[Federal Register Volume 66, Number 13 (Friday, January 19, 2001)]
[Rules and Regulations]
[Pages 5527-5706]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-995]



[[Page 5525]]

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Part II





Department of Labor





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Mine Safety and Health Administration



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30 CFR Part 72



Diesel Particulate Matter Exposure of Underground Coal Miners; Final 
Rule



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30 CFR Part 57



Diesel Particulate Matter Exposure of Underground Metal and Nonmetal 
Miners; Final Rule

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  Federal Register / Vol. 66, No. 13 / Friday, January 19, 2001 / Rules 
and Regulations  
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DEPARTMENT OF LABOR

Mine Safety and Health Administration

30 CFR Part 72

RIN 1219-AA74


Diesel Particulate Matter Exposure of Underground Coal 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 
coal mines that use equipment powered by diesel engines.
    This rule is designed to reduce the risks to underground coal 
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 coal mines would require that the 
dpm emissions from certain pieces of equipment be restricted to 
prescribed levels. Underground coal mine operators would also be 
required to train miners about the hazards of dpm exposure.
    By separate notice, MSHA will publish a rule to reduce dpm 
exposures in underground coal mines.

DATES: The provisions of the final rule are effective March 20, 2001. 
However, Sec. 72.500(b) will not apply until July 19, 2002; 
Sec. 72.501(b) will not apply until July 21, 2003; and, Sec. 72.501(c) 
will not apply until January 19, 2005.

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. Key Features of MSHA's Final Rule Limiting the Concentration of 
Diesel Particulate Matter (DPM) in Underground Coal Mines

(1) What are the requirements for permissible equipment?

    Permissible equipment must not emit more than 2.5 grams per hour of 
dpm, as measured in a laboratory test. Any permissible equipment that 
is added to a mine's inventory underground more than 60 days after the 
date this rule is published will have to meet this standard upon 
introduction. This includes newly purchased equipment, used equipment, 
or a piece of equipment receiving a replacement engine with a different 
serial number than the engine it is replacing, including engines or 
equipment coming from one mine into another. It does not include a 
piece of equipment whose engine was previously part of the mine's 
inventory and rebuilt.
    Within 18 months from the date the rule is issued, the entire 
permissible fleet must meet this standard.
    The rule leaves the choice of controls used to achieve the 
emissions limit to operators. Operators may use any combination of 
controls (e.g., cleaner engine, OCC, filter) to meet the emissions 
standard specified in this section.
    As a practical matter, MSHA expects that to comply with this 
standard, most permissible equipment will be equipped with a paper 
filter. As explained in Part IV of this preamble, MSHA has verified 
that there are commercially available paper filters which will allow 
99% of the existing 541 units in the permissible fleet to meet this 
requirement--including permissible units powered by the Deutz MWM 916, 
the Caterpillar 3304 and the Caterpillar 3306. Commercially available 
paper filters capable of bringing the emissions of these units into 
compliance include a model which can be installed directly on the 
exhaust coming from a water scrubber or on the exhaust coming from a 
heat exchanger, as well as the integrated DST system. Other 
filters which use paper with the same performance characteristics will 
also be acceptable. Control devices whose dpm removal efficiency has 
not been demonstrated by laboratory testing on a diesel engine can be 
evaluated following the procedures in 30 CFR 72.503 of this part added 
by this rulemaking. Moreover, the rule provides that MSHA may rely upon 
the test results of other organizations who perform equivalent tests.
    MSHA will publish on its web site a list of tested control devices 
and their performance. Compliance will be determined by reference to 
this data--there will be no in-mine testing.
    The only engine which might not be able to meet these requirements 
for dpm emissions from permissible equipment with a paper filter is the 
Isuzu QD-100. MSHA's inventory indicates there are currently only two 
units of permissible equipment using this engine; however, these two 
units can comply at a derated power setting.
    The engines currently approved for permissible use are generally 
high in particulate emissions. MSHA is committed to taking actions 
which will facilitate the approval for permissible use of the lower-
emission engines which have become available in recent years. These 
actions could include waiving test fees, contracting for the 
performance of such tests, or on an interim basis permitting the use of 
an engine approved for nonpermissible use in a permissible package. 
MSHA will solicit input from the mining community, through a Federal 
Register notice as it considers how to proceed in this regard.

(2) What are the requirements for heavy-duty non-permissible equipment?

    Non-permissible heavy duty equipment will ultimately not be 
permitted under the final rule to emit more than 2.5 grams per hour of 
dpm. For reasons of feasibility, this requirement will be implemented 
in phases.
    Any heavy duty equipment added to a mine's inventory more than 60 
days after the date of publication of this rule will have to comply 
with an interim emissions limit for that machine of 5.0 gr/hr. This 
includes newly purchased equipment, used equipment, or a piece of 
equipment receiving a replacement engine with a different serial number 
than the engine it is replacing, including engines or equipment coming 
from one mine into another. It does not include a piece of equipment 
whose engine was previously part of the mine's inventory and rebuilt.
    All heavy duty equipment in the fleet must meet the interim 
standard of 5.0 grams per hour of dpm in 30 months.
    Finally, another 18 months later (4 years in all), all 
nonpermissible heavy duty equipment in the fleet will have to meet the 
final standard of 2.5 grams per hour of dpm.
    As with permissible equipment, the rule leaves the choice of 
controls used to achieve the emissions limit to operators. Any 
combination of controls (e.g., cleaner engine, OCC, filter) can be used 
as long as compliance with the standard specified in this section is 
met.

[[Page 5527]]

    As a practical matter, MSHA believes that most existing heavy duty 
equipment will utilize commercially available hot gas filters (e.g., 
ceramic cell, wound fiber, sintered metal, etc.) to comply with the 
final limit. All the existing fleet can reach the interim limit with 
such a filter; some will not need one. MSHA determined that all but a 
few can reach the final limit with such a filter.
    The rule provides that MSHA may rely upon the test results of 
organizations who perform filtration efficiency tests. In this regard, 
MSHA will accept the results of filter tests performed by VERT. VERT is 
an acronym for Verminderung der Emissionen von Realmaschinen in 
Tunnelbau, a consortium of several European agencies conducting diesel 
emission research in connection with major planned tunneling projects 
in Austria, Switzerland and Germany. VERT was established to advance 
hot gas filter technology due to concerns in Europe about dpm levels. 
This gave VERT the opportunity to acquire the necessary filter 
evaluation expertise. A wide range of commercially available hot gas 
filters have been tested by VERT and the filtration efficiency 
determined. The Secretary may also accept filter efficiency test 
results from other testing organizations that can demonstrate a high 
level of expertise in filter evaluation (see Sec. 72.503(c) of the 
final rule).
    Operators using the DST'' system with the catalytic convertor on 
heavy duty equipment, or the Jeffrey dry exhaust system, will also be 
deemed in compliance with the final rule, since test results conducted 
in the same manner as the requirement in the final rule demonstrate 
that those systems can reduce the emissions from all existing heavy 
duty engines to below the limit. Filtration devices whose filter 
efficiency has not been demonstrated by testing on a diesel engine can 
be evaluated following the procedures in 30 CFR 72.503 of this part 
added by this rulemaking.
    MSHA will publish on its web site a list of tested control devices 
and their performance. Compliance will be determined by reference to 
this data--there will be no in-mine testing.
    The standard may also be met through the use of newer, cleaner 
engines in some heavy duty equipment with low horsepower engines. There 
are already many engines approved for non-permissible use in 
underground coal mines that will enable heavy duty equipment to limit 
emissions, thus allowing the use of lower efficiency filters. MSHA is 
also considering approaches that would expedite the approval of 
additional engines based on evidence that such engines meet EPA 
standards which ensure the engines are at least as clean as required 
under MSHA approval standards.

(3) What are the requirements for generators and compressors?

    The final rule provides that generators and compressors meet the 
same dpm emissions standards as heavy duty equipment. Thus, generators 
and compressors will ultimately not be permitted to emit more than 2.5 
grams per hour of dpm. Generators and compressors introduced into the 
fleet of an underground coal mine more than 60 days after the final 
rule is published will have to meet an interim emissions limit of 5.0 
g/hr. Generators and compressors in the existing fleet will have 30 
months to meet the interim standard of 5.0 grams per hour of dpm. After 
an additional 18 months (4 years in all), all generators and 
compressors underground will have to meet the final standard of 2.5 
grams per hour of dpm.
    Although the proposed rule would not have covered generators and 
compressors, MSHA explicitly asked the mining community if there were 
types of light duty equipment that should, because of operating 
characteristics, be treated like heavy duty equipment. Generators and 
compressors generate more dpm emissions than other light-duty equipment 
based on their known duty cycle and type of work for which they are 
designed; indeed, they use engines whose horsepower often exceeds that 
in permissible equipment. Accordingly, MSHA has determined they should 
be covered by this rulemaking.
    MSHA's inventory indicates that the 34 generators and 29 
compressors constitute less than 3% of the underground light duty 
diesel fleet. The existing compressors are using engines which should 
meet the standard's interim and final requirements with a commercially 
available hot gas filter.
    Generators and compressors will be able to utilize the same 
technologies as heavy duty machines to comply with this standard. This 
will include hot gas filters or paper filters, as appropriate. Smaller 
generators and compressors may utilize the clean engine technologies.

(4) What are the requirements for other nonpermissible equipment?

    The final rule provides that any piece of nonpermissible light-duty 
equipment introduced into an underground coal mine more than 60 days 
after the date of publication of the rule must not emit more than 5.0 
grams per hour of dpm. This includes newly purchased equipment, used 
equipment, or a piece of equipment receiving a replacement engine with 
a different serial number than the engine it is replacing, including 
engines or equipment coming from one mine into another, but it does not 
include a piece of equipment whose engine was previously part of the 
mine's inventory and rebuilt.
    The final rule does not impose any new requirements on the existing 
nonpermissible light-duty fleet (except for generators and compressors 
as noted above).
    While new light duty equipment would not have been covered by the 
proposed rule, MSHA explicitly asked the mining community if it would 
be feasible to cover such new light duty equipment, even if it were not 
feasible to set limits for all light duty equipment. MSHA has 
determined that it is feasible to require that newly introduced light 
duty equipment meet the same 5 gr/hr standard as new heavy duty 
equipment.
    To facilitate compliance with this standard, light duty equipment 
which uses an engine meeting certain EPA standards listed in the MSHA 
rule will be deemed to automatically meet the MSHA dpm standard for 
newly introduced light-duty equipment. For example, any ``heavy duty 
highway engine'' produced after 1994 will be deemed to meet this dpm 
standard. The agency has determined that there are already MSHA 
approved engines available in a full range of horsepower sizes that can 
meet the EPA standards listed in this final rule.
    In practice, what this rule does is simply ensure that very old 
engines with few, if any, emission controls are not added to a mine's 
current light duty fleet, thus accelerating the turnover to a newer 
generation of technology.

(5) Is there a summary of the applicable requirements and effective 
dates?

    All of the emissions standards established by MSHA's final rule are 
summarized in Table I-1.

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(6) What other requirements are contained in the final rule for 
underground coal mines?

    Miners have to be trained annually in the risks of dpm exposure and 
in control methods being used at the mine. Also, certain information 
about diesel engines and aftertreatment devices has to be added to the 
mine ventilation plan. The paperwork requirements added by this rule 
are small--on average, less than 7 hours in the first year and 4 hours 
per year thereafter for a mine operator that uses diesel powered 
equipment. Furthermore, manufacturers of diesel powered equipment will 
incur burden hours only during the first year that the rule is in 
effect in order to amend existing MSHA approvals. During the first year 
that the rule is in effect the average manufacturer will incur 70 
paperwork burden hours.

(7) Will the final rule eliminate any health risks to miners resulting 
from the use of diesel powered equipment underground?

    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 after the rule is fully implemented.
    MSHA considered establishing stricter standards for certain types 
of equipment, and covering more light duty equipment, but concluded 
that such actions would either be technologically or economically 
infeasible for the coal mining industry as a whole at this time. As 
MSHA takes actions to facilitate the introduction of newer and cleaner 
engines underground, and as control technologies continue to develop, 
additional reductions in dpm levels may become feasible for the 
industry as a whole. MSHA will continue to monitor developments in this 
area.

(8) What are the costs and benefits of the final rule?

Costs
    Table I-2 summarizes the compliance costs to mine operators that 
use diesel powered equipment for each section of the rule; total 
compliance costs are about $7 million a year. Table I-3 summarizes the 
compliance costs to mine operators that use diesel powered equipment by 
mine size (i.e., mines employing fewer than 20 workers, mines employing 
between 20 and 500 workers, and mines employing more than 500 workers). 
In addition, there is a total annualized cost to diesel equipment 
manufacturers of $30,030.
    MSHA's full Regulatory Economic Analysis, (REA) from which Tables 
I-2 and I-3 are derived, provides considerable detail on the 
assumptions MSHA used in developing these cost estimates, and on the 
costs associated with the controls required for particular engines in 
the current fleet. For example, MSHA is estimating that for a 
Caterpillar 3304 PCNA in a heavy duty piece of equipment, an operator 
will have to spend about $4,500 a year to achieve compliance with the 
limits for that equipment (hot gas filter, cost annualized, plus annual 
costs of regeneration). Copies of MSHA's full (REA) analysis are in the 
record and are available to the mining community upon request.

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Benefits
    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 1.8 lung cancer deaths will be avoided per year.\1\
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    \1\ 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 13 lung 
cancer deaths avoided per year.
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    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.

(9) What actions has MSHA taken, and what additional actions does it 
plan to take, to facilitate compliance with this rule?

    This rule is a continuation of efforts by MSHA to help the mining 
community deal with the use of diesel engines in mining. The diesel 
equipment rule, now in effect, has itself contributed to the reduction 
of diesel exhaust emissions through the use of low sulfur diesel fuel, 
the requirement that all engines underground be approved, and improved 
maintenance. In one case, testimony was presented by a mine operator 
that timely engine maintenance, triggered by the weekly undiluted 
exhaust emissions test required by the new regulation, has greatly 
reduced carbon monoxide emissions from diesel equipment. These properly 
tuned engines will generate less particulate. MSHA has devoted 
workshops specifically to dpm control, issued a Toolbox of control 
methods to assist the mining community in this regard, and developed a 
computerized Estimator to help individual mines evaluate the impact of 
alternative approaches of controlling dpm emissions. The agency has 
verified the efficiency of the current generation of paper filters, and 
has sponsored work on the measurement of dpm in ambient mine 
atmospheres.
    This final rule includes certain provisions to facilitate 
compliance--e.g., authorizing MSHA to rely on the testing requirements 
of organizations like VERT, and permitting compliance with certain EPA 
requirements to be deemed as compliance with the requirements in this 
rule for newly introduced light duty equipment. The agency is, as 
described above, planning to take action in consultation with the 
mining community to facilitate the approval, and in particular the 
approval for permissible use, of a newer, cleaner generation of diesel 
engines. The agency will be preparing a compliance guide for this rule, 
and posting a variety of useful information on its web site. If 
necessary, additional workshops may be scheduled. In addition, MSHA is 
ready to provide special technical assistance to those who are planning 
to bring new engines or equipment underground in the next few months.

(10) Are surface mines addressed in this rule?

    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.

II. Background Information

    This part provides the context for this preamble. The nine topics 
covered are:
    (1) The role of diesel-powered equipment in underground coal mining 
in the United States;
    (2) The composition of diesel exhaust and diesel particulate matter 
(dpm);
    (3) The difficulties in measuring ambient dpm in underground coal 
mines;
    (4) Limiting the public's exposure to diesel and other fine 
particulates--ambient air quality standards;
    (5) The impact on emissions of MSHA approval standards and 
environmental tailpipe standards;
    (6) Methods for controlling dpm emissions in underground coal 
mines;
    (7) Existing standards for underground coal mines that limit miner 
exposure to diesel emissions;
    (8) Information on how certain states are restricting occupational 
exposure to diesel particulate matter; 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 17501 et seq.). This version has 
been updated to reflect the record, to discuss certain issues relevant 
to underground coal mines in more detail, and reorganized as 
appropriate.

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

    Diesel engines, first developed about a century ago, now power a 
full range of mining equipment. However at this time, less than 20% of 
underground coal mines (fewer than 150 underground coal mines) utilize 
this technology. Equipment powered by other sources (electrical power 
delivered by cable or trolley, and battery power) continues to 
predominate in this mining sector. Moreover, unlike in other mining 
sectors, most of the current diesel fleet in underground coal mines 
consists of light-duty support vehicles, and only limited numbers of 
the equipment used in digging or hauling coal is powered by diesel 
engines.
    Many in the mining industry believe that diesel-powered equipment 
has productivity and safety advantages over equipment powered by other 
sources. Others cite evidence to the contrary, and several key 
underground coal mining states continue to ban or significantly 
restrict the use of diesel-powered equipment in underground coal mines. 
The use of diesel engines to power equipment in underground coal mining 
is increasing and appears likely to continue to do so absent 
significant improvement in other power technologies.
    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

[[Page 5532]]

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 an efficient lightweight diesel power 
unit was developed. 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 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 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 coal mines. 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, load-haul-dump units, face drills, and explosives trucks. Diesel 
engines are also used in support equipment including generators and air 
compressors, ambulances, crane trucks, ditch diggers, foam machines, 
forklifts, graders, locomotives, longwall component carriers, lube 
units, mine sealant machines, personnel carriers, hydraulic power 
units, rock dusting machines, roof drills, tractors, utility trucks, 
water spray units, and welders.
    Current Patterns of Diesel Power Use in Underground Coal Mining. 
The underground coal mining sector is not as reliant upon diesel power 
as are other mining sectors. While nearly all underground metal and 
nonmetal mines, and nearly all surface mines, use diesel-powered 
equipment, less than 20% of underground coal mines use it. Table II-1 
provides further information on the current inventory.

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    The great majority of the diesel engines used in underground coal 
mines are used to power support equipment, rather than production 
equipment. This is in sharp contrast to other sectors. For example, in 
underground metal and nonmetal mines, of the approximate 4,100 pieces 
of diesel equipment normally in use at the time of MSHA's proposal, 
nearly half of the units were estimated to be used for loading and 
hauling. By contrast, of the approximately 3,000 pieces of diesel 
equipment in use in underground coal mines, MSHA estimates that fewer 
than 10% are used for coal loading and haulage. Moreover, because of 
space constraints and other operating conditions in underground coal 
mines, virtually all coal loading and hauling equipment has engines 
less than 200 horsepower; by contrast, virtually all such equipment in 
metal and nonmetal mines has engines greater than 200 horsepower and 
ranging to more than 750 horsepower or greater. As a result, the 
average horsepower of diesel engines powering equipment in underground 
coal mines is much less than the average engine in underground metal 
and nonmetal mines and all surface mines. This is significant because, 
other things being equal, lower horsepower engines are going to produce 
less dpm emissions by mass than higher horsepower engines.
    The engines in underground coal mines can be divided into three 
categories recognized under existing MSHA regulations: ``permissible'', 
``heavy-duty nonpermissible'', and ``light-duty nonpermissible.'' In 
this final dpm rule, MSHA is establishing different requirements for 
each of these categories. Accordingly, some background on this 
categorization is needed.
    Use of Diesel Engines in Permissible Equipment. Under existing 
regulations, equipment, whether powered by diesel engines or 
electricity, that is used in areas of the mine where methane gas is 
likely to be present in dangerous concentrations must be MSHA-approved 
``permissible'' equipment.

[[Page 5533]]

Permissible diesel powered equipment for use in coal mines is provided 
with special equipment to prevent the ignition of methane. This special 
equipment includes flame arresters and special treatment of flanges and 
joints. Since diesel engines normally have very hot surface 
temperatures and hot exhaust gas that can constitute an ignition 
source, permissible diesels must be provided with a means to maintain 
the temperatures of surfaces and the exhaust gas below 302 deg.F.
    MSHA regulations are very specific in defining those areas of the 
mine where permissible equipment is required. Generally, permissible 
equipment is required where the coal mining is actually being 
performed, because the mining process typically liberates methane. 
These areas are commonly referred to as ``inby'' areas. In some cases, 
however, permissible equipment is required to be used in other areas of 
the mine. For example, only permissible diesel-powered equipment may be 
used in return aircourses. The permissible equipment provides an 
additional level of fire protection because of the strict temperature 
controls on the equipment surface and exhaust. This increased 
protection is required because of the potential for the accumulation of 
dangerous levels of methane in these aircourses.
    MSHA's January 2000 inventory indicates that of the 3,121 diesel 
powered pieces of equipment in underground coal mines, 528 units are 
permissible pieces. The emissions generated by permissible equipment 
make a significant contribution to dpm concentrations in the mines 
where they are functioning. This is because the equipment has large 
engines, works hard and continuously in locations generally far from 
ventilation sources, and in close quarters with miners.
    Moreover, the engines which have to date been approved for 
permissible use are among those which emit the highest levels of dpm 
(in grams/hour): the Caterpillar 3304, Caterpillar 3306 (available in 
two horsepower sizes), the Deutz D916-6, and the Isuzu QD-100. The 
Deutz D916-6 is still used in underground coal mines, however, it is no 
longer in production. MSHA recently approved the Caterpillar 3306PCTA 
permissible, the first approved turbocharged engine.
    Diesel engines in the horsepower ratings required to power 
permissible equipment are now available in new low emissions technology 
engines. However, none of them has been approved for use on permissible 
equipment because no applications for MSHA approval have been received. 
This situation may reflect a lack of adequate incentives for engine and 
equipment manufacturers to incur the development costs to meet MSHA 
permissibility requirements or to pay the fees required for approval.
    MSHA is developing programs that would facilitate the availability 
of engines that utilize the latest technologies to reduce gaseous and 
particulate emissions for use in permissible equipment. Current engine 
designs that utilize low emissions technologies are currently approved 
by MSHA in nonpermissible form.
    One of the programs that MSHA is considering would follow the 
precedent established in the recently published diesel equipment rule. 
To facilitate compliance with this dpm rule, MSHA is considering 
funding the additional emissions testing needed to gain permissibility 
approval, previously approved, non-permissible engines that utilize low 
emissions technology engines, or waiving the normal fees that the 
Agency charges for the administrative and technical evaluation portion 
of the approval process.
    Alternatively, MSHA may relax, as an interim measure, the 
requirement that engine approvals be issued only to engine 
manufacturers. Under this program an equipment manufacturer could 
utilize an engine, approved by MSHA as nonpermissible, in a permissible 
power package. MSHA would ensure that the additional emissions tests 
required for permissible engines are conducted as part of the power 
package approval process. Provisions of the two programs could be 
combined.
    While the availability of cleaner engines would help reduce the dpm 
emissions from the permissible fleet, there are aftertreatment filters 
available for such equipment that are both highly efficient and 
relatively low cost. As discussed in more detail in section 6 of this 
part, because the exhaust temperature of these permissible pieces of 
equipment must be cooled for safety reasons, aftertreatment devices 
whose filtration media consists of paper can be directly installed on 
this equipment. Paper filters exposed to uncooled exhaust pose a fire 
and ignition hazard.
    Use of Diesel Engines in Nonpermissible Equipment. In those areas 
of an underground coal mine where methane concentrations can be limited 
through the control of ventilation air, permissible equipment is not 
required. Generally, this is the case in areas away from the face, 
often referred to as ``outby'' areas. Most equipment operating in 
underground coal mines is ``nonpermissible'' equipment.
    Nonpermissible equipment is divided into several categories for 
purposes of the diesel equipment rules that currently apply in 
underground coal mines (30 CFR part 75). In pertinent part, those rules 
provide:

Sec. 75.1908  Nonpermissible diesel-powered equipment; categories

    (a) Heavy-duty diesel-powered equipment includes--
    (1) Equipment that cuts or moves rock or coal;
    (2) Equipment that performs drilling or bolting functions;
    (3) Equipment that moves longwall components;
    (4) Self-propelled diesel fuel transportation units and self-
propelled lube units; or
    (5) Machines used to transport portable diesel fuel 
transportation units or portable lube units.
    (b) Light-duty diesel-powered equipment is any diesel-powered 
equipment that does not meet the criteria of paragraph (a) * * *
    (c) * * *.
    (d) Diesel-powered ambulances and fire fighting equipment are a 
special category of equipment that may be used underground only in 
accordance with the mine fire fighting and evacuation plan * * *.

    MSHA's inventory indicates that of the 3,121 diesel powered pieces 
of equipment, 497 are heavy duty nonpermissible pieces, 66 are 
generators and air compressors, and 2,030--that is, about two-thirds of 
the total underground coal diesel fleet at present--are other light 
duty nonpermissible pieces.
    The rationale for the division of nonpermissible dieselized 
equipment into these classes requires some background here because in 
this rulemaking on dpm, MSHA proposed making a significant distinction 
between the requirements applicable to each class.
    The division resulted from MSHA's 1996 regulation establishing 
safety rules for the use of dieselized equipment in underground coal 
mines (the general history and purpose of which are summarized in 
section 9 of this Part). As discussed in the preamble to the final 
diesel safety rule (61 FR 55459-61), the purpose of the categorization 
was to take the diversity of nonpermissible equipment into account in 
establishing regulatory requirements relevant to safety. The final 
categorization scheme for nonpermissible equipment developed over the 
course of time in response to public comments to the proposed rule.
    Equipment falling within the heavy duty category is typically used 
for extended periods during a shift on a continuous, rather than an 
intermittent,

[[Page 5534]]

basis. Heavy duty equipment also moves heavy loads or performs 
considerable work. Accordingly, to ensure such equipment could operate 
in a safe manner, the safety rule required that each piece of heavy 
duty equipment:

* * * has to be equipped with an automatic fire suppression system 
addressing the additional fire risks resulting from the way this 
equipment is used. Heavy-duty equipment also produces greater levels 
of gaseous contaminants, and under the final rule is therefore 
subject to weekly undiluted exhaust emissions tests * * * and is 
included in the air quantity calculation of ventilation of diesel-
powered equipment * * *. (61 FR 55461)

    It is important to note that there are other types of underground 
coal mining equipment which, although they have operating 
characteristics much like heavy duty equipment, were not designated as 
such under the diesel equipment rule. That is because such equipment 
(e.g., generators and compressors) is considered as portable equipment 
and special requirements were established in that rule to address the 
hazards presented by that equipment.
    Ambulances and fire-fighting equipment which use diesel engines 
have operating characteristics like light-duty equipment, but under the 
diesel equipment rule are considered a special category of equipment 
that does not have to meet the requirements of that rule. The equipment 
in this category must only be used in emergencies or fire drills and in 
compliance with fire fighting and evaluation plan requirements. 
Consequently, such equipment is not required to have an approved engine 
or power package or comply with the design and performance requirements 
of Secs. 75.1909 and 75.1910 (61 FR 55461).
    Under the diesel equipment rule, heavy-duty equipment may be used 
to perform light-duty work; but equipment that is classified as light-
duty may not be used, even intermittently, to perform the functions 
listed in paragraphs (a)(1) through (a)(5) of 30 CFR 75.1908 because it 
is not required to have the automatic fire suppression system that MSHA 
determined was necessary for such kinds of work. (Id.) As noted in the 
preamble, two machines of the same model could fall into different 
equipment categories depending on how they are used. Although of the 
same design, they do not present the same risk of fire because of the 
way in which they are used, nor do they produce the same quantities of 
exhaust contaminants:

``* * * machines that are operated for extended periods of time 
under heavy load generate more contaminants than machines that are 
not.'' (Id.)

    It was for this reason--the rate of contaminant generation--that in 
proposing a rule to limit the concentration of dpm in underground coal 
mines, MSHA proposed making a distinction between heavy-duty equipment 
and light-duty equipment. MSHA proposed requiring heavy-duty 
nonpermissible equipment and permissible equipment to be equipped with 
filters capable of removing 95% of the dpm emitted by the engines in 
those pieces of equipment. The proposal did not include any controls 
for the dpm emitted from light-duty equipment nor for ambulances and 
fire-fighting equipment. As noted in section 9 of this part, the Agency 
asked the mining community to comment on the Agency's assumptions and 
consider some options in this regard. The record on this matter and 
MSHA's final decision are discussed in Part IV.
    Whether categorized as heavy-duty or light-duty, the engine exhaust 
from nonpermissible equipment is not required to be cooled for safety 
reasons like exhaust from permissible equipment. Accordingly, this 
means that paper-type filters cannot be added directly to 
nonpermissible equipment without first adding a water scrubber or heat 
exchanger; otherwise, the paper would burn. As a result, control 
devices that are designed to filter hot exhaust gases (e.g., ceramic 
filters) provide a cost effective alternative for dpm control with 
nonpermissible equipment.
    Does Diesel Power Have Advantages Over Alternative Sources of Power 
for Equipment Used in Underground Coal Mines? As pointed out by a 
commenter, a number of power sources for mining equipment have been 
tried in the mining industry only to be rejected for various reasons 
(e.g., gasoline engines, cables, and compressed air). Today, this 
commenter continued, there are three general ways of powering mining 
equipment: electric power (delivered by electric trailing cables or by 
trolley wires), on-board battery power, and diesel. Table II-2 
reproduces a list provided by this commenter as to his view of some of 
the ``advantages and challenges'' of these power sources; MSHA is 
reproducing this list as a convenient summary, but does not necessarily 
agree or disagree with each specific entry.

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[[Page 5536]]

    Some in the mining industry strongly favor the use of diesel 
engines to power equipment in underground coal mines. A representative 
of a company with four underground coal mines testified that it has 200 
pieces operated by diesel power, and is continuing to add more. Another 
commenter stated that diesel is the power source of choice for moving 
personnel and supplies in large underground mines where coal is moved 
by conveyor belt.
    A number of commenters asserted that diesel-powered equipment has 
productivity and safety advantages over electrically-powered and 
battery-powered equipment.
    One commenter argued that diesel reduces the risks associated with 
the use of electrical equipment by eliminating the need for trolley 
wires, trolley poles and trailing cables that cause injuries, accidents 
and fatalities--shocks, electrocutions, burns, fires, tripping or being 
struck by trolley poles, and also reduce the number of material 
handling injuries. This commenter also argued that unlike electrical 
power, diesel use does not restrict mining plans or the mining cycle 
because operations are not hampered by cable length or time consuming 
power moves, provide greater flexibility in underground travel routes, 
and make equipment moves from one area of a mine to another more 
efficient. This commenter further claimed that compared to battery-
powered mining equipment (which arguably provides the same 
flexibility), diesels can haul coal more efficiently over longer 
distance, provide more power, and eliminate time-consuming battery 
change-out time.
    Another commenter noted the increased potential for fatalities and 
injuries in underground coal mines when trolley wires are present, and 
further that trolley wires restrict ventilation in one entry.
    Another commenter noted the difficulties of evacuating miners in 
the event of emergencies over the large distances in some underground 
mines using sources of power that were more prone to failure than 
diesel.
    Another commenter asserted that all of the 18 employees who had 
died since 1972 as a result of exposed overhead direct current trolley 
lines could have lived if diesel power had been in use, and pointed to 
examples of fires initiated by trolley wires with associated loss of 
productivity. This commenter also noted that battery powered equipment 
has been known to cause injuries, and explosions both from its 
production of hydrogen gas and from sparks igniting methane in the mine 
atmosphere.
    Commenters also note that many asserted safety risks associated 
with the use of diesel powered equipment in underground coal mines have 
now been addressed as a result of MSHA's safety rules.
    Other commenters, however, pointed out that there are a number of 
the nation's most productive underground coal mines (including both 
those using longwall and those using room and pillar mining techniques) 
which do not use this technology. These commenters challenged industry 
claims that diesel power is necessary for business to survive. Some 
also noted that miners are trained to protect themselves better from 
safety hazards that accompany the use of electrical power, like 
tripping on cables and electrical hazards, but are not able to protect 
themselves from health hazards they cannot see. In this regard, the 
hearing transcripts are replete with reminders by underground coal 
miners of their concern about what they are breathing in light of the 
tragic experience with black lung disease.
    As indicated by MSHA in the preamble to the proposed rule (63 FR 
17503), not many studies done recently address the contentions that 
diesel power provides safety and/or productivity advantages, and the 
studies which have been reviewed by MSHA do not clearly support this 
hypothesis.

Outlook for Use of Diesel Engines To Power Equipment in Underground 
Coal Mines

    The use of diesel engines to power equipment in underground coal 
mining is increasing. In fact, since this rulemaking was proposed, 
MSHA's inventory has recorded an increase of about 5% in the number of 
diesel-powered pieces of equipment at the roughly 145 coal mines using 
diesel power underground. This trend appears likely to continue, absent 
significant improvement in other power technologies.
    Several key underground coal mining states--Ohio, Pennsylvania and 
West Virginia--continue to ban or significantly restrict the use of 
diesel-powered equipment in underground coal mines (as discussed in 
section 8 of this Part). There are 339 underground coal mines in these 
states. If the current restrictions in these States were relaxed, in 
accordance with the expressed interest of industry groups toward this 
end, many of these underground coal mines are likely to begin using 
diesel to power some equipment.
    Full implementation of MSHA's recent rules for the safe use of 
diesel-powered equipment in underground coal mines (discussed in 
section 7 of this part), is also likely to lead to increased diesel use 
because they resolve certain safety concerns that discouraged the 
mining community from using such equipment more widely. Another factor 
suggesting that the use of diesel power will expand is that both miners 
and mine operators are concerned about the future of their industry.
    On the other hand, operators as well as miners have acknowledged 
that potential health hazards associated with the use of diesel power 
must be addressed if its use is to become widespread. 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 after the rule 
is fully implemented. As explained in Part V of this preamble, however, 
MSHA has concluded that the underground coal mining sector as a whole 
cannot feasibly reduce dpm concentrations further at this time. 
Nevertheless, the efforts by US and overseas environmental regulators 
to restrict dpm and other diesel emissions into the environment, 
discussed in sections 4, 5 and 6 of this Part, are leading to 
technological improvements in engines, fuel and filters that will help 
reduce this risk.
    Currently, diesel power faces only a limited number of competitive 
power sources. It is unclear how quickly new ways to generate energy to 
run mobile vehicles will be available for use in underground mining 
activities. New hybrid electric automobiles have been introduced this 
year by two manufacturers (Honda and Toyota); these 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, and are reviewed for safe use underground, 
MSHA assumes that the mining community's interest in the use 
underground of diesel-power as an

[[Page 5537]]

alternative to direct electric power is likely to 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 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., phenanthrene, 
fluoranthene). The oxides of nitrogen (NOX) merit particular 
mention because in the atmosphere they can precipitate onto particulate 
matter. Thus, reducing the emissions of NOX is a 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. Most 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 
consisting mainly of elemental carbon. They also have a very surface-
rich morphology. This extensive surface absorbs many other toxic 
substances, that are transported with the particulates, and can 
penetrate deep into the lungs. More than 1,800 different organic 
compounds have been identified as absorbed onto the elemental carbon 
core. A portion of this hydrocarbon material results from incomplete 
combustion of fuel; however, most is derived from engine lubrication. 
In addition, the diesel particles contain a fraction of non-organic 
adsorbed materials. Figure II-1 illustrates the composition of dpm.

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    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 using 
equipment powered by diesel engines (Cantrell and Rubow, 1992). The 
vertical axis represents relative dpm concentration, and the horizontal 
axis the particle diameter.
    As can be seen, the distribution is bimodal, with dpm generally 
less than 1 m in size, and dust generated by the mining 
process greater than 1 m.

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[[Page 5538]]


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BILLING CODE 4510-43-C

    As shown on Figure II-3 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.

BILLING CODE 4510-43-P


[[Page 5539]]


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BILLING CODE 4510-43-C

    The particles in the nuclei mode, also know as nanoparticles, are 
being investigated for their health hazard relevance. Interest in these 
particles has been sparked by the finding that newer ``low polluting'' 
engines emit higher numbers of small particles than the old engine 
technology engines. Although the exact composition of diesel 
nanoparticles is not known, it is thought 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 dpm, with particular 
reference to very tiny particles known as nanoparticles, is discussed 
further in section 5 of this Part.

(3) The Difficulties of Measuring Ambient DPM in Underground Coal 
Mines.

    As it indicated in its notice of proposed rulemaking to limit the 
concentrations of dpm in underground coal mines (63 FR 17498, 17500), 
MSHA decided not to propose a rule to require the measurement of 
ambient dpm levels in underground coal mines in order to determine 
compliance. The Agency observed that while there are a number of 
methods which can measure ambient dpm at high concentrations in 
underground coal mines with reasonable accuracy. When the purpose is 
exposure assessment, MSHA does not believe any of these methods provide 
the accuracy that would be required to measure ambient dpm levels in 
underground coal mines at lower concentrations.
    In particular, MSHA expressed concern about potential difficulties 
in using the available methods to distinguish between dpm and submicron 
coal mine dust (63 FR 17506-17507). While the use of an available 
impactor device can prevent larger particles from entering the sampler 
(e.g., carbonates), albeit at the expense of eliminating the larger 
fraction of dpm as well, there are limits on the extent to which it can 
help MSHA distinguish how much of the fine particulate reaching the 
sampler is coal dust and how much is dpm. To make the distinction 
analytically, NIOSH method 5040 would have to be adjusted so that only 
the elemental carbon is determined. However, as MSHA noted, there are 
no established relationships between the concentration of elemental 
carbon and total dpm under various operating conditions. The organic 
carbon component of dpm can vary with engine type and duty cycle; 
hence, the amount of whole dpm present for a measured amount of 
elemental carbon may vary. Accordingly, MSHA concluded that it was 
``not confident that there is a measurement method for dpm that will 
provide accurate, consistent and verifiable results at lower 
concentration levels in underground coal mines'' (63 FR 17500).
    Since there has been no disagreement with MSHA's initial conclusion 
about the current availability of an accurate, consistent and 
verifiable method of measuring dpm concentration levels in underground 
coal mines, the final rule is not dependent on ambient air 
measurements. MSHA has proposed using such a method for underground 
metal and nonmetal mines, and the validity of the measurement was the 
subject of much comment; accordingly, a more complete discussion of 
this topic will be found in the preamble of the final rule for 
underground metal and nonmetal mines.

[[Page 5540]]

(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 emits 
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 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 an 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 used in 
highway vehicles, 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 ``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 measurements of pollution 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''.
    Total Suspended Particulates (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 mix 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.
    Particulates Less than 10 Microns in Diameter (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 PM10 limit of 50 g/
m3, and a 24-hour PM10 limit of 150 g/
m3. 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 
the levels of 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 (Shea, 
1995; comments of Newmont Gold Company, March 11, 1997, EPA docket 
number A-95-54, IV-D-2346).
    Particulate Less than 2.5 Microns in Diameter (PM2.5). 
The next EPA scientific review was completed in 1996. A proposed rule 
was published in November of 1996, and, after public hearings and 
review by the Office of 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, the new rule would 
establish a NAAQS for ``fine particulate matter'' that is less than 2.5 
microns in size. The PM2.5 annual limit was set at 15 
g/m3, with a 24-hour ceiling of 65 g/
m3.
    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).

[[Page 5541]]

    A majority of the DC Circuit Court, however, 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 Court ordered EPA to develop a new standard for this size 
category.
    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; 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). 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, 
and did so in its risk assessment (see part III of this preamble). 
Comments on the appropriateness of this conclusion by MSHA, are 
reviewed in Part III.
    (5) The impact on emissions of MSHA approval standards and 
environmental tailpipe standards.
    MSHA requires that the gaseous emissions from all diesel engines 
used in underground coal mines meet certain minimum standards of 
cleanliness; only engines which meet those standards are ``approved'' 
for use in underground coal mines. The 1996 diesel equipment safety 
rule required that all engines in the underground mining fleet be 
approved engines. Thus, these rules set a ceiling for various types of 
diesel gas emissions. But diesel engines do not have to meet a dpm 
emissions standard to be ``approved'' for underground use.
    Engine emissions of dpm are however, restricted by Federal 
environmental regulations, supplemented in some cases by State 
restrictions. Over time, these regulations have required, and are 
continuing to require, that new diesel engines meet tighter and tighter 
standards on dpm emissions. As these cleaner engines replace or 
supplement older engines in underground coal mines, they can lead to a 
significant reduction in the amount of dpm emitted by the underground 
fleet.
    This section reviews developments in this area. Although this 
subject was discussed in the preamble of the proposed dpm rule (63 FR 
17507), this review here updates the relevant information.
    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 (discussed in more detail in 
section 7 of this Part) made significant changes to diesel engine 
requirements for underground coal mines. The new rule required the 
entire underground coal fleet to convert to approved engines no later 
than November 1999. Accordingly, by the time this rule to limiting dpm 
exposure goes into effect, all diesel engines in underground coal mines 
are expected to be approved engines.
    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.
    Unlike the ventilation rate set for each engine, 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 diesel equipment rule was issued, MSHA explicitly 
deferred the question of whether to require engines used in mining 
environments to meet a specific PI (61 FR 55420-21, 55437). While the 
matter was discussed during the diesel equipment rulemaking, the 
approach taken in the final rule was to adopt the multi-level aproach 
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 are 
included 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 
manufacturers can use the particulate index to design and install 
exhaust after-treatments (61 FR 55421). So that the PI for any engine 
is known to the mining community, MSHA reports the index in the 
approval letter, posts the PI and ventilating air requirement for all 
approved engines on its website, and publishes the index containing its 
lists of approved engines.
    In the proposed dpm rule, MSHA indicated that given that the 
equipment rule was recently promulgated, it did not yet have enough 
information to determine the feasibility of a requirement that certain 
engines meet a specific PI in order to be used underground (63 FR 
17564). MSHA received comments on this subject during the hearings and 
thereafter; the Agency's response to these comments is included in Part 
IV of this preamble.
    Authority for Environmental Engine Emission Standards. The Clean 
Air Act authorizes the federal Environmental Protection Agency (EPA) to 
establish nationwide standards for mobile sources of air pollution, 
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 these standards. 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

[[Page 5542]]

situations, the California standards may be more stringent than federal 
standards.
    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., trucks under 8500 lbs GVWR, which 
include pick-up trucks and SUVs. EPA has also established a class of 
``medium duty passenger vehicles'' which include passenger vehicles 
over 8500 lbs. These vehicles, mostly large SUVs, are treated like 
light-duty trucks for the purposes of emission standards; (2) heavy 
duty highway engines (i.e., those designed primarily to power trucks) 
greater than 8500 lbs GVWR) which range from the largest pick-up trucks 
to over the road trucks); and (3) nonroad vehicles (i.e., those engines 
designed primarily to power small equipment, construction equipment, 
locomotives, farm equipment and other non-highway uses).
    The terms ``heavy duty'' and ``light duty'' are used differently by 
EPA and MSHA. The category of an engine for purposes of environmental 
regulations is not the same as the category of mining equipment in 
which it is used. The engine categories used by EPA have been 
established with reference to normal transportation uses. But as 
explained in section 1 of this Part, MSHA has established a 
classification system for underground coal mining equipment based on 
how that equipment is used in mining. This system includes 
``permissible'' equipment (required where explosive methane gas may be 
present in significant quantities) and two categories of 
``nonpermissible'' equipment known as ``heavy duty nonpermissible'' and 
``light duty nonpermissible''. Accordingly, ``heavy duty'' engines 
might be used in ``light duty'' nonpermissible equipment.
    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. 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. Although vehicle engines in these categories are not currently 
approved for use in underground coal mines, it might be sought in the 
future. Accordingly, some information about the applicable 
environmental regulations is provided here.\2\
---------------------------------------------------------------------------

    \2\ The discussion focuses on the particulate matter 
requirements for light duty trucks, although the current pm 
requirement for all 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 2009, 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 commercial mining 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 0.5g/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 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 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

[[Page 5543]]

hydrocarbons, carbon monoxide, NOX, and dpm. The limits were 
phased in over 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 to 2004, the standards limit pm emissions to 0.45 
g/bhp-hr and 0.54 g/bhp-hr respectively; 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 adopted 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. Further, they establish 
Tier II particulate matter limits for all other land-based nonroad 
engines (other than locomotives which previously had Tier II 
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 
required levels are feasible; EPA has indicated that in the context of 
that review, it intends to consider further limits for particulate 
matter. Because of the phase-in of these Tier II 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 II pm engines in some sizes available, but it is likely to be a 
few years before a full size range of Tier II pm nonroad engines is 
available.
    Table II-3 provides a full list of the EPA required particulate 
matter limitations on nonroad diesel engines for tier 1 and 2. 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 MSHA and EPA Engine Emission Standards on the 
Underground Coal Mining Fleet. In the mining industry, engines and 
equipment are often purchased in used condition, and frequently 
rebuilt. Thus, many of the diesel engines in an underground coal mine's 
fleet today may only meet older environmental emission standards, or no 
environmental standards at all. Although the environmental tailpipe 
requirements on dpm are already bringing about a reduction in the 
overall contribution of dpm to the general atmosphere, the beneficial 
effects of the EPA regulations on mining atmospheres will be slower 
absent incentive or regulatory actions that accelerate the turnover of 
mining fleets to engines that emit less dpm. Moreover, while the 
requirement that all underground coal mine engines be ``MSHA approved'' 
is leading to a less polluting fleet than would otherwise be the case, 
there are many approved engines that do emit significant levels of 
pollution, and in particular dpm. As noted in the discussion of MSHA's 
approval requirements, the Agency is taking internal actions to ensure 
that these requirements do not inadvertently slow the introduction of 
cleaner engine technology.
    It should be noted that in theory, underground mines can still 
purchase certain types of new engines that do not have to meet EPA 
standards. For example, the current rules on nonroad diesel engines 
state that they do not apply to engines intended to be used in 
underground coal and metal and nonmetal mines (40 CFR 89.1(b)). 
Moreover, it is not uncommon for engine manufacturers to take a model 
submitted for EPA testing and adjust the horsepower or other features 
for use in a mining application. In recent years, however, engine 
manufacturers have significantly cut back on such adjustments because 
the mining community is not a major market. Accordingly, MSHA believes 
that most of the diesel engines that will be available for underground 
mines in the future will meet the applicable EPA standard. In addition, 
many of the recently approved engines by MSHA currently meet the tier 
II nonroad pm standards.
    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 less than 50 nanometers (nm) in diameter.
    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

[[Page 5544]]

that nanoparticles emitted from the engine could be removed effectively 
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. He believed that this reduction in mass was attributed to those 
particles 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 scientific information on the potential health 
effects resulting from exposure to nanoparticles, this commenter did 
not believe that potential the risk of 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 under development by the engine 
industry to meet the standards accordingly focuses on reducing the mass 
of dpm 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 emitted from 
the engine.
    Figure II-3, repeated here from section 2 of this Part, illustrates 
this situation (Majewski, W. Addy, Diesel Progress, June, 1998).

BILLING 4510-43-P

[GRAPHIC] [TIFF OMITTED] TR19JA01.007


BILLING CODE 4510-43-C
    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.
    A number of studies have demonstrated that the size of the 
particles emitted from the newer low emission diesel engines, has 
shifted toward the generation of nuclei mode particles. One study 
(cited by Majewski) compared a 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 (see diesel.net 
technology guide). Mayer, while pointing out that nanoparticle 
production was a problem with older engines as well, concurs that the 
technology 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. This is because as dpm is released into the

[[Page 5545]]

atmosphere the diesel particulate undergoes very complex changes. In 
addition, current sampling procedures produce artificial particulates, 
which otherwise would not exist under atmospheric conditions. 
Experimental work conducted at West Virginia University (Bukarski) 
indicate that nanoparticles are not generated during the combustion 
process, but rather during other physical and chemical processes which 
the exhaust undergoes in aftertreatment systems.
    While current medical research findings indicate that small 
particulates, particularly those below 2m in diameter, may be 
more harmful to human health than the larger ones, much more medical 
research and diesel emission studies are needed to fully characterize 
diesel nanoparticles emissions and their influence on human health. If 
nanoparticles are found to have an adverse health impact by virtue of 
size or 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 can increase the number of nanoparticles.
    As discussed in Part III, the available evidence on the risks for 
dpm exposure do not currently include enough data to draw conclusions 
about the risks of exposure to significant numbers of very small 
particles. Research on nanoparticles and their health effects is 
currently a topic of investigation. As there have been few measurements 
of the number of particles emitted (as opposed to mass), it will be 
very difficult for epidemiologists to extrapolate information in this 
regard.
    Based on the comments received and a review of the literature 
currently available on the nanoparticle issue, MSHA believes that 
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 answered for some time because of the 
extensive research required to address the questions raised. MSHA's 
rules will require the application of exhaust aftertreatment devices on 
nearly all of the most polluting engines. The application of these 
measures will reduce the number of nanoparticles as well as the mass of 
the larger particles to which a miner will be exposed--miners wanted 
aftertreatment on all machines for this purpose.

(6) Other Methods for Controlling DPM in Underground Coal Mines

    As discussed in the last section, the introduction of new engines 
underground will play a significant role in reducing the concentration 
of dpm in underground coal mines. There are, however, other approaches 
to reducing dpm concentrations in underground coal mines. Among these 
are: use of aftertreatment devices to eliminate particulates emitted by 
an engine; altering fuel composition to minimize engine particulate 
emission; use of 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 use of 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, these control methods were 
discussed.
    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 have required that certain equipment 
be equipped with high-efficiency particulate filters; the 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 lessen dpm levels. Accordingly, information 
about them 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 their 
implications 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 also 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 which have water scrubbers installed which cool 
the exhaust. However, another alternative that is now used in coal 
mines is ``dry system technology'' which cools the diesel exhaust with 
a heat exchanger and then uses a paper filter. In addition, ``oxidation 
catalytic converters,'' devices used to limit the emission of diesel 
gases, and ``water scrubbers,'' devices used to cool the emission of 
diesel gases, are discussed here as well, because they also can have 
effect on limiting particle emission.
    Water Scrubbers. Water scrubbers are devices added to the exhaust 
system of diesel equipment. Water scrubbers are essentially metal boxes 
containing water through which the diesel exhaust gas passes. The 
exhaust gas is cooled, generally to below 170 degrees F. A small 
fraction of the unburned hydrocarbons is condensed and remains in the 
water with some 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. However, MSHA has no definitive evidence on the amount of 
dpm reduction that can be achieved with a particular water scrubber. 
The water scrubber does not remove the carbon monoxide, the oxides of 
nitrogen, or other gaseous emission that remains a gas at room 
temperature, so their effectiveness as aftertreatment devices is 
limited.
    The water scrubber serves as an effective spark and flame arrester 
and as a means to cool the exhaust gas. Consequently, it is used in 
most 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 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

[[Page 5546]]

to change resulting in water blowing out the exhaust pipe. Control 
devices can be 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 primary dpm control device on nonpermissible 
equipment.
    Oxidation Catalytic Converters (OCCs). 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 (Haney, Saseen, Waytulonis, 1997). Their use has 
been widespread. It has been estimated that more than 10,000 OCCs have 
been put into the mining industry over the last several years 
(McKinnon, dpm Workshop, Beckley, WV, 1995).
    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, in effect, by 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 a 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 where the 
exhaust gas came into contact with the catalyst. Designs have evolved, 
and now 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 due to 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 it 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 (0.05 percent) 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 required that this low sulfur fuel be used. 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 (0.0015 percent) for on-highway use in 2006.
    The particulate removal 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 CFR 85.1403). Aftertreatment manufacturers developed 
catalytic converter systems capable of reducing dpm by 20%. 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, or have 
unfavorable gaseous phase reactions increasing toxicity, and that the 
positive effects are irrelevant for construction site diesel engines. 
He concludes that the negative effects outweigh the benefits (Mayer).
    The Phase 1 interim data report of the Diesel Emission Control-
Sulfur Effects (DECSE) Program (a joint government-industry program 
established to explore lower sulfur content that is discussed in more 
detail later in this section) similarly indicates that testing of 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 under certain operating temperatures, and that 
oxidation is a part of aftertreatment systems approaches like the 
DST and some ceramic traps. But this commenter asserts that 
the sulfate production occurs at an operating mode that is seldom seen 
in real operation.
    Other commenters during the rulemaking strongly supported the use 
of OCCs to reduce particulate and other diesel emissions. They argue 
that the OCCs result in significant reductions in dpm and in dpm 
generating gases. One commenter noted that with a clean engine, an OCC 
might well reduce particulates enough to meet any requirements 
established by MSHA.

[[Page 5547]]

    However, another commenter noted that OCCs and ceramic traps can 
fail when used at higher altitude mines due to the lower oxygen content 
in the exhaust system. Another commenter asserted that OCCs are not 
effective at low temperature, although they are improving. Accordingly, 
this commenter indicated that OCCs have an impact only on light duty 
equipment when the equipment is working, not when it is idling, and are 
virtually useless on permissible equipment because of the low exhaust 
temperatures achieved through cooling. Despite a specific request from 
MSHA at the rulemaking hearings, no data were provided by OCC advocates 
to demonstrate that they can perform well at the lower temperatures 
normally found in light duty equipment.
    Hot gas particulate traps. 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 filter. 
Hot gas filter refers to the current commercially available particulate 
filters such as ceramic cell, woven fiber filter, sintered metal 
filter, etc.
    Following publication of EPA rules in 1985 limiting diesel 
particulate emissions from heavy duty diesel engines, development of 
aftertreatment devices capable of more significant reductions in 
particulate levels began to be developed for Comerica applications.
    The wall flow type ceramic honeycomb diesel particulate filter 
system was initially the most promising approach (SAE, SP-735, 1988). 
This consisted of a ceramic substrate encased in a shock-and vibration-
absorbing material 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 placing 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 exhaust from 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. One commenter noted 
that a total exhaust, wall-flow, ceramic filter developed in Canada in 
collaboration with a US firm has been successfully demonstrated 
underground with a reduction of between 60% and 90% of particulate 
matter.
    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 were found to be unnecessary 
for compliance with the EPA standards of the time for vehicle engines.
    These devices proved to be quite effective in 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 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 study 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.
    Reservations regarding their usefulness and practicality remain. 
One commenter stated 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.'' Another 
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. 
Another reported that ceramics would not work at higher altitudes 
because of lower oxygen content in the exhaust system. Another 
commenter pointed out that elevated operating temperatures in certain 
engine modes can result in sulfates adding as much as 50% to total 
particulate mass, and asserted that ceramic traps alone were unable to 
offset this effect on their own.
    In response to the proposed rule, MSHA received information and 
claims about the current efficiency of such technologies. One 
commenter, representing those who manufacture emissions controls, and 
referring to technologies other than low temperature paper filters--
such as higher temperature disposable paper filters, ceramic monolith 
diesel particulate filters, wound ceramic fiber filters, and metal 
fiber filters--asserted that there were technologies which could 
achieve in excess of 95% filtration efficiency under ``many operating 
conditions.'' Another commenter submitted copies of information 
provided to that commenter by individual manufacturers of emission 
control systems, many of which made similar claims. Another commenter, 
however, questioned manufacturer claims, asserting big differences had 
been observed between such claims an independent 8-mode tests.
    It appears that two groups in particular have been doing some 
research comparing the efficiency of recent ceramic models: the 
University of West Virginia, 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 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 (in both cases, background on the regulatory efforts of 
the jurisdictions involved is discussed in section 8 of this part).
    The legislature of the State of West Virginia enacted the West 
Virginia Diesel Act, which created the West Virginia Diesel Commission 
and set forth an administrative vehicle to allow and regulate the use 
of diesel equipment in underground coal mines in that state. West 
Virginia University was appropriated funds to test diesel exhaust 
controls, as well as an array of

[[Page 5548]]

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.
    The University provided 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, 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. The University reported problems 
with this system that would account for the lower than expected 
efficiency for a paper filter type system. A commenter who spoke for 
the Commission at MSHA's public hearing expressed serious reservations 
of the 95% collection efficiency of MSHA's proposed rule and believed 
it was not achievable with technology based on the University's current 
work. The WV Commission also provided MSHA a detailed proposal for 
setting a laboratory diesel particulate standard of 0.5 milligram per 
cubic meter. As discussed in part IV, this is similar to the 
Pennsylvania standard, but without a strict filter efficiency value, 
and as further discussed in part IV, MSHA's approach in this final rule 
is similar.
    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 (Mayer et al., March 1999, and 
Mayer, April 1999). 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 using an 
external power source 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%) by particulate count, but a lower rate of reduction in terms 
of mass.
    Subsequently, VERT has evaluated additional commerically available 
filter systems. A list of recently evaluated hot gas filters are shown 
in Table II-4. 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.

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    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 in complying with the requirements of the standards for heavy 
duty equipment, generators and compressors, are discussed in Part IV of 
this preamble.
    The accumulated dpm must be removed from 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 to self regenerate.
    Techniques are available to lower the temperature needed 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 is 
similar to that of 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 does not occur in systems 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. In attempting 
to maintain a surface temperature less than the 300 degrees Fahrenheit 
(required for permissibility purposes) the exhaust gas was 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 when the 
engine is 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. The 
burner is 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.
    Equipment owners may choose to remove the particle trap from the 
machine to perform the regeneration. Particle traps are available with 
quick release devices. 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. Investigators are 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 undergoing 
testing utilizes an electrical heating element installed in the filter 
system to provide the heated air for regeneration of the filter. This 
heating element requires connection of the filter to an external 
electrical source at the end of the shift. Initial tests have been 
successful.
    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. MSHA believes this information is relevant to 
coal and metal/nonmetal mining because the tunneling equipment on which 
these filters are installed is similar to metal/nonmetal equipment and 
can be applied to heavy duty equipment in coal mining operations. 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 Oberland-Mangold company has 
approximately 1,000 systems in the field. They 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 Unikat company has introduced in Switzerland 
over 250 traps since 1989 and 3,000 worldwide with some operating more 
than 20,000 hours. In German industry,

[[Page 5551]]

approximately 1,500 traps in forklifts are installed annually.
    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, 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, 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 
were conducted. It was determined, by in mine ambient gravimetric 
sampling, that the particulate filter reduced dpm emissions by 95 
percent compared with the same machine without the filter. The test 
results showed 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.
    Despite the initial reports on the high efficiency of paper 
filters, during the hearings and in the comments on this rulemaking a 
number of commenters 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 paper filter to reduce the dpm 
generated by a typical engine used in permissible equipment. The 
results of this verification investigation are reviewed in Part IV of 
this preamble. They confirmed that commercially available paper filters 
are capable of achieving very high efficiencies.
    Another commenter noted that the volatile fraction of particulate 
is not trapped by hot gas filters, but rather passes through the filter 
in gaseous form. The volatile fraction consists of, among other 
components, gaseous forms of sulfur compounds, lube oil and the high 
boiling point fraction of unburned fuel. These components condense in 
the mine atmosphere as diesel particulate. The commenter asserted that 
the process of volatilization is reduced in the water cooled exhaust, 
but it is present nevertheless.
    MSHA recognizes that the volatile fraction of dpm passes through 
hot gas filters. This volatile fraction later condenses in the mine 
atmosphere and is collected on particulate samplers. This is not the 
case with hot gas filters that utilize a catalytic converter. The 
volatile fraction is oxidized in the catalytic converter and the gases 
produced do not condense as particulate. Paper filters are typically 
used with water scrubbers or heat exchangers, both of which condense 
the volatile fraction into dpm before the exhaust gas reaches the paper 
filter. This allows the paper filter to trap the condensed volatile 
fraction.
    Dry systems technology. The 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 from being discharged. The 
surfaces 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 controlling dpm emissions, 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 on diesel haulage equipment, 
longwall component carriers, longwall component extraction equipment, 
and in nonpermissible form, on locomotives. However, as pointed out by 
commenters, requiring the use of a dry system on all mining equipment 
would be expensive, cumbersome, and in many cases would require 
considerable engineering measures that might render them infeasible.
    Although the dry systems were originally designed for permissible 
equipment applications, they can also be used directly on outby 
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.
    Two manufacturers have received approval for diesel power packages 
that are configured as described above; Paas Technologies, (under 
various corporate designations including Minecraft and a registered 
trade name, Dry Systems Technology, or DST ) and Jeffrey 
Mining Equipment Company (currently Long-Airdox-Jeffrey).
    The design of the dry system manufactured by DST  
includes a catalytic converter. However, with respect to the basic Paas 
Technologies system, without a catalytic converter, the initial 
reported laboratory reductions in dpm were dramatic: up to 98%.
    During the hearings, however, there were many questions about the 
applicability of the early results to MSHA's proposed requirement that 
emissions of certain equipment be reduced 95% by mass. It was indicated 
by a commenter that the original Paas Technology dry system tests with 
a paper filter were performed at West Virginia University used high 
sulfur fuel which is currently prohibited in underground coal mines. 
The commenter stated that the University tested different fuels 
containing varying sulfur contents and the results indicated a 
fluctuation in overall dpm emission results. The commenter stated the

[[Page 5552]]

difference in dpm collection efficiency by the filter was on the order 
of 12 to 15%. Another commenter stated the difference in dpm reduction 
using a 0.37 percent fuel sulfur and a 0.04 percent fuel sulfur was 
about 22 percent. This commenter further stated that other published 
papers from Europe report the same dpm reductions with varying fuel 
sulfur levels, approximately 15 to 20 percent reduction.
    As was stated ealier, Paas Technologies has further developed its 
system by the adding a catalytic converter in the exhaust before the 
particulate paper filter. Paas Technologies have developed a technique 
whereby the catalytic converter is mounted so that the exhaust gas 
temperature remains high enough for the converter to operate 
effectively while complying with the MSHA surface temperature 
requirement. In addition to removing most of the carbon monoxide, the 
catalytic converter removes most of the unburned hydrocarbons before 
they are cooled and condensed. This feature extends the operating life 
of the filter. Any sulfate formed in the catalytic converter or in the 
engine combustion process condenses to a solid form as the exhaust gas 
passes through the heat exchanger and is collected in the particulate 
filter.
    Paas Technologies submitted a detailed set of test results on a 
94hp MWM D-916-6 test engine equipped with a Model M38 DST  
Management System, which included the catalytic converter, for the 
rulemaking record. These tests were conducted by Southwest Research 
Institute using an 8-mode test, with ASTM No. 2-D diesel fuel. Both the 
test cycle and test fuel (low sulfur) conformed with the test procedure 
detailed in the proposed rule and in this final rule. In idle mode, the 
dpm emissions were reduced about 90%; in mode 5, the dpm emissions were 
down 99%; on average of the 8 modes, the dpm emissions were reduced by 
97%.
    The Jeffrey system, which does not utilize a catalytic converter, 
was the subject of the MSHA verification initiative, noted in part IV. 
The verification was conducted in such a way as to test filter 
efficiency separately from whole system, with the low sulfur fuel 
required for coal mine use and without a catalytic converter. The 
verification confirmed that the paper filter has a dpm removal 
efficiency greater than 95 percent.
    This data submitted to the rulemaking record demonstrates that 
paper filters used on dry systems can achieve a filtration efficiency 
that allows equipment to meet the 2.5 gm/hr standard with low sulfur 
diesel fuel both with and without a catalytic converter in the system.
    Reformulated fuels. It has long been known that sulfur content can 
have a big effect on dpm emissions. In the diesel equipment rule, MSHA 
requires that 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 is the type of diesel 
fuel 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 from 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. Such standards may be difficult to meet without 
advanced catalyst technologies that in turn are likely to require 
sulfur reductions in the fuel.
    Moreover, planned Tier 3 standards for nonroad vehicles would 
require similar action (64 FR 26143). (For more information on the EPA 
planned engine standards, see section 5 of this Part). 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, 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 were fairly intolerant of fuel sulfur. Accordingly, the agency 
hopes to gather information on whether or not low sulfur fuel was 
needed for effective PM control (64 FR 26150). EPA's proposed rule was 
published in May 2000 and EPA 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 2006-2009.
    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 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.
    Several commenters in this rulemaking suggested other fuel 
formulations which could have a beneficial effect on dpm emissions. One 
commenter encouraged the use of FRF, Fire Resistant Fuel, which has 
various safety features as well as lower NOX and PM, and 
noted it is under study for use by the military.
    Another commenter noted the development of a catalytic ignition 
system that permits the engines to operate on alternative fuels which 
greatly reduce harmful emissions. For example, using a water-methanol 
mix, the commenter noted dramatic reductions in harmful emissions of 
NOX, CO and HC over a gasoline, spark ignition engine. This 
commenter also noted that the ignition system could operate on a diesel 
engine, but provided no information about emissions reductions by its 
use.
    Meyer reports the results of a test by VERT of a special synthetic 
fuel containing neither sulfur nor bound nitrogen nor aromatics, with a 
very high

[[Page 5553]]

Cetane index. The fuel performed very well, but produced only abut 10% 
fewer particulates than low sulfur diesel fuel, nor did it show any 
improvement in diminishing nonparticulate emissions.
    Cabs. Even though cabs are not the type of control device that is 
attached to 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.
    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 as found in some surface operations. 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.
    To be effective, a cab should be tightly sealed with windows and 
doors closed. Rubber seals around doors and windows should be in good 
condition. Door and window latches must operate properly. In addition 
to being well sealed, the cab should have an air filtration and 
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. Several different types of filter media have 
been tested in underground mines. These include standard filter paper 
and high efficiency filter paper. Filter papers can reduce diesel 
particulate exposures by 60 percent to 90 percent. When changing filter 
media, it is necessary to make sure that the airflow into the cab is 
not reduced and that the airflow through an air conditioning system is 
not reduced.
    Although the installation of a cab does not relieve the mine 
operator from the responsibility of complying with the equipment dpm 
limits, cabs provide assistance in complying with noise and respirable 
dust regulations. Cabs protect the equipment operator protection from 
dpm, respirable dust and noise exposures.

(7) Existing Standards for Underground Coal Mines That Assist in 
Limiting Miner Exposure to Diesel Emissions

    MSHA already has in place various requirements that indirectly help 
to control miner exposure to diesel emissions in underground mines--
including exposure to diesel particulate. The first such requirements 
were developed in the 1940's; the most recent went into full effect 
only in November, 1999. It is important to understand these 
requirements because they form the base upon which this new rule is 
overlaid.
    Early developments. In 1944, part 31 established procedures for 
limiting the gaseous emissions from diesel powered equipment and 
establishing the recommended dilution air quantity for mine locomotives 
that use diesel fuel. In 1949, part 32 established procedures for 
testing of mobile diesel-powered equipment for non-coal mines. In 1961, 
part 36 was added to provide requirements for the use of diesel 
equipment in gassy noncoal mines, in which engines must be temperature 
controlled to prevent explosive hazards. These rules were drafted in 
response to research conducted by the former Bureau of Mines.
    Continued research by the former Bureau of Mines in the 1950s and 
1960s led to refinements of its ventilation recommendations, 
particularly when multiple engines are in use. An airflow of 100 to 250 
cfm/bhp for engines that have a properly adjusted fuel to air ratio was 
recommended (Holtz, 1960). An additive ventilation requirement was 
recommended for operation of multiple diesel units, which could be 
relaxed based on the mine operating procedures. This approach was 
subsequently refined to become a 100-75-50 percent guideline (MSHA 
Policy Memorandum 81-19MM, 1981). Under this guideline, when multiple 
pieces of diesel equipment are operated, the required airflow on a 
split of air would be the sum of: (a) 100 percent of the approval plate 
quantity for the vehicle with the highest approval plate air quantity 
requirement; (b) 75 percent of the approval plate air quantity 
requirement of the vehicle with the next highest approval plate air 
quantity requirement; and (c) 50 percent of the approval plate airflow 
for each additional piece of diesel equipment.
    Limitations on Diesel Gasses. MSHA has limits on some of the gasses 
produced in diesel exhaust. These are listed in Table II-5, 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).

BILLING CODE 4510-43-P


[[Page 5554]]


[GRAPHIC] [TIFF OMITTED] TR19JA01.009


BILLING CODE 4510-43-C
    To change an MSHA exposure limit, regulatory action is required 
because 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 air 
quality rule (concerning control of drill dust and blasting) was 
promulgated. As a result of a recent legal action, MSHA's efforts to 
revise the specific limits for those gases emitted by diesel engines 
have been placed under the continued supervision of a federal court of 
appeals. This action is discussed in more detail in section 9 of this 
Part.
    Diesel Equipment Rule for 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'' (61 FR 55412). The history of 
this ``diesel equipment rule'' (sometimes referred to here as the 
``diesel safety rule'' to help distinguish it from this rulemaking 
which is oriented toward health) is set forth as part of the history of 
this rulemaking (see 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.
    Certain requirements were included in the diesel equipment rule 
that are directly related to reducing diesel emissions. For example, 
the diesel equipment rule requires that the emissions of permissible 
and heavy duty equipment be tested weekly. The tests are conducted 
using instrumentation and the tests are conducted with the engines 
operated at a loaded condition which is representative of actual 
operation. The results are monitored and recorded. Higher than normal 
emissions readings indicate that the engines and equipment are not 
being maintained in approved condition. Although some of these 
requirements help reduce dpm emissions, they were not included in the 
rule for that specific purpose.
    Lower-emission engines. The diesel equipment rule requires that 
virtually all diesel-powered engines used in underground coal mines be 
approved by MSHA; see 30 CFR part 7, (approval requirements), part 36 
(permissible machines defined), and part 75 (use of such equipment in 
underground coal mines). The approval requirements, among other things, 
require clean-burning engines in diesel-powered equipment (61 FR 
55417). In promulgating the final rule, MSHA recognized that clean-
burning engines are ``critically important'' to reducing toxic gasses 
to levels that can be controlled through ventilation. To achieve the 
objective of clean-burning engines, the rule sets performance standards 
which must be met by virtually all diesel-powered equipment in 
underground coal mines.

[[Page 5555]]

    As noted in section 5 of this part, the technical requirements for 
approved diesel engines focus on limiting the amount of various gases 
that an engine can emit, including undiluted exhaust limits for carbon 
monoxide and oxides of nitrogen (61 FR 55419). The limits for these 
gasses are derived from existing 30 CFR part 36.
    The diesel equipment rule also provides that the particulate matter 
emitted by approved engines be determined during the testing required 
to gain approval. The particulate index (or PI), calculated under the 
provisions of 30 CFR 7.89, indicates what air quantity is necessary to 
dilute the diesel particulate in the engine exhaust to 1 milligram of 
diesel particulate matter per cubic meter of air. The purpose of the PI 
requirement is discussed in more detail in section 5 of this part.
    Gas Monitoring. The diesel equipment rule also addresses the 
monitoring and control of gaseous diesel exhaust emissions (30 CFR part 
70; 61 FR 55413). In this regard, the rule requires that mine operators 
take samples of carbon monoxide and nitrogen dioxide as part of 
existing onshift workplace examinations (61 FR 55413, 55430-55431). 
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 (30 CFR part 70, 61 FR 55413).
    Engine Maintenance. The diesel equipment rule requires that diesel-
powered equipment be maintained in safe and approved condition (30 CFR 
75.1914; 61 FR 55414). 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 (61 FR 55413-55414).
    The rule also requires the weekly examination of diesel-powered 
equipment (30 CFR 75.1914(g)). To determine if more extensive 
maintenance is required, the rule further requires a weekly check of 
the gaseous CO emission levels on permissible and heavy duty outby 
machines. 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, operators are required to establish programs to ensure 
that those performing maintenance on diesel equipment are qualified (61 
FR 55414).
    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 (30 CFR 75.1910(a); 61 FR 55413). 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 (30 CFR 75.1916(d).
    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 generally 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 stated 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. The diesel equipment 
rule is helping the mining community use diesel-powered equipment more 
safely in underground coal mines. Moreover, the diesel equipment rule 
has many features which reduce the emission and concentration of 
harmful diesel emissions in underground coal mines--including the 
particulate component of these emissions.
    During the public hearings on the equipment rule, miners complained 
about the high concentrations of diesel emissions at the section 
loading point and in the areas of the mine where longwall equipment is 
being installed or removed. Accordingly, MSHA established, in that 
rule, provisions which would address miners' concerns.
    The equipment rule required that the approval plate ventilation 
quantity be provided at the section loading point. The loading point is 
also identified as a location where regular air quality samples are 
required to be taken. Corrective action is required if the samples of 
CO and NO2 exceeded more than one half the allowable 
concentration limit of these gases.
    Longwall equipment installations and removals are handled in a 
similar manner. The diesel emissions from all of the equipment in the 
area of the mine where the longwall move is being made are required to 
be considered in establishing the amount of ventilation air to be 
provided. A specific location where that quantity is to be measured is 
established. Additionally, the same air quality sampling program 
required for section loading points is required for areas of the mine 
where the longwall move is to take place.
    Permissible haulage vehicles contribute the largest quantities of 
emissions at the section loading point. Longwall moves are typically 
carried out by permissible and heavy duty equipment such as shield 
carriers, mules, and locomotives which produce large quantities of 
diesel emissions. Emissions from these vehicles are reduced by the use 
of approved engines, low sulfur fuel, the loaded repeatable engine 
condition testing, regular maintenance by trained personnel and the 
ventilation and sampling provisions of the diesel equipment rule.
    Because the effective dates for provisions of the diesel equipment 
regulations are staggered, the full impact of the new rules was not 
known at the time the dpm hearings were held. MSHA expects that the 
concentrations of diesel emissions at the section loading point and 
during longwall moves will be reduced as these provisions are fully 
implemented.
    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

[[Page 5556]]

at the health risks of dpm exposure. (61 FR 55420).

(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 but no request was ever 
approved.
    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 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 the difficulty in efforts to get a small 
outby unit approved under the current Pennsylvania law. Accordingly, 
the industry has indicated that it would seek additional changes in the 
Pennsylvania diesel law. Commenters representing miners indicated that 
they were also 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 appropriate parts in 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/m3 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

[[Page 5557]]

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 safety 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 such advisory 
committees as it deems appropriate, the agency appointed an advisory 
committee ``to provide advice on the complex issues concerning the use 
of diesel-powered 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 had 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

[[Page 5558]]

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, among other things, addressed, and in 
fact followed, the MSHA Diesel Advisory Committee's recommendation that 
MSHA promulgate regulations requiring the approval of diesel engines 
(54 FR 40951), limiting gaseous pollutants from diesel equipment, 
(Id.), establishing ventilation requirements based on approval plate 
dilution air quantities (54 FR 40990), requiring equipment maintenance 
(54 FR 40958), requiring that trained personnel work on diesel-powered 
equipment, (54 FR 40995), establishing fuel requirements, (Id.), 
establishing gaseous contaminant monitoring (54 FR 40989), and 
requiring that a particulate index indicating the quantity of air 
needed to dilute particulate emissions from diesel engines be 
established. (54 FR 40953).
    On January 6, 1992, MSHA published an Advance Notice of Proposed 
Rulemaking (ANPRM) indicating it was in the early stages of developing 
a rule specifically addressing miners exposure to diesel particulate 
(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 (57 FR 501). 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 (57 FR 501). 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 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

[[Page 5559]]

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.
    Proposed Rulemaking on Dpm. 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).
    MSHA went to some lengths to ensure the mining community would be 
able to review and comment on the proposed rule. The agency made copies 
of the proposal available for review by the mining community at each 
district and field office location, at the National Mine Safety and 
Health Academy, and at each technical support center. MSHA also 
provided the opportunity for comments to be accepted from the mining 
community at each of those locations, as well as through mail,
e-mail and fax to the national office. MSHA also distributed the 
proposal to all underground mines, to mining associations and other 
interested parties. A copy was also posted on MSHA's website.
    In order to further facilitate participation by the mining 
community, MSHA developed as an introduction to its preamble explaining 
the proposed rule a ``plain language'' questions and answers section.
    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 17512-17514). 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 17580 et seq.). 
A complete description of the Estimator, and several examples, were 
also presented in the preamble of the proposed dpm rule (63 FR 17565 et 
seq.).
    The proposed dpm rule was fairly simple. In addition to miner 
training, the proposed rule would have required aftertreatment filters 
on all permissible equipment and, subsequently, on all heavy duty 
nonpermissible equipment.
Throughout the preamble, MSHA discussed a number of other approaches 
that might have merit in limiting the concentration of dpm in 
underground coal mines. MSHA made it very clear to the mining community 
that the rule being proposed represented only one of the approaches 
which might ultimately be required by the final rule and on which 
comment was being solicited by the proposed rulemaking notice.
    For example, the agency noted the following:

    ``MSHA recognizes that a specification standard does not allow 
for the use of future alternative technologies that might provide 
the same or enhanced protection at the same or lower cost. MSHA 
welcomes comment as to whether and how the proposed rule can be 
modified to enhance its flexibility in this regard * * *. (There 
are) two alternative specification standards which would provide 
somewhat more flexibility for coal mine operators. Alternative 1 
would treat the filter and engine as a package that has to meet a 
particular emission standard. Instead of requiring that all engines 
be equipped with a high-efficiency filter, this approach would 
provide some credit for the use of lower-polluting engines. 
Alternative 2 would also provide credit for mine ventilation beyond 
that required.'' (63 FR 17498)

These alternatives were further discussed in a separate Question and 
Answer (#12). The agency was also clear it would welcome comment on 
``whether there are some types of light-duty equipment whose dpm 
emissions should, and could feasibly, be controlled'', and ``whether it 
would be feasible for this sector to implement a requirement that any 
new light-duty equipment added to a mine's fleet be filtered'' Question 
and Answer (#6) (63 FR 17556).
    MSHA also discussed and welcomed comment on a number of other 
alternatives: e.g., restricting the exposure of underground coal mines 
to all fine particulates regardless of source (63 FR 17495); and the 
use of administrative controls (e.g., rotation of personnel) and 
personal protective equipment (e.g., respirators) to reduce the dpm 
exposure of miners. The Agency also sought comments on its risk 
assessment, presented in full in the preamble to the proposed rule 
(Part III). As noted therein, this was the first risk assessment ever 
performed by the agency to be peer reviewed. Such a review is not 
required under the agency's statute, but MSHA took the time to obtain 
such a review in this instance due to significant disagreement within 
the mining community about the health risks of exposure to dpm (63 FR 
17521).
    MSHA also asked for comment on its economic assumptions in the 
preamble. Two of the Questions and Answers (#5 and #7) were 
specifically devoted to cost impacts, including those on small mines. 
MSHA also specifically requested all members of the mining community to 
consider using the Estimator in developing comments on the proposed 
rulemaking (63 FR 17565).
    On July 14, 1998, in accordance with the National Environmental 
Protection Act, MSHA published a notice in the Federal Register seeking 
comment on its preliminary determination that the proposed rule would 
not have a significant environmental impact (63 FR 37796).
    The initial comment period was scheduled to last for 120 days until 
August 7, 1998. In response to requests from the public, on August 5, 
1998, MSHA extended the initial comment period on the proposed rule 
(and the comment period on its preliminary determination of no 
significant environmental impact) for an additional 60 days, until 
October 9, 1998 (63 FR 41755). That notice also announced MSHA's intent 
to hold public hearings on the proposal.
    On October 19, 1998, MSHA announced in the Federal Register 
locations of four public hearings on the proposed rule. The agency 
further announced that the close of the post-hearing comment period and 
rulemaking record would be on February 16, 1999 (63 FR 55811).
    In November 1998, MSHA held hearings in Salt Lake City, Utah and 
Beckley, West Virginia. In December 1998, hearings were held in Mt. 
Vernon, Illinois, and Birmingham, Alabama.
    These hearings were well attended. Testimony was presented by 
individual miners, representatives of miners, individual coal 
companies, 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 engage in direct dialogue

[[Page 5560]]

with members of MSHA's rulemaking committee-responding to questions and 
asking questions on 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 but not limited to those on which MSHA 
specifically sought comment), and the technological and economic 
feasibility of various alternatives.
    During the hearings, MSHA made a number of requests that 
information provided at the hearing be supplemented by submission of 
cited sources, additional data, and in particular for data to support 
assertions made about various control technologies. MSHA again 
solicited information concerning the agency's cost assumptions, for the 
results of studies using MSHA's Estimator model, and also asked for any 
data on a number of other points. For example, the agency requested 
further information on the size distribution of particles from cleaner 
engines, on the viability of a fine particulate standard in lieu of a 
dpm standard, for a list of any studies concerning the risks of dpm or 
lack thereof, and data on equipment upgrades.
    On February 12, 1999, (64 FR 7144) MSHA published a notice in the 
Federal Register announcing: (1) The availability of three additional 
studies discussed in the preamble of the proposed rule but not 
available at the time of publication; and (2) the extension of the 
post-hearing comment period and close of record for 60 additional days, 
until April 30, 1999.
    On April 27, 1999, in response to requests from the public, MSHA 
extended the post-hearing comment period and close of record for 90 
additional days, until July 26, 1999 (64 FR 22592).
    On July 8, 1999, 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 by July 26, 1999, the close of 
the rulemaking record (64 FR 36826). The Estimator model was 
subsequently published in the literature.
    The rulemaking record closed on July 26, 1999, fifteen months after 
the date the proposed rule was published for public notice. The 
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 MSHA's paper filter verification studies and the recent 
information from VERT on the performance of hot gas filters mentioned 
in section 6 of this Part. In addition, the notice provided an 
opportunity for comment on additional documents being placed in the 
rulemaking record for a related rulemaking for underground metal and 
nonmetal mines, 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.
    Other Related Activity. On September 3, 1999, the United States 
Court of Appeals for the District of Columbia Circuit issued its 
decision on writ of mandamus sought by the United Mine Workers to 
compel MSHA to issue final regulations controlling gaseous emissions in 
the exhaust of diesel engines used in underground coal mines. (190 F.3d 
545.) The UMWA argued that such action should have been completed some 
years before as part of MSHA's air quality rulemaking to update 
emissions limits on hundreds of exposure limits. The Court found that 
the Agency was in violation of the statute's requirement that the 
Secretary must either promulgate final regulations, or explain her 
decision not to promulgate them, within ninety days of the 
certification of the record of a hearing if one is held or the close of 
the public comment period if a hearing is not held 30 U.S.C. 811(a)(4). 
However, the Court declined to immediately issue the mandamus order 
sought in this case because, among other factors: (a) The UMWA agreed 
that the diesel equipment rules alone may have the desired effect of 
reducing exposure to these gases; (b) the UMWA further agreed that the 
control of diesel particulate matter and respirable mine dust rank as 
higher rulemaking priorities for MSHA; and (c) MSHA submitted a 
tentative schedule for such rulemaking that the court found to be 
reasonable. However, the court retained jurisdiction of the case to 
ensure MSHA would move forward on this matter, and ordered several 
reports by the agency on its progress on December 31, 1999, June 30, 
2000, December 31, 2000, and December 31, 2001.

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

[[Page 5561]]

    (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
    (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.\3\ 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.
---------------------------------------------------------------------------

    \3\ 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/m 3 (respirable combustible dust), 
with maximum measurements ranging from 1020 to 3100 g/m 
3 (Gangel and Dainty, 1993). Among 622 full shift 
measurements collected since 1989 in German underground noncoal 
mines, 91 (15%) exceeded 400 g/m 3 (total 
carbon) (Dahmann et al., 1996). As explained elsewhere in this 
preamble, 400 g/m 3 (total carbon) corresponds 
to approximately 500 g/m 3 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

[[Page 5562]]

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 coal and surface mines.\4\ 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.
---------------------------------------------------------------------------

    \4\ 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.\5\ 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.
---------------------------------------------------------------------------

    \5\ 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 error of
                        Mine type                           Number of mines   Number of samples    Mean exposure    mean  (g/   Exposure range
                                                                                                 (g/m\3\)        m\3\)        (g/m\3\)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Surface..................................................                 11                 45                 88                 11              9-380
Underground Coala........................................                 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

[[Page 5563]]

normally consisted of collecting samples on the continuous miner 
operator and coal haulage vehicle 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.

BILLING CODE 4510-43-P
[GRAPHIC] [TIFF OMITTED] TR19JA01.010


BILLING CODE 4510-43-C
    As stated in the proposed risk assessment, no statistically 
significant difference was observed in mean dpm concentration between 
the personal and area samples.\6\ 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.
---------------------------------------------------------------------------

    \6\ 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.\7\ At five of the twelve mines, 
all dpm measurements were 300 g/m3 or greater in 
the absence of after-treatment filters.
---------------------------------------------------------------------------

    \7\ 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

[[Page 5564]]

employment of WMFs, the mean 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/m3 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.\8\ 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.
---------------------------------------------------------------------------

    \8\ 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.\9\ 
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.
---------------------------------------------------------------------------

    \9\ 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/m3 to more than 3500 g/m3. 
Exposure measurements based on area samples ranged from less than 100 
g/m3 to more than 3000 g/m3. 
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.\10\ 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.
---------------------------------------------------------------------------

    \10\ 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.\11\ 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.
---------------------------------------------------------------------------

    \11\ 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,\12\ 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.'' \13\ 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.''
---------------------------------------------------------------------------

    \12\ 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.
    \13\ 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 conditions at underground M/NM mines. MSHA believes that 
results at these mines, as depicted in Figure III-2, in fact fairly 
reflect the variety 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
[GRAPHIC] [TIFF OMITTED] TR19JA01.012

MSHA considers this degree of uncertainty to be acceptable, given that 
the overall mean concentration observed exceeded 800 g/m\3\.
---------------------------------------------------------------------------

    \14\ 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 727 
g/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%.''

[[Page 5567]]

    MSHA considers the size-selective, gravimetric method capable of 
providing reasonably accurate measurements when the dpm concentration 
is greater than 200 g/m3, 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/
m3. 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/m3) 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
                           Commodity                                of
                                                                  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/m3 at all mines sampled. The 
maximum dpm concentration observed was less than or equal to 200 
g/m3 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/
m3.

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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,\15\ 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/m3, as measured by submicrometer elemental 
carbon (EC) (NIOSH, 1990). Reported geometric mean concentrations of 
submicrometer EC ranged from 2.0 to 7.0 g/m3 for 
truck drivers and from 4.8 to 28 g/m3 for truck 
mechanics, depending on weather conditions (Zaebst et al., 1991).
---------------------------------------------------------------------------

    \15\ 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 5570]]

    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).\16\ 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.
---------------------------------------------------------------------------

    \16\ 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/m\3\, 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/m\3\, 6.8 to 24 g/m\3\, 18 to 102 g/m\3\ 
and 49 to 191 g/m\3\. The range of median dpm concentrations 
observed at different underground coal mines is 55 to 2100 g/
m\3\, with filters employed at mines showing the lower 
concentrations.\17\ For underground M/NM mines, the corresponding range 
is 68 to 1835 g/m\3\, and for surface mines it is 19 to 160 
g/m\3\. 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.
---------------------------------------------------------------------------

    \17\ 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/m\3\, 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/m\3\, 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,\18\ 
and up to 10 times as high as median exposures estimated for the most 
heavily exposed workers in other occupational groups.
---------------------------------------------------------------------------

    \18\ 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.\19\
---------------------------------------------------------------------------

    \19\ 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/m\3\ 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 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 5573]]

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 researches use epidemiologic and animal studies ``* * * to help 
understand different aspects of the carcinogenic process.'' \20\ 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.
---------------------------------------------------------------------------

    \20\ This risk assessment is not limited to cancer effects, but 
the commenter's point can be generalized.
---------------------------------------------------------------------------

    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 a legitimate indicators of the carcinogenicity of Dpm in 
humans.'' The Nevada Mining Association, endorsing Dr. Valberg's 
comments, added:

    * * * 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

[[Page 5574]]

findings and to identify and explore potential mechanisms of toxicity.
    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

[[Page 5575]]

m in diameter) are more strongly associated than ``coarse'' 
respirable particulates (i.e., particles greater than 2.5 m 
but less 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 the 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

[[Page 5576]]

developed due to ``idiosyncracy of his lungs that respond to any type 
of a respiratory irritant.'' The manager suggested that this incident 
should not be generalized to other situations but provided no evidence 
that the miner's lungs were unusually susceptible to irritation.\21\
---------------------------------------------------------------------------

    \21\ 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.\22\
    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.
---------------------------------------------------------------------------

    \22\ 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.
---------------------------------------------------------------------------

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.''\23\ Furthermore, 
despite its reservations about anecdotal evidence:
---------------------------------------------------------------------------

    \23\ 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. 
MSHA completed an analysis of the impact of the 1996 diesel regulations 
for underground coal mines (See Part II, Section 7). We do expect that 
the concentrations of diesel emissions at the section loading point and 
during longwall moves will be reduced as these provisions are fully 
implemented. These dpm levels, though reduced, are still above the 
exposures expected to cause sensory irritations and respiratory 
symptoms (See Section 3(d)(5)). 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 when developing the 1996 
underground coal diesel regulations. It was understood that the agency 
would be taking a separate look at the health risks of dpm exposure. In 
addition, the NMA did not 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 exposures 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

[[Page 5577]]

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 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 
/m\3\ to 1000 /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

[[Page 5578]]

300 g dpm into their nostrils.\24\ 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.\25\ 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 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.
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    \24\ Assuming that a working miner inhales approximately 1.25 
m\3\ of air per hour, this dose corresponds to a 1-hour exposure at 
a dpm concentration of 240 g/m\3\.
    \25\ 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.
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    Salvi et al. (1999) exposed healthy human volunteers to diluted 
diesel exhaust at a dpm concentration of 300 g/m\3\ 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/m3. The EPA also concluded that relatively 
small, but statistically significant increases in mortality risk exist 
at particulate (but not SO2) levels below 500 g/
m3, 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 g/
m3, 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/m3 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 of mortality in the general population increases by about 2.6 to 
5.5 percent per 25 g/m3 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/
m3 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/m3 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 29  10 ml decrease in 1-second 
Forced Expiratory Volume (FEV1) per 50 g/
m3 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/m3, Dusseldorp et al. (1995) found 45 and 77 ml/
sec decreases, respectively, for evening and morning Peak Expiratory 
Flow Rate (PEFR) per 50 g/m3 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

[[Page 5582]]

no correlation with their estimated level of exposure.
    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 are not conclusive but 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.\26\ 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

[[Page 5583]]

their own reviews of many of these studies. In arriving at its 
conclusions, MSHA considered all of these reviews, including those of 
the commenters, as well as the 47 source studies available to MSHA.
---------------------------------------------------------------------------

    \26\ 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 64 FR 7144. Saverin et al. (1999) is the published 
English version of a Germany 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'' \27\ of the epidemiologic literature: Lipsett and Campleman 
(1999) thru \28\ and Bhatia et al. (1998).\29\ 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.
---------------------------------------------------------------------------

    \27\ 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.
    \28\ 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.
    \29\ 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.\30\ 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.
---------------------------------------------------------------------------

    \30\ 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.\31\
---------------------------------------------------------------------------

    \31\ 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 
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(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 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 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.\32\ 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.
---------------------------------------------------------------------------

    \32\ 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.\33\ 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.
---------------------------------------------------------------------------

    \33\ 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

[[Page 5593]]

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.'' \34\ 
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 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.
---------------------------------------------------------------------------

    \34\ 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. \35\
---------------------------------------------------------------------------

    \35\ 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.\36\ Both of these recent cohort studies 
took smoking habits into account. These

[[Page 5594]]

studies both reported an excess risk of lung cancer associated with dpm 
exposure.
---------------------------------------------------------------------------

    \36\ 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.\37\ 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.
---------------------------------------------------------------------------

    \37\ 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.\38\ With one exception (Benhamou et al. 1988), these studies 
also presented evidence of increased age-adjusted risk for workers with 
longer exposures and/or latency periods.
---------------------------------------------------------------------------

    \38\ 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 \39\ and (2) it must have allowed for 
sufficient exposure, latency, and follow-up time to have detected an 
existing relationship.
---------------------------------------------------------------------------

    \39\ 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.\40\
---------------------------------------------------------------------------

    \40\ 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

[[Page 5595]]

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 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.\41\ 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,

    * * * 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,

[[Page 5596]]

demonstrate adequate control for smoking without applying a smoking 
adjustment.
---------------------------------------------------------------------------

    \41\ 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.
---------------------------------------------------------------------------

    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.\42\ 
In practice, it is not usually possible to obtain detailed information, 
and the effects of smoking and other known confounders cannot be 
precisely quantified.
---------------------------------------------------------------------------

    \42\ 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.
---------------------------------------------------------------------------

    Stoaber 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 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.\43\ 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.
---------------------------------------------------------------------------

    \43\ 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

[[Page 5597]]

``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:

------------------------------------------------------------------------
                                                          Estimate of RR
                                                           of death from
               Study on cigarette smoking                 cardiovascular
                                                              disease
------------------------------------------------------------------------
British doctors.........................................           1.6
Males in 25 states:                                       ..............
    Ages 45-64..........................................           2.08
    Ages 65-79..........................................           1.36
U.S. Veterans...........................................           1.74
Japanese study..........................................           1.96
Canadian veterans.......................................           1.6
Males in nine states....................................           1.70
Swedish males...........................................           1.7
Swedish females.........................................           1.3
California occupations..................................           2.0
------------------------------------------------------------------------
Source: U.S. Department of Health and Human Services (1989).

    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.\44\ 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,\45\ 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.\46\ 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.
---------------------------------------------------------------------------

    \44\ 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.
    \45\ 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).
    \46\ 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

[[Page 5598]]

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 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) \47\ 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.''
---------------------------------------------------------------------------

    \47\ 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

[[Page 5599]]

identified with diesel exhaust exposure, including mining. After 
adjusting for smoking patterns,\48\ 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.
---------------------------------------------------------------------------

    \48\ 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.
    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.\49\ 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.
---------------------------------------------------------------------------

    \49\ 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)

[[Page 5600]]

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 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

[[Page 5601]]

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, 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.\50\
---------------------------------------------------------------------------

    \50\ 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\.\51\ Estimates of RR based on 
the mine-unadjusted model would substitute 1.227 for 1.156 in these 
calculations.
---------------------------------------------------------------------------

    \51\ 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).\52\ 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.
---------------------------------------------------------------------------

    \52\ 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

[[Page 5602]]

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/m\3\ for production, 230 g/m\3\ 
for maintenance, and 120 g/m\3\ 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/m\3\.
    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/m\3\ 
of occupational TC exposure, the relative risk of lung cancer was 
estimated to increase by the following multiplicative factor: \53\
---------------------------------------------------------------------------

    \53\ 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/m \3\ refers to the average TC concentration 
experienced over a year's worth of 8-hour shifts.

------------------------------------------------------------------------
                                                       RR per  mg-yr/m3
                                                     -------------------
                       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.\54\ 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/m3. The resulting RR values were reported as 
follows:
---------------------------------------------------------------------------

    \54\ 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.

------------------------------------------------------------------------
                                                      RR for 4.9  mg-yr/
                                                              m3
                       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 
this study at the May 27, 1999, public

[[Page 5603]]

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:

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    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:

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[[Page 5606]]

    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.

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[[Page 5608]]

    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 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.'' \55\ (EPA, 1999, p. 7-13) No 
objection to this conclusion was raised in the most recent CASAC review 
of the EPA draft (CASAC, 2000).
---------------------------------------------------------------------------

    \55\ 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

[[Page 5609]]

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).

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    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 pack-years), 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.

[[Page 5610]]

    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.

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[[Page 5612]]

    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 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.''

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    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 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

[[Page 5614]]

of the type employed in this study, random errors in a multivariate 
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 
date.

    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 tested 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, 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

[[Page 5615]]

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).\56\ 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.''

    \56\ 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.
---------------------------------------------------------------------------

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    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:

[[Page 5616]]

* * * 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 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

[[Page 5617]]

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 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/m3 (i.e., 1 g-hr/m3) were calculated by MSHA 
based on the regression coefficients reported by the authors. The 
conversion from mg-hr/m3 to mg-yr/m3 assumes 
1,920 occupational exposure hours per year. Although 6.1 mg-yr/
m3 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).

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    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

[[Page 5618]]

published report (ibid., p.420). The approximate equivalency between 
4.9 mg-yr/m3 TC and 6.1 mg-yr/m3 dpm assumes 
that, on average, TC comprises 80 percent of dpm.

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    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 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

[[Page 5619]]

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/m\3\, with a median value of 373 g-yr/m\3\. The 
estimates of relative risk (expressed as odds ratios) presented for EC 
exposures of 373 g-yr/m\3\, 1000 g-yr/m\3\, and 2450 
-yr/m\3\ 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 5620]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.034


BILLING CODE 4510-43-C

[[Page 5621]]

    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 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.\57\
---------------------------------------------------------------------------

    \57\ 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 lung 
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

[[Page 5622]]

(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 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

[[Page 5623]]

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 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

[[Page 5624]]

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 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.
(v) 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

[[Page 5625]]

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).
    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).\58\ 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).\59\ 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.
---------------------------------------------------------------------------

    \58\ 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.
    \59\ 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 PM 
2.5 (which includes dpm) and age-adjusted total 
mortality.\60\ 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.\61\ 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.
---------------------------------------------------------------------------

    \60\ 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.
    \61\ 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 PM 2.5 
exposure and morbidity in adults show effects that are difficult to 
separate from measures of PM 10 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

[[Page 5626]]

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 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,

[[Page 5627]]

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 
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 
mghr/m\3\ 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).

BILLING CODE 4510-43-P

[[Page 5628]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.035


BILLING CODE 4510-43-C

[[Page 5629]]

    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 5630]]

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/m3 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. (1996) 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 5631]]

exhaust.\62\ 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.
---------------------------------------------------------------------------

    \62\ 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.\63\ 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.
---------------------------------------------------------------------------

    \63\ The only details provided for this calculation pertained to 
ajusting 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.
---------------------------------------------------------------------------

    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 5632]]

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/m3 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.\64\
---------------------------------------------------------------------------

    \64\ NIOSH commented as follows: ``Data cited by MSHA in support 
of this statement are not comparable. Rats were exposed to dpm at 4 
mg/m3 for 2 years (Mauderly et al. 1987; Brightwell et 
al. 1989), in contrast to rats exposed to Ti02 at 250 mg/
m3 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 Ti02 
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 Ti02 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.,

[[Page 5633]]

1996) did not pertain to questions of 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 Ti02 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., 1996). 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/m3. 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/m3 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

[[Page 5634]]

inducing lung cancer by a mechanism 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/
m3 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/
m3), 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/m3 continuous lifetime 
exposure (approximately 2,500 g/m3 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 5635]]


    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/m3 would have 
different implications for lung clearance than 24 hours at 200 
g/m3.
    (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.'' \65\ MARG continued as follows:
---------------------------------------------------------------------------

    \65\ 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).

    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 5636]]


    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.\66\ 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.
---------------------------------------------------------------------------

    \66\ 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

[[Page 5637]]

relationship between PAHs and other products of fossil fuel combustion 
found 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/m\3\ 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

[[Page 5638]]

the studies had limitations when viewed in isolation. MSHA nevertheless 
concluded (in the proposal) that the best available epidemiologic 
studies, supported by experimental data 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

[[Page 5639]]

means that the collective results, showing increased risk for exposed 
workers, are statistically significant at a very high confidence 
level--regardless 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.\67\ 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.
---------------------------------------------------------------------------

    \67\ With respect to 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 Systematic 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-9 and III-
10, 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 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. 
(1990,1992,1998), 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 5643]]

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).\68\ 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.
---------------------------------------------------------------------------

    \68\ 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 5644]]

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),\69\ 
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/m3 is the multiplicative product of 
exposure duration and dpm concentration for the most highly exposed 
workers in each of these two studies.

    \69\ Listed in Table III-5 under Swanson et al., 1993.
---------------------------------------------------------------------------

BILLING CODE 4510-43-P

[GRAPHIC] [TIFF OMITTED] TR19JA01.038

    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

[[Page 5645]]

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, 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

[[Page 5646]]

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.\70\ One commenter observed that--
---------------------------------------------------------------------------

    \70\ 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,\71\ 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)
---------------------------------------------------------------------------

    \71\ 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.'' \72\
---------------------------------------------------------------------------

    \72\ 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 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,

[[Page 5647]]

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-analyzes 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, metaanalyses, 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 
(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

[[Page 5648]]

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,\73\ there is no reason to expect that such effects will 
consistently bias results in the same direction, across all occupations 
and geographic regions.
---------------------------------------------------------------------------

    \73\ 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 
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.

[[Page 5649]]

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.\74\ 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.
---------------------------------------------------------------------------

    \74\ 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/
m3. 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 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

[[Page 5650]]

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/m\3\, then we can assign the 0.49 excess risk 
(Bhatia's meta-analysis result) to the 5-50 g/m\3\ 
exposure. Hence, dpm concentrations for miners in the range of 100-
2,000 g/m\3\ 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 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 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

[[Page 5651]]

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. However, the truck 
drivers 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. Truck drivers commonly congregate 
in parking areas and sleep 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/m\3\ 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/m\3\ and 644 g/m\3\ 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/m\3\--corresponding to a mean dpm concentration of about 490 
g/m\3\. 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/m\3\ to 370 
g/m\3\. 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.\75\ 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.
---------------------------------------------------------------------------

    \75\ 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 
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.
    (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

[[Page 5652]]

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]


[[Page 5653]]


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]''
    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

[[Page 5654]]

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 
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%

[[Page 5655]]

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.
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-11 presents a similar comparison, based on the 
highest mean dpm level observed at

[[Page 5656]]

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.\76\ As shown in Figure III-11, 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.
---------------------------------------------------------------------------

    \76\ 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.

---------------------------------------------------------------------------
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BILLING CODE 4510-43-C

    Given the significant increases in mortality and other acute health 
effects associated with increments of 25 g/m\3\ 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.
    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 coal 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

[[Page 5657]]

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 
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. In 1996 MSHA published 
the diesel equipment safety rule that focused primarily on 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. In developing this 
diesel equipment safety rule for underground coal mines, however, MSHA 
did not explicitly consider the health 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. It was 
understood that the agency would be evaluating the health risks of dpm 
exposure at a later date. (61 FR 55420). With the implementation of the 
diesel safety rule in underground coal mines, MSHA believes that dpm 
concentrations may have declined, in the past two to three years. 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/
m\3\--less than half of the average level that MSHA observed in its 
field studies. However, MSHA also believes that a reduction in exposure 
of more than 50 percent is highly implausible, even with the safety 
standard implemented. It is also important to note that the diesel 
equipment rule applied only to underground coal mines and not 
underground metal/nonmetal mines.

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BILLING CODE 4510-43-C

    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.

[[Page 5658]]

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/m3 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/
m3. For example, for a PM2.5 concentration of 40 
g/m3, 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/m3 over an 8-hour workshift are at least as great 
as those at a concentration of 20 g/m3 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/m3 
would correspond to a short-term occupational exposure to dpm at a 
concentration of 60 g/m3. Consequently, the RR at 
an occupational exposure level of Y g/m3 would 
equal the RR calculated for an ambient exposure level of 20  x  (Y/60) 
g/m3. For example, the relative risk (RR) of acute 
lower respiratory symptoms at an occupational exposure level of 300 
g/m3 dpm would, at a minimum, correspond to the RR 
at an ambient exposure level equal to 5  x  20 g/m3 
PM2.5. (See Table III-3) A dpm concentration of 300 
g/m3 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/m3 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/
m3 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/m3 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 
``coarse'' respirable 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

[[Page 5659]]

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/m3 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. A lower bound on the increased risk expected at 
an occupational dpm concentration greater than 30 g/
m3, 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/m3. For a 
concentration of 300 g/m3, 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 conducted on 
populations whose average exposure is estimated to be less than 200 
g/m3--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/m3 EC at an individual dock (NIOSH, 
1990). As explained in Subsection 1.d of this risk assessment, this 
corresponds to

[[Page 5660]]

less than 150 g/m3 of dpm, on average. Published 
dpm measurements for railworkers have generally also been less than 150 
g/m3 (measured as respirable particulate matter 
other than cigarette smoke). The reported mean of 224 g/
m3 for hostlers displayed in Figure III-11 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/m3--corresponding to a mean dpm concentration of 
about 490 g/m3. As shown in Table III-1, the mean 
dpm exposure level MSHA observed in underground production areas and 
haulageways was 644 g/m3 for coal mines and 808 
g/m3 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.\77\
---------------------------------------------------------------------------

    \77\ 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.
---------------------------------------------------------------------------

    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

[[Page 5661]]

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 
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).

[[Page 5662]]

    Assuming that, on average, EC comprises 40 percent of total 
dpm,\78\ the formula for calculating a relative risk (RR) using 
Steenland's simple cumulative exposure model is
---------------------------------------------------------------------------

    \78\ The assumption is that, on average, EC = TC/2 and TC = 
0.8 x dpm.
---------------------------------------------------------------------------

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.\79\
---------------------------------------------------------------------------

    \79\ 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.\80\ 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.
---------------------------------------------------------------------------

    \80\ 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 
Saverin 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.

Exposure-Response Relationships Obtained From Two Studies on Underground
                                 Miners
------------------------------------------------------------------------
                                                                Unit RR
                 Study and statistical model                   per mg-yr/
                                                                m\3\ dpm
------------------------------------------------------------------------
Saverin et al. (1999):
  Poisson, full cohort.......................................      1.024
  Cox, full cohort...........................................      1.089
  Poisson, subcohort.........................................      1.110
  Cox, subcohort.............................................      1.176
Johnston et al. (1997):
  15-year lag, mine-adjusted.................................      1.321
  15-year lag, mine-unadjusted...............................     1.479
------------------------------------------------------------------------
 Unit RR calculated from Tables III and IV, assuming TC = 0.8 x
  dpm.
 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/m\3\. Then each year of occupational 
exposure would contribute 0.5 mg-yr/m\3\ to the miner's cumulative dpm 
exposure. Suppose also that this miner's occupational exposure

[[Page 5663]]

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/m\3\ 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/m\3\ 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)\45x0.644\ = 
2.0 at a mean concentration of 644 g/m\3\ or RR = 
(1.024)\45x0.808\ = 2.4 at mean concentration of 808 g/m\3\. 
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/m\3\. The second-lowest estimate of relative risk, for 
example, is RR = (1.089)\45x0.644\ = 11.8, predicted by Saverin's full 
cohort Cox model.\81\
---------------------------------------------------------------------------

    \81\ 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/m\3\. 
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/m\3\, 500 g/m\3\, 644 g/m\3\ 
(the mean dpm concentration observed by MSHA at underground coal mines, 
as shown in Table III-1), and 808 g/m\3\ (the mean dpm 
concentration observed by MSHA at underground M/NM mines, as shown in 
Table III-1).

BILLING CODE 4510-43-P


[[Page 5664]]


[GRAPHIC] [TIFF OMITTED] TR19JA01.041


BILLING CODE 4510-43-C

    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/m\3\, the range of excess risks shown in Table III-7 
is nearly identical to the range (50 to 810 g/m\3\) 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/m\3\), 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/m\3\), 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.
    Even though the coal rule is an equipment based standard limiting 
emissions to 5.0 gm/hr and 2.5 gm/hr dpm output, MSHA estimates that 
these emissions limits will result in ambient dpm concentration in an 
underground coal mines of approximately 200 g/m\3\. MSHA 
believes this is a reasonable estimate to use in light of several 
sample calculations which indicate that using available controls in 
underground mining sections with dirty equipment can reduce emissions 
to that level or further. For example, in part IV of this preamble, 
MSHA discusses the comparison of the machine-based standard in this 
final rule with the State of Pennsylvania's diesel law. MSHA provides 
data showing that a permissible engine equipped with a 95% filter and 
using the approval plate air quantity will result in a calculated 
ambient concentration of dpm of 142 g/m\3\. In part V of this 
preamble, MSHA uses the ``Estimator''--a computerized spreadsheet 
designed to calculate dpm ambient levels from given engine emissions 
and mine ventilation rates and the impact of various controls on those 
ambient levels. Table V-3 of part V presents Estimator results using 
another permissible engine to show that the ambient levels would be 
approximately 200 g/m\3\ when applying various filters and 
using various intake dpm concentrations.
    An alternative approach to estimating exposures once the rule is 
implemented is to look at the factors affecting dpm production. Dpm 
exposure is related to the emissions from engines, ventilation,

[[Page 5665]]

and engine duty cycle. If emissions drop from 25 and 50 gm/hr (dpm 
concentration range emitted from current permissible engines) to 2.5 
and 5.0 gm/hr (as required under the rule), there would be a ten-fold 
reduction in exposure. With current ventilation required for the diesel 
equipment, the ambient concentrations would also be reduced 
accordingly. Thus, assuming that emissions will be reduced down to 200 
g/m\3\ is a conservative approach in estimating benefits.
    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. As explained in Part IV of the preamble, the 
rule is expected to limit dpm concentrations to which miners in 
underground coal mines are exposed to approximately 200 g/
m\3\. Assuming that, in the absence of this rule, underground coal 
miners would be occupationally exposed to dpm for 45 years at a mean 
level of 644 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 
644 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 644 g/m \3\ to 200g/m
                                   \3\
------------------------------------------------------------------------
                                                             Expected
                                                           reduction in
                                                            lung cancer
               Study and statistical model                  deaths per
                                                           1000 affected
                                                          miners
------------------------------------------------------------------------
Saverin et al. (1999):
  Poisson, full cohort..................................            46
  Cox, full cohort......................................           352
  Poisson, subcohort....................................           470
  Cox, subcohort........................................           579
Johnston et al. (1997):
  15-year lag, mine-adjusted............................           457
  15-year lag, mine-unadjusted..........................          298
------------------------------------------------------------------------
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 coal 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 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

[[Page 5666]]

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. Discussion of Final Rule

    This part of the preamble describes each of the provisions of the 
final rule. As appropriate, this part references discussions in other 
parts of this preamble: In particular, the background discussions and 
controls in part II, and the feasibility discussions in part V.
    Table IV-1 will be referenced throughout this discussion. The table 
provides information about each engine approved by MSHA for use in 
underground coal mines. This table reflects the emission results based 
on the MSHA approval data.
    The top rows of the table provide information about permissible 
configurations, designated by the MSHA approval numbers which contain 
an ``A''; the remainder of the table provides information about 
nonpermissible configurations, designated by the MSHA approval numbers 
which contain a ``B''. Within each engine grouping, the permissible 
engines are listed in order of MSHA approval number, and the 
nonpermissible engines are listed in increasing ``Rated Horsepower''.
    The table has ten columns. The first column gives the MSHA approval 
number. The second and third column lists the engine manufacturer and 
the engine model designation. The fourth column lists the rated 
horsepower of the engine as approved by MSHA. The fifth column gives 
the Particulate Index (PI) expressed in cubic feet per minute (cfm), 
the sixth column lists the DPM emissions expressed in gm/hr--weighted 
average over the 8 mode test cycle specified in 30 CFR 7.89, the 
seventh column weighted average horsepower, the eighth is the dpm 
expressed in grams per bhp-hr (calculated by dividing column six by 
column seven), the ninth column gives the filter efficiency needed to 
meet a 5.0 gm/hr standard, and the tenth column gives the filter 
efficiency needed to meet a 2.5 gm/hr standard.

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[[Page 5669]]

    The final rule would add six new sections to 30 CFR part 72 on 
March 20, 2001.

Section 72.500  Emission Limits for Permissible Diesel Powered 
Equipment.

    Organization. As with the proposed rule, this section establishes 
the controls applicable to permissible equipment. As proposed, 30 CFR 
72.500 also had included other requirements--controls for 
nonpermissible heavy-duty vehicles in 30 CFR 72.500(b) and requirements 
for the maintenance of such controls in 72.500(c). In this final rule, 
MSHA has retained the requirements for dpm reduction for permissible 
equipment in this section but has moved the requirements for 
nonpermissible heavy-duty vehicles to a new 30 CFR 72.501. MSHA has 
also moved the maintenance requirements for emission controls to a new 
30 CFR 72.503. These organizational changes were made to make it easier 
for the mining community to locate specific requirements in the final 
rule.
    Summary of final rule. The final rule requires all permissible 
equipment to meet an emissions limit of 2.5 grams of dpm per hour. The 
existing fleet has 18 months to meet this limit. In addition, any 
permissible engine introduced into the fleet of an underground coal 
mine after the effective date of this rule will have to meet that 
standard upon being introduced into the mine. MSHA means by 
``introduced'' any equipment added to the mine's diesel equipment 
inventory. This includes newly purchased equipment, used equipment, or 
a piece of equipment receiving a replacement engine with a different 
serial number than the engine it is replacing. It also includes engines 
or equipment coming from one mine into another. It does not include a 
piece of equipment whose engine was previously part of the mine's 
inventory and rebuilt.
    Infeasibility of a concentration limit for underground coal mines. 
The preamble accompanying the proposed rule explained why the Agency 
was not proposing an ambient concentration limit for underground coal 
mines as it was proposing for underground metal and nonmetal mines. The 
Agency was not confident at the time the rule was proposed that there 
was a measurement method for dpm that provided accurate, consistent and 
verifiable results at lower concentration levels in underground coal 
mines. The available measurement methods for determining dpm 
concentrations in underground coal mines were carefully evaluated by 
the Agency, including field testing, before the Agency reached this 
conclusion. The Agency continued to collect data and has consulted with 
NIOSH in an attempt to resolve questions about the measurement of dpm 
in underground coal mines. There were no comments received that 
objected to the fact that the Agency was not proposing an ambient 
concentration limit for underground coal mines as it was proposing for 
underground metal and nonmetal mines.
    Why dpm emissions from permissible equipment need to be controlled. 
The preamble accompanying the proposed rule also explained why the 
agency was proposing to limit the emissions from permissible equipment 
in particular. Dpm concentration samples taken in the field indicate 
that permissible equipment used for face haulage makes the largest 
contribution to high dpm levels. Dpm samples taken in the intake air to 
working sections where diesel face haulage was used showed relatively 
low dpm levels. When diesel particulate filters were not used, dpm 
samples taken on the working section and in returns from those sections 
generally showed dpm levels in excess of 500 g/m\3\.
    Other permissible equipment can also generate significant dpm 
emissions because this equipment utilizes the same engines as used in 
face haulage equipment. Since the time of the proposal, the diesel 
inventory for permissible machines has not changed significantly. The 
same four permissible engines that were available at the time the 
proposal was written continue to be the power source for the current 
permissible fleet. Table IV-1 shows that these four engines produce 
higher dpm emissions on a gm/hr basis than nonpermissible engines with 
the same horsepower. Commenters did not present evidence that dpm 
concentrations in areas where permissible equipment is used have 
decreased since the proposed rule was published.
    Why the final rule uses a machine based emission limit instead of a 
requirement for the addition of a filter with a specified filtration 
efficiency. The final rule for permissible equipment is different from 
that proposed. As proposed by MSHA (63 FR 17491 et.seq.), 30 CFR 
72.500(a) would have required mine operators to install on permissible 
vehicles a system capable of removing on average, at least 95% of dpm 
by mass. Operators were required to complete these filter installations 
within 18 months from the date of publication of the final rule; no 
action to control emissions from permissible equipment was required 
before that date.
    The use of an emissions limit for permissible machines in the final 
rule stems directly from an alternative which MSHA placed before the 
mining community in the preamble to the filter-efficiency based rule 
that was proposed. In that preamble, the agency also described a number 
of alternative approaches considered, and asked the mining community to 
comment on whether there were other approaches for the control of dpm 
from permissible equipment that might accomplish the same task with 
more flexibility. 63 FR 17498, 17499, 17556, 17563. The agency also 
described the approach being taken by the State of Pennsylvania that 
combined a filter efficiency standard with a tailpipe limit.
    The Agency emphasized that it was particularly interested in 
comment on an alternative approach it described that would establish a 
machine based limit on emissions in lieu of a filter efficiency 
requirement (see, e.g., 63 FR 17556, 17563). In fact, a separate 
``Question and Answer'' was included in the preamble to highlight this 
alternative, immediately after the description of the proposed rule. 63 
FR 17501, 17653.
    Based on the record, MSHA has concluded that the original proposal 
had deficiencies which are avoided by this alternative approach.
    MSHA received many comments objecting to exclusive reliance on 
filters. Commenters stated that MSHA was denying operators the benefit 
of the full range of available dpm controls outlined in MSHA's Toolbox 
(the history and content of which are described in Part II of this 
preamble). These commenters stated that mine operators should be 
allowed to chose the combination of controls that best suit their 
operations.
    On the other hand, other commenters favored requiring a filter on 
all underground mining equipment (including permissible equipment). 
Some of these commenters noted that controls are only effective if 
properly maintained, and some asserted that filters are easier to 
monitor in this regard than engines. Similarly, commenters argued that 
in the absence of a requirement for a filter on each piece of 
equipment, operators would rely primarily on increased ventilation to 
control dpm concentrations, and asserted that the industry had a very 
poor record of maintaining ventilation controls. Also, one commenter 
asserted that filters were the only known control that would limit the 
number of nanoparticles emitted as well as reducing the mass of dpm 
discharged, whereas newer diesel engines designed to produce less dpm 
mass may actually

[[Page 5670]]

increase the number of nanoparticles emitted.
    A number of commenters pointed out that even if filters were 
required, relying on a filter-efficiency standard would be 
inappropriate. These commenters noted that even if a particular 
efficiency (e.g., 95%) is achievable with a ``dirty'' engine like those 
currently composing the underground coal permissible fleet, such 
efficiency may not be feasible on the modern, clean burning engines 
that will eventually take their place. That is, if the emissions from a 
``cleaner'' engine are lower to begin with, the filter mounted on a 
machine with such an engine would have to be much more efficient than 
the one mounted on a machine with a dirtier engine to remove the same 
percentage of dpm. Commenters stated that since it might not be 
possible to meet the proposed requirement for a 95% efficient filter 
with a newer engine, MSHA's proposed rule might well inhibit the 
introduction of cleaner engines into underground coal mines, and thus 
force operators to rely on older and dirtier engines which would 
require more maintenance.
    There was also considerable discussion at the hearings and in the 
written comments about the experience of Pennsylvania, which has a 95% 
filter efficiency standard for permissible and other diesel equipment, 
as well as a requirement that each piece of equipment meet an emissions 
standard. Commenters clarified the development of that approach, its 
requirements and procedures, and implementation issues to date; many 
noted problems in meeting the standard as currently set forth. Other 
commenters noted that what might be feasible for Pennsylvania, a state 
which heretofore has not permitted diesel equipment underground, might 
not be feasible for operators in other states with existing fleets.
    As proposed, the rule would have ensured that the emissions from 
the most polluting, commonly used engine (Caterpillar 3306PCNA, 150 
horsepower, 45.88 gm/hr) would be reduced by 95%, resulting in tailpipe 
emissions of 2.29 gm/hr (5% of 44.88 gm/hr). After carefully 
considering all of the discussion at the hearings and the written 
comments, MSHA has concluded that the alternative approach on which it 
initially invited comment, a dpm emissions limit for each machine, has 
a number of advantages over the approach initially proposed. While MSHA 
has evidence that there are filters readily available for the existing 
permissible fleet which are 95% efficient it lacks evidence of the 
technological feasibility of filter performance at a 95% level for the 
cleaner engines which will eventually replace the current fleet. 
Moreover, the same problem exists at any filter efficiency rating. 
Changing the proposed rule to require that filters on permissible 
equipment must only be 70% efficient, as suggested by a commenter, does 
not guarantee they can provide this efficiency for future engines. At 
the same time it sets a limit for the current fleet that is far below 
what can be achieved. Thus, while a requirement for a high filter 
efficiency could have the perverse effect of inhibiting the 
introduction of cleaner engine technologies or other technologies that 
could be forthcoming that could make substantial reductions in dpm 
levels, a low filter efficiency requirement fails to provide protection 
for miners from dpm emissions from engines in today's fleet. 
Accordingly, MSHA has concluded that requiring a specific filter 
efficiency is not a good idea, either by itself or (as is the case in 
Pennsylvania) as a supplement to a machine emissions limit.
    The machine emission limit specified in this final rule achieves 
the desired goal of significantly reducing the mass of dpm emitted from 
the permissible machines without specifying a filter efficiency. Using 
the 2.5 gm/hr emission limit provides a consistent target and resolves 
the issue relative to lower filter efficiency or cleaner engines.
    With this final rule, MSHA is allowing the mine operator a wide 
choice of approaches from the toolbox to control dpm such as low 
emission diesel engines, aftertreatment controls (catalytic converters 
and/or dpm filters), fuel with a very low level sulfur content, 
alternative fuels, and fuel additives in order to meet the machine 
emission limit. Other aspects of the MSHA toolbox are already a 
requirement in underground coal mines such as the use of approved 
diesel engines, fuel with a sulfur content less than 500 ppm, optional 
EPA approved fuel additives, regular maintenance by qualified 
mechanics, prohibition of unnecessary idling, and training of mechanics 
and equipment operators. In practice, however, MSHA expects all 
permissible equipment to need filtration to achieve the required limit.
    The final rule does not, however, permit operators to satisfy the 
requirements for permissible equipment by increasing ventilation or by 
using enclosed cabs, although the Toolbox describes both as methods for 
reducing miner exposure to dpm. While MSHA encourages operators to take 
such steps, the Agency concluded that it would not be appropriate to 
make an adjustment to or an exemption from the machine emissions limit 
when such controls are used.
    In the case of ventilation, while increasing mine ventilation does 
reduce dpm concentrations in the ambient air, such a change does not 
impact a requirement based strictly on the emissions emitted from an 
individual machine. One variation of the alternative proposed by MSHA 
would have allowed a credit for added ventilation in determining 
whether a machine met the required emissions limit. However, after 
careful consideration the agency has concluded that this approach is 
inappropriate. It should be noted that while the agency acknowledges 
the evidence offered by many commenters that reliance upon ventilation 
as a primary dpm control is inappropriate in light of the record of 
violations of ventilation standards--even though not all of the data 
supplied supported the general conclusion being expressed and does not 
reflect the implementation of the new diesel equipment rule--this is 
not the basis on which the agency has determined not to allow operators 
a credit for increasing ventilation. Rather, MSHA concluded that such 
an approach would not be necessary in light of its conclusion about the 
capabilities of paper filters alone to enable the permissible fleet to 
meet the requirement. Controlling engine emissions to the required 
levels would have called for a ventilation rate of five times the 
engine particulate index air quantity. This quantity would have been 
specified in the Approved Ventilation Plan. Such a ventilation rate is 
achievable in only a few mines. At the same time, once the proper 
filter is installed, the emissions are controlled to the required 
levels; allowing a credit for ventilation makes no difference in 
practice given the range of available filters. While providing a 
ventilation credit would allow operators to use a less efficient 
filter, this would reduce dpm emissions less in such mines; and since 
the use of more efficient filters is feasible, the Act requires MSHA to 
pick the more protective approach. Moreover, due to the mobility of the 
equipment, a ventilation credit for outby equipment would be difficult 
to monitor and enforce. The Agency has indirectly allowed for 
ventilation by allowing a higher outby emission rate. The higher outby 
emission rate for light-duty equipment was based on the duty cycle and 
the normally higher ventilation rates in outby areas. Additionally, 
allowing for a ventilation credit based on the specific air volume 
would have become too complicated to administer

[[Page 5671]]

in certain cases (for example, permissible equipment in multi-entry 
systems, or permissible equipment used in outby areas). Ventilation 
regulations for single and multiple units of permissible diesel 
equipment are based primarily on the approval plate quantity. Depending 
on a ventilation quantity other than that on the approval plate would 
have complicated an already complex issue.
    While enclosed cabs or booths can be used to lower exposures for a 
machine operator, cabs do not currently exist for permissible 
underground coal mining equipment. Even if developed for permissible 
equipment, these enclosures would not provide protection for other 
miners working in that same area. Moreover, there will be no sampling 
to assure that the miners are protected. Consequently, the final rule 
requires that even if a cab were developed for permissible equipment, 
dpm emission limits would have to be maintained the same as other 
permissible equipment.
    Having made the determination that an emissions limit is preferable 
to a filter efficiency requirement, and not to provide credit for 
ventilation or an exemption for the use of cabs, MSHA turned to the 
question of whether filters should always be required. Some commenters 
noted that controls are only effective if properly maintained, and 
asserted that filters are easier to monitor in this regard than 
engines. Also, one commenter asserted that filters were the only known 
control that would limit the number of nanoparticles emitted as well as 
dpm mass, whereas newer diesel engines designed to produce less dpm 
mass may actually increase the number of nanoparticles emitted.
    With respect to maintenance, MSHA notes that while the provisions 
of the recently promulgated diesel equipment regulations dealing with 
maintenance and the training of qualified maintenance personnel were in 
effect at the time of the hearing, the full effect of implementation of 
these provisions may not have been apparent to the commenters. These 
regulations when fully implemented, should address many of the concerns 
expressed by the commenters in this regard.
    With respect to nanoparticles, section 5 of Part II of this 
preamble notes that there is very little information at this time about 
the possible risk of such particles. Moreover, the evidence on whether 
filters can protect against such particles is unclear. In any event, it 
will be some time before the newest generation of diesel engines 
becomes commonplace in underground mines.
    Accordingly, MSHA has concluded that at this time, it is not 
necessary to require filters that specifically limit nanoparticles. 
MSHA will, however, continue to monitor the situation. If it becomes 
apparent that the evidence warrants further action, the agency will not 
hesitate to act upon that information. In practice, as noted above, 
current permissible equipment will have to be filtered to meet the 
emissions standard.
    In this regard, one commenter stated that if MSHA does not require 
filters on all equipment underground, it would be more difficult for 
the individual states to require filters on all diesel equipment. MSHA 
does not agree with the commenter. States can impose a more stringent 
standard than MSHA's requirements. While MSHA recognizes that 
Pennsylvania and West Virginia and other States are going to take a 
close look at the Federal government's standard, each State faces 
different circumstances--e.g., the number and nature of diesel powered 
equipment already underground, the economic situation of the state's 
coal industry, etc. MSHA's discussion of the risks of dpm exposure in 
Part III suggest that further controls would be warranted where it is 
technologically and economically feasible for the underground coal 
mining industry as a whole to implement such controls; and while MSHA 
has concluded this is not feasible for the US industry as a whole, an 
individual State might well conclude it is feasible for the situation 
that exists in that State.
    Some commenters requested that some or all of the State of 
Pennsylvania approach be adopted by MSHA. The Pennsylvania law requires 
an MSHA approved engine, a catalytic converter, and a 95% filter. 
Additionally, Pennsylvania establishes a ventilating air requirement 
calculated to dilute the dpm emitted from the filter to 120/
m3. With respect to permissible equipment, MSHA's 
requirement for a machine dpm emission limit of 2.5 grams per hour is 
essentially equivalent to the emissions standard required under 
Pennsylvania law.
    MSHA did not adopt a calculated ambient dpm concentration based on 
the approval plate air quantity. Instead, MSHA set the emission 
standard to represent the dpm emitted from the individual machine. 
However, since MSHA already requires an approval plate quantity based 
on the gaseous emissions, an ambient dpm concentration can be 
calculated from the engine's dpm emission data, the filter efficiency, 
and the approval plate air quantity. For example, as noted on Table IV-
1, the Caterpillar 3306 PCNA engine produces 45.88 gm/hr of dpm from 
the Category A, permissible configuration. This engine has an approval 
plate quantity of 9500 cfm or 269m3/minute of air. When 
equipped with a 95% dpm filter, the resultant calculated laboratory 
ambient quantity for a single machine using the Caterpillar 3306 PCNA 
engine would be 142/m3. This is based on the 
following formula: (dpm,gm/hr) / 60 * ((100-95%)* 1000 / (approval 
plate quantity, m3/minute)* 1000. To reduce the emissions of 
this engine to the level specified in the Pennsylvania law would 
require additional air or a higher efficiency filter.
    One commenter presented data from a laboratory test conducted on 
different filter media. The data indicated that the highest efficiency 
achieved was 81% using the ISO 8178, C1 test cycle. This commenter 
suggested that MSHA adopt an approach similar to the Pennsylvania 
approach but establish a 0.5 milligram per cubic meter ( mg/
m3) calculated ambient concentration instead of the 
120/m3 (0.120 mg/m3). This commenter's 
approach included the use of a minimum 70% efficient filter and a 
recalculation of the approval plate air quantity to achieve the 
500/m3 (0.5 mg/m3) concentration.
    As with the Pennsylvania approach, MSHA basically agrees with the 
commenter's general approach. The dpm emission limits specified in this 
final rule limits the machine's dpm output, requiring the mine operator 
to choose an engine and aftertreatment device, if necessary, to meet 
the standard. This approach as previously stated significantly reduces 
dpm emissions and is based on laboratory testing of the engine and 
filter. However, since MSHA currently has a requirement for the use of 
approval plate air quantities in underground coal, MSHA did not impose 
an additional calculated approval plate air quantity as suggested by 
the commenters. MSHA is not imposing a minimum filter efficiency as 
suggested by the commenters because MSHA believes that the mine 
operator should be able to use all the available tools to meet the 
standards. MSHA believes that all of the current permissible engines 
will require filtration to meet the standard; however with this 
approach taken in the final rule, MSHA is not limiting future 
technologies.
    A commenter asked why the Agency had not chosen to utilize the 
particulate index established during the MSHA approval process for each 
engine as the basis of any dpm regulation.
    As discussed in Part II of this preamble, the requirement for 
determining the particulate index was

[[Page 5672]]

contained in the Agency's diesel equipment regulations. It implemented 
a recommendation of the Diesel Advisory Committee which called for a 
particulate index to be set for approved diesel engines. The 
particulate index specifies the quantity of air needed to dilute the 
particulate generated by the engine to 1 milligram of diesel 
particulate matter per cubic meter of air and is based on data 
collected under the engine approval test described in 30 CFR 7.89.
    MSHA established the particulate index to be used as a guide to the 
mining community in making certain decisions about the control of dpm 
while the Agency finalized regulations that specifically addressed dpm. 
This information is available to the mining industry from the 
manufacturer and MSHA. The particulate index enables the mining 
community to compare the particulate levels generated by different 
engines in terms of a ventilating air quantity. For example, if the 
particulate indices for diesel engines of the same horsepower were 
established as 7,500 cubic feet of air per minute (cfm) and 12,000 cfm 
respectively, an equipment manufacturer, mine operator, and MSHA 
personnel can use this 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 certain decisions. A mine operator can 
use this information when choosing an engine to roughly estimate an 
engine's contribution of diesel particulate to the mine's total 
respirable dust. MSHA would use this information when evaluating mine 
dust control plans. Equipment manufacturers can use the particulate 
index to design and install exhaust after-treatments. MSHA posts this 
information on its website at http://www.msha.gov/S&HINFO/DESLREG/1907b5.HTM for permissible engines and at http://www.msha.gov/S&HINFO/DESLREG/1909a.HTM for nonpermissible engines.
    Had the Agency decided to take an approach in this regulation 
similar to the approach taken by the state of Pennsylvania in its 
diesel law, or to establish an ambient dpm concentration limit, the 
particulate index could have been used directly to compute an estimated 
level of dpm that could be achieved with various quantities of 
ventilation air. Instead, as was discussed above, the Agency chose to 
limit the quantity of dpm emitted from the machine, and is therefore 
expressing the standard in that fashion.
    Nevertheless, there is a relationship between the PI and the 
machine limits established under this rule. The determination of the 
quantity of dpm emitted from the machine is based on the same 
information from the engine approval tests in 30 CFR 7.89 as was used 
to establish the particulate index. Both means of expressing the dpm 
characteristics of the machine start with determining the permissible 
fleet. With the exception of the Isuzu QD100 engine which is only used 
in two machines in the permissible fleet, the Caterpillar 3306 PCNA 
meets this criteria. The Caterpillar engine emits approximately 46 
grams of dpm per hour based on the MSHA approval test for part 7, 
Category A. Accordingly a 90% reduction would limit emissions to 5.0 
grams an hour; and a 95% reduction would limit emissions to 2.5 grams 
an hour. If a filter could reduce the dpm emitted from the Caterpillar 
engine to these levels, it could reduce the emissions of any other 
permissible engine in the fleet to that level.
    A number of commenters stated that they had been unable to 
substantiate the agency's contention that there are filters 
commercially available that meet such high efficiency requirements. 
Moreover they asserted that the only system which allegedly came close 
to this requirement, a system known as the DST, was a system 
that would be economically infeasible to install on the entire current 
fleet of permissible equipment.
    The DST system is described in section 6 of Part II. Data 
was submitted for the record that the DST system does indeed 
reduce the dpm emissions from an engine by more than 95% (i.e., below 
2.5 grams per hour) when tested on the ISO 8178,C1 test cycle. The 
engine tested with the DST was a MWM916-6 diesel engine which 
emits 25.5 gm/hr based on the MSHA approval test for part 7, Category 
A. The system is composed of several components; a paper filter and a 
catalytic converter, with a heat exchanger used to reduce the 
temperature of the exhaust to the levels required by MSHA for 
permissible equipment. The low exhaust gas temperature enables the use 
of a paper filter without igniting the filter. Most permissible 
equipment uses water scrubbers to cool the exhaust temperature; hence, 
switching to the dry system would involve considerable expense.
    The agency has reviewed the evidence to determine whether a 
commercially available paper type filter, mounted at the outlet of the 
water scrubber used to cool the exhaust of most permissible machines, 
can achieve comparable reductions in dpm emissions. Filter kits are 
readily available for most permissible machines, and the costs of 
equipping the fleet in this fashion is significantly lower than 
converting everything to a dry system.
    MSHA had good reason to think that paper filters alone could do the 
job. In the early 1990's, equipment manufacturers along with the then 
Bureau of Mines installed paper filters to the exhaust of water 
scrubbers for dpm reduction. These systems proved to be very effective 
in dpm removal. Some mines have used these filters on permissible 
equipment successfully since the early 1980s. Anecdotal experience was 
also supportive. For example, a miner commented very favorably about 
improvement in emissions from a diesel equipped with a paper filter. 
The miner was referring to a dry system other than DST. 
Moreover, based upon what it knows about the components of the 
DST system discussed above, MSHA had reason to believe that 
based upon the extent to which the heat exchanger and catalytic 
convertor can themselves reduce dpm concentrations, that the main 
reason for the extensive dpm reduction of the system might well be the 
paper filter. However, although the record could support such a 
conclusion, the record contained no specific filter efficiency data. 
Moreover, some asserted that the DST results were due to all 
of its components working together. Other commenters challenged the 
agency's assumption that a 95% reduction of emissions from the 
permissible engines that produce the highest dpm concentrations was 
feasible. Such a filter efficiency would be necessary to satisfy an 
emissions limit of 2.5 grams per hour.
    In order to dispel any doubts about the matter and verify whether 
the addition of a paper filter alone could achieve such a significant 
reduction in dpm, MSHA had an analysis performed by an independent 
laboratory. MSHA has placed a full report of this verification analysis 
in the record. The analysis was performed on an engine that is 
representative of the permissible engines in the fleet that produce the 
most dpm.
    The part 7 approval information indicates that three engines--the 
Caterpillar 3306 PCNA, 3304 PCNA, and the MWM 916-6--are basically of 
the same design. The Caterpillar 3306 PCNA used for the analysis is 
representative of the three engines' emissions performance. The Isuzu 
QD 100 is approved by MSHA and is used in a small number of permissible 
machines that can emit higher levels of dpm than the Caterpillar engine 
tested. This occurs when the Isuzu engine is

[[Page 5673]]

adjusted to the highest horsepower rating approved by MSHA. However, 
this engine can be derated to an existing lower horsepower MSHA 
approval rating which is only 5.5% lower than maximum rating. The two 
machines of which MSHA is aware that are currently using this engine 
are operated in a two entry mine through a petition for modification. 
The petition for modification requires these machines to be 
permissible. If this was not the case, the two machines that are 
currently using this engine would be considered light-duty equipment. 
In a light-duty equipment application, the lower horsepower adjustment 
for this engine would not be as critical as when installed in a heavy 
duty machine.
    MSHA contracted with Southwest Research Institute (SwRI) to 
determine the efficiency of a paper filter when installed on a Jeffrey 
dry system equipped with a Caterpillar 3306 PCNA diesel engine. 
Jeffrey's permissible system incorporates a heat exchanger and a 
synthetic type filter, but no oxidation catalytic converter. For the 
purpose of this verification test, a paper filter was substituted for 
the synthetic filter. In the setup for the verification test, as 
described below, the paper filter efficiency was determined.
    Although most permissible equipment is cooled by a water scrubber, 
MSHA did not ask SwRI to verify filter performance with a water 
scrubber system actually in place. The agency has concluded that such 
verification is not feasible at this time. Laboratory testing of dpm 
removal efficiency with a water scrubber is very difficult due to the 
high moisture content of the exhaust. The high moisture content would 
cause interference in the measurement methods using laboratory dilution 
tunnels. Others have attempted this type of work on a limited basis, 
but in most cases, were not successful or the investigators did not 
repeat previous attempts. Accordingly, as noted under the next heading, 
MSHA will assume for compliance purposes that a paper filter whose 
efficiency is measured with a heat exchanger will work just as well 
when used with a water scrubber.
    The paper filter installed on the Jeffery power package was 
acquired from Donaldson Filter Corporation. The filter paper was a 
standard primary air filter media, Donaldson Part No. EN0701026. When 
tested by Donaldson for use as a standard primary air filter media for 
many applications including diesel engine intake air filter, the paper 
has a particle removal efficiency of 32% for 0.5 micron particles, 60% 
for 1.0 micron particle, and 97% for 3.0 microns particles. This 
information was derived from data using neutralized KCL aerosol and on 
a test bench which complies with SAE J1669 requirements. The test was 
conducted on flatsheet media at 10.5 fpm face velocity. However, since 
the application of this paper filter media is unique to mining, the 
verification tests determined the efficiency when used in the cooled 
diesel exhaust stream (less than 300 deg.F) to filter whole dpm (less 
than 1 micorn in size). The paper filter media used had performance 
specifications equivalent to the paper filter used on the DST 
system. Moreover, it also is the same paper media which is used on the 
kits sold by Jeffrey and Wagner for installation of a paper filter on 
the exhaust of a water scrubber.
    A standard ISO 8178, C1 eight-mode emission, test which is 
identical to the tests required by this final, rule was performed in 
three component configurations. The first configuration consisted of 
measuring engine-out emissions with no heat exchanger or filter 
attached to the engine. This was considered baseline dpm emission data. 
The second configuration consisted of routing the engine exhaust 
through the heat exchanger and filter housing with no filter installed. 
The third configuration consisted of installing a filter into the 
filter housing and routing the exhaust through the heat exchanger and 
then through the filter. The difference between the mass of diesel 
particulate measured at the outlet of the filter, and the baseline dpm 
emissions, enabled the collection efficiency of the filter to be 
determined.
    The results of the verification conducted by Southwest Research 
Institute confirmed that a paper filter, without a catalytic converter, 
can reduce the dpm emissions of a Caterpillar 3306PCNA by 95%, down to 
a machine emissions rate of 2.3 gm/hr, thus meeting the 2.5 gm/hr 
standard. When the efficiency of the paper filter, as determined in the 
Southwest verification is applied to MSHA's approval data for these 
three permissible engines which make up almost all of the current 
permissible fleet, the 2.5 gm/hr standard is met. This is illustrated 
in the part of Table IV-1 dealing with permissible engines.
    As can be seen in that table, machines equipped with the Isuzu QD-
100 engine cannot meet the standard as currently operated. However, 
these engines can be derated from the highest power setting to a lower 
power setting and, with a paper filter, meet the emissions limit as 
shown by the second rating for that engine in the table. Since the 
paper filter used in the test has the same paper media as is generally 
used for dpm filters, MSHA has verified that the installation of a 
paper type filter alone will reduce the dpm concentration on all 
permissible machines currently in usage in underground coal mines.
    A commenter who reviewed the report of the verification test 
conducted by SwRI raised two issues about relying upon the results.
    One issue involves the dpm reduction from the heat exchanger. The 
results of the SwRI test indicated that there was a 9% reduction in dpm 
attributable to the heat exchanger. The commenter questioned whether 
the 9% attributed to the heat exchanger was also reported in the 95% 
reduction in dpm for the disposable paper filter. The test procedures 
required particulate measurements be made on bare engine emissions, 
with the heat exchanger in-line, and with the heat exchanger and 
disposable paper filter in-line. Comparing the particulate measurements 
made with the heat exchanger and filter installed to the measurements 
with only the heat exchanger installed, a 95% reduction in dpm 
concentration was observed.
    The commenter also questioned the validity of the SwRI test because 
the results of two tests were different with the filter installed. MSHA 
is aware of the minor difference in test results. However, MSHA's 
interest is in the efficiency of a clean filter, not a used filter. The 
efficiency of a used filter is typically greater than the efficiency of 
a clean filter. The second test was the 8-mode test using the same 
filter tested in the first test. The filter was exposed to dpm for 
approximately four hours (time incurred in running the first test). 
MSHA expected this second test to perform similarly. In fact, on a 
percentage basis, the results were close, 94% versus 96%, as shown in 
figure 4 of the SwRI report. However, MSHA does agree with the 
commenter that the results would be expected to be closer. Although not 
documented on the SwRI report, the raw data did show an increase in the 
filter weight from the first 8 mode test. SwRI and MSHA hypothesize 
that a ``chunk'' of dpm may have dislodged from the filter paper during 
the test and biased the filter weight. As with any lab testing, further 
studies could have been done to investigate the difference. However, as 
noted in the next section, MSHA intends to use the results of this test 
as the basis for accepting as evidence of compliance with the standard 
for permissible equipment the use of a paper filter like that tested; 
accordingly, the agency believes it can proceed without this 
confirmatory data.

[[Page 5674]]

    One commenter suggested that a standard adopted by MSHA would have 
to be adjusted with respect to equipment used at high altitude. This 
commenter stated that high altitude has an extreme effect on these 
types of filtration systems. This commenter's experience appeared to be 
related to catalytic converters. The commenter did not supply any data 
in supporting his position.
    MSHA is aware of the effect of altitude on engine performance. 
Engine deration must be performed on most engines to compensate for the 
decrease in the density of air at increasing altitudes to maintain the 
proper fuel-air ratio. However, the effect on aftertreatment controls 
specifically claimed by the commenter is not supported by any 
scientific principle. MSHA has experience with the former BOM on the 
use of paper filters on permissible machines at a high altitude mine. 
These were very successful tests. MSHA is not aware of any problems 
with other types of filters, including ceramic filters. If a self 
regeneration problem is noted by a mine, then the mine could use 
acceptable alternative regeneration devices to clean the ceramic 
filters. MSHA believes that the machine's dpm emission levels specified 
in this final rule are feasible at high altitude mines and the mine 
operator has many options available to meet the standards. Moreover, as 
discussed in the next section, if an operator is using a paper filter 
that is consistent with that already tested by MSHA, the agency will 
find the machine in compliance. There is no requirement in the final 
rule for an ambient air test; the laboratory test will be used.
    MSHA wishes to note that it did receive comments from some in the 
industry acknowledging that it was appropriate for the agency to force 
technology; and also received some comments from filter manufacturers 
to the effect that they could meet whatever requirements MSHA set. 
Moreover, many miners commented that the costs of controlling dpm 
should not factor into the human cost of overexposure to dpm.
    In light of these comments, and the statute, MSHA did consider 
whether it would be feasible for the underground coal mining industry 
to meet tighter requirements than the 2.5 gm/hr standard chosen. 
However, as discussed in Part V concerning feasibility, MSHA recognizes 
that the underground coal mining community has certain other relatively 
new standards with which to comply and others pending; moreover, the 
dpm exposure generated by permissible equipment is only one dpm source 
in many mines that needs to be addressed. Accordingly, the agency 
believes that an effort to force technology on paper filters at this 
time would not be warranted.
    How the mining community can go about implementing this 
requirement, and how MSHA can help. As explained above, MSHA has 
verified that a commercially available paper filter can reduce the 
emissions of any permissible piece of equipment to 2.5 grams per hour, 
and so has set the limit at that point. But the rule itself provides 
flexibility of controls, and there are many aftertreatment products on 
the market. Thus both MSHA and operators need a way to know whether a 
particular combination of controls will limit emissions to 2.5 grams 
per hour.
    The emission rate of a machine will be determined by the engine 
baseline dpm concentration determined during the MSHA engine approval 
process. The engine baseline dpm data for each MSHA approved engine is 
already known to the Agency. For the convenience of the mining 
community, the Agency is adding this information to its approval 
listings currently on the agency's web site. This information for 
permissible engines is located at http://www.msha.gov/S&HINFO/DESLREG/1907b5.HTM.
    Under the final rule, an operator can purchase any commercially 
available aftertreatment device and would, upon a request from MSHA, 
have to provide evidence that the device would reduce the emissions of 
the machine on which it is to be installed to the emission standard. 
However, in a majority of cases the mine operator will not be required 
to submit any data nor have any aftertreatment device tested. This is 
because MSHA will accept as evidence of compliance the use of any paper 
filter which meets or exceeds the specifications of the paper filter 
used in the verification described above; and, as noted in the 
discussion of that test, it appears that most current paper filters 
designed to reduce dpm use exactly the same paper as that used in the 
system tested. Thus, a mine operator can add almost any current paper 
filter to permissible machines without additional filter tests and be 
in compliance with the machine emission limit.
    It should be remembered, however, that the agency has established 
criteria for filter media intended for use on permissible equipment 
that go beyond filtration efficiency. These criteria were established 
to ensure that the addition of the filter would not compromise the 
permissibility features of the machine. MSHA will continue to apply 
these criteria in conjunction with this rule. A list of paper filters 
meeting the permissibility criteria and which have the required 
efficiency will be posted on the MSHA web site as this information 
becomes available.
    As noted above, MSHA's verification was conducted on a system whose 
exhaust was cooled by a heat exchanger, not a system whose exhaust was 
cooled by a water scrubber. MSHA recognizes that most permissible 
equipment is cooled by a water scrubber, and that MSHA has not verified 
filter performance with a water scrubber system actually in place. For 
the reasons noted, the agency has concluded that such verification is 
not feasible at this time. Since such verification is not feasible at 
this time, for purposes of implementing the rule, MSHA will assume that 
the results achieved with a filter tested on a dry exhaust cooling 
system apply equally to a system in which the exhaust is cooled by a 
water scrubber.
    The modifications required for the addition of a paper filter to 
the permissible machines can be made without any additional filter 
efficiency tests being conducted by the mine operator or machine 
manufacturer. The addition of a paper filter to the exhaust of the 
existing permissible machines would be evidence that those machines 
meet the 2.5 gm/hr standard. The mine operator would simply purchase a 
paper filter kit from the manufacturer of the permissible machine or 
perform a field modification to add an equivalent paper filter to the 
permissible machines. Since the machines are permissible, any 
modifications would have to be evaluated to make sure that the 
permissibility aspects of the diesel power package are not affected. 
This would normally involve evaluation of the machine's total 
backpressure and the addition of a high temperature exhaust gas sensor 
to the safety shutdown system.
    The process that mine operators may elect to follow to demonstrate 
compliance with the dpm standard is very similar to the process MSHA 
established for existing permissible machines when the 1996 diesel 
equipment rule was implemented. MSHA had four engines tested to 
determine a gaseous ventilation rate and particulate index for those 
engines. Mine operators only needed to update the machine approval 
plate to show the newly determined gaseous ventilation rate to continue 
to operate the existing permissible machine. The machine manufacturer 
normally supplied the updated plate.

[[Page 5675]]

    To demonstrate compliance with the dpm rule, the mine operator need 
only add a filter kit supplied by the equipment manufacturer. Filter 
kits which have been evaluated for permissibility are available from 
machine manufacturers for approximately 222 out of the 481 permissible 
machines that are not already equipped with filters. In the event that 
a kit is not available for a particular machine, the mine operator may 
work with the machine manufacturer to adapt an existing kit, or 
fabricate a special kit. MSHA will expedite the evaluation of field 
modifications submitted by mine operators to add such kits.
    One commenter stated that MSHA has not done enough with its 
knowledgeable personnel and research facility, and indicated that 
industry would welcome the opportunity to develop with MSHA research 
and development programs in the area of dpm filtration. MSHA has worked 
with NIOSH, labor representatives, and the industry in the past and is 
committed to continue to work with these groups on projects which 
promote a safer mining environment. The Diesel Toolbox arose out of 
just such an effort, as described in part II. But the Agency must also 
act to require the use of existing technology when it determines that 
miners are at significant risk of a material impairment to their 
health.
    One concern expressed by the mining community about more extensive 
reliance upon paper filtration systems is the increased potential for 
fires if, for example, water scrubbers run dry and the exhaust gases 
then become hot enough to ignite the paper filters. Several commenters 
expressed concerns about reports of fires that occurred on permissible 
diesel powered equipment on which paper particulate filters had been 
installed. Commenters told of fires on equipment in both western and 
eastern mines and further stated that the fires were the result of a 
lack of maintenance. While MSHA is concerned about all fires in 
underground mines, fires on permissible equipment are of particular 
concern because that equipment may operate in areas of the mine where 
methane may be present.
    Shortly after particulate filters were introduced, MSHA received 
reports of a filter fire in an underground mine and at a surface 
facility of a second mine. In the latter incident, the machine operator 
was unaware that a filter had been installed and continued to operate 
the equipment on the surface without water in the water scrubber. After 
looking into the incidents, MSHA issued a Program Information Bulletin 
informing the mining community of the importance of maintaining those 
components of permissible diesel power packages that limit the exhaust 
gas temperature below 170 degrees Fahrenheit. This PIB, P92-17, was 
published on October 23, 1992, and was given wide distribution 
throughout the country.
    Until the public hearings on this rule, MSHA was not aware of any 
additional filter fires. MSHA has no additional information concerning 
incidents of fires in mines involving permissible diesel equipment with 
particulate filters. Maintenance personnel at one mine had related that 
several filters had been exposed to high exhaust gas temperatures and 
that the filter media had started smoldering. The smoldering had been 
accompanied by significant amounts of smoke which alerted the equipment 
operators. The equipment operators removed the filters and extinguished 
the smoldering material before any actual fire broke out. According to 
mine maintenance personnel, these incidents had occurred several years 
ago, and since improved maintenance procedures were established and 
additional training had been provided, no additional problems had been 
noted.
    MSHA has continued to investigate this matter because of the 
potential consequences of a filter fire underground. MSHA is aware of a 
filter media used in Australia for the same application on permissible 
diesel equipment. The media is called Filtrete and is manufactured by 
3M. The media is polypropylene and when exposed to a heat source, the 
media reportedly melts away rather than burns. Reportedly, the filter 
media is as effective at removing diesel particulate as the filters 
currently used on diesels with water scrubber systems. MSHA is in 
contact with the filter manufacturer, and with Australian mine 
regulatory authorities, and mine operators concerning their experience 
with the filters. MSHA has also reviewed the flammability 
characteristics of the filter media used on dry type permissible 
diesels. One such media is a fiberglass/polyester fabric which seems to 
have flammability characteristics similar to the Filtrete media.
    As noted by at least one commenter, observing the recent diesel 
equipment maintenance requirements should minimize the already small 
potential for any problems. Nevertheless, MSHA will continue to look at 
alternative media, if for no other reason that to ascertain if they 
perform better than paper filters in removing dpm from the engine 
emissions.
    Although operators can comply with this requirement by using a 
paper filter, MSHA would like to encourage the introduction of cleaner 
engines in permissible equipment. The rule does not deal directly with 
factors which may be discouraging operators from using engines which 
incorporate the latest technologies to reduce dpm emissions. In order 
for an engine to be used in underground coal mines in permissible 
equipment, the engine has to be approved by MSHA for permissible 
applications, and this process operates at the initiative of engine 
manufacturers rather than mine operators. MSHA notes that even though 
engine manufacturers are producing significantly cleaner diesel 
engines, engine manufacturers have not submitted applications to MSHA 
to have these newer engines approved for permissible applications prior 
to this final rule. There are 528 permissible diesel powered machines 
in underground coal mines. The majority of the permissible machines use 
the Caterpillar 3306 PCNA, Caterpillar 3304 PCNA, or the Deutz-MWM 916-
6 diesel engines as stated previously. These engines are of older 
technology design and produce almost 10 times the dpm emissions as 
modern engines. However, due to the costs of obtaining approval of an 
engine for permissible applications, which are borne by the applicant, 
and low sales volumes in underground coal for permissible machines, 
engine manufacturers are understandably reluctant to submit new 
technology engines for approval as permissible.
    MSHA is developing programs that would facilitate the availability 
of engines that utilize the latest technologies to reduce gaseous and 
particulate emissions for use in permissible equipment. Current engine 
designs that utilize low emissions technologies are currently approved 
by MSHA in nonpermissible form. Particulate emissions are currently 
being determined by third parties testing under 30 CFR, Part 7. MSHA is 
in the process of purchasing an engine particulate testing system. Once 
this system is installed, MSHA will be able to facilitate testing and 
defer some of the cost of diesel engine particulate emission testing at 
its Approval and Testing Center. MSHA is considering a number of other 
programs that could aid the industry with emission tests.
    One of the programs that MSHA is considering would follow the 
precedent established in the recently published diesel equipment rule. 
To facilitate compliance with this dpm rule, MSHA is considering 
funding the additional emissions testing needed to gain approval as 
permissible, certain

[[Page 5676]]

previously approved, non-permissible engines that utilize low emissions 
technology engines. Additionally, MSHA is considering waiving the 
normal fees that the Agency charges for the administrative and 
technical evaluation portion of the approval process.
    Alternatively, MSHA may relax, as an interim measure, the 
requirement that engine approvals be issued only to engine 
manufacturers. This requirement, stated in part 7, is intended to 
ensure that the party to whom the engine approval is granted has the 
ability to ensure that the engine is manufactured in the approved 
configuration. MSHA is considering a program in which an equipment 
manufacturer may utilize an engine, approved by MSHA as nonpermissible, 
in a permissible power package. MSHA would ensure that the additional 
emissions tests required for permissible engines are conducted as part 
of the power package approval process. The use of an engine previously 
approved as nonpermissible is a critical element of the program. For 
those engines, the engine manufacturer has already made the commitment 
to manufacture the engine in an approved configuration. The permissible 
configuration would be the same as the nonpermissible configuration. 
Provisions of the two programs could be combined. MSHA will solicit 
input from the mining community as it continues to develop these 
program concepts.
    In response to comments, MSHA also took another look at the other 
components added to diesel engines of permissible equipment. One such 
control on permissible equipment is the device used to cool the hot 
gases emitted by diesel engines to the temperatures required for 
permissible applications. Specifically, in order to use a paper filter, 
a means of cooling the exhaust gas must be installed upstream of the 
paper filter to reduce the exhaust temperature below the ignition 
temperature of filter media. This is accomplished on permissible 
machines with either a water scrubber or a heat exchanger. The water 
scrubber allows the water to contact the exhaust, thus cooling the 
exhaust to less than 170 deg. F. The heat exchanger cools without 
direct contact between water and the exhaust, thus providing a dryer 
exhaust. Research conducted by others has shown that water scrubbers 
can lower dpm concentrations by 20-30%. The Southwest verification 
showed that a heat exchanger can remove approximately 9% of the dpm. 
Either cooling method would reduce dpm to some degree; however MSHA is 
confident, and the SwRI tests clearly showed, that the majority of the 
filtering comes from the paper filter.
    One commenter asserted that the most important emissions control 
that could be placed on a piece of diesel equipment is a catalytic 
converter. While there is some evidence in the record suggesting that 
OCCs can remove up to 20% of dpm emissions, this commenter's assertions 
about the importance of this control appear to stem from the view that 
the hazards to miners from diesel emissions come primarily from diesel 
gases rather than the particulate emissions. As indicated in MSHA's 
risk assessment, the risks which MSHA is acting to prevent in this case 
are from particulate emissions. Catalytic converters alone could not 
reduce dpm emissions from permissible equipment to levels that MSHA 
deems necessary.
    Time frames for implementation. Commenters were also concerned that 
the 18-month time frame established in the proposed rule to bring 
existing fleets into compliance would not be feasible.
    In part, these concerns stemmed from technological feasibility--
that controls did not yet exist which would be available by the 
required time. Also, these concerns related to economic feasibility. As 
noted above, some commenters thought they would have to replace wet 
systems with a dry system package in order to comply with the proposed 
rule; such a changeover would be expensive and, given the amount of 
work involved, take time. Others were concerned about the availability 
of filtration systems that would fit existing systems and the time 
necessary to develop or rig systems to fit on a variety of existing 
machines underground.
    The evidence discussed above addresses these concerns. MSHA is not 
pushing technology with the proposed emissions limit; rather, the 
technology is already here and for many pieces of equipment already in 
kit form for ready installation. The costs to the industry as a whole 
of adding paper filter to the permissible fleet after 18 months are 
economically feasible as well.
    Moreover, the final rule requires that a permissible piece of 
equipment being ``introduced'' underground for the first time 60 days 
after this rule is promulgated will have to be so equipped.
    MSHA means by ``introduced'' any equipment added to the mine's 
diesel equipment inventory. That inventory, and any changes to it, must 
be recorded by an operator as a result of this rulemaking and be 
maintained pursuant to new 30 CFR 72.520. ``Introduced'' means newly 
purchased equipment, used equipment, or a piece of equipment receiving 
a replacement engine with a different serial number than the engine it 
is replacing, including engines or equipment coming from one mine into 
another. It does not include a piece of equipment whose engine was 
previously part of the mine's inventory and rebuilt.
    As a result of the information discussed above, MSHA has determined 
that this requirement is both technologically and economically feasible 
to require any newly introduced equipment to have the filter in place 
(see MSHA's REA for additional information). MSHA recognizes that in 
some areas, longwall moving equipment may be shared among mines, and 
that in one or two cases a scheduled longwall move could be impacted by 
this effective date; however, MSHA has concluded that by working with 
machine manufacturers, operators who find themselves in such a 
situation can avoid any disruptions.
72.501  Emission Limits for Nonpermissible Heavy Duty Diesel Powered 
Equipment, Generators, and Compressors
    Organization. MSHA proposed limits on the dpm emitted by 
nonpermissible heavy-duty vehicles as part of 30 CFR 72.500, but in the 
final rule MSHA moved these requirements to a new 30 CFR 72.501. Also, 
this section now contains requirements for two types of light-duty 
equipment whose operating characteristics produce large quantities of 
dpm.
    Summary of final rule. In the final rule, MSHA has adopted a 
machine emission limit for heavy duty diesel powered equipment, as 
defined by Sec. 75.1908(a), just as it is doing with permissible 
equipment pursuant to Sec. 72.500 of this final rule. It also applies 
this limit to generators and compressors.
    Paragraph (a) specifies a machine emission limit for dpm at 5.0 gm/
hr for heavy-duty equipment, generators or compressors introduced into 
an underground area of an underground coal mine more than 60 days after 
the date of publication of this final rule. ``Introduced'' means any 
equipment added to the mine's diesel equipment inventory.
    Paragraph (b) provides that the fleet of such equipment already in 
a mine must reach a machine emission limit for dpm at 5.0 gm/hr within 
30 months.
    Paragraph (c) provides that the emission limit for all such 
equipment is further reduced to 2.5 gm/hr after 4 years.
    Paragraph (d) exempts from the requirements of the rule any 
generator

[[Page 5677]]

or compressor that discharges its exhaust directly into intake air that 
is coursed directly into a return air course, or discharges its exhaust 
directly into a return air course.
    Why dpm emissions from heavy-duty equipment, generators and 
compressors need to be controlled. 
    As discussed in connection with Sec. 72.500, MSHA determined that 
it could not establish a dpm concentration limit for underground coal 
mines, and therefore needed to focus its attention on the control of 
dpm emissions from specific types of equipment.
    The preamble accompanying the proposed rule also explained why the 
agency was proposing to limit the emissions from heavy-duty equipment 
in particular. MSHA discussed earlier in the permissible section that 
engines used in permissible equipment generated large quantities of 
dpm. Many pieces of heavy-duty equipment utilize the same engines as 
permissible equipment and consequently produce similar high levels of 
dpm. MSHA closely examined the dpm emission rates from engines used in 
other heavy-duty equipment and found them to be as high as those rates 
found in permissible equipment. Furthermore, heavy-duty equipment is 
used in areas of the mine where the ventilation quantities may be less 
than those provided where permissible equipment is used. Equipment that 
moves long wall components is known to work at a high duty cycle, in 
close proximity to miners, and in areas of the mine where there are 
frequent ventilation interruptions. Numerous commenters stated that 
diesel emissions continue to be the cause of air quality problems 
during long wall moves. Even though newer engines are being added to 
the heavy duty fleet, additional controls are needed to further reduce 
the dpm levels to which miners are exposed. As shown in table IV-1, 
engines like the Deutz BF4M1012EC rated at 113hp and the Detroit Diesel 
Series 40 DDEC rated at 230 horsepower are low emission engines that 
have been designed to meet current EPA standards. However, the gm/hr 
levels are still higher than the MSHA standards and would require 
aftertreatment controls.
    The proposed rule did not cover generators and compressors. 
However, the extension of the heavy duty requirements to generators and 
compressors stems directly from a question MSHA placed before the 
mining community. In reviewing alternative approaches considered by the 
Agency, the preamble of the proposed rule (63 FR 17564) noted that 
light-duty equipment does contribute to the total particulate 
concentration in underground coal mines, and explored the possibility 
of requiring light-duty equipment to be treated like permissible and 
heavy-duty equipment. The agency noted that it had tentatively 
concluded that requiring controls for the whole light duty fleet may 
not be feasible for the underground coal sector at this time. In this 
regard, it should be noted that light-duty equipment in underground 
coal mines makes up approximately \2/3\ of the whole fleet: 2,030 
engines out of the total MSHA inventory of 3121.
    The Agency stated that it welcomed ``information about light-duty 
equipment which may be making a particularly significant contribution 
to dpm emissions in particular mines or particular situations, and 
which is likely to continue to do so after full implementation of the 
approval requirements of the diesel equipment rule.'' The Agency went 
on to say that: ``MSHA will consider including in the final rule 
filtration requirements that may be necessary to address any such 
identified problem.'' This discussion was repeated in the section by 
section review of the proposed rule. (63 FR 17556) The Agency 
reiterated its request for comments in this regard in its Questions and 
Answers (Q and A #10, 63 FR 17499).
    As discussed below, based on the record, MSHA has concluded that 
generators and compressors, while considered light-duty equipment for 
purposes of the diesel equipment rule, in fact have operating 
characteristics that produce large quantities of dpm, and should be 
controlled in the same manner as heavy-duty equipment.
    Numerous commenters spoke on the issue of whether light-duty 
equipment, as defined by the diesel equipment rule, should be subject 
to dpm emissions standards. However, the record is divided between 
those who asserted that this type of equipment really operates much 
like heavy-duty equipment--i.e., works many hours during a shift at 
high loads--and those who asserted that the equipment is normally used 
at low loads and very little during the day. Very limited data was 
provided by proponents of either position; not enough for MSHA to make 
a clear determination of which position to adopt when looking at light-
duty equipment as a whole.
    Based on the record, MSHA believes that light-duty equipment is 
used in a variety of ways dependent on individual mine situations. The 
engine loading dependent on mine conditions can play an important role 
in the emissions from the diesel. Two different mining conditions with 
identical equipment could experience vastly different emission levels 
from these engines due to the engine load that must be produced to 
complete the work. Therefore the commenters may be correct for their 
individual mines where the light-duty equipment must work at higher 
engine loads to complete the work. However, other miners with identical 
equipment may not experience the same degree of engine load which could 
result in lower levels of exhaust emissions.
    However, the situation becomes much clearer when the focus narrows 
to specific types of light-duty equipment. For example, one commenter 
noted that some light-duty equipment (such as air compressors) which 
was exempt from requirements in the proposed rule, emitted high levels 
of dpm as determined by emission analyzers. Another commenter stated 
that larger engines that have heavy duty loads produce more dpm per 
hour and should be controlled. The commenter specifically recommended 
an OCC, adequate ventilation, and soot (dpm) filters.
    After a review of the information available, MSHA has concluded 
that air compressors and generators emit more dpm in the mine 
environment than other light-duty equipment because their engines are 
operated continuously under high-load conditions when they are running. 
Generators are designed to run under a loaded condition to produce 
electricity and air compressors work at full load to produce compressed 
air. In both cases, these engines are operating at a high load, which 
contributes to high dpm emissions. Based on the information provided by 
a commenter that the gaseous emissions levels from air compressors were 
high, this would correlate with high engine load and also would be 
related to higher dpm emissions. In addition, generators and 
compressors can use very large horsepower engines, i.e. above 200 
horsepower; by comparison, permissible equipment generally does not 
exceed 150 horsepower. In fact, some of the highest horsepower engines 
in underground coal mines are in generators and compressors. For 
example, in Table IV-1 engines that are known to be used in generators 
and compressors are represented by approval numbers B018, B037, and 
B036 and have horsepower ratings of 500, 275, and 220, respectively. 
Accordingly, in the final rule MSHA requires that air compressors and 
the generators meet the same engine emission limits as established for 
heavy-duty equipment. MSHA's inventory indicates that there are 66 air

[[Page 5678]]

compressors and generators out of a total of 3,121 pieces of diesel-
powered equipment in underground coal mines--about 3% of the 2,096 
light duty units.
    Why the final rule uses a machine-based emission limit instead of 
requiring for a high-efficiency filtration system.
    The proposed rule would have required mine operators by 30 months 
from the date of publication of the final rule to install, on 
nonpermissible heavy-duty vehicles, a system capable of removing, on 
average, at least 95% of dpm by mass.
    The use of a machine emissions limit in the final rule stems 
directly from an alternative which MSHA placed before the mining 
community in the preamble to the filter-efficiency based proposed rule. 
In that preamble, the Agency requested comment on an alternative 
approach that would establish a machine based limit on emissions in 
lieu of a filter efficiency requirement (see, e.g., 63 FR 17556, 
17563). In fact, a separate ``Question and Answer'' was included in the 
preamble to highlight this alternative, immediately after the 
description of the proposed rule. 63 FR 17501, 17653. Based on the 
record, MSHA has concluded that the original proposal had deficiencies 
(such as a credit for clean engines and a variety of filter 
efficiencies) which are avoided by the alternative approach.
    As explained in connection with Sec. 72.500, based on the record 
developed, the Agency concluded that a machine based emissions limit 
avoids a number of problems with the approach initially proposed. The 
explanation provided in that discussion as to (1) why MSHA moved to a 
machine based emissions limit for permissible equipment; (2) why it 
decided not to make adjustments for ventilation or permit an exemption 
for enclosed cabs; and (3) the flexibility in choice of controls 
provided to operators, is fully applicable for heavy-duty equipment, 
and accordingly is not repeated.
    Why MSHA concluded that the emissions limit for heavy-duty 
equipment, generators and compressors should ultimately be 2.5 grams 
per hour. As with permissible equipment, the emissions limit for this 
type of equipment was determined with reference to technological and 
economic feasibility. As is evident from the final rule, the emissions 
limit is 2.5 grams/hour, the same as the permissible limit; and, like 
permissible equipment, 2.5 grams/hour represents a 95% reduction in the 
dpm emissions of the engine that produced the most dpm emission in this 
category.
    MSHA wishes to emphasize that despite this fact, the limit in the 
final rule was not merely a determination to use the proposed rule in 
another form, or to have an equivalency between permissible equipment 
and this equipment. Rather, once MSHA decided to use an emissions limit 
approach, it reviewed the record to determine what could feasibly be 
achieved with the controls available for this type of equipment. 
Instead of using paper filters as with permissible equipment, this kind 
of equipment would generally be filtered by ceramic or other hot gas 
filters--or systems that lower the temperature of the emissions so that 
paper filters can be used. Ceramic filters cost more than paper 
filters, require regeneration, and have certain other associated costs. 
On the other hand, unlike the permissible fleet, the fleet of heavy-
duty equipment, generators and compressors has many choices of approved 
engines available for use, many of them modern technology engines with 
significantly lower emission rates than the engines currently utilized 
in this equipment.
    Table IV-1 shows the current dpm emissions from MSHA's inventory of 
heavy-duty equipment, generators and compressors based on engine 
approval data, and shows the filter efficiency required to reduce those 
emissions to the interim and final limits required by the final rule. 
Based on information about the current efficiencies of hot gas filters 
(discussed in the next section), MSHA believes that a significant 
percentage of the current fleet can immediately meet a limit of 2.5 
grams/hour with such filters alone--and all of the current fleet, 
except equipment powered by the Caterpillar 3306PCTA, can move 
immediately to meet a limit of 5.0 grams/hour with filters of only that 
efficiency. And even in the highly unlikely case that filter efficiency 
does not continue to improve to meet new demands in Europe and for over 
the road hauling in the United States, operators can bring the 
remainder of the fleet into compliance with new engines and filters 
with present day performance capabilities. In fact, the only reason for 
the two-tiered approach adopted in the final rule is to ensure that 
implementation of the rule will be economically feasible.
    Some commenters stated that the proposed rule is technology forcing 
and would require manufacturers to conduct approval tests to market new 
products, although some commenters who made this observation conceded 
that MSHA had the legal right to force technology. Another commenter 
stated that all heavy-duty equipment would require heat exchangers or 
equivalent means to allow for the use of paper filters since these, in 
the views of that commenter, appear at present to have higher filter 
efficiencies.
    These comments have some credibility with respect to the original 
proposal, which would in essence have required the engines that produce 
the most dpm emission in this category to achieve a limit of 2.5 grams/
hour with filters alone; although as noted above, there are already 
some hot gas filters that are approaching this result. However, the 
machine emission limits set forth in this section are clearly feasible 
with current technology, as cleaner, approved nonpermissible engines 
are available should a piece of equipment not be able to reduce dpm to 
the required limit with filter alone.
    A number of commenters argued that MSHA should not establish a rule 
which might rely heavily on the availability of ceramic filters because 
such systems have not performed well from either a practical or 
efficiency standpoint. MSHA has been aware that in many cases the 
industry, especially the metal/nonmetal mining sector, has had problems 
with the use of ceramic filters. However, these problems were reported 
over 10 years ago when the ceramic filter technology was originally 
being developed for the on-highway truck engines. When the highway 
truck sector did not need ceramic traps to comply with the on-highway 
EPA regulations, significant work on these trap systems was abandoned 
for the on-highway sector.
    More recently, the European directive requiring filters on diesels 
in confined areas, Canadian mines research with dpm filters, and the 
continued US efforts to reduce dpm emissions in the environment, have 
led filter manufacturers to improve the performance and reliability of 
ceramic filters. Some M/NM mines have reported favorably on the use of 
ceramic traps. Aftertreatment control vendors, mine operators and VERT 
have reported filter life of over 8000 hours. After a review of the 
information in the record in this regard, as was described in more 
detail in section 6 of Part II, MSHA has concluded that the more recent 
work with ceramic traps has shown they are feasible for use by the 
underground coal mining industry.
    How the mining community can go about implementing this 
requirement, and how MSHA can help. While the rule provides flexibility 
of controls to reach the required limit (controls that reduce engine 
emissions, that is), most operators are going to utilize hot gas 
(ceramic) filters to comply. In some

[[Page 5679]]

cases, however, installation of a cleaner engine or the DST 
or similar modified dry system (one without the permissibility 
components) may be more cost effective, and will be permitted under 
this machine based rule. Therefore to determine whether a particular 
machine is in compliance, MSHA will generally need to know the 
emissions from the engine in the equipment and the filtration 
efficiency of the filter.
    The dpm emission rate of an engine will be established by the dpm 
concentration determined during the engine approval process. The engine 
baseline dpm data for each MSHA approved non-permissible engine will be 
posted on the MSHA homepage at http://www.msha.gov/S&HINFO/DESLREG/1909a.HTM.
    Unlike the situation at present with permissible engines, in which 
none of the cleaner technology engines manufactured in recent years has 
been submitted for approval for permissible use, engine manufacturers 
have been submitting applications for approval of nonpermissible 
engines which meet EPA standards for both on road and nonroad 
applications. Thus, mine operators have the option of significantly 
reducing dpm emissions from heavy-duty equipment, generators and 
compressors by switching to cleaner approved engines. Moreover, MSHA is 
planning to accelerate the process of approving such engines so as to 
ensure that equipment of all sizes and shapes can utilize the cleanest 
engines available.
    MSHA is developing a program which 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.
    As noted in the prior section, MSHA expects that most operators 
will turn first to hot gas filters to reach the interim or even the 
final limit. Technically, an operator using a commercial filtration 
device would, upon a request from MSHA, have to provide evidence that 
the device is capable of reducing the emissions of the machine on which 
it is to be installed to the emission standard. The procedures by which 
a mine operator will demonstrate compliance with the rule are described 
in detail in the discussion of 30 CFR 72.503 of this part. However, the 
particulate removal efficiency of many commercially available hot gas 
filters is evaluated by VERT. VERT is a joint project of several 
European regulatory agencies, and private companies involved in the 
tunneling industry. VERT maintains facilities for the testing and 
evaluation of diesel engine aftertreatment devices for use on equipment 
used in tunneling. MSHA will accept dpm filtration efficiencies 
determined by VERT under the provisions of 30 CFR 72.503(c) of this 
rule.
    VERT evaluates the filtration efficiency of candidate devices using 
a diesel engine with an average dpm production of 0.08 gr/hp-hr. This 
engine produces less dpm than the majority of engines approved by MSHA. 
As further discussed in section 72.503, the test must be conducted on 
an engine that emits no more dpm than the engine that the 
aftertreatment device will be used on in the machine. This is to ensure 
that ``dirty'' engines are not used to over estimate a filter 
efficiency. The VERT engine used is considered a clean engine by 
current production standards and clean when compared to many engines in 
the current underground fleet. The assigned filter efficiencies from 
VERT would not be considered over-rated and would be consistent with 
expected efficiencies when used on current underground engines. 
Consequently, the filter efficiency determined by VERT test can be used 
to establish the machine dpm level in order to comply with 
72.503(b)(i).
    MSHA received some comments suggesting the agency could not rely 
upon the most recent VERT test data (listed in Table II-4) because not 
enough is known about how those results were derived. MSHA agrees that 
more information about the test data would be useful; however, given 
the purposes for which the agency is relying upon the data, the agency 
believes the information it currently has on the test data are 
adequate. This information is discussed in section 6 of Part II. The 
VERT data is generated through procedures as stringent as those MSHA is 
requiring in the tests which are being established in the final rule 
for filters not tested by such an organization. While the results noted 
in Table II-4 have not been incorporated into a published article and 
has references that are in other sources, MSHA's review of other VERT 
papers shows that VERT is using the same nomenclature in all their 
reports and the pertinent information needed from the table is 
available from these other VERT papers. The table shows VERT results on 
filters tested ``new'' and after field test. MSHA is only concerned 
with the ``new'' filter efficiency data for applying a filter 
efficiency number to the baseline engine emission data in order to 
determine if the machine meets the machine emission limit specified in 
this final rule. The range of filter efficiencies is not critical since 
the operator can choose a filter system based on the need for the 
engine for each individual machine.
    MSHA will maintain a list of dpm filtration devices and their 
filtration efficiencies on its website at www.msha.gov to assist the 
mining community. Where the particulate reduction capability of an 
aftertreatment device is not known, the operator would have to have the 
system tested at a laboratory capable of performing the tests as 
described in 30 CFR 72.503 of this rule to obtain the necessary data. 
However, in a majority of cases the mine operator will not be required 
to submit any data nor have the aftertreatment device tested. Since 
ceramic filters are used in general industry and automotive 
applications worldwide, extensive information on filter efficiency is 
available and a variety of hot gas filters are commercially available.
    The two tier machine emission limits provide operators with a 
choice when making initial control decisions--whether to select a 
control that will bring the equipment into compliance with the interim 
limit first, or whether to go ahead and purchase controls that will be 
required in any event by the final emissions limit. MSHA envisions that 
the mine operator will in most cases make a single decision as to the 
options to select to bring the machine into compliance. If the machine 
is old

[[Page 5680]]

and is expected to reach the end of its useful life in 4 years or less, 
the mine operator may choose a less costly set of options with the 
intention to scrap the machine when the lower emission level is 
effective. However, if the machine has a life expectancy beyond four 
years, then the mine operator may choose to install a filter system/
engine combination that will meet the 2.5 gm/hr standard immediately. 
Moreover, MSHA has reviewed the VERT list and it identifies several 
filter systems that can be purchased that have sufficient efficiency 
ratings to meet the 2.5 gm/hr standard when matched to the majority of 
the MSHA approved engines in heavy-duty equipment, generators and 
compressors. MSHA anticipates that more such high efficiency filters 
will become available before the final emissions limit must be reached. 
Accordingly, some operators may be able to satisfy the requirements in 
this fashion.
    Yet another alternative that can currently enable heavy-duty 
equipment to reach the 2.5 gm/hr final limit is the DST 
system. Test data was submitted for the record showing an overall 
system efficiency of greater than 95%. While more costly than hot gas 
filters, this approach might in some cases be cheaper than a high 
efficiency hot gas filter and a new engine.
    The final rule prohibits any piece of nonpermissible heavy duty 
diesel powered equipment, generator or compressor, from exceeding 5.0 
grams per hour of diesel particulate emissions. MSHA believes that by 
working with manufacturers of aftertreatment systems, filters can be 
installed so that newly manufactured machines comply with this 
requirement. MSHA expects that new equipment, or any equipment with an 
expected service greater than four years will be provided with a filter 
capable of meeting the 2.5 gm/hr machine standard.

Section 72.502  Requirements for nonpermissible light-duty diesel 
powered equipment other than generators and compressors

    Organization. The proposed rule did not contain specific provisions 
for light-duty diesel powered equipment. However, in the preamble to 
the rule, the agency asked the mining community if light-duty equipment 
should be subject to provisions that would address dpm emissions. This 
section is new in the final rule and is based on the large response 
from the mining community to that question.
    Summary of final rule. Paragraph (a) of this section provides that 
light-duty equipment (other than generators or compressors, which are 
covered by 30 CFR 72.501) introduced into an underground area of an 
underground coal mine more than 60 days after the issuance of the final 
rule cannot emit more than 5.0 grams/hour of dpm. MSHA means by 
``introduced'' any equipment added to the mine's diesel equipment 
inventory. That inventory, and any changes to it, must be recorded by 
an operator as a result of this rulemaking and be maintained pursuant 
to new 30 CFR 72.520. This includes newly purchased equipment, used 
equipment, or a piece of equipment receiving a replacement engine with 
a different serial number than the engine it is replacing, including 
engines or equipment coming from one mine into another, but it does not 
include a piece of equipment whose engine was previously part of the 
mine's inventory and rebuilt. MSHA will exempt newly manufactured 
light-duty equipment from meeting the requirements in 30 CFR 72.502, if 
the equipment is received after the 60 day time frame as long as a mine 
operator can present evidence that the equipment was ordered prior to 
the date of publication of this final rule.
    Paragraph (b) provides that an engine will be deemed to be in 
compliance with this requirement if it meets or exceeds certain EPA dpm 
emission requirements listed in Table 72.502-1 which appears in the 
rule.
    Paragraph (c) excludes any diesel-powered ambulance or fire 
fighting equipment that is being used in accordance with the mine fire 
fighting and evacuation plan from the requirements of this section.
    Why the final rule covers newly introduced light-duty equipment. 
The final rule's coverage of newly introduced light-duty equipment 
stems directly from an alternative which MSHA placed before the mining 
community in the preamble to the filter-efficiency based rule that was 
proposed.
    In reviewing alternative approaches considered by the Agency, the 
preamble of the proposed rule (63 FR 17564) noted that light-duty 
equipment does contribute to the total particulate concentration in 
underground coal mines, and explored the possibility of requiring 
light-duty equipment to be treated like permissible and heavy-duty 
equipment. The agency noted that it had tentatively concluded that 
requiring controls for the whole light duty fleet may not be feasible 
for the underground coal sector at this time. In this regard, it should 
be noted that this type of equipment in underground coal mines makes up 
approximately \2/3\ of the whole fleet: 2096 engines out of the total 
MSHA inventory of 3121.
    The preamble further stated that the Agency welcomed ``information 
about light-duty equipment which may be making a particularly 
significant contribution to dpm emissions in particular mines or 
particular situations, and which is likely to continue to do so after 
full implementation of the approval requirements of the diesel 
equipment rule''. As noted in connection with 30 CFR 72.501, the record 
on this point led MSHA to treat light duty generators and compressors 
the same way as heavy duty nonpermissible equipment in the final rule.
    The preamble to the proposed rule also indicated MSHA's specific 
interest in exploring whether it would be feasible to require controls 
on just the new equipment being added to the light duty fleet. ``The 
Agency would also welcome comment on whether it would be feasible for 
this sector to implement a requirement that any new light-duty 
equipment added to a mine's fleet be filtered.'' The Agency further 
noted that limiting a filtering requirement to just this portion of the 
light duty fleet was a different issue in terms of economic feasibility 
than filtering the whole fleet. ``By way of rough cost estimate, if 
turnover is only 10% a year, for example, the cost of such an approach 
would be only about a tenth of that for filtering all light-duty 
outby.'' 63 FR 17564. This discussion was repeated in the section by 
section review of the proposed rule. (63 FR 17556) The Agency 
reiterated its request for comments in this regard in its Questions and 
Answers (Q and A #10, 63 FR 17499).
    As noted in the discussion of 30 CFR 72.501 of this part, MSHA 
received considerable comment on whether the light duty fleet as a 
whole should be covered. In a significant number of mines, the light 
duty fleet may work under heavy loads for considerable periods of time, 
resulting in localized intensive exposures. But it would also appear 
that in other mines this is not the case; moreover, many of the 
experiences with localized exposures may have been due to maintenance 
problems, as the diesel equipment rule with its requirements for 
maintenance had yet to go into effect.
    Also, many miners commented that large numbers of light-duty 
equipment were in the same area of the mine on occasion and their 
emissions were not adequately diluted by the ventilation air provided. 
MSHA believes these comments were made based on experience gained 
before the effective date of the ventilation requirements under the 
diesel equipment rule.

[[Page 5681]]

Section 70.1900(a)(4) of the diesel equipment rule now allows the 
district manager to establish areas in the mine where air quality 
samples for gases must be collected to identify and correct problems 
such as those described. Even though the focus in 30 CFR 70.1900(a)(4) 
is on gaseous emissions, the point is that a buildup of gaseous 
emissions would be an indication of a build up of diesel emissions 
generally and thus, of the inadequate ventilation that was the concern 
of the commenters.
    The comments about the light duty fleet as a whole were not 
particularly helpful in evaluating the agency's specific request for 
comment on whether it would be feasible for this sector to implement a 
requirement that the emissions from any new light-duty equipment added 
to a mine's fleet be limited. Nevertheless, as noted in Part III, the 
best available evidence is that a significant risk of adverse health 
effects due to dpm exposures will remain even after this rule will be 
implemented. Since the Agency is under a legal obligation to eliminate 
significant risks to the extent feasible, the Agency determined it 
should conduct a further analysis of the feasibility of limiting 
emissions from newly introduced light-duty equipment into underground 
coal mines. The service life of light-duty equipment (e.g., pickup 
trucks) is roughly ten years--much shorter than other types of 
equipment which is often rebuilt underground. Accordingly, if the 
engines in the new equipment are cleaner than the ones in the old 
equipment, the dpm emissions in the mine can be lowered over this 
period of time without the need to place controls on the existing 
fleet.
    MSHA then examined the kinds of engines that were likely to be in 
new light-duty equipment, as compared with the engines in the current 
light duty fleet. It turns out that there is likely to be a major 
difference. Many of the engines in the current fleet were designed and 
produced before the advent of EPA emission standards. Almost all of 
those engines likely to be available for introduction underground in 
the future will be subject to such standards. Accordingly, MSHA has 
determined that if newly introduced light duty engines or equipment are 
limited to more recent models, the dpm emissions from the new light 
duty fleet will eventually be significantly less than from the current 
fleet. The service life of light-duty equipment (e.g., pickup trucks) 
is roughly ten years--much shorter than other types of equipment which 
is often rebuilt underground. As explained in the next section of this 
discussion, MSHA determined that requiring all light-duty equipment 
introduced underground in the future to comply with these standards is 
feasible; the engines required to meet the requirement are available in 
all types and sizes. Accordingly, the agency decided that the record 
warranted adoption of the alternative it had placed before the mining 
community, and the final rule establishes emission standards for newly 
introduced light-duty equipment.
    How did MSHA determine the emissions limit for newly introduced 
light-duty equipment? MSHA examined whether it could establish the 
standard for newly introduced light-duty equipment at the same level as 
the standard it is establishing for newly introduced heavy-duty 
equipment, generators and compressors. In this regard, the agency 
looked at two sets of existing requirements to determine what types of 
engines used in light-duty equipment are readily available today, and 
then set the standard accordingly. First, the agency looked at current 
MSHA approval standards, and then it looked at current EPA standards.
    The record indicated that equipment in the light duty fleet may be 
used to the extent that the dpm emissions from these vehicles could 
contribute to overall mine air quality in a manner similar to heavy-
duty equipment. However, an equal number of commenters stated that 
light-duty vehicles are not used very much except for transporting 
miners in, out, and around the mine on a limited basis. MSHA believes 
that mines utilizes their light duty fleet in various ways depending on 
the individual mine conditions, fleet management, and standard 
operating practices. Also MSHA believes that many light-duty vehicles 
are operated in areas of the mine where the ventilation rate exceeds 
the approval plate quantities. Because MSHA did not receive sufficient 
information to establish the need to control dpm emissions from light-
duty equipment to the same degree as required for heavy duty or 
permissible equipment, MSHA established a new approach. MSHA determined 
that no action needs to be taken to modify equipment in the existing 
light duty fleet. However, MHSA wanted to ensure that steps be taken to 
limit the dpm emissions from any light-duty equipment introduced into 
mines. The steps would include purchasing equipment that uses engines 
representative of the state-of-the-art in emission control that are 
commercially available. These engines would be the type that are being 
manufactured to comply with the current EPA standards for diesel 
engines for both on-highway and nonroad applications. MSHA also 
recognized that manufacturers of mine specific vehicles currently 
utilize engines of older design that would not meet the EPA standards. 
Manufacturers of this equipment could continue to use these engines 
with appropriate after treatment of the exhaust to limit the dpm 
emissions.
    In its deliberations to determine the emissions standard that was 
required to be met by heavy-duty equipment, MSHA also determined that 
engines in existing light-duty equipment could be provided with 
commercially available aftertreatment controls to reduce the dpm 
emissions to 5.0 gm/hr. In fact, some light-duty equipment with 
relatively low horsepower engines can meet a 5.0 gm/hr standard without 
any aftertreatment controls.
    Some existing light-duty equipment built specifically for mine use 
is representative of equipment that will probably continue to be 
introduced into the mines. This type of light-duty equipment will 
continue to use engines that would not meet the EPA dpm standards. 
Hence for any such equipment introduced into an underground coal mine 
after the effective date, aftertreatment will be required.
    Consequently, MSHA established the 5.0 gm/hr standard for any 
light-duty equipment introduced into mines after the effective date of 
the rule.
    As stated above, part of the approach established by MSHA for 
light-duty equipment was to ensure that introduced light-duty equipment 
would be provided with engines representative of the state of the art 
in emission control that are commercially available. These engines 
would be the type that are being manufactured to comply with the 
current EPA standards for diesel engines for both on-highway and 
nonroad applications.
    As noted in section 5 of Part II, the EPA emission standards are 
established for light-duty vehicles and trucks, heavy duty highway 
engines, and nonroad engines. These requirements take effect for new 
production runs of engines at various times depending on engine type 
and size. MSHA recognizes that introduced equipment provided with these 
engines may exceed the 5.0 gm/hr standard. However, the engines being 
built to meet the EPA standards represent the state of the art in 
emission controls that are feasible to limit diesel exhaust emissions 
for those sizes of engines. MSHA did not intend to require 
aftertreatment controls on introduced light-duty equipment. MSHA 
believes that as long as mine

[[Page 5682]]

operators purchase equipment with these new engines, the in-mine dpm 
concentrations will be reduced as the existing light-duty equipment 
fleet is replaced.
    MSHA has established an exception in 30 CFR 72.502(b) that would 
allow mine operators to introduce equipment powered by engines that 
meet the EPA standards listed in Table 72.502-1 in lieu of meeting the 
5.0 gm/hr standard given in 72.502(a). MSHA also knows that the EPA 
intends to tighten the emission standards for new diesel engines. As 
engines meeting these future requirements are produced, they will also 
become available for use in mining equipment, thus the overall 
contribution of dpm from the in-mine light-duty equipment should 
decrease even further.
    MSHA has already approved engines produced by a variety of engine 
manufacturers in a wide range of horsepowers that meet the EPA 
standards listed in Table 72.502-1 of this part. These engines are 
shown on Table IV-1 by an asterisk (*).
    Many pickup trucks used in underground coal mines use engines that 
would be classified by the EPA as ``heavy duty highway engines''. 
Consequently, if the engine was produced after 1994, it has met the EPA 
emissions standard of 0.1 g/bhp-hr shown in table 72.502-1. MSHA 
believes that the mining community is not likely to have any problem 
finding a pickup truck that meets the standard. Many pickup trucks can 
be moved from mine to mine and meet the standard.
    This is basically the same for any on-highway engine the EPA 
classifies as a ``light-duty vehicle'' or ``light duty trucks''. If 
manufactured in or after model year 1994, the vehicle or truck must be 
limited to a dpm output of 0.1 gr/mile and meets the EPA requirement. 
However, there are no such vehicles currently in use in mines.
    Mine operators frequently purchase equipment for use in underground 
coal mines that come with engines which are categorized by EPA as 
nonroad engines for use in underground coal mines. This includes both 
industrial equipment and mine specific equipment such as forklifts, 
rockdusters, tractors, pumps, manlifts, personnel carriers, and 
welders. EPA's requirements on nonroad engines vary by horsepower. As 
discussed in part II of this preamble, EPA originally regulated these 
engines at standards referred to as tier 1. The most recent standards 
that are scheduled to become effective for these engines are designated 
as tier 2 standards. Many of the engines used in this equipment will 
soon be meeting the EPA tier 2 dpm limits as a result of the 1998 
rulemaking by that agency. MSHA chose the tier 2 standards in 30 CFR 
72.502(b) of this part since they will represent the most advanced 
technologies for emission controls. As previously stated, some nonroad 
engines are already being produced which meet the tier 2 requirements 
and have been approved by MSHA. Approximately two-thirds of the 
nonpermissible MSHA approved engines meet the tier 2 standards. The 
exact EPA emission limits for each tier for each engine size category 
are listed in Table 72.502-1 of the final rule which is reproduced here 
in the preamble for reference:

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    In this final rule, operators have the option to meet the 
requirements of the standard by installing filters on newly introduced 
light-duty equipment. For example, an operator wishing to take an 
existing piece of light-duty equipment whose emissions exceed 5.0 
grams/hour from one mine and use it in another mine could do so if the 
machine is equipped with a filter or catalytic converter efficient 
enough to bring the emissions down to 5.0 grams/hour. MSHA anticipates 
that the majority of mine operators will choose to purchase equipment 
with MSHA approved engines meeting the EPA dpm standards. Some models 
of small utility equipment might be difficult to filter, so the mine 
operator will probably choose to introduce this type of equipment with 
an engine that meets EPA requirements. However in some cases where an 
engine which complies with the 5.0 g/hr standard or the EPA 
requirements is too expensive or hard to use for a specific machine 
application, a filter system can be designed in during the construction 
of the vehicle instead of a retrofit.
    The Agency wishes to emphasize that it is not barring operators 
from introducing used equipment into an underground coal mine simply 
because it is used. As noted in the examples above, many of these EPA 
requirements have been in place for a while, so operators should have a 
wide choice of equipment from which to choose, and in other cases there 
are MSHA approved engines that will meet the standards.
    MSHA will undertake other actions to further facilitate compliance 
with this standard. As noted above, MSHA is enabling operators to 
comply with this standard by selecting engines or equipment that comply 
with various EPA standards. However, under the diesel equipment rule, 
all engines used underground have to be approved by MSHA. Accordingly, 
MSHA is reviewing actions that could be taken to facilitate the 
approval process when an engine meets EPA standards.
    As was described earlier in the discussion of the heavy-duty 
equipment requirements, MSHA is developing a program which 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 testing procedures 
(currently used only in the certification program for nonroad engines). 
MSHA will announce the specifics of the program when it is finalized. 
This program, when implemented, will assure mine operators and mining 
equipment manufacturers of the availability of low emissions engines, 
approved by both MSHA and EPA, in a wide range of horsepowers with 
which they can easily comply with the dpm requirements for light-duty 
equipment.
    Exemption for ambulances and fire fighting equipment. Paragraph (c) 
of this section excludes from these requirements diesel powered 
ambulance and fire fighting equipment being used in accordance with the 
mine fire fighting and evacuation plan under 30 CFR 75.1101-23. This is 
done in the same manner as MSHA excluded this type of equipment in the 
diesel equipment rule. This exclusion ensures consistency between this 
rule and the diesel equipment rule.

Section 72.503  Determination of Emissions; Filter Maintenance

    Organization. This section is added to the final rule to specify 
the means to determine and maintain compliance with the machine 
emission limits established in this part. The requirements of this 
section revise and refine provisions included in the proposal under 
72.500(c) and (d). The requirements have been moved to a separate 
section because they are relevant to the requirements of several other 
sections--30 CFR 72.500, 72.501 and 72.502.
    Engine emissions. Section 72.503(a) of the final rule specifies 
that the amount of dpm emitted by a particular engine shall be 
determined from the engine approval pursuant to 30 CFR 
7.89(a)(9)(iii)(B) or 7.89(a)(9)(iv)(A), except for those engines in 
light-duty equipment deemed to be in compliance with the requirements 
of this rule pursuant to 30 CFR 72.502(b).
    This approach using part 7 engine approval data was inherent in the 
requirements of proposed 30 CFR 70.500(d). The current formulation 
refines the requirement to make it more clear and extends coverage to 
the EPA approval program.
    MSHA currently lists all part 7 engine approvals on the Internet. 
The web addresses have been previously listed in this section. To 
assist mine operators in complying with the provisions of this rule, 
MSHA will add the dpm grams per hour number for each approved engine 
based on the approval test data. This number is calculated from the 
equations in 30 CFR 7.89(a)(9)(iii)(B) or 7.89(a)(9)(iv)(A) which are 
direct results of tests conducted for determination of the particulate 
index. This value will be used as an engine's baseline dpm 
concentration; the efficiency of the filter will then be multiplied by 
this baseline dpm number to establish compliance with the machine's 
emission limit under the appropriate section of this rule. MSHA will 
use the gm/hr data obtained from the MSHA approval data and not the gm/
hr data determined from other filter tests that determine the 
efficiency of the filter being tested. Results from different engine 
configurations or different laboratories could give results that could 
prevent the mine operator from showing compliance. The data could also 
be different if the tests were run differently from the approval test.
    Laboratory test procedures for testing aftertreatment devices; MSHA 
acceptance of results of other organizations. Section 72.503(b) of this 
final rule provides that the efficiency of an aftertreatment device is 
to be established by a laboratory test with a device representative of 
that to be used--and not by an actual test at the mine site on a 
particular filter. The test of the aftertreatment device is to be on an 
approved engine that emits no more dpm than the engine in the machine 
on which the aftertreatment device is to be used. If the filter test 
were run on an engine with higher emissions, the filter is likely to be 
rated as having a higher efficiency than it does when installed on an 
engine that produces lower emissions. This is consistent with the views 
of those commenters who objected to the proposal to establish a 95% 
efficient filter standard on the grounds that they would not be able to 
maintain such an efficiency as cleaner engines are introduced. The 
engine is to be run on the same test cycle used for MSHA approvals. The 
test procedure to follow must be appropriate to the filter media being 
tested. Furthermore the test is to be done by a laboratory capable of 
testing engines in accordance with MSHA approval requirements, to 
ensure consistency among testing and results.
    Although these requirements provide the specifications for filter 
efficiency tests, MSHA does not believe that many filter tests will 
need to be run in order for mine operators to comply with the 
requirements of this rule. A key reason is that 30 CFR 72.503(c) allows 
the Secretary to accept the results of tests conducted or certified by 
an organization whose testing standards are deemed by the Secretary to 
be as rigorous as those set forth in 30 CFR 72.503(b). Also, the 
Secretary may accept the results of tests for one aftertreatment device 
as evidencing the efficiency of another aftertreatment device which the 
Secretary determined to be essentially identical to the one tested.
    With respect to hot gas filters, the agency has already indicated 
(in the discussion of 30 CFR part 72.501) its intention to accept the 
efficiency results of any filter tested by VERT--

[[Page 5685]]

notwithstanding their use of somewhat different test procedures. MSHA 
will provide additional information on how mine operators can easily 
obtain the filter efficiency data from VERT in the compliance guide for 
this rule.
    Moreover, the record of this rulemaking contains data establishing 
the efficiency of both the DST system and paper filters. Both 
of these were tested by SwRI in tests meeting the requirements of this 
section. MSHA has indicated (in the discussion of proposed section 
72.500 of this part) that it will accept as having the same efficiency 
as the paper filter it tested, any filter using the same or equivalent 
media. Such filter paper appears to be used for the production of a 
variety of filters. Consequently, effective filters will be readily 
available.
    The filter efficiency test procedure stated in this final rule is 
basically the same as that procedure specified in the proposal. This 
test procedure follows the test cycle specified in part 7, subpart E, 
for determination of the particulate index. This test is similar to the 
test procedure used by VERT. VERT has streamlined their test procedure 
to minimize testing time but retained the main dpm producing modes on 
the steady state test cycle. The MSHA test procedures in part 7, 
subpart E were originally adapted from the ISO 8178 procedures. VERT 
actually follows the test procedures in ISO 8178.
    Several commenters questioned whether the ISO 8178 is an 
appropriate test for performing the filter efficiency tests, but 
offered no suggestions as to a cycle which should be used. Other 
commenters stated that the ISO 8178 is the best test at this point in 
time for conducting the filter efficiency test since no other cycle is 
available. Because ISO 8178 is an internationally accepted test cycle 
for evaluating diesel engine emissions, MSHA is retaining the ISO 8178 
test procedure in this final rule. However the rule does allow the 
Secretary to accept data from tests.
    MSHA will maintain a list (posted on its web site) of additional 
sources from which mine operators and inspectors can obtain the 
necessary information, including aftertreatment manufacturers who 
follow testing procedures MSHA deems meet its requirements. Mine 
operators will have to show evidence that for each particular machine, 
the engine baseline data multiplied by the filter efficiency will meet 
the appropriate standard. Any questions on acceptance of a filter 
manufacturer should be made prior to purchasing of the filter media. 
The mine operator may want to contact MSHA's approval and certification 
center located at Triadelphia, WVA to determine that the filter 
efficiency data is acceptable prior to purchasing, especially if the 
filter data is not from VERT or from a source listed by MSHA.
    One commenter stated that industry was concerned that laboratory 
tests of filters may give invalid indication of filter efficiency. MSHA 
believes that the filter test should be appropriate to the media; that 
is the aftertreatment device should be tested with the contaminant that 
is being controlled. The aftertreatment industry has been testing 
filters in the laboratory for many years in development of their 
products. In the case of ceramic type filters, MSHA is not aware of any 
types of tests performed on ceramics that does not use dpm from the 
diesel exhaust. Aftertreatment control manufacturers that build dpm 
control devices test their systems for various applications worldwide, 
through both laboratory and field work.
    Other types of filter media (e.g., paper) have been developed by 
the mining industry for use on permissible equipment which is specific 
to mining. General industry does not use paper for dpm reduction due to 
the high exhaust gas temperatures from diesels. Paper filters are 
mainly produced as intake air cleaners and industry test standards for 
determining air cleaner efficiency are followed. Since these filters 
are mainly used for intake air filters, MSHA believes that industry 
standard intake air filter tests could be representative tests for this 
type of filter media when used for dpm reduction. MSHA would compare 
the paper specifications to determine equivalency. If the papers were 
equivalent, then air filter type tests would be acceptable to the 
Secretary for this type of media.
    Aftertreatment device maintenance requirements. Section 72.503(d) 
of this rule states that any aftertreatment device installed on a piece 
of diesel equipment, upon which the operator relies to remove dpm, 
shall be maintained in accordance with manufacturer specifications and 
shall be free of observable defects. Except for the last phrase, which 
was added by MSHA in order to clarify the requirement for the mining 
community, this requirement was specified in the proposal under section 
72.500(d).
    One commenter requested that MSHA also require an on board engine 
performance and diagnostic system. MSHA is aware that some permissible 
machines have added electronic type shut down systems and electronic 
controlled fire suppression systems. On some newer nonpermissible 
engines, especially larger engines, engine manufacturers use electronic 
controls to regulate the engine's fuel injection timing and governing. 
Engines equipped with these electronic devices typically have complete 
diagnostic capability. MSHA believes as engine technologies develop, 
more engines will have diagnostic systems built in from the 
manufacturer. MSHA is not requiring in this final rule on board engine 
performance and diagnostic systems on equipment. However, MSHA will 
work with engine manufacturers under the part 7 approval process to 
evaluate new electronic controls, especially for permissible engines.
    Other commenters stated that maintenance is part of the toolbox 
approach, and therefore ought not to be specifically included. MSHA has 
a requirement in the current diesel equipment rule to maintain diesel 
powered equipment in approved and safe condition or be removed from 
service. This final rule is extending the requirements for maintenance 
specifically to aftertreatment controls added to the machines to reduce 
dpm.

Section 72.510  Miners Health Training

    Paragraph (a) of this section requires annual hazard awareness 
training of underground coal miners who can reasonably be expected to 
be exposed to dpm. Paragraph (b) includes provisions on records 
retention, access and transfer.
    Section 72.510(a) of this rule would require any underground coal 
miner ``who can reasonably be expected to be exposed to diesel 
emissions'' be trained annually in: (1) The health risk associated with 
exposure to diesel particulate matter; (2) the methods used in the mine 
to control diesel particulate matter concentrations; (3) identification 
of the person responsible for maintaining those controls; and (4) 
actions miners must take to ensure the controls operate as intended. 
The final rule is the same as that proposed.
    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 must be reminded of the dpm hazard to make them active and 
committed partners in implementing actions that will reduce that risk.
    Several commenters expressed concern about which miners will be 
required to be trained. MSHA believes the rule is clear on this issue. 
The training need only be provided to underground miners who can 
reasonably be expected to be exposed to

[[Page 5686]]

dpm at the mine. The training is to be provided by the operator; hence, 
it is to be without cost to the miner.
    The rule places no constraints on how the operator should conduct 
this training. MSHA believes that the required training can be provided 
with minimal cost and with minimal disruption. This final rule does not 
require any special qualifications for instructors, nor does it specify 
the hours of instruction.
    One-on-one discussions that cover the required topics is one 
approach that can be used. Alternatively, instruction could take place 
at safety meetings before the shift begins. Several of the training 
requirements can be covered by simply providing miners with a copy of 
MSHA's ``toolbox.'' Operators may determine how the ``toolbox'' can be 
used at their mine.
    The Agency requested comments concerning inclusion of dpm training 
in the required part 48 training plan. The only comment received 
suggested that this training be included in the part 48 training and 
removed from this rule. MSHA considered whether the requirements of 
part 48 were adequate to ensure the training required under the final 
diesel particulate standard. After careful consideration, MSHA 
concluded that available information provided to miners under current 
part 48 training would be inadequate to fully convey information under 
the diesel particulate final rule. MSHA will, however, accept part 48 
training for compliance with diesel particulate training requirements 
under this section, provided mine operators fully integrate the 
requirements of diesel particulate training into their existing 
program.
    Section 115 of the Federal Mine Safety and Health Act of 1977 and 
30 CFR part 48, ``Training and Retraining of Miners,'' requires 
operators to submit to MSHA and obtain its approval of training plans 
under which miners are provided training, primarily through initial and 
annual refresher training courses. Part 48, among other things, also 
specifies qualifications for training instructors, minimum training 
hours for miners and instruction on particular topics which must be 
covered within the specified minimum training time. Existing section 
48.8(a) establishes a minimum of eight hours of annual refresher 
training for underground miners. Section 48.8(b), specifies that 
underground miners must be trained on a minimum of eleven different 
subjects, none of which MSHA believes would cover the specific 
requirements for diesel particulate training.
    Nevertheless, MSHA believes compliance with this proposal can in 
many cases be fulfilled at the same time as scheduled part 48 training. 
The Agency, however, does not believe special language is required in 
this final rule to permit this action under part 48. If incorporated 
into part 48, mine operators would, however, be required to submit a 
revised training plan to the appropriate MSHA district office for 
approval. Some mine operators, however, may not be able to incorporate 
these topics in their part 48 plans. MSHA has endeavored to make the 
training requirements as simple as possible. If conducted separately 
from part 48 training, there are no specifications on trainer 
qualifications, no minimal training time, nor any training plans. If, 
however, the training is incorporated into part 48, then all applicable 
part 48 requirements will have to be met.
    A commenter expressed concerns about individual MSHA inspectors 
determining their own set of health risks for training purposes and 
then trying to cite a company for not training on those health risks. 
They also suggested that the Agency develop a ``Question and Answer'' 
document to address this problem. To address the mine operators concern 
about the training requirements, MSHA intends to develop an instruction 
outline that mine operators can use as a guide for training personnel. 
Instruction materials will also be provided with the outline. MSHA 
believes this will not only provide guidance to the mining industry but 
also to MSHA inspectors.
    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.
    Section 72.510(a)(1) of this rule requires the operator to train 
underground miners who can reasonably be expected to be exposed to 
diesel emissions in the health risk associated with dpm exposure. 
Several commenters disagreed with this requirement. They do not believe 
the health risks associated with exposures to diesel emissions have 
been sufficiently identified. ``If the health effects have not been 
identified, how can effective training be provided to the effected 
miners?'' MSHA disagrees with this comment. MSHA believes, as throughly 
discussed in Part III of this preamble, that the health effects 
associated with diesel emissions have been well documented. Comments 
received during this rulemaking further support MSHA's position 
concerning health effects associated with diesel emissions. Therefore, 
the requirements for training underground miners who can be reasonably 
be expected to be exposed to diesel emissions have been retained in the 
final rule.
    Section 72.510(a)(3) of this rule requires the operator to identify 
personnel responsible for maintaining the methods used to control dpm 
in the mine. Some commenters suggested removing this provision from the 
rule. These commenters objected to identifying the personnel 
responsible for maintaining the methods used to control dpm. Because 
they were concerned about having the employee, ``singled out from the 
remaining workforce.'' Another commenter, asked how MSHA wanted the 
operator to identify the employee responsible for maintaining dpm 
controls; is the name to be posted, made available to interested 
persons, put in the training plan, etc? While there is no provision in 
this final rule for posting the information on the mine bulletin board 
or in any other location, this information is required to be presented 
to any underground miner who can reasonably be expected to be exposed 
to diesel emissions. The final rule requires this information to be 
presented at least annually but does not specify any specific method 
for presenting the information. The operator has the option of 
presenting this information orally or in written form.
    The Agency believes this provision is consistent with the 
requirements contained in 30 CFR 75.1915(c). 30 CFR 75.1915(c) requires 
the operator to maintain a record of persons qualified to perform 
maintenance, repairs, examinations and tests on diesel-powered 
equipment. The operator is also required by Sec. 75.1915(c) to include 
a copy of the training program used to qualify persons to perform 
maintenance, repairs, examinations and tests in their records. Section 
75.1915(c) also requires the operator to make this record available for 
inspection by an authorized representative of the Secretary of Labor. 
All records that would need to be maintained concerning the 
qualification of personnel responsible for maintaining dpm controls are 
contained in Sec. 75.1915(c). The individuals identified by 
Sec. 75.1915(c) would also be the individuals identified in 
Sec. 72.510(a)(3). The requirement to identify personnel qualified to 
perform specialized tasks is not a novel approach. Therefore, 
Sec. 72.510(a)(3) has not been changed or deleted from the final rule.

[[Page 5687]]

    Section 72.510(b)(1) of this rule requires that any log or record 
produced signifying that the training has taken place would be retained 
for one year. A commenter stated other records are not required to be 
maintained and should not be required by this rule. Numerous training 
records are required to be maintained for a variety of training 
requirements throughout 30 CFR, and MSHA believes that retention of the 
record for one year is important for documentation purposes. Therefore, 
Sec. 72.510(b)(1) of this rule was not changed from the proposed rule 
and is incorporated in this final rule.
    The training records need to be where an inspector can view them 
during the course of an inspection, as the information in the record 
may determine how the inspection proceeds. If the mine site has a fax 
machine or computer terminal, MSHA would permit the record to be 
maintained elsewhere so long as they are readily accessible. This 
approach is consistent with the Office of Management and Budget 
Circular A-130 and 30 CFR 75.1915(c).
    Paragraph (b)(2) of section 72.510 of this rule requires mine 
operators to provide prompt access to the training records upon request 
from an authorized representative of the Secretary of Labor, the 
Secretary of Health and Human Services, or from an authorized 
representative of the miners. If an operator ceases to do business, all 
training records of employees are expected to be transferred to any 
successor operator. The successor operator is expected to maintain 
those training records for the required one year period unless the 
successor operator has undertaken to retrain the employees. There were 
no comments received concerning the maintenance of records by a 
successor operator. Therefore, the final rule has adopted the wording 
as published in the proposed rule.

Section 72.520  Diesel Equipment Inventory

    Proposed Sec. 75.371(qq) would have required, ``A list of diesel-
powered units used by the mine operator together with information about 
any unit's emission control or filtration system.'' One commenter 
stated that the proposal was vague and overly burdensome. The commenter 
also stated that exhaustive, detailed technical specifications were not 
needed in the approved ventilation plan. MSHA agrees with the comments 
and has changed the final rule to reflect what MSHA believes is 
necessary information to help evaluate the effectiveness of dpm 
controls in underground coal mines. By specifying the information 
required, MSHA has provided uniform guidance to the mining community as 
to the information required to be submitted in the diesel equipment 
inventory.
    Another commenter suggested the information be provided and posted 
at the mine and made available to a representative of the Secretary and 
other interested person. Another commenter was concerned with the time 
delay in submitting an addendum to the ventilation plan and the 
approval of the plan. The commenter stated that this was not required 
of other equipment used underground and should not be required of 
diesel-powered equipment. Concerns were raised by several commenters 
about delays in the approval of revisions to the ventilation plan.
    MSHA has taken these comments into consideration and in the final 
rule has removed the diesel equipment inventory provision from the 
Approved Ventilation Plan and established it as a separate requirement 
Sec. 72.520. There was no intent to require that the inventory be 
approved, but rather to require the information to be provided to MSHA 
and the representatives of the miners. The final rule requires each 
mine operator to prepare and submit a diesel equipment inventory to the 
District Manager. It also clarifies the information that must be 
included in the inventory. This information must be accurate so that 
the appropriate emission controls can be matched with an engine and to 
ensure that the required emission rates during the phase-in period are 
met. If there are modifications to the inventory, such as equipment 
being added or deleted, or changes to emission control systems, these 
modification must be submitted to the District Manager within 6 months. 
If no changes to the inventory are made, there is no need to update the 
diesel equipment inventory. The final rule also requires that mine 
operators provide a copy the diesel equipment inventory to the 
representative of the miners within 3 days.

Effective Dates

    The final rule provides that unless otherwise specified, its 
provisions take effect 60 days after the date of promulgation. Some 
provisions of the final rule contain delayed effective dates that 
provide more time for technical assistance to the operators. Table I-1 
presents the effective dates of various provisions of the final rule is 
reproduced below for convenience.

BILLING CODE 4510-43-P


[[Page 5688]]


[GRAPHIC] [TIFF OMITTED] TR19JA01.045


BILLING CODE 4510-43-C

    The final rule stipulates that any piece of diesel-powered 
equipment introduced into an underground coal mine 60 days after the 
promulgation date of this final rule is required to meet specific 
emission limits. For equipment that is currently used in underground 
coal mines, the compliance dates vary with regards to the type of 
diesel-powered equipment used in underground coal mines. MSHA includes 
in the category of equipment currently in use in underground coal mines 
any equipment that is ordered on or before the promulgation date of 
this final rule, even if the delivery date is more that 60 days from 
the promulgation date. By treating equipment on order as equipment 
already in use, the Agency is allowing the operator to use the 
equipment as delivered by the equipment supplier. A valid purchase 
order would be required of the operator as evidence that the diesel-
powered equipment was ordered on or before the promulgation date of the 
final rule.
    The time frame of 60 days after the promulgation date of the final 
rule also applies to newly introduced diesel-powered equipment as a 
result of explicit effective dates in 30 CFR 72.500, 72.501, and 72.502 
of this rule. Diesel-powered equipment that is introduced in an 
underground coal mine 60 days after the promulgation date of the final 
rule must emit no more than 2.5 grams per hour of dpm. The term 
``introduced'' is defined in Sec. 72.503(e) and is explained in the 
appropriate Section-by-Section discussion in this preamble.
    Section 72.500(b) of this rule allows the operator 18 months from 
the promulgation date of the final rule to meet emission limits for 
permissible diesel-powered equipment currently in use in underground 
coal mines. Several commenters stated the 18 month time frame was 
insufficient to comply with the proposed rule. They suggested 
increasing the effective date to between 2 and 4 years from the 
promulgation date of the final rule. The proposed rule would have 
required, in part, a system capable of removing, on average, at least 
95% of diesel particulate matter by mass. The only system reportedly 
available that achieved the filtration efficiency necessary, was the 
DST system. As discussed elsewhere in this preamble, the 
final rule sets emission limits on diesel-powered equipment and allows 
the operator to use whatever diesel particulate reducing technologies 
available to meet the limits. Information submitted during the rule 
making process and verification testing conducted for MSHA, has 
identified that readily available paper filters can achieve the 
emission limits set for permissible diesel-powered equipment. 
Therefore, MSHA has retained the 18 month effective date for diesel-
powered equipment currently in use in underground coal mines.
    Section 72.501 of this rule addresses emission limits for 
nonpermissible heavy-duty diesel-powered equipment, generators and 
compressors. There are 3 time tables associated with these pieces of 
diesel-powered equipment. As with permissible diesel-powered equipment, 
all nonpermissible heavy-duty diesel powered equipment, generators and 
compressors introduced into an underground coal mine 60 days from the 
promulgation date of the final rule would be required to meet a 
specific dpm emission limit. As stated the final rule differs from the 
proposed rule, however, the compliance date for newly introduced 
diesel-powered equipment has not been changed.
    The final rule allows 30 months from the promulgation date for the 
operator to reduce the emission levels to the levels required for newly 
introduced diesel-powered equipment. Some commenters believe this time 
frame should be increased to 3 to 4 years.

[[Page 5689]]

Another commenter stated the time frame for complying with the standard 
should be shortened. Based upon information obtained during the rule 
making process, MSHA believes the 30 month time table is adequate and 
reasonable to install the necessary particulate controls to comply with 
the required emission limits.
    Section 72.501(c) of this final rule requires all nonpermissible 
heavy-duty diesel-powered equipment, generators and compressors to meet 
a stricter emission limit within 4 years after promulgation of the 
final rule. The proposed rule would have allowed 6 years to achieve 
these stricter limits. After reviewing the record, particularly 
information submitted by aftertreatment device manufacturers, MSHA has 
concluded that these stricter standards can be met in a shorter time 
frame. Discussions on these emission limits are covered in greater 
detail elsewhere in this preamble. Therefore, the effective date for 
the stricter emission limits was reduced from 6 years to 4 years.
    Section 72.503 of this final rule addresses nonpermissible light-
duty diesel-powered equipment other than generators and compressors. 
The proposed rule did not address nonpermissible light-duty diesel-
powered equipment. As discussed earlier in the preamble, nonpermissible 
light-duty diesel-powered equipment has been included in this final 
rule. The final rule only addresses nonpermissible light-duty diesel-
powered equipment that is introduced 60 days after the promulgation 
date of this final rule. Equipment currently in use in underground coal 
mines is excluded from meeting emission limits. Based upon information 
gathered during the rule making process, MSHA believes 60 days after 
the promulgation date of the final rule is reasonable and this 
requirement has been added to the final rule.

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 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 coal mining industry.
    The discussion then turns to the main component of the rule being 
promulgated by the Agency for underground coal mines. MSHA is requiring 
that mine operators limit the emissions of dpm to defined quantities 
for various categories of diesel equipment underground. This part 
evaluates the rule to ascertain if, as required by the statute, it 
achieves the highest degree of protection for underground coal miners 
that is both technologically and economically feasible for mine 
operators.
    About half a dozen regulatory alternatives to the final rule were 
also reviewed by MSHA in light of the record. After considerable study, 
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 
coal mining 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 the Secretary 
of Labor (Secretary) in promulgating mandatory standards dealing with 
toxic materials or harmful physical agents under the Act, shall set 
standards when most:

    * * * [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, in promulgating 
these mandatory standards, must 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:

    * * * This 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 the agency 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 a reasonable assessment of 
the likely range of costs that a new standard will have on the 
industry. The agency must also show that a reasonable probability 
exists that the typical firm in the 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

[[Page 5690]]

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. The industry profile provides background 
information describing the structure and economic characteristics of 
the coal mining industry. This information was considered by MSHA in 
reaching its conclusions about the economic feasibility of various 
regulatory alternatives.
    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.
    MSHA 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 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--for mining, 500 or fewer employees--when determining a 
rule's economic impact.
    Table V-1 presents the total number of small and large coal mines 
and the corresponding number of miners, excluding contractors, for the 
coal mining segment. This table uses three mine size categories based 
on the number of employees: (1) fewer than 20 employees (MSHA's 
traditional definition of small), (2) 20 to 500 employees (small 
according to SBA's definition) and (3) more than 500 employees. Table 
V-1 further disaggregates data by surface mines and underground mines, 
as well as (for employees) office workers. Table V-2 presents 
corresponding data on the number of independent contractors and their 
employees working in the coal mining segment.
    Although this particular rulemaking does not apply to the surface 
coal sector, information about surface coal mines is provided here in 
order to give context for the discussions on underground mining.

 Table V-1.--Distribution of Coal Mine Operations and Employment (Excluding Contractors) by Mine Type and Size a
----------------------------------------------------------------------------------------------------------------
                                                                                   Mine type
                                                             ---------------------------------------------------
       Size of coal mine b                                                                  Office
                                                                Underground     Surface     workers   Total coal
----------------------------------------------------------------------------------------------------------------
Fewer Than 20 Employees..........  Mines....................             382       1,058  ..........       1,438
                                   Employees................           3,751       6,491         487      10,729
20 to 500 Employees..............  Mines....................             522         492  ..........       1,014
                                   Employees................          39,566      31,731       3,389      74,692
Over 500 Employees...............  Mines....................               6           1  ..........           7
                                   Employees................           3,459         510         189       4,158
All Coal Mines...................  Mines....................             910       1,549  ..........       2,459
                                   Employees................          46,776      38,738       4,065     89,579
----------------------------------------------------------------------------------------------------------------
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 1, p. 5.
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 Contractors and Contractor Employment by Size of Operation a
----------------------------------------------------------------------------------------------------------------
                                                                                  Contractors
                                                             ---------------------------------------------------
       Size of contractor b                                                                 Office
                                                                Underground     Surface     workers      Total
----------------------------------------------------------------------------------------------------------------
Fewer Than 20 Employees..........  Mines....................           1,077       2,403  ..........       3,480
                                   Employees................           4,078       9,969       1,064      15,111
20 to 500 Employees..............  Mines....................              79         242  ..........         321
                                   Employees................           4,131      11,618       1,192      16,941
Over 500 Employees...............  Mines....................  ..............  ..........  ..........  ..........
                                   Employees................  ..............  ..........  ..........  ..........
Total Contractors................  Mines....................           1,156       2,645  ..........       3,801
                                   Employees................           8,209      32,052       2,256     30,052
----------------------------------------------------------------------------------------------------------------
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 5, p. 20.

[[Page 5691]]

 
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.

    Agency data (Table V-1) indicate that there were about 2,459 coal 
mines in 1998. When applying MSHA's definition of a small mine (fewer 
than 20 workers), 1,438 (about 58%) were small mines and 1,021 (about 
42%) were large.\82\ Using SBA's definition, only 7 coal mines (0.3 
percent) were large. These data show that employment at coal mines in 
1998 was about 89,600, of which (by MSHA's definition) about 10,700 (12 
percent) worked at small mines and 78,900 (88 percent) worked at large 
mines.\83\ Using SBA's definition, 95 percent of coal miners worked at 
small mines and 5 percent worked at large mines. Using MSHA's 
definition, small coal mine average 7 employees, and large coal mines 
average 77 employees. Using SBA's definition, there are, on average, 35 
employees in each small coal mine and 594 employees in each large coal 
mine. MSHA classifies the U.S. coal mining segment into two major 
commodity groups: bituminous and anthracite. About 92 percent of total 
coal production is bituminous. The remaining 8 percent is the product 
of lignite and anthracite mines.\84\
---------------------------------------------------------------------------

    \82\ U.S. Department of Labor, MSHA, 1998 Final MIS data CM441 
cycle 1998/198.
    \83\ U.S. Department of Labor, MSHA, 1998 Final MIS data CM441 
cycle 1998/198.
    \84\ U.S. Department of Energy, Energy Information 
Administration, Annual Energy Review 1998, July 1999, p. 191.
---------------------------------------------------------------------------

    Mines east of the Mississippi accounted for about 49% of coal 
production in 1998. For the period 1949 through 1998, coal production 
east of the Mississippi River fluctuated relatively little, from a low 
of 395 million tons in 1954 to a high of 630 million tons in 1990; 1998 
production was estimated at 571 million tons. Coal production west of 
the Mississippi, by contrast, increased each year from a low of 20 
million tons in 1959 to a record high of 548 million tons in 1998.\85\ 
The growth in western coal has been due, in part, to environmental 
concerns that led to increased demand for low-sulfur coal, which is 
abundant in the West.
---------------------------------------------------------------------------

    \85\ U.S. Department of Energy, Energy Information 
Administration, Annual Energy Review 1998, July 1999, p. 191.
---------------------------------------------------------------------------

    In addition, surface mining, with its higher average productivity, 
is much more prevalent in the West. Surface mining methods for coal, 
which include drilling and blasting, are also practiced in surface 
mines for other commodity types. Most surface mines use front-end 
loaders, bulldozers, shovels, or trucks for haulage.
    The U.S. coal sector produced a record 1.12 billion short tons of 
coal in 1998, at an average price of $17.58 per ton. The total value of 
U.S. coal production in 1998 was estimated as $19.7 billion. Small 
mines (by MSHA's definition) produced about 4 percent (40 million tons) 
of domestic coal production valued at $0.7 billion, and large mines (by 
MSHA's definition) produced about 96 percent (1.08 billion tons) valued 
at $19.0 billion.\86\
---------------------------------------------------------------------------

    \86\ U.S. Department of Energy, Energy Information 
Administration, Annual Energy Review 1998, July 1999, p. 203, U.S. 
Department of Energy, Energy Information Administration, Coal 
Industry Annual 1997, December 1998, pp. ix and 154, and U.S. 
Department of Labor, Mine Safety and Health Administration, Division 
of Mining Information Systems, 1998 Final MIS data (quarter 1-
quarter 4) CM441 cycle 1998/198.
---------------------------------------------------------------------------

    The U.S. coal industry enjoys a fairly constant domestic demand. 
Over 90 percent of U.S. coal demand was accounted for by electric 
utilities in 1998.\87\ Due to the high conversion costs of changing a 
fuel source, MSHA does not expect a substantial change in coal demand 
by utility power plants in the near future.\88\
---------------------------------------------------------------------------

    \87\ U.S. Department of Energy, Energy Information 
Administration, Annual Energy Review 1998, July 1999, p. 187.
    \88\ U.S. Department of Energy, Energy Information 
Administration, Annual Energy Outlook 2000, p. 68.
---------------------------------------------------------------------------

    Adequacy of Miner Protection Provided by the Rule for Underground 
Coal Mines. In evaluating the protection provided by the rule, it 
should be noted that MSHA has measured dpm concentrations in production 
areas and haulageways of underground coal mines which exceed 
2500DPM g/m3 with a mean concentration 
of 644DPM g/m3. See Table III-1 and 
Figure III-1 in part III of this preamble. As discussed in detail in 
part III of this preamble, these concentrations place underground coal 
miners at significant risk of material impairment of their health, and 
the evidence supports the proposition that reducing the exposure 
reduces the risk.
    The final rule would require operators to limit the emissions of 
dpm emitted by various categories of equipment in underground coal 
mines--permissible, heavy duty (and compressors and generators), and 
other light duty. Equipment added to a mine's inventory more than 60 
days after the rule is promulgated (or equipment already in the 
inventory but equipped with a new engine after that time), would have 
to comply with the appropriate standard. In addition, operators would 
have 18 months to bring the existing fleet of permissible diesel 
equipment into compliance with a 2.5 gr/hr emission standard. Operators 
would have an additional year (30 months from date of promulgation) to 
bring the existing fleet of heavy duty equipment (and generators and 
compressors) into compliance with a 5.0 gr/hr emission standard, and up 
to 4 years in all to bring that fleet down to a standard of 2.5 gr/hr.
    As an example of how these emission standards can reduce dpm 
concentration levels in a section of an underground coal mine, take the 
case of a single-section mine with three Ramcars (94hp, indirect 
injection) and a section airflow of 45,000 cfm. MSHA measured 
concentrations of dpm in this mine at 610DPM g/
m3. Of this amount, 25DPM g/
m3 was coming from the intake to the section, and the 
remaining 585 DPM g/m3 was emitted by 
the engines. Reducing the engine emissions by 95% through the use of 
commercially available paper filters would reduce the dpm emitted to 
29DPM g/m3. With an intake amount of 
25DPM g/m3, the ambient concentration 
would be about 54DPM g/m3. Similarly, 
dramatic results can be achieved in almost any situation by adding high 
efficiency aftertreatment filters or by replacing current engines in 
the fleet with a more recent generation.
    While the reductions in section concentration from the controls 
required by the final rule can be significant, it is important to 
recognize that the actual reductions in a section will vary depending 
upon a number of factors.
    In the first place, unlike the proposed rule, the final rule does 
not require current dpm emissions from each machine to be reduced by 
95%. While the existing permissible fleet, and much of the existing 
heavy duty fleet, will need to reduce engine emissions significantly to 
come into compliance with the final standard, this will be feasible in 
many cases with a less efficient filter. A detailed table illustrating 
by how much the emissions from each current engine in the inventory 
must be reduced to achieve compliance is shown in table IV-1.
    Second, while aftertreatment filters currently available are 
capable in laboratory tests of achieving a very significant reduction 
in dpm mass, and this has been confirmed in some field tests, the 
Agency has not tested filter efficiency under a variety of actual 
mining conditions. Therefore, actual performance may be different in 
the field due to individual mining

[[Page 5692]]

conditions (e.g., ventilation changes, changes of the equipment due to 
maintenance, and the type of engine used).
    Third, the impact on a mine section of reduced emissions from a 
particular machine depends upon the ventilation rate and the ambient 
dpm intake into the section. If ventilation levels drop below the 
requirements established to control gaseous emissions, or if many 
pieces of equipment throughout the mine create a high ambient level of 
dpm, implementation of the rule may not bring concentrations down as 
effectively as suggested in the prior example. On the other hand, if 
the ventilation rate is maintained at a higher level, the emissions 
would be better diluted and the ambient concentration could offset any 
decrease in control efficiency under actual mining conditions. The 
intake of dpm to any section depends on what emissions are upstream. In 
this regard, it should be noted that the final rule does not require 
controls on the existing fleet of light-duty equipment, except for 
generators and compressors; hence, mines with significant light duty 
equipment will have this exhaust as an ``intake'' in such calculations.
    Table V-3 summarizes information from a series of simulations 
designed to illustrate some of these variables. The simulations were 
performed using MSHA's ``Estimator''--a computerized spreadsheet 
designed to calculate dpm ambient levels from given equipment, and the 
impact of various controls on those ambient levels. (The Estimator was 
discussed in detail in an Appendix to the preamble to the proposed rule 
and has since been published (Haney and Saseen, April 2000)). The 
example simulated here involves a mine section with a 94 horsepower 
engine, with a 0.3 gm/hp-hr dpm emission rate and a nameplate airflow, 
5500 cfm. The engine was operated during an eight hour shift. The 
Estimator was used to calculate the section concentrations with a paper 
filter at full laboratory efficiency (95%) and two lower filter 
efficiencies. The same results would be obtained for multiple pieces of 
equipment provided that the nameplate airflow is additive for each 
piece of equipment.

BILLING CODE 4510-43-P

[[Page 5693]]

[GRAPHIC] [TIFF OMITTED] TR19JA01.046


BILLING CODE 4510-43-C
    In Table V-3, the intake dpm (second column) increases after every 
fourth row. Within each group of four rows, the ventilation (first 
column) increases

[[Page 5694]]

from one row to the next. The last 3 columns display the ambient dpm 
concentration with a particular filter efficiency.
    The first four rows represent a situation where there is no intake 
dpm. If the mine is ventilated with four times the nameplate airflow 
(row 4), the ambient dpm concentration using a filter operating at 95% 
(last column) is reduced to 38DPM g/m\3\. If the 
filter in this situation only works in practice at 85% efficiency in 
removing dpm, the ambient dpm concentration is only reduced to 
113DPM g/m\3\. And if the ventilation is reduced to 
the nameplate airflow (first column) and the filter is only 85% 
efficient, the ambient dpm climbs to 452DPM g/m\3\.
    The last four rows display the parallel situation but with an 
ambient intake concentration to the section of 75DPM 
g/m\3\. In this situation, depending on ventilation and filter 
effectiveness, the ambient dpm concentration ranges from 
113DPM to 527DPM g/m\3\.
    In the example discussed above--a single section mine with three 94 
hp Ramcars--the airflow of 45,000 cfm represents three times the 
current nameplate requirements. Many underground coal mines may use 
more than the nameplate ventilation to lower methane concentrations at 
the face. But if this airflow were reduced to the current nameplate 
requirements, the ambient dpm would have been 1620DPM 
g/m\3\, and would have been reduced by 95% effective filters 
to 105DPM g/m\3\.
    Based on its experience as to the general effects of mining 
conditions on the expected efficiency of equipment, and on ventilation 
rates, MSHA has concluded that the rule for this sector will 
substantially reduce the concentrations of dpm to which underground 
coal miners are exposed.
    Alternatives considered. In order to ensure that the maximum 
protection that is feasible for the underground mining industry as a 
whole is provided, the Agency has considered some alternatives. Most 
are discussed elsewhere in this preamble, but are briefly repeated here 
and illustrate the extensive thought MSHA gave to this issue.
    (1) Establish a Concentration Limit. MSHA considered establishing a 
dpm concentration limit for this sector, as it is doing for underground 
metal and nonmetal mines. A concentration limit provides operators with 
flexibility to select any combination of controls that keep ambient dpm 
concentrations below the limit.
    The agency has concluded that it is not yet technologically 
feasible to establish a dpm concentration limit for underground coal 
mines. The problem is that significant questions remain as to whether 
there is a sampling and analytical system that can provide consistent 
and accurate measurements of dpm in areas of underground coal mines 
where there is a heavy concentration of coal dust. The Agency is 
continuing to work on the technical issues involved, and should it 
determine that these technological problems have been resolved, it will 
notify the mining community and proceed accordingly.
    (2) 95% Filters on Defined Categories of Equipment. This is what 
the agency initially proposed for this sector. It has the advantage of 
ensuring that all controlled equipment is filtered, which some assert 
is easier to keep in proper shape through observation, and others 
believe provides more protection against nanoparticles. On the other 
hand, such an approach may quickly become technologically infeasible as 
newer, cleaner engines are introduced underground; removing 95% (or any 
defined percentage) of the lower emissions of these engines is likely 
to prove much more difficult. Moreover, this approach could act as a 
disincentive to introduce cleaner engines underground, and thus slow 
the reduction of dpm that such a replacement fleet might make possible. 
Finally, the Agency determined that at this time, there is not enough 
evidence about the risks of nanoparticles to regulate on that basis. 
Accordingly, the agency rejected this approach in order to avoid the 
problems associated with its implementation over the long term.
    (3) A machine-based emissions limit with credit for extra 
ventilation used in the mine. Under this approach, if the bench test of 
the combined engine and filter package was conducted at the approval 
plate ventilation, a mine's use of more than that level of ventilation 
would be factored into the calculation of what package would be 
acceptable. So if, for example, an engine equipped with a ceramic 
filter can reduce emissions to 5.0 grams/hour in a test using the 
approval plate ventilation, and the mine actually ventilates at twice 
the name plate ventilation, the system would be deemed to reach 2.5 
grams/hour under that circumstance. This alternative, however, is less 
protective than the rule adopted by the agency, as it would not require 
dpm emissions to be reduced as much. Accordingly, since the more 
protective alternative is feasible as well, it would be inappropriate 
under the law for the agency to adopt this alternative.
    (4) Adjust the Time-Frame for Implementation of the Final Rule. The 
final rule will not be fully implemented for several years. The 
existing permissible fleet is given a full 18 months to comply, even 
though the agency has determined that there are readily available paper 
filters which can bring this equipment into compliance. The 
implementation schedule for the existing heavy duty fleet (and 
compressors and generators) extends for 4 years from the date of 
promulgation, even though the agency has concluded that there are hot 
gas filters readily available which can bring most of this equipment 
into compliance with the final emissions limit. Accordingly, the agency 
has considered whether a faster implementation schedule is feasible.
    Cutting the 18 month time-frame for permissible equipment does not 
appear to be practicable for the industry. Eighteen months to obtain 
and install a relatively new technology is a reasonable time. Time is 
needed for operators to familiarize themselves with this technology. 
Also, mine personnel have to be trained in how to maintain control 
devices in working order. Moreover, MSHA needs time to work with the 
mining community to develop a revised approach to approving engines for 
use in permissible equipment in order to accelerate the introduction of 
a cleaner generation of engines into the permissible fleet.
    With respect to the heavy duty fleet, the four years permitted to 
meet the final emissions limit is actually two years faster than 
originally proposed by the agency when 95% filters were being proposed. 
As indicated in section 6 of Part II of this preamble, the development 
of high efficiency hot gas filters has proceeded much faster than 
expected, so that it is technologically feasible to comply more quickly 
with this requirement than originally proposed. Moreover, MSHA has 
determined that the cost differential to the industry of reaching the 
final 2.5 micrograms/hour emission limit in 4 years instead of 6 is 
minor (see REA). However, MSHA has concluded that moving up the 
timeline further would create unwarranted difficulties for operators in 
terms of installing the required engines and filters, and accordingly 
has determined that further acceleration of this schedule would be 
infeasible.
    (5) Require Machine Emission Limits on all Diesel Equipment in 
Underground Coal Mines. The final rule would not immediately apply to 
more than 60% of the fleet--light-duty equipment other than generators 
and compressors. Over time, the final rule would have an impact on the 
remaining light duty fleet through controls on any new equipment 
introduced underground, but it will take

[[Page 5695]]

many years before mine workers get the benefits of this approach. By 
contrast, the Commonwealth of Pennsylvania has recently adopted 
legislation for universal high-efficiency filtration based on an 
agreement in the mining community of that state. The Pennsylvania law 
requires that all diesel-powered equipment introduced into underground 
coal mines in that state (essentially all equipment, given the past 
ban), meet an emissions limit requirement (as well as a separate filter 
requirement).
    One reason asserted for not covering all light duty equipment is 
that this equipment may run only intermittently, and under light loads, 
hence producing less dpm than other kinds of equipment. This 
proposition was supported by industry representatives during the 
rulemaking, and disputed by miners during the rulemaking proceedings. 
The Agency has not been able to draw any conclusions based on the mixed 
evidence as to the light duty fleet as a whole; as noted previously, it 
has carved out the 3% of the light duty fleet that clearly works like 
heavy duty equipment, and is covering them in this rule (generators and 
compressors).
    A second issue is costs. The Agency decided to consider what it 
would take to bring the rest of the industry up to the standard 
established under the Pennsylvania agreement of universal coverage. 
MSHA has calculated that such a requirement would cost the underground 
coal industry an additional $9.7 to $17.4 million a year. This would be 
an increase of 135-240% of the cost of the rule for the underground 
coal mining industry. Since drawing conclusions concerning the level of 
dpm actually produced by light duty equipment in underground coal mines 
is difficult, the Agency has decided to take the approach of phasing in 
emission controls for light duty outby equipment over a period of five 
years. This approach significantly reduces the cost of the rule. 
Eventually, dpm exposures will be reduced for all miners in all areas 
of the mine.
    (6) Requiring certain engines to meet defined particulate emission 
standards. As discussed in part II of this preamble, the Mine Safety 
and Health Advisory Committee on Standards and Regulations for Diesel-
Powered Equipment in Underground Coal Mines recommended the 
establishment of a particulate index (PI), and MSHA did so in its 
diesel equipment rule. Under that rule, the PI establishes the amount 
of air required to dilute the dpm produced by an engine (as determined 
during its approval test under subpart E of part 7) to 1000 g/
m3.
    In the preamble of the diesel equipment rule, MSHA noted that mine 
operators and machine manufacturers would find it useful to consider 
the engine PI in selecting and purchasing decisions. The agency 
explicitly deferred until this rulemaking the question of whether to 
require engines used in mining environments to meet a particular PI.
    In its final rule, the Agency is, in fact, using a significant 
portion of the concepts embodied in the particulate index. The 
determination of the quantity of dpm emitted from the machine is based 
on the information from the engine approval tests in 30 CFR 7.89 as was 
used to establish the particulate index. Both means of expressing the 
dpm characteristics of the machine begin with determining the total 
amount of dpm, expressed in grams/hour, produced by the engine over the 
test cycle described in ISO 8178. The particulate index is determined 
by calculating the quantity of air required to dilute that particulate 
to a concentration of 1 mg/m\3\. The quantity of dpm emitted from the 
machine is determined by multiplying the quantity of dpm emitted from 
the engine by the filtration efficiency of the aftertreatment device.
    Had the agency been able to utilize a concentration limit in this 
sector, the particulate index could have been used directly to compute 
an estimated level of dpm that could be achieved with various 
quantities of ventilation air. As noted above, however, that approach 
was found to be infeasible.
    Feasibility of final rule for underground coal mining sector. The 
Agency has carefully considered both the technological and economic 
feasibility of the rule for the underground coal mining sector as a 
whole.
    Although some doubts were expressed about this during the 
rulemaking proceedings, it is clear now that the technology exists to 
implement the final rule's requirements. As this preamble explains in 
overview in section 6 of Part II, and reiterates in connection with the 
specific requirements of the rule in Part IV, there are available 
emission controls which can bring all existing and contemplated future 
diesel equipment into compliance with the requirements of the rule. 
Paper filters have now been verified to reduce emissions from the 
dirtiest permissible engines to the required limit of 2.5 grams per 
hour. Ceramic filters have been certified by VERT to have the 
efficiency required to reduce emissions from the dirtiest heavy duty 
engines to the interim limit of 5.0 grams/hour, and for all but one 
engine to the final limit of 2.5 grams/hour. Approved engines that meet 
the emissions limit for newly introduced light duty equipment are 
available for all categories. And as MSHA and the mining industry work 
together to address aspects of the approval process that may be 
inhibiting the introduction of the newer generations of engines into 
underground mines, there should be no technological nor practical 
barriers to further emission limit reductions.
    The economic feasibility of this rule has also been carefully 
considered by MSHA. The total for the final rule for underground coal 
mines will be about $7 million per year. The costs per dieselized mine 
are expected to be about $48,000 a year. MSHA has calculated that the 
costs of the final rule amount to less than one-quarter of one percent 
(0.23 percent) of the annual revenues of the dieselized underground 
coal mining sector. (The methodology for this calculation is discussed 
in Chapter IV of the Agency's REA). After reviewing the economic 
profile of that sector, and taking into account the cost of 
implementing the related diesel equipment rule, MSHA has concluded that 
the rule is economically feasible for this sector as a whole.
    Conclusion: Underground Coal Mines. Based on the best evidence 
available to it at this time, the Agency has concluded that the final 
rule for the underground coal sector meets the statutory requirement 
that it 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 coal 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

[[Page 5696]]

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 Certification.

    The Regulatory Flexibility Act (RFA) requires regulatory agencies 
to consider a rule's economic impact on small entities. Under the RFA, 
MSHA must use the Small Business Administration's (SBA's) criterion for 
a small entity in determining a rule's economic impact unless, after 
consultation with the SBA Office of Advocacy, MSHA establishes an 
alternative definition for a small mine and publishes that definition 
in the Federal Register for notice and comment. For the mining 
industry, SBA defines ``small'' as a mine with 500 or fewer workers. 
MSHA traditionally has considered small mines to be those with fewer 
than 20 workers. To ensure that the final rule conforms with the RFA, 
MSHA has analyzed the economic impact of the final rule on mines with 
500 or fewer workers (as well as on those with fewer than 20 workers).
    MSHA has determined that the final rule would not have a 
significant economic impact on small mines, whether a small mine is 
defined as one with 500 or fewer workers or one with fewer than 20 
workers.
    Using the Agency's traditional definition of a small mine, which is 
one employing fewer than 20 workers, the estimated yearly cost of the 
final rule on small underground coal mines will be about $7,400. This 
estimated annualized cost for small mines compares to estimated annual 
revenues of approximately $9.1 million for the class of small 
underground coal mines.
    Using SBA's definition of a small mine, which is one employing 500 
or fewer workers, the estimated yearly cost of the final rule for all 
small underground coal mines would be about $6.1 million. This 
estimated cost for small mines compares to estimated annual revenues of 
approximately $2.95 billion for small underground coal mines, using 
SBA's criteria.
    Based on its analysis, MSHA has determined that the final rule 
would not have a significant economic impact on a substantial number of 
small mines. MSHA has so certified these findings to the Small Business 
Administration. The factual basis for this certification is discussed 
in Chapter V of the REA for this rule.

(C) 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.

(D) 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 
paperwork burden hours on underground coal mine operators that use 
diesel powered equipment and on manufacturers of diesel powered 
equipment. For mine operators that use diesel powered equipment, the 
final rule imposes two types of burden hours. 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). 
Manufacturers of diesel equipment that are affected by this rule, will 
incur only first year burden hours.

Mine Operators

First Year Burden Hours
    In the first year that the rule takes effect, mine operators will 
incur 997 burden hours, which is composed of 349 first year burden 
hours (from Table VI-1) and 648 annual burden hours (from Table VI-
1(a)). The related costs to mine operators will be $33,049, of which 
$12,627 is related to first year burden hours (from Table VI-1) and 
$20,422 is related to annual burden hours (from Table VI-1(a)).
Burden Hours After the First Year
    Beginning in the second year the rule takes effect and continuing 
every year thereafter, mine operators will incur 648 burden hours and 
related costs of $20,422 (from Table VI-1(a)).

Manufacturers

First Year Burden Hours
    In the first year that the rule is in effect, manufacturers will 
incur 700 burden hours and related costs of $35,000 (from Table VI-2). 
After the first year, manufacturers will not incur any burden hours or 
related costs.

                                                  Table VI-1.--Mine Operators--First Year Burden Hours
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                         20 emp.           20 to 500 emp.           >500 emp.               Total
                             Detail                              ---------------------------------------------------------------------------------------
                                                                     Hrs.      Costs       Hrs.      Costs       Hrs.      Costs       Hrs.      Costs
--------------------------------------------------------------------------------------------------------------------------------------------------------
75.1915/72.503..................................................        1.0        $28         50     $1,299        1.0        $14         52     $1,341
72.510..........................................................        0.6         29         11        568        0.1          4         12        602
72.520..........................................................        9.0        399        267     10,027        9.0        257        285     10,684
                                                                 ---------------------------------------------------------------------------------------
      Total.....................................................       11.0        456        329     11,895       10.0        276        349     12,627
--------------------------------------------------------------------------------------------------------------------------------------------------------


                                                   Table VI-1(a).--Mine Operators--Annual Burden Hours
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                        20 emp.            20 to 500 emp.           >500 emp.               Total
                             Detail                             ----------------------------------------------------------------------------------------
                                                                    Hrs.       Costs       Hrs.      Costs       Hrs.      Costs       Hrs.      Costs
--------------------------------------------------------------------------------------------------------------------------------------------------------
72.510.........................................................         5.0       $167        563    $17,971       28.0       $922        597    $19,061
72.1915/72.503.................................................         0            0          4         76        0.3          5          4         82
72.520.........................................................         0.3          8         43      1,177        3.5         94         47      1,279
                                                                ----------------------------------------------------------------------------------------
      Total....................................................         5.0        176        610     19,225       32.0      1,021        648     20,422
--------------------------------------------------------------------------------------------------------------------------------------------------------


[[Page 5697]]


             Table VI-2.--Manufacturers--Annual Burden Hours
------------------------------------------------------------------------
                    Detail                          Hrs.        Costs
------------------------------------------------------------------------
Amended Applications..........................          700      $35,000
------------------------------------------------------------------------

    The paperwork provisions for the proposed rule were approved under 
OMB Control Number 1219-0124. Our paperwork submission summarized above 
is explained in detail in the final 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. This final 
rule is being submitted to OMB under the same control number. 
Respondents are not required to respond to any collection of 
information unless it displays a current valid OMB control number.

(E) 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.

(F) 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.

(G) 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.

(H) Executive Order 12988  Civil Justice Reform

    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.

(I) 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.

(J) 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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[[Page 5703]]

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[[Page 5704]]

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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.
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News Release, ``ARB Identifies Diesel Particulate Emissions as a 
Toxic Air Contaminant,'' August 27, 1998.
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Properties of 3 Environmental Classes of Diesel Oil and Their 
Indicator Dyes,'' Contact Dermatitis, 34:309-315, 1996.
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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 72

    Coal, Health standards, 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:

PART 72--[AMENDED]

    1. The authority citation for Part 72 continues to read as follows:

    Authority: 30 U.S.C. 811, 813(h), 957, 961.

    2. Part 72 is amended by adding Subpart D to read as follows:

Subpart D--Diesel Particulate Matter--Underground Areas of 
Underground Coal Mines

72.500   Emission limits for permissible diesel-powered equipment.
72.501   Emission limits for nonpermissible heavy-duty diesel-
powered equipment, generators and compressors.
72.502   Requirements for nonpermissible light-duty diesel-powered 
equipment other than generators and compressors.
72.503   Determination of emissions; filter maintenance; definition 
of ``introduced''.
72.510   Miner health training.
72.520   Diesel equipment inventory.

Subpart D--Diesel Particulate Matter--Underground Areas of 
Underground Coal Mines


Sec. 72.500  Emission limits for permissible diesel-powered equipment.

    (a) Each piece of permissible diesel-powered equipment introduced 
into an underground area of an underground coal mine after March 20, 
2001 must not emit no more than 2.5 grams per hour of diesel 
particulate matter.
    (b) As of July 19, 2002, each piece of permissible diesel-powered 
equipment operated in an underground area of an underground coal mine 
must not emit no more than 2.5 grams per hour of diesel particulate 
matter.


Sec. 72.501  Emission limits for nonpermissible heavy-duty diesel-
powered equipment, generators and compressors.

    (a) Each piece of nonpermissible heavy-duty diesel-powered 
equipment (as defined by Sec. 75.1908(a) of this part), generator or 
compressor introduced into an underground area of an underground coal 
mine after March 20, 2001 must not emit no more than 5.0 grams per hour 
of diesel particulate matter.

[[Page 5705]]

    (b) As of July 21, 2003, each piece of nonpermissible heavy-duty 
diesel-powered equipment (as defined by Sec. 75.1908(a) of this part), 
generator or compressor operated in an underground area of an 
underground coal mine must not emit no more than 5.0 grams per hour of 
diesel particulate matter.
    (c) As of January 19, 2005, each piece of nonpermissible heavy-duty 
diesel-powered equipment (as defined by Sec. 75.1908(a) of this part), 
generator or compressor operated in an underground area of an 
underground coal mine must not emit no more than 2.5 grams per hour of 
diesel particulate matter.
    (d) Notwithstanding the other provisions of this section, a 
generator or compressor that discharges its exhaust directly into 
intake air that is coursed directly to a return air course, or 
discharges its exhaust directly into a return air course, is not 
subject to the applicable requirements of this section.


Sec. 72.502  Requirements for nonpermissible light-duty diesel-powered 
equipment other than generators and compressors.

    (a) Each piece of nonpermissible light-duty diesel-powered 
equipment (as defined by Sec. 75.1908(b) of this part), other than 
generators and compressors, introduced into an underground area of an 
underground coal mine after March 20, 2001 must not emit no more than 
5.0 grams per hour of diesel particulate matter.
    (b) A piece of nonpermissible light-duty diesel-powered equipment 
must be deemed to be in compliance with the requirements of paragraph 
(a) of this section if it utilizes an engine which meets or exceeds the 
applicable particulate matter emission requirements of the 
Environmental Protection Administration listed in Table 72.502-1, as 
follows:

                             Table 72.502-1
------------------------------------------------------------------------
       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)............  Tier 2 nonroad......  Varies by power:
                              kW (hp11)...........  0.80 g/kW-hr (0.60 g/
                                                     bhp-hr).
                              8kW19      0.80 g/kW-hr (0.60 g/
                               (11hp25).  bhp-hr).
                              19kW37     0.60 g/kW-hr (0.45 g/
                               (25hp50).  bhp-hr).
                              37kW75     0.40 g/kW-hr (0.30 g/
                               (50hp100   bhp-hr).
                               ).
                              75kW130    0.30 g/kW-hr (0.22 g/
                               (100hp17   bhp-hr).
                               5).
                              130kW225   0.20 g/kW-hr (0.15 g/
                               (175hp30   bhp-hr).
                               0).
                              225kW450   0.20 g/kW-hr (0.15 g/
                               (300hp60   bhp-hr).
                               0).
------------------------------------------------------------------------
 Notes: ``g'' means grams; ``kW'' means kilowatt; ``hp'' means
  horsepower; ``g/kW-hr'' means grams/kilowatt-hour; ``g/bhp-hr'' means
  grams/brake horsepower-hour.

    (c) The requirements of this section do not apply to any diesel-
powered ambulance or fire fighting equipment that is being used in 
accordance with the mine fire fighting and evacuation plan under 
Sec. 75.1101-23.


Sec. 72.503  Determination of emissions; filter maintenance; definition 
of ``introduced''.

    (a) MSHA will determine compliance with the emission requirements 
established by this part by using the amount of diesel particulate 
matter emitted by a particular engine determined from the engine 
approval pursuant to Sec. 7.89(a)(9)(iii)(B) or Sec. 7.89(a)(9)(iv)(A) 
of this title, with the exception of engines deemed to be in compliance 
by meeting the EPA requirements specified in Table 72.502-1 
(Sec. 72.502(b)).
    (b) Except as provided in paragraph (c) of this section, the amount 
by which an aftertreatment device can reduce engine emissions of diesel 
particulate matter as determined pursuant to paragraph (a) must be 
established by a laboratory test:
    (1) on an approved engine which MSHA has determined, pursuant to 
paragraph (a) of this section, to emit no more diesel particulate 
matter than the engine being used in the piece of diesel-powered 
equipment in question;
    (2) using the test cycle specified in Table E-3 of Sec. 7.89 of 
this title, and following a test procedure appropriate for the 
filtration system, by a laboratory capable of testing engines in 
accordance with the requirements of Subpart E of part 7 of this title; 
and
    (3) with an aftertreatment device representative of that being used 
on the piece of diesel-powered equipment in question.
    (c) In lieu of the laboratory tests required by paragraph (b), the 
Secretary may accept the results of tests conducted or certified by an 
organization whose testing standards are deemed by the Secretary to be 
as rigorous as those set forth by paragraph (b) of this section; and 
further, the Secretary may accept the results of tests for one 
aftertreatment device as evidencing the efficiency of another 
aftertreatment device which the Secretary determines to be essentially 
identical to the one tested.
    (d) Operators must maintain in accordance with manufacturer 
specifications and free of observable defects, any aftertreatment 
device installed on a piece of diesel equipment upon which the operator 
relies to remove diesel particulate matter from diesel emissions.
    (e) For purposes of Secs. 72.500(a), 72.501(a) and 72.502(a), the 
term ``introduced'' means any piece of equipment whose engine is a new 
addition to the underground inventory of engines of the mine in 
question, including newly purchased equipment, used equipment, and 
equipment receiving a replacement engine that has a different serial 
number than the engine it is replacing. ``Introduced'' does not include 
a piece of equipment whose engine was previously part of the mine 
inventory and rebuilt.


Sec. 72.510  Miner health training.

    (a) Operators must provide annual training to all miners at a mine 
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)(1) An operator must keep a record of the training at the mine 
site for one year after completion of the training. An

[[Page 5706]]

operator may keep the record elsewhere if the record is immediately 
accessible 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 such training record. Whenever an operator ceases 
to do business, that operator must transfer the training records, or a 
copy, to any successor operator who must maintain them for the required 
period.


Sec. 72.520  Diesel equipment inventory.

    (a) The operator of each mine that utilizes diesel equipment 
underground, shall prepare and submit in writing to the District 
Manager, an inventory of diesel equipment used in the mine. The 
inventory shall include the number and type of diesel-powered units 
used underground, including make and model of unit, type of equipment, 
make and model of engine, serial number of engine, brake horsepower 
rating of engine, emissions of engine in grams per hour or grams per 
brake horsepower-hour, approval number of engine, make and model of 
aftertreatment device, serial number of aftertreatment device if 
available, and efficiency of aftertreatment device.
    (b) The mine operator shall make changes to the diesel equipment 
inventory as equipment or emission control systems are added, deleted 
or modified and submit revisions, to the District Manager, within 7 
calendar days.
    (c) If requested, the mine operator shall provide a copy of the 
diesel equipment inventory to the representative of the miners within 3 
days of the request.

[FR Doc. 01-995 Filed 1-18-01; 8:45 am]
BILLING CODE 4510-43-P