    (a) After July 19, 2002 and until January 19, 2006, any mine 
operator covered by this part must limit the concentration of diesel 
particulate matter to which miners are exposed in underground areas of 
a mine by restricting the average eight-hour equivalent full shift 
airborne concentration of total carbon, where miners normally work or 
travel, to 400 micrograms per cubic meter of air (400TC 
g/m\3\).
    (b) After January 19, 2006, any mine operator covered by this part 
must limit the concentration of diesel particulate matter to which 
miners are exposed in underground areas of a mine by restricting the 
average eight-hour equivalent full shift airborne concentration of 
total carbon, where miners normally work or travel, to 160 micrograms 
per cubic meter of air (160TC g/m\3\).
    (c)(1) If, as a result of technological constraints, a mine 
requires additional time to come into compliance with the limit 
specified in paragraph (b) of this section, the operator of the mine 
may file an application with the Secretary for a special extension.
    (2) No mine may be granted more than one special extension, nor may 
the time otherwise available under this section to a mine to comply 
with the limit specified in paragraph (b) be extended by more than two 
years.
    (3) The application for a special extension may be approved, and 
the additional time authorized, only if the application includes 
information adequate for the Secretary to ascertain:
    (i) That diesel-powered equipment was used in the mine prior to 
October 29, 1998;
    (ii) That there is no combination of controls that can, due to 
technological constraints, bring the mine into full compliance with the 
limit specified in paragraph (b) within the time otherwise specified in 
this section;
    (iii) The lowest achievable concentration of diesel particulate, as 
demonstrated by data collected under conditions that are representative 
of mine conditions using the method specified in Sec. 57.5061; and
    (iv) The actions the operator will take during the duration of the 
extension to:
    (A) Maintain the lowest concentration of diesel particulate; and
    (B) Minimize the exposure of miners to diesel particulate.
    (4) The Secretary may approve an application for a special 
extension only if:
    (i) The mine operator files, the application at least 180 days 
prior to the date the mine must be in full compliance with the limit 
established by paragraph (b) of this section; and
    (ii) The application certifies that the operator has posted one 
copy of the application, at the mine site for 30 days prior to the date 
of application, and has provided another copy to the authorized 
representative of miners.
    (5) A mine operator must comply with the terms of any approved 
application for a special extension, and post a copy of an approved 
application for a special extension at the mine site for the duration 
of the special extension period.
    (d)(1) Mine operators may permit miners engaged in inspection, 
maintenance, or repair activities, and only in such activities, with 
the advance approval of the Secretary under the circumstances and 
conditions defined in paragraphs (d)(2) through (d)(4) of this section, 
to work in concentrations of diesel particulate matter exceeding the 
applicable concentration limit under paragraph (a) or (b) of this 
section.
    (2) The Secretary will only provide advance approval:
    (i) For inspection, maintenance or repair activities to be 
conducted:
    (A) In areas where miners work or travel infrequently or for brief 
periods of time;
    (B) In areas where miners otherwise work exclusively inside of 
enclosed and environmentally controlled cabs, booths and similar 
structures with filtered breathing air; or
    (C) In shafts, inclines, slopes, adits, tunnels and similar 
workings that the operator designates as return or exhaust air courses 
and that miners use for access into the mine or egress from the mine;
    (ii) When the Secretary determines that it is not feasible to 
reduce the concentration of dpm in the areas where the inspection, 
maintenance or repair activities are to be conducted to those otherwise 
applicable under paragraph (a) or (b) of this section; and
    (iii) When the Secretary determines that the mine operator will 
employ adequate safeguards to minimize the dpm exposure of the miners.
    (3) The Secretary's determinations under paragraph (d)(2) of this 
section will be based on evaluating a plan prepared and submitted by 
the operator no less than 60 days before the commencement of any 
inspection, maintenance or repair activities. The mine operator must 
certify in the plan that one copy of the application has been posted at 
the mine site for 30 days prior to the date of submission, and another 
copy has been provided to the authorized representative of miners. The 
plan must identify, at a minimum, the types of anticipated inspection, 
maintenance, and repair activities that must be performed for which 
engineering controls sufficient to comply with the concentration limit 
are not feasible, the locations where such activities could take place, 
the concentration of dpm in these locations, the reasons why 
engineering controls are not feasible, the anticipated frequency and 
duration of such activities, the anticipated number of miners involved 
in such activities, and the safeguards that the operator will employ to 
limit miner exposure to dpm, including, but not limited to the use of 
respiratory protective equipment. The approved plan must include a 
program for selection, maintenance, training, fitting, supervision, 
cleaning and use of personal protective equipment and must meet the 
minimum requirements established in Sec. 57.5005 (a) and (b).
    (4) An advance approval by the Secretary for employees to engage in 
inspection, maintenance, or repair activities will be valid for no more 
than one year. A mine operator must comply with the conditions of the 
approved plan [which was the basis of the approval], and must post a 
copy of the approved plan at the mine site for the duration of its 
applicability.
    (e) Other than pursuant to the conditions required in paragraphs 
(c) or (d) of this section, an operator must not

[[Page 5908]]

utilize personal protective equipment to comply with the requirements 
of either paragraph (a) or paragraph (b) of this section.
    (f) An operator must not utilize administrative controls to comply 
with the requirements of this section.


Sec. 57.5061  Compliance determinations.

    (a) A single sample collected and analyzed by the Secretary in 
accordance with the requirements of this section shall be an adequate 
basis for a determination of noncompliance with an applicable limit on 
the concentration of diesel particulate matter pursuant to 
Sec. 57.5060.
    (b) The Secretary will collect samples of diesel particulate matter 
by using a respirable dust sampler equipped with a submicrometer 
impactor and analyze the samples for the amount of total carbon using 
the method described in NIOSH Analytical Method 5040, except that the 
Secretary also may use any methods of collection and analysis 
subsequently determined by NIOSH to provide equal or improved accuracy 
for the measurement of diesel particulate matter. Copies of the NIOSH 
5040 Analytical Method are available by contacting MSHA's, Pittsburgh 
Safety and Health Technology Center, P.O. Box 18233, Cochrans Mill 
Road, Pittsburgh, PA 15236.
    (c) The Secretary will determine the appropriate sampling strategy 
for compliance determination, utilizing personal sampling, occupational 
sampling, and/or area sampling, based on the circumstances of the 
particular exposure.


Sec. 57.5062  Diesel particulate matter control plan.

    (a) In the event of a violation by the operator of an underground 
metal or nonmetal mine of the applicable concentration limit 
established by Sec. 57.5060, the operator, in accordance with the 
requirements of this section, must--
    (1) Establish a diesel particulate matter control plan for the mine 
if one is not already in effect, or modify the existing diesel 
particulate matter control plan, and
    (2) Demonstrate that the new or modified diesel particulate matter 
control plan controls the concentration of diesel particulate matter to 
the applicable concentration limit specified in Sec. 57.5060.
    (b) A diesel particulate control plan must describe the controls 
the operator will utilize to maintain the concentration of diesel 
particulate matter to the applicable limit specified by Sec. 57.5060. 
The plan also must include a list of diesel-powered units maintained by 
the mine operator, information about any unit's emission control 
device, and the parameters of any other methods used to control the 
concentration of diesel particulate matter. The operator may 
consolidate the plan with the ventilation plan required by 
Sec. 57.8520. The operator must retain a copy of the current diesel 
particulate matter control plan at the mine site during its duration 
and for one year thereafter.
    (c) An operator must demonstrate plan effectiveness by monitoring, 
using the measurement method specified by Sec. 57.5061(b), sufficient 
to verify that the plan will control the concentration of diesel 
particulate matter to the applicable limit under conditions that can be 
reasonably anticipated in the mine. The operator must retain a copy of 
each verification sample result at the mine site for five years. The 
operator monitoring must be in addition to, and not in lieu of, any 
sampling by the Secretary pursuant to Sec. 57.5061.
    (d) The records required by paragraphs (b) and (c) of this section 
must be available for review upon request by the authorized 
representative of the Secretary, the authorized representative of the 
Secretary of Health and Human Services, or the authorized 
representative of miners. In addition, upon request by the District 
Manager or the authorized representative of miners, the operator must 
provide a copy of any records required to be maintained pursuant to 
paragraph (b) or (c) of this section.
    (e)(1) A control plan established as a result of this section must 
remain in effect for 3 years from the date of the violation which 
caused it to be established, except as provided in paragraph (e)(3) of 
this section.
    (2) A modified control plan established as a result of this section 
must remain in effect for 3 years from the date of the violation which 
caused the plan to be modified, except as provided in paragraph (e)(3) 
of this section.
    (3) An operator must modify a diesel particulate matter control 
plan during its duration as required to reflect changes in mining 
equipment or circumstances. Upon request from the Secretary, an 
operator must demonstrate the effectiveness of the modified plan by 
monitoring, using the measurement method specified by Sec. 57.5061, 
sufficient to verify that the plan will control the concentration of 
diesel particulate matter to the applicable limit under conditions that 
can be reasonably anticipated in the mine.
    (f) The Secretary will consider an operator's failure to comply 
with the provisions of the diesel particulate matter control plan in 
effect at a mine or to conduct required verification sampling to be a 
violation of this part without regard for the concentration of diesel 
particulate matter that may be present at any time.


Sec. 57.5065  Fueling and idling practices.

    (a) Diesel fuel used to power equipment in underground areas must 
not have a sulfur content greater than 0.05 percent. The operator must 
retain purchase records that demonstrate compliance with this 
requirement for one year after the date of purchase.
    (b) The operator must only use fuel additives registered by the 
U.S. Environmental Protection Agency in diesel powered equipment 
operated in underground areas.
    (c) Idling of mobile diesel-powered equipment in underground areas 
is prohibited except as required for normal mining operations.


Sec. 57.5066  Maintenance standards.

    (a) Any diesel powered equipment operated at any time in 
underground areas must meet the following maintenance standards:
    (1) The operator must maintain any approved engine in approved 
condition;
    (2) The operator must maintain the emission related components of 
any non-approved engine to manufacturer specifications; and
    (3) The operator must maintain any emission or particulate control 
device installed on the equipment in effective operating condition.
    (b)(1) A mine operator must authorize and require each miner 
operating diesel powered equipment underground to affix a visible and 
dated tag to the equipment at any time the miner notes any evidence 
that the equipment may require maintenance in order to comply with the 
maintenance standards of paragraph (a) of this section.
    (2) A mine operator must ensure that any equipment tagged pursuant 
to this section is promptly examined by a person authorized by the mine 
operator to maintain diesel equipment, and that the affixed tag not be 
removed until the examination has been completed.
    (3) A mine operator must retain a log of any equipment tagged 
pursuant to this section. The log must include the date the equipment 
is tagged, the date the equipment is examined, the name of the person 
examining the equipment, and any action taken as a result of the 
examination. The operator must retain the information in the log for 
one year

[[Page 5909]]

after the date the tagged equipment was examined.
    (c) Persons authorized by a mine operator to maintain diesel 
equipment covered by paragraph (a) of this section must be qualified, 
by virtue of training or experience, to ensure that the maintenance 
standards of paragraph (a) of this section are observed. An operator 
must retain appropriate evidence of the competence of any person to 
perform specific maintenance tasks in compliance with those standards 
for one year after the date of any maintenance, and upon request must 
provide the documentation to the authorized representative of the 
Secretary.


Sec. 57.5067  Engines.

    (a) Any diesel engine introduced into an underground area of a mine 
covered by this part after March 20, 2001, other than an engine in an 
ambulance or fire fighting equipment which is utilized in accordance 
with mine fire fighting and evacuation plans, must either:
    (1) Have affixed a plate evidencing approval of the engine pursuant 
to subpart E of Part 7 of this title or pursuant to Part 36 of this 
title; or
    (2) Meet or exceed the applicable particulate matter emission 
requirements of the Environmental Protection Administration listed in 
Table 57.5067-1, as follows:

                             Table 57.5067-1
------------------------------------------------------------------------
       EPA requirement            EPA category            PM limit
------------------------------------------------------------------------
40 CFR 86.094-                light duty vehicle..  0.1 g/mile.
 8(a)(1)(i)(A)(2).
40 CFR 86.094-                light duty truck....  0.1 g/mile.
 9(a)(1)(i)(A)(2).
40 CFR 86.094-                heavy duty highway    0.1 g/bhp-hr.
 11(a)(1)(iv)(B).              engine.
40 CFR 89.112(a)............  nonroad (tier, power  varies by power
                               range).               range:
                              tier 1 kW8 (hp11)...  1.0 g/kW-hr (0.75 g/
                                                     bhp-hr).
  ..........................  tier 1 8kW19 (11hp25).
  ..........................  tier 1 19kW37 (25hp50).
  ..........................  tier 2 37kW75 (50hp100).
  ..........................  tier 2 75kW130 (100hp175).
  ..........................  tier 1 130kW225 (175hp300).
  ..........................  tier 1 225kW450 (300hp600).
  ..........................  tier 1 450kW560 (600hp750).
  ..........................  tier 1 kW560 (hp750).
------------------------------------------------------------------------
Notes:
``g'' means grams.
``hp'' means horsepower.
``g/bhp-hr'' means grams/brake horsepower-hour.
``kW'' means kilowatt.
``g/kW-hr'' means grams/kilowatt-hour.

    (b) For purposes of paragraph (a):
    (1) The term ``introduced'' means any engine added to the 
underground inventory of engines of the mine in question, including:
    (i) An engine in newly purchased equipment;
    (ii) An engine in used equipment brought into the mine; and
    (iii) A replacement engine that has a different serial number than 
the engine it is replacing; but
    (2) The term ``introduced'' does not include engines that were 
previously part of the mine inventory and rebuilt.


Sec. 57.5070  Miner training.

    (a) Mine operators must provide annual training to all miners at a 
mine covered by this part who can reasonably be expected to be exposed 
to diesel emissions on that property. The training must include--
    (1) The health risks associated with exposure to diesel particulate 
matter;
    (2) The methods used in the mine to control diesel particulate 
matter concentrations;
    (3) Identification of the personnel responsible for maintaining 
those controls; and
    (4) Actions miners must take to ensure the controls operate as 
intended.
    (b) An operator must retain a record at the mine site of the 
training required by this section for one year after completion of the 
training.


Sec. 57.5071  Environmental monitoring.

    (a) Mine operators must monitor as often as necessary to 
effectively determine, under conditions that can be reasonably 
anticipated in the mine--
    (1) Whether the concentration of diesel particulate matter in any 
area of the mine where miners normally work or travel exceeds the 
applicable limit specified in Sec. 57.5060; and
    (2) The average full shift airborne concentration of diesel 
particulate matter at any position or on any person designated by the 
Secretary.
    (b) The mine operator must provide affected miners and their 
representatives with an opportunity to observe exposure monitoring 
required by this section. Mine operators must give prior notice to 
affected miners and their representatives of the date and time of 
intended monitoring.
    (c) If any monitoring performed under this section indicates that 
the applicable concentration limit established by Sec. 57.5060 has been 
exceeded, an operator must promptly post notice of the corrective 
action being taken, initiate corrective action by the next work shift, 
and promptly complete such corrective action.
    (d)(1) The results of monitoring for diesel particulate matter, 
including any results received by a mine operator from sampling 
performed by the Secretary, must be posted on the mine bulletin board 
within 15 days of receipt and must remain posted for 30 days. The 
operator must provide a copy of the results to the authorized 
representative of miners.
    (2) The mine operator must retain for five years (from the date of 
sampling), the results of any samples the operator collected as a 
result of monitoring under this section, and information about the 
sampling method used for obtaining the samples.


Sec. 57.5075  Diesel particulate records.

    (a) The table entitled ``Diesel Particulate Recordkeeping 
Requirements'' lists the records the operator must retain pursuant to 
Secs. 57.5060 through 57.5071, and the duration for which particular 
records need to be retained. The table follows:

[[Page 5910]]



                                  Diesel Particulate Recordkeeping Requirements
----------------------------------------------------------------------------------------------------------------
             Record                                  Section reference                         Retention time
----------------------------------------------------------------------------------------------------------------
1. Approved application for                                              Sec.  57.5060(c)  1 year beyond
 extension of time to comply                                                                duration of
 with final concentration limit.                                                            extension.
2. Approved plan for miners to                                           Sec.  57.5060(d)  For duration of plan.
 perform inspection,
 maintenance or repair actions
 in areas exceeding the
 concentration limit.
3. Control plan................                                          Sec.  57.5062(b)  1 year beyond
                                                                                            duration of plan.
4. Compliance plan verification                                          Sec.  57.5062(c)  5 years from sample
 sample results.                                                                            date.
5. Purchase records noting                                               Sec.  57.5065(a)  1 year beyond date of
 sulfur content of diesel fuel.                                                             purchase.
6. Maintenance log.............                                          Sec.  57.5066(b)  1 year after date any
                                                                                            equipment is tagged.
7. Evidence of competence to                                             Sec.  57.5066(c)  1 year after date
 perform maintenance.                                                                       maintenance
                                                                                            performed.
8. Annual training provided to                                           Sec.  57.5070(b)  1 year beyond date
 potentially exposed miners.                                                                training completed.
9. Sampling method used to                                               Sec.  57.5071(d)  5 years from sample
 effectively evaluate mine                                                                  date.
 particulate concentration, and
 sample results.
----------------------------------------------------------------------------------------------------------------

    (b)(1) Any record listed in this section which is required to be 
retained at the mine site may, notwithstanding such requirement, be 
retained elsewhere if the mine operator can immediately access the 
record from the mine site by electronic transmission.
    (2) Upon request from an authorized representative of the Secretary 
of Labor, the Secretary of Health and Human Services, or from the 
authorized representative of miners, mine operators must promptly 
provide access to any record listed in the table in this section.
    (3) An operator must provide access to a miner, former miner, or, 
with the miner's or former miner's written consent, a personal 
representative of a miner, to any record required to be maintained 
pursuant to Sec. 57.5071 to the extent the information pertains to the 
miner or former miner. The operator must provide the first copy of a 
requested record at no cost, and any additional copies at reasonable 
cost.
    (4) Whenever an operator ceases to do business, that operator must 
transfer all records required to be maintained by this part, or a copy 
thereof, to any successor operator who must maintain them for the 
required period.

[FR Doc. 01-996 Filed 1-18-01; 8:45 am]
BILLING CODE 4510-43-P