[Federal Register Volume 61, Number 214 (Monday, November 4, 1996)]
[Rules and Regulations]
[Pages 56746-56856]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-27791]
[[Page 56745]]
_______________________________________________________________________
Part II
Department of Labor
_______________________________________________________________________
Occupational Safety and Health Administration
_______________________________________________________________________
29 CFR Part 1910, et al.
Occupational Exposure to 1,3-Butadiene; Final Rule
Federal Register / Vol. 61, No. 214 / Monday, November 4, 1996 /
Rules and Regulations
[[Page 56746]]
DEPARTMENT OF LABOR
Occupational Safety and Health Administration
29 CFR Parts 1910, 1915 and 1926
[Docket No. H-041]
RIN 1218-AA83
Occupational Exposure to 1,3-Butadiene
AGENCY: Occupational Safety and Health Administration (OSHA),
Department of Labor.
ACTION: Final rule.
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SUMMARY: This final standard amends the Occupational Safety and Health
Administration's (OSHA) occupational standard that regulates employee
exposure to 1,3-Butadiene (BD). The basis for this action is a
determination by the Assistant Secretary, based on animal and human
data, that OSHA's current permissible exposure limit (PEL) which
permits employees to be exposed to BD in concentrations up to 1,000
parts BD per million parts of air (1,000 ppm) as an eight-hour time-
weighted average (TWA) does not adequately protect employee health.
OSHA's new limits reduce the PEL for BD to an 8-hour TWA of 1 ppm and a
short term exposure limit (STEL) of 5 ppm for 15 minutes. An ``action
level'' of 0.5 ppm as an 8-hour TWA is included in the standard as a
mechanism for exempting an employer from some administrative burdens,
such as employee exposure monitoring and medical surveillance, in
instances where the employer can demonstrate that the employee's
exposures are consistently at very low levels. In order to reduce
exposures and protect employees, OSHA's BD standard includes
requirements such as engineering controls, work practices and personal
protective equipment, measurement of employee exposures, training,
medical surveillance, hazard communication, regulated areas, emergency
procedures and recordkeeping.
DATES: The effective date of these amendments is February 3, 1997.
Start-up date for engineering controls is November 4, 1998, and for the
exposure goal program November 4, 1999. Affected parties do not have to
comply with the information collection requirements in
Sec. 1910.1051(d) exposure monitoring, Sec. 1910.1051(f) methods of
compliance, Sec. 1910.1051(g) exposure goal program, Sec. 1910.1051(h)
respiratory protection, Sec. 1910.1051(j) emergency situations,
Sec. 1910.1051(k) medical screening and surveillance, Sec. 1910.1051(l)
communication of BD hazards to employees; and Sec. 1910.1051(m)
recordkeeping until the Department of Labor publishes a Federal
Register notice informing the public that OMB has approved these
information requirements under the Paperwork Reduction Act of 1995.
Other Dates: Written comments on the paperwork requirements of this
final rule must be submitted on or before January 3, 1997.
ADDRESSES: In accordance with 28 U.S.C. 2112(a), the Agency designates
the following party to receive petitions for review of this regulation:
Associate Solicitor for Occupational Safety and Health, Office of the
Solicitor, Room S-4004, U.S. Department of Labor, 200 Constitution
Ave., NW., Washington, DC 20210. These petitions must be filed no later
than the 59th calendar day following promulgation of this regulation;
see section 6(f) of the Occupational Safety and Health Act of 1970 (OSH
Act), 29 CFR 1911.18(d), and United Mine Workers of America v. Mine
Safety and Health Administration, 900 F.2d 384 (D.C. Circ. 1990).
Comments regarding the paperwork burden of this regulation, which
are being solicited by the Agency as required by the Paperwork
Reduction Act of 1995, are to be submitted to the Docket Office, Docket
No. ICR 96-13, U.S. Department of Labor, Room N-2625, 200 Constitution
Ave., NW., Washington, DC 20210, telephone (202) 219-7894. Written
comments limited to 10 pages or less in length may also be transmitted
by facsimile to (202) 219-5046.
FOR FURTHER INFORMATION CONTACT: Ms. Anne Cyr, OSHA Office of Public
Affairs, United States Department of Labor, Room N-3641, 200
Constitution Avenue, NW., Washington, DC. 20210, Telephone (202) 219-
8151. Copies of the referenced information collection request are
available for inspection and copying in the Docket Office and will be
mailed to persons who request copies by telephoning Vivian Allen at
(202) 219-8076. For electronic copies of the 1,3-Butadiene Information
Collection Request, contact OSHA's WebPage on Internet at http://
www.osh.gov/.
I. Collection of Information; Request for Comment
This final 1,3-Butadiene standard contains information collection
requirements that are subject to review by the Office of Management and
Budget (OMB) under the Paperwork Reduction Act (PRA95) 44 U.S.C. 3501
et seq. (see also 5 CFR part 1320). PRA95 defines collection of
information to mean, ``the obtaining, causing to be obtained,
soliciting, or requiring the disclosure to third parties or the public
of facts or opinions by or for an agency regardless of form or
format.'' (44 U.S.C. 3502(3)(A))
The title, the need for and proposed use of the information, a
summary of the collections of information, description of the
respondents, and frequency of response required to implement the
required information collection is described below with an estimate of
the annual cost and reporting burden (as required by 5 CFR
1320.5(a)(1)(iv) and 1320.8(d)(2)). Included in the estimate is the
time for reviewing instructions, gathering and maintaining the data
needed, and completing and reviewing the collection of information.
OSHA invites comments on whether the proposed collection of
information:
Ensures that the collection of information is necessary
for the proper performance of the functions of the agency, including
whether the information will have practical utility;
Estimates the projected burden accurately, including
whether the methodology and assumptions used are valid;
Enhances the quality, utility, and clarity of the
information to be collected; and
Minimizes the burden of the collection of information on
those who are to respond, including the use of appropriate automated,
electronic, mechanical, or other technological collection techniques or
other forms of information technology, e.g., permitting electronic
submissions of responses.
Title: 1,3-Butadiene, 29 CFR 1910.1051.
Description: The final 1,3-Butadiene (BD) Standard is an
occupational safety and health standard that will minimize occupational
exposure to BD. The standard's information collection requirements are
essential components that will protect employees from occupational
exposure. The information will be used by employers and employees to
implement the protection required by the standard. OSHA will use some
of the information to determine compliance with the standard.
Summary of the Collection of Information: The collections of
information contained in the standard include the provisions concerning
objective data; exposure monitoring records and employee notification
of exposure monitoring results; written plans for compliance,
respiratory protection, exposure goal, emergency situations;
information to the physician; employee medical exams and medical
[[Page 56747]]
records; respirator fit-testing records; record of training program;
employee access to monitoring and medical records; and transfer of
records to NIOSH.
Respondents: The respondents are employers whose employees may have
occupational exposure to BD above the action level. The main industries
affected are 1,3-Butadiene Polymer Production, Monomer purification of
1,3-Butadiene, Stand-Alone Butadiene Terminals, and Crude 1,3-Butadiene
Producers.
Frequency of Response: The frequency of monitoring and notification
of monitoring results will be dependent on the results of the initial
and subsequent monitoring events and the number of different job
classifications with BD exposure. The Compliance Plan is required to be
established and updated as necessary and reviewed at least annually.
The Exposure Goal Program, Respiratory Protection Program, and
Emergency Plans are required to be established and updated as
necessary. For those using respirators, respirator fit testing is
required initially, and at least annually thereafter. The frequency of
the medical examinations will be dependent on the number of employees
who will be exposed at or above the action level, or in emergency
situations. A record of the training program is required to be
maintained. Those employers using objective data in lieu of monitoring
must maintain records of the objective data relied upon. The employer
must maintain exposure monitoring and medical records, which includes
information provided to the physician or other licensed health care
professional, in accordance with 29 CFR 1910.20. Fit-Test records must
be maintained for respirator users until the next fit test is
administered.
Total Estimated Cost: First Year $820,388; Second Year $658,949;
and Third and Recurring Years $75,890.
Total Burden Hours: The total burden hours for the first year is
estimated to be 8,077; for the second year, the burden is estimated to
be 5,342; and for the third and recurring years, the burden is
estimated to be 1,587. The Agency has submitted a copy of the
information collection request to OMB for its review and approval.
Interested parties are requested to send comments regarding this
information collection to the OSHA Docket Office, Docket No. ICR 96-13,
U.S. Department of Labor, Room N-2625, 200 Constitution Avenue, NW,
Washington, DC 20210. Written comments limited to 10 pages or fewer may
also be transmitted by facsimile to (202) 219-5046.
Comments submitted in response to this notice will be summarized
and included in the request for Office of Management and Budget
approval of the final information collection request; they will also
become a matter of public record.
Copies of the referenced information collection request are
available for inspection and copying in the OSHA Docket Office and will
be mailed to persons who request copies by telephoning Vivian Allen at
(202) 219-8076. Electronic copies of the 1,3-Butadiene information
collection request are available on the OSHA WebPage on the Internet at
http://www.osha.gov/.
Federalism
This standard has been reviewed in accordance with Executive Order
12612, 52 FR 41685 (October 30, 1987), regarding Federalism. This Order
requires that agencies, to the extent possible, refrain from limiting
State policy options, consult with States prior to taking any actions
only when there is clear constitutional authority and the presence of a
problem of national scope. The Order provides for preemption of State
law only if there is a clear Congressional intent for the Agency to do
so. Any such preemption is to be limited to the extent possible.
Section 18 of the Occupational Safety and Health Act (OSH Act),
expresses Congress' clear intent to preempt State laws with respect to
which Federal OSHA has promulgated occupational safety or health
standards. Under the OSH Act, a State can avoid preemption only if it
submits, and obtains Federal approval of, a plan for the development of
such standards and their enforcement. Occupational safety and health
standards developed by such State Plan-States must, among other things,
be at least as effective in providing safe and healthful employment and
places of employment as the Federal standards. Where such standards are
applicable to products distributed or used in interstate commerce, they
may not unduly burden commerce and must be justified by compelling
local conditions. (See section 18(c)(2).)
The final BD standard is drafted so that employees in every State
will be protected by general, performance-oriented standards. States
with occupational safety and health plans approved under section 18 of
the OSH Act will be able to develop their own State standards to deal
with any special problems which might be encountered in a particular
state. Moreover, the performance nature of this standard, of and by
itself, allows for flexibility by States and employers to provide as
much leeway as possible using alternative compliance.
This final rule of BD addresses a health problem related to
occupational exposure to BD which is national in scope.
Those States which have elected to participate under section 18 of
the OSH Act would not be preempted by this regulation and will be able
to deal with special, local conditions within the framework provided by
this performance-oriented standard while ensuring that their standards
are at least as effective as the Federal Standard.
State Plans
The 23 States and 2 territories with their own OSHA-approved
occupational safety and health plans must adopt a comparable standard
within 6 months of the publication of this final standard for
occupational exposure to 1,3-butadiene or amend their existing
standards if it is not ``at least as effective'' as the final Federal
standard. The states and territories with occupational safety and
health state plants are: Alaska, Arizona, California, Connecticut (for
State and local government employees only), Hawaii, Indiana, Iowa,
Kentucky, Maryland, Michigan, Minnesota, Nevada, New Mexico, New York
(for State and local government employees only), North Carolina,
Oregon, Puerto Rico, South Carolina, Tennessee, Utah, Vermont,
Virginia, the Virgin Islands, Washington, and Wyoming. Until such time
as a State standard is promulgated, Federal OSHA will provide interim
enforcement assistance, as appropriate, in these states and
territories.
SUPPLEMENTARY INFORMATION:
I. Table of Contents
The preamble to the final standard on occupational exposure to BD
discusses events leading to the final rule, physical and chemical
properties of BD, manufacture and use of BD, health effects of
exposure, degree and significance of the risk presented, an analysis of
the technological and economic feasibility, regulatory impact and
regulatory flexibility analysis, and the rationale behind the specific
provisions set forth in the proposed standard. The discussion follows
this outline:
I. Table of Contents
II. Pertinent Legal Authority
III. Events Leading to the Final Standard
IV. Chemical Identification, Production, and Use
A. Monomer
B. Polymers
V. Health Effects
[[Page 56748]]
A. Introduction
B. Carcinogenicity
1. Animal Studies
2. Epidemiologic Studies
C. Reproductive Effects
D. Other Relevant Studies
VI. Quantitative Risk Assessment
VII. Significance of Risk
VIII. Summary of the Final Economic Analysis
IX. Environmental Impact
X. Summary and Explanation of the Proposed Standard
A. Scope and Application
B. Definitions
C. Permissible Exposure Limits
D. Exposure Monitoring
E. Regulated Areas
F. Methods of Compliance
G. Exposure Goal Program
H. Respiratory Protection
I. Personal Protective Equipment
J. Emergency Situations
K. Medical Screening and Surveillance
L. Hazard Communication
M. Recordkeeping
N. Dates
O. Appendices
XI. Final Standard and Appendices
Appendix A: Substance Safety Data Sheet for 1,3-Butadiene
Appendix B: Substance Technical Guidelines for 1,3-Butadiene
Appendix C: Medical Screening and Surveillance for 1,3-Butadiene
Appendix D: Sampling and Analytical Method for 1,3-Butadiene
Appendix E: Respirator Fit Testing Procedures
Appendix F: Medical Questionnaires
II. Pertinent Legal Authority
The purpose of the Occupational Safety and Health Act, 29 U.S.C.
651 et seq. (``the Act'') is to ``assure so far as possible every
working man and woman in the nation safe and healthful working
conditions and to preserve our human resources.'' 29 U.S.C. 651(b). To
achieve this goal, Congress authorized the Secretary of Labor to
promulgate and enforce occupational safety and health standards. U.S.C.
655(a) (authorizing summary adoption of existing consensus and federal
standards within two year of Act's enactment), 655(b) (authorizing
promulgation of standards pursuant to notice and comment), 654(b)
(requiring employers to comply with OSHA standards.)
A safety or health standard is a standard ``which requires
conditions, or the adoption or use of one or more practices, means,
methods, operations, or processes, reasonably necessary or appropriate
to provide safe or healthful employment or places of employment.'' 29
U.S.C. 652(8).
A standard is reasonably necessary or appropriate within the
meaning of Section 652(8) if it substantially reduces or eliminates
significant risk, and is economically feasible, technologically
feasible, cost effective, consistent with prior Agency action or
supported by a reasoned justification for departing from prior Agency
actions, supported by substantial evidence, and is better able to
effectuate the Act's purposes than any national consensus standard it
supersedes. See 58 FR 16612-16616 (March 30, 1993).
The Supreme Court has noted that a reasonable person would consider
a fatality risk of 1/1000 over a 45-year working lifetime to be a
significant risk. Industrial Union Dep't v. American Petroleum
Institute, 448 U.S. 607, 646 (1980) (benzene standard). OSHA agrees
that a fatality risk of 1/1000 over a working lifetime is well within
the range of risk that reasonable people would consider significant.
See e.g., International Union, UAW v. Pendergrass, 878 F.2d 389 (D.C.
Cir. 1989) (formaldehyde standard); Building and Constr. Trades Dep't,
AFL-CIO v. Brock, 838 F.2d 1258, 1265 (D.C. Cir. 1988) (asbestos
standard).
A standard is technologically feasible if the protective measures
it requires already exist, can be brought into existence with available
technology, or can be created with technology that can reasonably be
expected to be developed. American Textile Mfrs. Institute v. OSHA, 452
U.S. 490, 513 (1981) (``ATMI''), American Iron and Steel Institute v.
OSHA, 939 F.2d 975, 980 (D.C. cir. 1991) (``AISI'').
A standard is economically feasible if industry can absorb or pass
on the cost of compliance without threatening its long term
profitability or competitive structure. See ATMI, 452 U.S. at 530 n.
55; AISI, 939 F. 2d at 980.
A standard is cost effective if the protective measures it requires
are the least costly of the available alternatives that achieve the
same level of protection. ATMI, 453 U.S. at 514 n. 32; International
Union, UAW v. OSHA, 37 F. 3d 665, 668 (D.C. Cir. 1994) (``LOTO III'').
All standards must be highly protective. See 58 FR 16614-16615;
LOTO III, 37 F. 3d at 668. However, health standards must also meet the
``feasibility mandate'' of Section 6(b)(5) of the Act, 29 U.S.C.
655(b)(5). Section 6(b)(5) requires OSHA to select ``the most
protective standard consistent with feasibility'' that is needed to
reduce significant risk when regulating health hazards. ATMI, 452 U.S.
at 509.
Section 6(b)(5) also directs OSHA to base health standards on ``the
best available evidence,'' including research, demonstrations, and
experiments. 29 U.S.C. 655(b)(5). OSHA shall consider ``in addition to
the attainment of the highest degree of health and safety protection *
* * the latest scientific data * * * feasibility and experience gained
under this and other health and safety laws.'' Id.
Section 6(b)(7) of the Act authorizes OSHA to include among a
standard's requirements labeling, monitoring, medical testing and other
information gathering and transmittal provisions. 29 U.S.C. 655(b)(7).
Finally, whenever practical, standards shall ``be expressed in
terms of objective criteria and of the performance desired.'' Id.
III. Events Leading to the Final Standard
The standard adopted for BD by OSHA in 1971 pursuant to Section
6(a) of the OSH Act, 29 U.S.C. 655 from an existing Walsh-Healey
Federal Standard required employers to assure that employee exposure
does not exceed 1,000 ppm determined as an 8-hour TWA (29 CFR
1910.1000, Table Z-1). The source of the Walsh-Healey Standard was the
Threshold Limit Value (TLV) for BD developed in 1968 by the American
Conference of Governmental Industrial Hygienists (ACGIH). This TLV was
adopted by the ACGIH to prevent irritation and narcosis.
In 1983, the National Toxicology Program (NTP) released the results
of an animal study indicating that BD causes cancer in rodents. (Ex.
20) Based on the strength of the results of this animal study, ACGIH in
1983 classified BD as an animal carcinogen and in 1984 recommended a
new TLV of 10 ppm. (Ex. 2-4) Based on the same evidence, on February 9,
1984, the National Institute for Occupational Safety and Health (NIOSH)
published a Current Intelligence Bulletin (CIB) recommending that BD be
regarded as a potential occupational carcinogen, teratogen and a
possible reproductive hazard. (Ex. 23-17) On January 5, 1984, OSHA
published a Request for Information (RFI) jointly with the
Environmental Protection Agency. (EPA) (49 FR 844) EPA also announced
the initiation of a 180 day review under the authority of section 4(f)
of the Toxic Substance Control Act (TSCA) (49 FR 845) to determine
``whether to initiate appropriate action to prevent or reduce the risk
from the chemical or to find that the risk is not unreasonable.''
Comments were to be submitted to OSHA by March 5, 1984. On April 4,
1984, OSHA extended the comment period until further notice. (49 FR
13389)
Petitions for an Emergency Temporary Standard (ETS) of 1 ppm or
less for workers' exposure to BD were submitted to OSHA on January 23,
1984, by the United Rubber, Cork, Linoleum and
[[Page 56749]]
Plastic Workers of America (URW), the Oil, Chemical and Atomic Workers
(OCAW), the International Chemical Workers Union (ICWU), and the
American Federation of Labor and Congress of Industrial Organizations
(AFL-CIO). (Ex. 6-4) On March 7, 1984, OSHA denied the petitions on the
ground that the Agency was still evaluating the health data to
determine whether regulatory action was appropriate.
Based on its 180-day review of BD, EPA published, on May 15, 1984,
an Advance Notice of Proposed Rulemaking (ANPR) (49 FR 20524) to
announce the initiation of a regulatory action by the EPA to determine
and implement the most effective means of controlling exposures to the
chemical BD under the TSCA. EPA was working with OSHA because available
evidence indicated that exposure to BD occurs primarily within the
workplace.
Information received in response to this ANPR was used by EPA to
develop risk assessments. Subsequently, EPA identified BD as a probable
human carcinogen (Group B2) according to EPA's classification of
carcinogens, and concluded that current exposures during the
manufacturing of BD and its processing into polymers presented an
unreasonable risk of injury to human health. (Ex. 17-4) Additionally,
EPA determined that the risks associated with exposure to BD may be
reduced to a sufficient extent by action taken under the OSH Act.
Following these findings, EPA, in accordance with section 9(a) of TSCA,
on October 10, 1985 (50 FR 41393), referred BD to OSHA to give this
Agency an opportunity to regulate the chemical under the OSH Act. EPA
requested that OSHA determine whether the risks described in the EPA
report may be prevented or reduced to a sufficient extent by action
taken under the OSH Act and then if such a determination is made, OSHA
issue an order declaring whether the manufacture and use of BD
described in the EPA report present the risk therein described. EPA
asked OSHA to respond within 180 days, by April 8, 1986. (50 FR 41393)
On December 27, 1985, OSHA published a notice soliciting public
comments on EPA's referral report. (50 FR 52952) Based on all the
available information, OSHA, on April 11, 1986, responded to the EPA
referral report by making a preliminary determination (50 FR 12526)
that a revised OSHA standard limiting occupational exposure to BD could
prevent or reduce the risk of exposure to a sufficient extent and that
such risks had been accurately described by EPA in the report. On
October 1, 1986, OSHA published an ANPR (51 FR 35003) to initiate a
rulemaking within the meaning of section 9(a) of TSCA. The Agency
requested that comments be submitted by December 30, 1986. Twenty-four
comments, some of them containing new information, were received in
response to the ANPR. (Ex. 28-1 to 28-24) Six additional comments were
received after the deadline. (Ex. 29-1 to 29-6)
OSHA reviewed the available data and conducted risk assessment,
regulatory impact and flexibility analyses. These analyses demonstrate
that the proposed standard was technologically and economically
feasible and substantially reduced the significant risk of cancers and
other adverse health effects.
On August 10, 1990, OSHA published its proposed rule to regulate
occupational exposure to 1,3-butadiene. (55 FR 32736) Based on the
Agency's review of studies of exposed animals and epidemiologic studies
and taking into account technologic and economic feasibility
considerations, OSHA proposed a permissible exposure limit (PEL) of 2
ppm as an 8-hour time-weighted average and a short term exposure limit
(STEL) of 10 ppm for a 15 minute sampling period. Also included in the
proposal was an ``action level'' of 1 ppm which triggered certain
provisions of the standard such as medical surveillance and training.
OSHA convened public hearings in Washington, DC., on January 15-23,
1991, and in New Orleans, Louisiana, on February 20-21, 1991. The post-
hearing period for the submission of briefs, arguments and summations
was to end July 22, 1991, but was extended by the Administrative Law
Judge to December 13, 1991, in order to give participants time to
review new data on low-dose exposures submitted by NTP and a
quantitative risk assessment done by NIOSH. The comment period closed
February 10, 1992.
In the Fall of 1992, the International Agency for Research on
Cancer (IARC) published the results of the Working Group on the
Evaluation of Carcinogenic Risks to Humans, which reviewed the
carcinogenic potential of BD and concluded that:
There is limited evidence for the carcinogenicity in humans of
1,3-butadiene * * * There is sufficient evidence for the
carcinogenicity in experimental animals * * * (Ex. 125)
IARC stated that its overall evaluation led it to conclude that ``1,3-
butadiene is probably carcinogenic to humans (Group 2A).'' (Ex. 125)
To assist OSHA in issuing a final rule for BD, representatives of
the major unions and industry groups involved in the production and use
of BD submitted the outline of a voluntary agreement reached by the
parties dated January 29, 1996, outlining provisions that they agreed
upon and recommended be included in the final rule. The letter
transmitting the agreement was signed by J.L. McGraw for the
International Institute of Synthetic Rubber Producers (IISRP), Michael
J. Wright for the United Steelworkers of America (USWA), and Michael
Sprinker (CWU). The committee that worked on the issues also included
Joseph Holtshouser of the Goodyear Tire and Rubber Company, Carolyn
Phillips of the Shell Chemical Company, representing the Chemical
Manufacturers Association, Robert Richmond of the Firestone Synthetic
Rubber and Latex Company, and Louis Beliczky (formerly of the URW) and
James L. Frederick of the SWA.
The agreement proposed a change in the permissible exposure limits,
additional provisions for exposure monitoring, and an exposure goal
program designed to reduce exposures below the action level. It also
set forth other modifications to the scope, respiratory protection,
communication of hazards, medical surveillance, and start-up dates
sections of the final rule.
On March 8, 1996 OSHA published the labor/industry joint
recommendations and re-opened the record for 30 days to allow the
public to comment. (61 FR 9381) In response to requests from the
parties to the agreement, the comment period was extended to April 26,
1996. (61 FR 15205)
At the beginning of the comment period, OSHA placed in the
rulemaking record an epidemiologic study of BD exposed workers by
Delzell, et al. sponsored by IISRP, along with IARC volume 127
``Butadiene and Styrene Assessment of Health Hazards,'' a published
paper by Santos-Burgoa, et al. entitled ``Lymphohematopoietic Cancer in
Styrene-Butadiene Polymerization Workers,'' and abstracts from a
symposium entitled ``Evaluation of Butadiene and Isoprene Health
Risks.'' (Ex. 117-1; 117-2; 117-3; 117-4) The epidemiological study had
also been submitted to the EPA in compliance with provisions of the
Toxic Substances Control Act.
In response to the re-opening of the BD record, 18 sets of comments
were received. The parties to the labor/industry agreement submitted a
draft regulatory text which put their recommendations into specific
requirements. The outline and the
[[Page 56750]]
subsequent draft regulatory text are solely the work product of the
negotiating committee. OSHA was neither a party to nor present at the
negotiations.
While the responses to the record re-opening helped clarify the
intent of the negotiating parties, the rationales behind several of the
changes were not fully explained.
On September 16, 1996, Judge John M. Vittone, for Judge George C.
Pierce who presided over the BD hearings, closed the record of the
public hearing on the proposed standard for 1,3-butadiene and certified
it to the Assistant Secretary of Labor. (Ex. 135)
IV. Chemical Identification, Production and Use
A. Monomer
The chemical 1,3-butadiene (BD) (Chemical Abstracts Registry Number
106-99-0) is a colorless, noncorrosive, flammable gas with a mild
aromatic odor at standard ambient temperature and pressure. It has a
chemical formula of C4H6, a molecular weight of 54.1, and a
boiling point of -4.7 deg.C at 760 mm Hg, a lower explosive limit of
2%, and an upper explosive limit of 11.5%. Its vapor density is almost
twice that of air. It is slightly soluble in water, somewhat soluble in
methanol and ethanol, and readily soluble in less polar organic
solvents such as hexane, benzene, and toluene. (Ex. 17-17) It is highly
reactive, dimerizes to 4-vinylcyclohexene, and polymerizes easily.
Because of its low odor threshold, high flammability and explosiveness,
BD has been handled with extreme care in the industry.
In the United States BD has been produced commercially by three
processes: Catalytic dehydrogenation of n-butane and n-butene,
oxidative dehydrogenation of n-butene, and recovery as a by-product
from the C4 co-product stream from the steam cracking process used
to manufacture ethylene, which is the major product of the
petrochemical industry. For economic reasons, almost all BD currently
made in the U.S. is produced by the ethylene co-product process.
In the steam cracking process for ethylene, a hydrocarbon feedstock
is diluted with steam then heated rapidly to a high temperature by
passing it through tubes in a furnace. The output stream, containing a
broad mixture of hydrocarbons from the pyrolysis reactions in the
cracking tubes plus unreacted components of feedstock, is cooled and
then processed through a series of distillation and other separation
operations in which the various products of the cracking operation are
separated for disposal, recycling or recovery.
The cracking process produces between 0.02 to 0.3 pounds of BD per
pound of ethylene, depending upon the composition of the feedstock. BD
is recovered from the C4 stream by the separation operations. The
C4 stream contains from 30 to 50% BD plus butane, butenes and
small fractions of other hydrocarbons. This crude BD stream from the
ethylene unit may be refined in a unit on site, or transferred to
another location, a monomer plant, owned by the same or a different
company, to produce purified BD.
Regardless of the source of the crude BD-ethylene co-product,
(dehydrogenation, or blending of C4 streams from other sources),
the processes used by different companies to refine BD for subsequent
use in polymer production are similar. Extractive distillation is used
to effect the basic separation of BD from butanes and butenes and
fractional distillation operations are used to accomplish other related
separations. A typical monomer plant process is described below.
C3 and C4 acetylene derivatives, present in the C4
co-product stream, are converted to olefins by passing the stream
through a hydrogenation reactor. The stream is then fed to an
extractive distillation column to separate the BD from butanes and
butenes. Several different solvents have been employed for this
operation, including n-methylpyrrolidone, dimethylformamide, furfural,
acetonitrile, dimethylacetamide, and cuprous ammonium acetate solution.
The BD, extracted by the solvent, is stripped from it in the solvent
recovery column, then fed to another fractionation column, the
methylacetylene column, to have residual acetylene stripped out. The
bottom stream from the methylacetylene column, containing the BD, is
fed to the BD rerun column, from which the purified BD product is taken
off overhead. The solvent, recovered in the solvent recovery column, is
recycled to the extractive distillation column with part of it
distilled to keep down the level of polymer. (Ex. 17-17)
A stabilizer is added to the monomer to inhibit formation of
polymer during storage. It is stored as a liquid under pressure,
sometimes refrigerated to reduce the pressure, generally stored in a
tank farm in diked spheres. It is shipped to polymer manufacturers and
other users by pipeline, barge, tank car, or tank truck.
BD is a major commodity product of the petrochemical industry.
Total U.S. production of BD in 1991 was 3.0 billion pounds. Although BD
is a toxic flammable gas, its simple chemical structure with low
molecular weight and high chemical reactivity make it a useful building
block for synthesizing other products. In ``1,3-Butadiene Use and
Substitutes Analysis,'' EPA identified 140 major, minor and potential
uses of BD in the chemical industry. (Ex. 17-15)
Over 60% of the BD consumed in the United States is used in the
manufacture of rubber, about 12% in making adiponitrile which in turn
is used to make hexamethylenediamine (HMDA), a component of Nylon,
approximately 8% in making styrene-butadiene copolymer latexes,
approximately 7% in producing polychloroprene, and about 6% in
producing acrylonitrile-butadiene-styrene (ABS) resins. Lesser amounts
are consumed in the production of rocket propellants, specialty
copolymer resins and latexes for paint, coatings and adhesive
applications, and hydrogenated butadiene-styrene polymers used as
lubricating oil additives. Some nonpolymer applications include the
manufacture of the agricultural fungicides, Captan and Captofol, the
industrial solvent sulfolane, and anthroquinone dyes.
B. Polymers
BD based synthetic elastomers are manufactured by polymerizing BD
by itself, by polymerizing BD with other monomers to produce
copolymers, and by producing mixtures of these polymers. The largest-
volume product is the copolymer of styrene and BD, styrene-butadiene
rubber, followed in volume by polybutadiene, polychloroprene, and
nitrile rubber. Polybutadiene is the polymer of BD monomer by itself.
Polychloroprene is made by polymerizing chloroprene, produced by
chlorination of BD. Nitrile rubbers are copolymers of acrylonitrile and
BD.
Four general types of processes are used in polymerizing BD and its
copolymers: emulsion, suspension, solution and bulk polymerization. In
emulsion and suspension polymerization, the monomers and the many
chemicals used to control the reaction are finely dispersed or
dissolved in water. In solution polymerization, the monomers are
dissolved in an organic solvent such as hexane, pentane, toluene. In
bulk polymerization, the monomer itself serves as solvent for the
polymer. The polymer product, from which end-use products are
manufactured, is produced in the form of polymer crumb (solid
particles), latex (a milky suspension in water), or cement (a
solution).
[[Page 56751]]
Emulsion polymerization is the principal process used to make
synthetic rubber. A process for the manufacture of styrene-butadiene
crumb is typical of emulsion processes. Styrene and BD are piped to the
process area from the storage area. The BD is passed through a caustic
soda scrubber to remove the inhibitors which were added to prevent
premature polymerization. The fresh BD monomer streams are mixed with
styrene, aqueous emulsifying agents, activator, catalyst, and modifier,
and then fed to the first of a train of reactors. The reaction proceeds
stepwise in the series of reactors to around 60% conversion of monomer
to polymer. In the cold process, the reactants are chilled and the
reactor temperature is maintained at 4 deg.C to 7 deg.C (40 deg.F to
45 deg.F) and pressure at 0 to 15 psig; in the hot rubber process,
temperature and pressure are around 50 deg.C (122 deg.F) and 40 to 60
psig, respectively.
The latex from the reactor train is flashed to evaporate unreacted
BD which is compressed, condensed and recycled. Uncondensed vapors are
absorbed in a kerosene absorber before venting and the absorbed BD is
steam stripped or recovered from the kerosene by some other operation.
The latex stream is passed through a steam stripper, operated under
vacuum, to remove and recover unreacted styrene. The styrene and water
in the condensate are separated by decanting. The styrene phase is
recycled to the process. Noncondensibles from the stripping column
contain some BD and are directed through the BD recovery operations.
Stripped latex, to which an antioxidant has been added, is pumped
to coagulation vessels where dilute sulfuric acid and sodium chloride
solution are added. The acid and brine mixture breaks the emulsion,
releasing the polymer in the form of crumb. Sometimes carbon black and
oil are added during the coagulation step since better dispersion is
obtained than by mixing later on.
The crumb and water slurry from the coagulation operation is
screened to separate the crumb. The wet crumb is pressed in rotary
presses to squeeze out most of the entrained water then dried with hot
air on continuous dry belt dryers. The dried product is baled and
weighed for shipment.
Production of styrene-butadiene latex by the emulsion
polymerization process is similar to that for crumb but is usually
carried out on a smaller scale with fewer reactors. For some but not
all products, the reaction is run to near completion, monomer removal
is simpler and recovery may not be practiced.
Polybutadiene rubber is usually produced by solution
polymerization. Inhibitor is removed from the monomer by caustic
scrubbing. Both monomer and solvent are dried by fractional
distillation, mixed in the desired ratio and dried in a desiccant
column. Polymerization is conducted in a series of reactors using
initiators and catalysts and is terminated with a shortstop solution.
The solution, called rubber cement, is pumped to storage tanks for
blending. Crumb is precipitated by pumping the solution into hot water
under violent agitation. Solvent and monomer are recovered by stripping
and distillation similar to those previously described. The crumb is
screened, dewatered, dried and baled.
Polychloroprene (neoprene) elastomers are manufactured by
polymerizing chloroprene in an emulsion polymerization process similar
to that used for making styrene-butadiene rubber. The monomer,
chloroprene (2-chloro-BD), is made by chlorination of BD to make 3,4-
dichlorobutene, and dehydrochlorination of the latter.
Nitrile rubbers, copolymers of acrylonitrile and BD, are produced
by emulsion polymerization similar to that used to make styrene-
butadiene rubber.
Substantial amounts of BD are used in the production of two other
large volume polymers: Nylon resins and ABS resin. Dupont manufactures
adiponitrile from BD and uses the product to make hexamethylenediamine
which is polymerized in making Nylon resins and fibers, including Nylon
6,6. Acrylonitrile, BD and styrene are the monomers used to make ABS
resin which is a major thermoplastic resin. Chemically complex
emulsion, suspension and bulk polymerization processes are used by
different producers to make ABS polymer.
V. Health Effects
A. Introduction
The toxicity of BD was long considered to be low and non-
cumulative. Thus, the OSHA standard for BD was 1,000 ppm on the basis
of its irritation of mucous membranes and narcosis at high levels of
exposure. However, in the 1980s, carcinogenicity studies indicated BD
is clearly a carcinogen in rodents. In 1986, the American Conference of
Governmental Industrial Hygienists (ACGIH) was prompted by these
studies to lower the workplace threshold limit value (TLV) from 1,000
to 10 ppm. (Ex. 2-5)
Rodent studies are now conclusive that BD is an animal carcinogen.
Further, a consistent body of epidemiologic studies have also shown
increased mortality from hematopoietic cancers associated with BD
exposure among BD-exposed production and styrene/BD rubber polymer
workers. Complementary studies of metabolic products and genotoxicity
support these cancer findings. OSHA was also concerned about evidence
that BD affects the germ cell as well as the somatic cell, and what
potential reproductive toxicity might result from exposure to BD. Since
BD itself does not appear to be carcinogenic, but must be metabolized
to an active form, OSHA also reviewed studies on the metabolism of BD
to determine wether they might help explain the observed differences in
cancer incidence among species.
The following sections discuss the effects of BD exposure, both in
human and animal systems.
B. Carcinogenicity
1. Animal Studies
In the proposed BD rule, OSHA discussed the results of two lifetime
animal bioassays, one on the Sprague-Dawley rat and one in the
B6C3F1 mouse. (55 FR 32736 at 32740) Both studies found evidence
of BD carcinogenicity, with the greater response in the mouse. The rat
study involved exposure levels of 0, 1000, or 8000 ppm BD, starting at
five weeks of age, to groups of 100 male and 100 female Sprague-Dawley
rats for 6 hours per day, five days per week, for 105 weeks. Mortality
was increased over controls in the 1,000 ppm exposed female rats and in
both of the male rat exposure groups. Significant tumor response sites
in the male rats included exocrine adenomas and carcinomas (combined)
of the pancreas in the highest exposure group (3, 1, and 11 tumors in
the 0, 1000, and 8000 ppm groups, respectively); and Leydig-cell tumors
of the testis (0, 3, and 8 in the same groups, respectively). In the
female rats, the significantly increased tumor response also occurred
in the highest exposure group; cancers seen included follicular-cell
adenomas and carcinomas (combined) of the thyroid gland (0,4, and 11
tumors in the three exposure groups, respectively), and benign and
malignant (combined) mammary gland tumors (50, 79, and 81 in the same
exposure groups). To a lesser degree there were also sarcomas of the
uterus (1, 4, 5 tumors in the three exposure groups), and Zymbal gland
(0, 0, 4 tumors in the same exposure groups, respectively). While only
high
[[Page 56752]]
exposure group tumor response for some of these sites was statistically
significant, trend tests were also significant.
In contrast to the generally less than 10% increase in tumor
response seen in the Sprague-Dawley rat at levels far above BD
metabolic saturation, the carcinogenic response to BD in the
B6C3F1 mouse in the National Toxicology Program study (NTP I) was
extensive. (Ex. 23-1) In this study, groups of 50 male and 50 female
mice were exposed via inhalation to 0, 625 or 1250 ppm BD for 6 hours
per day, 5 days per week in a study originally designed to last 2
years. However, the high carcinogenic response included multiple
primary cancers, with short latent periods, and led to early study
termination (60-61 weeks) due to high cancer mortality in both the 625
ppm and 1250 ppm exposure groups of both sexes. This mortality was due
mainly to lymphocytic lymphomas and hemangiosarcomas of the heart, both
of which were typically early occurring and quickly fatal. This large
and rapidly fatal carcinogenic response led to both the NTP and
industry to undertake additional studies to better understand the
mechanisms involved.
Some commenters have associated qualitative or quantitative
differences in mouse and rat BD carcinogenicity with the differences in
rat and mouse BD metabolism. Many studies published and submitted to
the BD record since the proposed rule have sought to better
characterize the metabolic, distributional, and elimination processes
involved, and some have attributed species differences (at least in
part) to the metabolic differences. These will be addressed separately
in the metabolism section.
Another factor hypothesized to account for differences between
mouse and rat BD carcinogenicity was the role of activation of
ecotropic retrovirus in hematopoietic tissues on tumor response in the
B6C3F1 mouse. This virus is endogenous to the B6C3F1 mouse
and was hypothesized to potentiate the BD lymphoma response in this
strain. To study this hypothesis Irons and co-workers exposed both (60)
B6C3F1 male (those with the endogenous virus) and (60) NIH Swiss
male (those without the endogenous virus) mice to either 0 or 1250 ppm
BD, for 6 hours./day, 5 days per week for 52 weeks. (Ex. 32-28D) A
third group of 50 B6C3F1 male mice received 1250 ppm for 12 weeks
only and was observed until study termination at 52 weeks. The results
of the study showed significantly increased thymic lymphomas in all
exposed groups but significantly greater response in the B6C3F1
mouse--1 tumor/60 (2%) in the control (zero exposure) group, 10/48
(21%) in the 12 week exposure group, and 34/60 (57%) in the 52 week
exposure group--vs. the NIH Swiss mice, which developed 0 tumors/60 in
the control group, and 8 tumors/57 (14%) in the BD exposed group.
Hemangiosarcomas of the heart were also observed in both strains
exposed to BD for 52 weeks--5/60 (8%) in the B6C3F1 mice vs. 1/57
in the NIH Swiss mice. (Ex. 32-28D). The B6C3F1 response was very
similar to the NTP I high exposure group response, verifying that
earlier study. The qualitatively similar lymphoma responses of the two
strains also confirmed that the mouse hematopoietic system is highly
susceptible to the carcinogenic effects of BD, although quantitatively
the strains may differ. The 21% 1-year lymphoma response in the 12-week
stop-exposure B6C3F1 group also increased concerns about high
concentration, short duration exposures.
NTP II Study
Concurrent with the industry studies, the NTP, in order to better
characterize the dose-response and lifetime experience, conducted a
second, much larger research effort over a much broader dose range.
(Ex. 90; 96) These toxicology and carcinogenesis studies included a
100-fold lower (6.25 ppm) low exposure group than NTP I, several
intermediate exposure groups, a study of dose-rate effects using
several high-concentration partial lifetime (stop-) exposure groups,
and planned interim sacrifice groups. Other parts of the study included
clinical pathology studies (with the 9- and 15-month interim
sacrifices, metabolism studies, and examination of tumor bearing
animals for activated oncogenes).
For the lifetime carcinogenesis studies, groups of 70 B6C3F1
mice of each sex were exposed via inhalation to BD at levels of 0,
6.25, 20, 62.5, 200, or 625 ppm (90 of each sex in this highest group)
for 6 hours per day, 5 days per week for up to 2 years. Up to 10
randomly selected animals in each group were sacrificed after 9 and 15
months of exposure, and these animals were assessed for both
carcinogenicity and hematologic effects.
For the stop-exposure study, different groups of 50 male mice were
exposed 6 hours per day, 5 days per week to concentrations of either
200 ppm for 40 weeks, 625 ppm for 13 weeks, 312 ppm for 52 weeks, or
625 ppm for 26 weeks. Following the BD exposure period, the exposed
animals were then observed for the remainder of the 2-year study. The
first two stop-exposure groups received a total exposure (concentration
times duration) of 8,000 ppm-weeks, while the latter two groups
received approximately 16,000 ppm-weeks of exposure. For the analysis
discussed below, groups are compared both with each other for dose-rate
effects and with the lifetime (2 year) exposure groups for recovery
effects.
Methodology
Male mice were 6-8 weeks old and female mice were 7-8 weeks old
when the exposures began. Animals were exposed in individual wire mesh
cage units in stainless steel Hazelton 2000 chambers (2.3 m\3\). The
exposure phase extended from January, 1986 to January, 1988. Animals
were housed individually; water was available ad libitum; NIH-07 diet
feed was also available ad libitum except during exposure periods.
Animals were observed twice daily for moribundity and mortality;
animals were weighed weekly for the first 13 weeks and monthly
thereafter. Hematology included red blood cell count (RBC), and white
blood cell count (WBC). The study was conducted in compliance with the
Food and Drug Administration (FDA) Good Laboratory Practice Regulations
with retrospective quality assurance audits.
The results of the study are presented below for the two-year and
stop-exposure study. Between study group comparisons are made where it
is deemed appropriate. Emphasis is placed on the neoplastic effects.
Results
Two-Year Study
While body weight gains in both exposed male and female mice were
similar to those of the control groups, exposure related malignant
neoplasms were responsible for decreased survival in all exposure
groups of both sexes exposed to concentrations of 20 ppm or above.
Excluding the interim sacrificed animals, the two-year survival
decreased uniformly with increasing exposure for females (37/50, 33/50,
24/50, 11/50, 0/50, 0/70), and nearly uniformly for males (35/50, 39/
50, 24/50, 22/50, 4/50, 0/70). As with the earlier NTP study, all
animals in the 625 ppm group were dead by week 65, mostly as a result
of lymphomas or hemangiosarcomas of the heart. The 200 ppm exposure
groups of both sexes also had much higher mortality, but significantly
less than that of the 625 ppm group. The survival of the lowest
exposure group (6.25 ppm) was slightly better than controls for the
male mice, slightly less for the female mice. Mean
[[Page 56753]]
survival for the males was an exposure-related 597, 611, 575, 558, 502,
and 280 days; for the females it was similarly 608, 597, 573, 548, 441,
and 320 days. This decreased survival with increasing exposure was
almost totally due to tumor lethality.
Carcinogenicity
Nine different sites showed primary tumor types associated with
butadiene exposures, seven in the male mice and eight in the female
mice. These were lymphoma, hemangiosarcoma of the heart, combined
alveolar-bronchiolar adenoma and carcinoma, combined forestomach
papilloma and carcinoma, Harderian gland adenoma and adenocarcinoma,
preputial gland adenoma and carcinoma (males only), hepatocellular
adenoma and carcinoma, and mammary and ovarian tumors (females only).
These are shown in Table V-1 adapted from Melnick et al. (Ex. 125) From
this table it is seen that six of these tumor sites are statistically
significantly increased in the highest exposed males and five were
statistically significantly increased in the highest exposed females.
Two additional sites which showed significant increases at lower
exposures showed decline at the highest exposures because other tumors
were more rapidly fatal. At 200 ppm preputial gland adenoma and
carcinoma combined were significantly increased in males (p<.05; 0/70
(0%) control vs. 5/70 (7%) in the 200 ppm group) and hepatocellular
adenoma and carcinoma were increased for both exposed males and
females. At the lowest exposure concentration, 6.25 ppm, only female
mouse lung tumors (combined adenoma and carcinoma) showed statistical
significance (p<.05; 4/70 (6%) in controls vs. 15/70 (21%) in the 6.25
ppm group); these tumors in female mice showed a monotonic increase
with increasing exposure up to 200 ppm. At 20 ppm female mouse
lymphomas and liver tumors also reached statistical significance
(lymphomas, p<.05; 10/70 (15%) in controls vs. 18/70 (26%) in the 20
ppm group; liver tumors, p<.05; 17/70 (24%) in controls vs. 23/70 (33%)
in the 20 ppm group), and at 62.5 ppm, tumors at several other sites
were also significantly increased. In general, while there were some
differences in amount of tumor response between the male and female
mice, there is fairly consistent pattern of tumor type in mice of both
sexes for the six non-sexual organ sites.
Table V-1.--Tumor Incidences (I) and Percentage Mortality-Adjusted Tumor Rates (R) in Mice Exposed to 1,3-Butadiene For up to 2 Years.
[Adapted from Ex. 125]
--------------------------------------------------------------------------------------------------------------------------------------------------------
Exposure concentration (ppm)
-----------------------------------------------------------------------------
Tumor Sex 0 6.25 20 62.5 200 625
-----------------------------------------------------------------------------
I Rc I R I R I R I R l R
--------------------------------------------------------------------------------------------------------------------------------------------------------
Lymphoma........................................ M 4/70 8 3/70 6 8/70 19 11/70 a25 9/70 a27 69/90 a97
F 10/70 20 14/70 30 a18/70 41 10/70 26 19/70 a58 43/90 a89
Heart--Hemangiosarcoma.......................... M 0/70 0 0/70 0 1/70 2 5/70 a13 20/70 a57 6/90 a53
F 0/70 0 0/70 0 0/70 0 1/70 3 20/70 a64 26/90 84
Lung--Alveolar-bronchiolar adenoma and carcinoma M 22/70 46 23/70 48 20/70 45 33/70 a72 42/70 a87 12/90 a73
Forestomach--Papilloma and carcinoma............ F 4/70 8 15/70 a32 19/70 a44 27/70 a61 32/70 a81 25/90 a83
Harderian gland--Adenoma and adenocarcinoma..... M 1/70 2 0/70 0 1/70 2 5/70 13 12/70 a36 13/90 a75
F 2/70 4 2/70 4 3/70 8 4/70 12 7/70 a31 28/90 a85
Preputial gland--Adenoma and carcinoma.......... M 6/70 13 7/70 15 11/70 25 24/70 a53 33/70 a77 7/90 a58
F 9/70 18 10/70 21 7/70 17 16/70 a40 22/70 a67 7/90 48
Liver--Hepatocellular adenoma and carcinoma..... M 0/70 0 0/70 0 0/70 0 0/70 0 5/70 a17 0/90 0
Mammary gland--Adenocarcinoma................... M 31/70 55 27/70 54 35/70 68 32/70 69 40/70 a87 12/90 75
Ovary--Benign and malignant granulosa-cell F 17/70 35 20/70 41 23/70 a52 24/70 a60 20/70 a68 3/90 28
tumors.
F 0/70 0 2/70 4 2/70 5 6/70 a16 13/70 a47 13/90 a66
F 1/70 2 0/70 0 0/70 0 9/70 a24 11/70 a44 6/90 44
--------------------------------------------------------------------------------------------------------------------------------------------------------
a Increased compared with chamber controls (0 ppm), p < 0.05, based on logistic regression analysis.
b The Working Group noted that the incidence in control males and females was in the range of that in historical controls (Haseman et al., 1985).
c Mortality adjusted tumor rates are adjusted for competing causes of mortality, such as death due to other tumors, whose rates differ by exposure
group.
Hemangiosarcoma of the heart, with metastases to other organs was
first observed at 20 ppm in 1 male (the historical controls for this
strain are 1/2373 in males and 1/2443 in females), in 5 males and 1
female at 62.5 ppm and in 20 males and 20 females at 200 ppm; at 625
ppm these tumor rates leveled off as other tumors, especially lymphomas
became dominant. Lymphatic lymphomas increased to statistical
significance first in females at 20 ppm and were usually rapidly fatal,
the first tumor appearing at week 23, most likely preempting some of
the later appearing tumors in the higher exposure groups. Because of
the plethora of primary tumors and the different time patterns observed
to onset of each type, several tumor dose-response trends do not appear
as strong as they would otherwise be.
Non-Neoplastic Effects
Several non-cancer toxic effects were noted in the exposed groups,
reflecting many of the same target sites for which the neoplastic
effects were seen. (Ex. 90; 96; 125).
Although the reported numbers differ slightly in the different
exhibits, generally dose-related increases in hyperplasia were observed
in the heart, lung, forestomach, and Harderian gland, both in the two-
year study (both sexes) and in the stop-exposure study (conducted in
males only). In addition, testicular atrophy was observed in both the
two-year and stop-exposure male mice, but remained in the 6%-10% range
except for the 2-year, 625 ppm
[[Page 56754]]
group where it was 74%. Ovarian germinal hyperplasia (2/49 (control),
3/49 (6.25 ppm), 8/48 (20 ppm), 15/50 (62.5 ppm), 15/50 (200 ppm), 18/
79 (625 ppm), ovarian atrophy (4/49, 19/49, 32/48, 42/50, 43/50, 69/
79), and uterine atrophy (1/50, 0/49, 1/50, 1/49, 8/50, 41/78) were
also dose related, with ovarian atrophy significantly increased at the
lowest BD exposure of 6.25 ppm. These toxic effects to the reproductive
organs are discussed in greater detail in the reproductive effects
section of this preamble. Bone marrow atrophy was noted only in the
highest exposure groups, occurring in 23/73 male mice and 11/79 female
mice.
Stop-Exposure Study
As with the 2-year study, the body weights of the four treated
groups in the stop-exposure study were similar to controls. All
exposure groups exhibited markedly lower survival than controls, and
only slightly better survival than that of the comparable full lifetime
exposure groups. Mortality appeared to be more related to total dose
than to exposure concentration. Most deaths were caused by tumors.
Neoplastic Effects
All of these stop-exposure groups exhibited a very similar tumor
profile to that of the lifetime high exposure groups, with the lone
exception of liver tumors, which were increased only in the lifetime
exposure group; all the other multiple primary tumors were observed at
significantly increased levels in both the stop- and lifetime-exposure
groups, Table V-2. (Ex. 125) In addition, the 625 ppm, 26 week exposure
group had higher rates for several of the tumor types compared to the
lifetime 625 ppm group, possibly because of the shorter exposure
group's slightly better survival. The most prevalent tumor type,
lymphoma, also showed a dose-rate effect, as the tumor incidence was
greater for exposure to short-term higher concentrations compared with
a lower long-term exposure (p=.01; 24/50 at 625 ppm for 13 weeks vs.
12/50 at 200 ppm for 40 weeks: p<.0001; 37/50 at 625 ppm for 26 weeks
vs. 15/50 at 312 ppm for 52 weeks). The same pattern was seen with
forestomach tumors and preputial gland carcinomas. Conversely, the
hemangiosarcomas of the heart and alveolar-bronchiolar tumors showed an
opposite trend, as lower exposures for a longer time yielded a
significantly higher incidence of these tumors than the same cumulative
exposures over a shorter time (survival-adjusted, as opposed to the raw
incidence lung tumor rates actually suggest no dose-response trends).
These inconsistent trends with the different tumor sites may be the
result of multiple mechanisms of carcinogenicity or partially due to
the rapid fatality caused by lymphocytic lymphomas in the short-term
high-exposure groups. As with the lifetime study, angiosarcomas of the
heart and lymphomas presented competing risks in the highly exposed
mice.
Table V-2.--Tumor Incidences (I) and Percentage Mortality-Adjusted Tumor Rates (R) in Male Mice Exposed to 1,3-
Butadiene in Stop-Exposure Studies. (After Exposures Were Terminated, Animals Were Placed in Control Chambers
Until the End of the Study at 104 Weeks.)
[Adapted from Ex. 125]
----------------------------------------------------------------------------------------------------------------
Exposure
----------------------------------------------------------------
0 200 ppm, 625 ppm, 312 ppm, 625 ppm,
Tumor ------------- 40 wk 13 wk 52 wk 26 wk
---------------------------------------------------
I R c I R I R I R I R
----------------------------------------------------------------------------------------------------------------
Lymphoma....................................... 4/70 8 12/50 a 3
5 24/50 a 6
1 15/50 a 5
5 37/50 a 9
0
Heart--Hemang-iosarcoma........................ 0/70 0 7/50 a47 7/50 a 3
1 33/50 a 8
7 13/50 a 7
6
Lung--Alveolar-bronchiolar adenoma and
carcinoma..................................... 22/70 46 35/50 a 8
8 27/50 a 8
7 32/50 a 8
8 18/50 a 8
9
Forestomach--Squamous-cell papilloma and
carcinoma..................................... 1/70 2 6/50 a 2
0 8/50 a 3
3 13/50 a 5
2 11/50 a 6
3
Harderian gland--Adenoma and adenocarcinoma.... 6/70 13 27/50 a 7
2 23/50 a 8
2 28/50 a 8
6 11/50 a 7
0
Preputial gland--Carcinoma..................... 0/70 0 1/50 3 5/50 a21 4/50 a 2
1 3/50 a 3
1
Kidney--Renal tubular adenoma.................. 0/70 0 5/50 a 1
6 1/50 5 3/50 a 1
5 1/50 11
----------------------------------------------------------------------------------------------------------------
From Melnick et al (1990).
AAaIncreased compared with chamber controls (0ppm), p<0.05, based on logistic regression analysis.
cMortality adjusted tumor rates are adjusted for competing causes of mortality, such as death due to other
tumors, whose rates differ by exposure group.
Activated Oncogenes
The presence of activated oncogenes in the exposed groups which
differ from those seen in tumors in the control group can help in
identifying a mechanistic link for BD carcinogenicity. Furthermore,
certain activated oncogenes are seen in specific human tumors and K-ras
is the most commonly detected oncogene in humans. In independent
studies, tumors from this study were evaluated for the presence of
activated protooncogenes. (Ex. 129) Activated K-ras oncogenes were
found in 6 of 9 lung adenocarcinomas, 3 of 12 hepatocellular cancers
and 2 of 11 lymphomas in BD exposed mice. Nine of these 11 K-ras
mutations, including all six of those seen in lung tumors, were G to C
conversions in codon 13. Activation of K-ras genes by codon 13
mutations has not been detected in lung or liver tumors or lymphomas in
unexposed B6C3F1 mice, but activation by codon 12 mutation was
observed in 1 of 10 lung tumors in unexposed mice. (Ex. 129)
Conclusion
All of the four animal bioassays (one rat, three mouse) find a
clear carcinogenic response; together they provide sufficient evidence
to declare BD a known animal carcinogen and a probable human
carcinogen. The three mouse studies, all with a positive lymphoma
response, further support a finding that the mouse is a good model for
BD related lymphatic/hematopoietic and other site tumorigenicity. The
most recent NTP II study confirms and strengthens the previous NTP I
and Irons et al. mouse studies, and presents clear evidence that BD is
a potent multisite carcinogen in B6C3F1 mice of both sexes. (Ex.
23-1;32-28D, Irons) The finding of lung tumors at exposures as low as
6.25 ppm, 100 fold lower than the lowest exposure of the NTP I study
and a level that is in the occupational exposure range, increases
concern for workers' health. Two other concerns
[[Page 56755]]
raised by both the second NTP and the Irons et al. studies are, (1)
substantial carcinogenicity is found with less-than-lifetime exposures
(as low as 12 or 13 weeks) for lymphomas and hemangiosarcomas, at least
at higher concentrations, and, (2) for lymphomas and at least two other
sites, there appears to be a dose-rate effect, where exposure to higher
concentrations for a shorter time yields higher tumor response (by a
factor of as much as 2-3) than a comparable total exposure spread over
a longer time. These findings suggest that even short-term exposures
should be as low as possible. Positive studies for genotoxicity and the
detection of activated K-ras oncogenes in several of these tumors
induced in mice, including lymphomas, liver, and lung, suggest a
mutagenic mechanism for carcinogenicity, and support reliance on a
linear low-dose extrapolation procedure (on the basis of the multistage
mutagenesis theory of carcinogenicity), at least for these tumor sites.
The finding of activated K-ras oncogenes in these mouse tumors may also
be relevant to humans, because K-ras is the most commonly detected
oncogene in humans.
The different dose-rate trends for different tumor sites suggest
that different mechanisms are involved at different sites. The
observation of a highly nonlinear exposure-response for lymphomas at
exposure levels of 625 ppm and above suggests a secondary high-exposure
mechanism as well, not merely a metabolic saturation, as is suspected
with the high-exposure saturation seen in Sprague-Dawley rats. (Ex. 34-
6, Owen and Glaister) The picture emerges of BD as a potent genotoxic
multisite carcinogen in mice, far more potent in mice than in rats.
With respect to appropriate tumor sites for risk extrapolation from
mouse to humans, Melnick and Huff have presented information comparing
animal tumor response for five known or suspected human carcinogens--
BD, benzene, ethylene oxide, vinyl chloride, and acrylonitrile. (Ex.
117-2) BD, benzene, and ethylene oxide all have strong occupational
epidemiology evidence of increased lymphatic/hematopoietic cancer (LHC)
mortality and all three cause both LHC, lung, Harderian gland, and
mammary gland tumors in mice, plus several other primary tumors (see
Table V-3). Only BD and vinyl chloride cause mouse hemangiosarcomas, BD
in the heart and vinyl chloride in the liver. In rats, while all five
carcinogens cause tumors at multiple sites, only brain and Zymbal gland
tumors are associated with as many as four of the compounds. In general
mice and rats are affected at different tumor sites by these
carcinogens. LHC, lung, Harderian gland, mammary gland and, possibly
hemangiosarcomas are sites in mice which correlate well with human LHC.
This suggests that mice, rats and humans may have different target
sites for the same carcinogen, but that compounds which are multisite
carcinogens in the mouse and rat are likely to be human carcinogens as
well. Based on BD's strong LHC association in humans, and its multisite
carcinogenicity in the mouse, including occurrence at several of the
same target sites seen with other carcinogens, OSHA concludes that the
mouse is a good animal model for predicting BD carcinogenesis in
humans.
Table V-3.--Sites at Which Neoplasms are Caused by 1,3-Butadiene in Mice and Rats: Comparison With Results of Studies With Benzene, Ethylene Oxide,
Vinyl Chloride and Acrylonitrile
[From Ex. 117-2]
--------------------------------------------------------------------------------------------------------------------------------------------------------
1,3-Butadiene Benzene Ethylene oxide Vinyl chloride Acrylonitrile
Site ---------------------------------------------------------------------------------------------------
Mice Rats Mice Rats Mice Rats Mice Rats Mice Rats
--------------------------------------------------------------------------------------------------------------------------------------------------------
Lymphatic/hematopoietic............................. NS
Lung................................................
Heart............................................... f a a
Harderian gland.....................................
Ovary...............................................
Mammary gland.......................................
Preputial gland.....................................
Brain...............................................
Zymbal gland........................................
Uterus..............................................
Pancreas............................................
Testis..............................................
Thyroid gland.......................................
--------------------------------------------------------------------------------------------------------------------------------------------------------
NS, not studied.
Hemangiosarcoma.
2. Epidemiologic Studies
(i) Introduction. OSHA has concluded that the epidemiologic studies
contained in this record, as well as the related hearing testimony and
record submissions, show that occupational exposure to BD is associated
with an increased risk of death from cancers of the Lymphohematopoietic
(LH) system. However, in contrast to the available toxicologic data,
our understanding of BD epidemiology is based on
observational studies, not experimental ones. In other words, the
investigators who conducted these epidemiologic studies did not have
control over the exposure status of the individual workers. They were,
nonetheless, able to select the worker populations and the
observational study design.
Cohort and case control studies are two types of observational
study designs. Each of these designs has strengths and weaknesses that
should be considered when the results are
interpreted. Cohort studies, for example, have the advantages of
decreasing the chance of selection bias regarding exposure status and
providing a more complete description of all health outcomes subsequent
to exposure. The disadvantages of cohort studies include the large
number of subjects that are needed to study rare diseases and the
potentially long duration required for follow-up. By comparison, case
control studies are well suited for the study of rare diseases and they
require fewer
[[Page 56756]]
subjects. The disadvantages of case control studies, however, include
the difficulty of selecting an appropriate control group(s), and the
reliance on recall or records for information on past exposures.
Regardless of the selected observational study design, the greatest
limitation of occupational epidemiologic studies is their ability to
measure and classify exposure.
In spite of the inherent limitations of observational epidemiologic
studies, guidelines have been developed for judging causal association
between exposure and outcome. Criteria commonly used to distinguish
causal from non-causal associations include: Strength of the
association as measured by the relative risk ratio or the odds ratio;
consistency of the association in different populations; specificity of
the association between cause and effect; temporal relationship between
exposure and disease which requires that cause precede effect; biologic
plausibility of the association between exposure and disease; the
presence of a dose-response relationship between exposure and disease;
and coherence with present knowledge of the natural history and biology
of the disease. These criteria have been considered by OSHA in the
development of its conclusion regarding the association between BD and
cancer of the LH system.
As stated previously, each type of epidemiologic study design has
strengths and weaknesses. Since epidemiologic studies are observational
and not experimental, each study will also have inherent strengths and
weaknesses; there is no perfect epidemiologic study. The most
convincing evidence of the validity and reliability of any
epidemiologic study comes with replication of the study's results.
There are six major epidemiologic studies in the record that have
examined the relationship between occupational exposure to BD and human
cancer. These studies include: A North Carolina study of rubber workers
(Ex. 23-41; 23-42; 23-4; 2-28; 23-27; 23-3); a Texaco study of workers
at a BD production facility in Texas (Ex. 17-33; 34-4; 34-4); a NIOSH
study of two plants in the styrene-butadiene rubber (SBR) industry (Ex.
2-26; 32-25); the Matanoski cohort study of workers in SBR
manufacturing (Ex. 9; 34-4); the nested case-control study of workers
in SBR manufacturing conducted by Matanoski and Santos-Burgoa (Ex. 23-
109); and a follow-up study of synthetic rubber workers recently
completed by Delzell et al. (Ex. 117-1). Several comments in the record
have concluded that these studies demonstrate a positive association
between occupational exposure to BD and LH cancers. However, OSHA has
been criticized by the Chemical Manufacturers Association (CMA) and the
International Institute of Synthetic Rubber Producers, Inc. (IISRP) for
its interpretation of these studies as showing a positive association;
the chief criticisms will be discussed below. (Ex. 112 and 113)
OSHA's final consideration of the BD epidemiologic studies is
organized and presented according to what have been identified as key
issues. These are the epidemiologic issues that were raised and
considered throughout the rulemaking. They are also the issues most
pertinent to OSHA's conclusions. These key issues surrounding BD
exposure and LH cancer are: Evidence of an association; observation of
a dose-response relationship; observation of short latency periods; the
potential role of confounding exposures and the observed study results;
the biological basis for grouping related LH cancers; relevance of
subgroup analyses; and appropriateness of selected reference
populations.
(ii) Evidence of an Association Between BD and LH Cancer. Each of
the studies listed above contributes to the epidemiologic knowledge
upon which OSHA's conclusion regarding the relationship of BD exposure
and LH cancer has been developed.
(a) North Carolina Studies. This series of studies was undertaken
to examine work-related health problems of a population of workers in a
major tire manufacturing plant. They were not designed to look
specifically at the health hazards of BD. (Tr. 1/15/91, p. 117)
However, in a work area that involved the production of elastomers,
including SBR, relative risks of 5.6 for lymphatic and hematopoietic
malignancies and 3.7 for lymphatic leukemia were found among workers
employed for more than five years. The International Agency for
Research on Cancer (IARC) evaluation concluded that this study suggests
an association between lymphatic and hematopoietic malignancy and work
in SBR manufacturing. (Tr. 1/15/91, p. 117) However, the IISRP asserted
that these studies do not provide ``meaningful evidence of an
association between butadiene and cancer.'' (Ex. 113, p. A-23) OSHA
recognizes that the researchers who conducted these studies
acknowledged that the workers may have had exposures to organic
solvents, including benzene, a known leukemogen, as pointed out by the
IISRP. (Ex. 113, p. A-24)
(b) Texaco Study. The two Texaco studies examined mortality of a
population of workers in a BD manufacturing facility in Texas. (Ex. 17-
33; 34-4 Vol. III, H-2; Divine 34-4, Vol. III, H-1) A qualitative
method of exposure classification, based on department codes and expert
consensus judgement, was used in the Downs study. (Ex. 17-33; 34-4,
Vol. III, H-2) From this methodology four exposure groups were defined:
Low exposure, which included utility workers, welders, electricians,
and office workers; routine exposure, which included process workers,
laboratory personnel, and receiving, storage and transport workers;
non-routine exposure, which included skilled maintenance workers; and
unknown exposure, which included supervisors and engineers. It is
OSHA's opinion that although this is a crude approach to exposure
classification, there are important findings in this study that
contribute to our understanding of BD epidemiology.
In the Downs study (Ex. 34-4, Vol. III, H-2) the standardized
mortality ratio (SMR) for all causes of death in the entire study
cohort was low (SMR 80; p < .05) when compared to national population
rates. However, a statistically significant excess of deaths was
observed for lymphosarcoma and reticulum cell sarcoma combined (SMR
235; 95% confidence interval (CI) = 101,463) when compared with
national population rates. (The issue of reference population selection
is discussed below in paragraph (viii).)
When analyzed by duration of employment, the SMR for the category
of all LH neoplasms was higher in workers with less than five years
employment (SMR = 167) than for those with more than five years
employment (SMR = 127). (Ex. 34-4, Vol. III, H-2) However, neither of
these findings was statistically significant. Alternatively, it has
been suggested that perhaps the short-term workers were wartime
workers, and that these workers were actually exposed to higher levels
of BD, albeit for a shorter time. (Tr. 1/15/91, p. 119)
Analyses of the four exposure groups also showed elevated but not
statistically significant SMRs. The routine exposure group had a SMR of
187 for all LH neoplasms, explained primarily by excesses in Hodgkin's
disease (SMR = 197) and other lymphomas (SMR = 282). (Ex. 34-4, Vol.
III, H-2) Those workers in the non-routine exposure group also had an
elevated SMR for all LH neoplasms (SMR = 167), with excess mortality
for Hodgkin's disease (SMR = 130), leukemias (SMR = 201), and other
[[Page 56757]]
lymphomas (SMR = 150) (Ex. 34-4, Vol. III, H-2).
These data were updated by Divine by extending the period of
follow-up from 1979 through 1985. (Ex. 34-4, Vol. III, H-1) The SMR for
all causes of mortality remained low (SMR = 84, 95% CI = 79,90), as it
did for mortality from all cancers (SMR = 80, 95% CI = 69,94). (Ex. 34-
4, Vol. III, H-1) However, the SMR for lymphosarcoma and
reticulosarcoma combined was elevated and statistically significant
(SMR = 229, 95% CI = 104,435). This finding was consistent with the
previous analyses done by Downs. (Tr. 1/15/91, p. 120).
Exposure group analyses were also consistent with the previous
findings by Downs. The highest levels of excess mortality from
lymphatic and hematopoietic malignancy were again seen in the routine
and non-routine exposure groups. The routine exposure group that was
``ever employed'' had a statistically significant excess of
lymphosarcoma (SMR = 561, 95% CI = 181,1310), that accounted for most
of the LH excess. (Ex. 34-4, Vol. III, H-1) The cohort of workers
employed before 1946 (wartime workers) also demonstrated a
statistically significant excess of mortality due to lymphosarcoma and
reticulosarcoma combined (SMR = 269, 95% CI = 108,555). (Ex. 34-4, Vol.
III, H-2)
In summary, the Texaco study provides several notable results. The
first of these is the consistently elevated mortality for
lymphosarcoma. This finding is consistent with excess lymphomas
observed in experimental mice. (Ex. 23-92) Second, the excess risk of
mortality was found in the routine and non-routine exposure groups.
Based on the types of jobs held by workers in these two exposure
groups, this finding suggests that the incidence of lymphatic
malignancy is highest in the groups with the heaviest occupational
exposure to BD. (Tr. 1/15/91, p. 121) The third notable result of this
study was the significantly elevated rate of malignancy in workers
employed for fewer than 10 years.
(c) NIOSH Study. The NIOSH study was undertaken in January 1976 in
response to the report of deaths of two male workers from leukemia.
(Ex. 2-26; 32-25) These workers had been employed in two adjacent SBR
facilities (Plant A and Plant B) in Port Neches, Texas. The hypothesis
tested by this study is that:
Employment in the SBR production industry was associated,
specifically, with an increased risk of leukemia and, more
generally, with an increased risk of other malignancies of
hematopoietic and lymphatic tissue. (Ex. 2-26)
This study did not specifically examine the association between BD and
all LH cancers. Thus, OSHA agrees with the criticism that this study by
itself did not demonstrate that occupational exposure to BD causes
cancer. (Ex. 113, pp. A-13, A-19) However, the findings in this study
are consistent with the patterns observed in the other epidemiologic
studies discussed in this section. In Plant A, the overall mortality
was significantly decreased (SMR=80, p<0.05). (Ex. 2-26) The SMR for
all malignant neoplasms was also decreased (SMR=78), but this result
was not statistically significant. (Ex. 2-26) The SMR for LH cancers
was elevated (SMR=155), as it was for lymphosarcoma and reticulum cell
sarcoma (SMR=181) and leukemia (SMR=203), but none of these results was
statistically significant. (Ex. 2-26)
The pattern of mortality for a subgroup of wartime workers was also
examined for the Plant A population. For this subgroup of white males,
employed at least six months between the beginning of January 1943 and
the end of December 1945, there was an elevated SMR for lymphatic and
hematopoietic neoplasms (SMR = 212) that was statistically significant
at the level of 0.050-19, 20-99, 100-199, and 200+,
respectively. (Ex. 117-1, pp. 68-69; 158) Poisson regression analyses
were also conducted using varying exposure categories of BD ppm-years.
These analyses demonstrated a stronger and more consistent relationship
between BD and leukemia than between styrene and leukemia. (Ex. 117-1,
p. 69, 159) Although a clearly positive relationship between BD ``peak-
years'' and leukemia was observed from additional Poisson regression
analyses, even after controlling for BD ppm-years, styrene ppm-years,
and styrene peak-years, the dose-response relationship was less clear.
(Ex. 117-1, pp. 71, 162)
In summary, one of the most important findings of the research of
Delzell et al. was strong and consistent evidence that employment in
the SBR industry produced an excess of leukemia. In the authors own
words:
This study found a positive association between employment in
the SBR industry and leukemia. The internal consistency and
precision of the result indicate that the association is due to
occupational exposure. The most likely causal agent is BD or a
combination of BD and [styrene]. Exposure to [benzene] did not
explain the leukemia excess. (Ex. 117-1, p. 85)
(g) Summary. These studies provide a current body of scientific
evidence regarding the association between BD and LH cancers. As
previously discussed, two of the criteria commonly used to determine
causal relationships are consistency of the association and strength of
the association. The consistency criterion for causality refers to the
repeated observation of an association in different populations under
different circumstances. Consistency is perhaps the most striking
observation to be made from this collection of studies: ``[E]very one
of these studies to a greater or lesser extent finds excess rates of
deaths from tumors of the lymphatic and hematopoietic system.'' (Tr. 1/
15/91, p. 129)
Strength of the association is determined by the magnitude and
precision of the estimate of risk. In general, the greater the risk
estimate, e.g., SMR or odds ratio, and the narrower the confidence
intervals around that estimate, the more probable the causal
association. In the nested case-control study, although the confidence
intervals were wide, the odds ratios provide evidence of a strong
association between leukemia and occupational exposure to BD.
(iii) Observation of a Dose-Response Relationship. A dose-response
relationship is present when an increase in the measure of effect
(response), e.g., SMR or odds ratio, is positively correlated with an
increase in the exposure, i.e., estimated dose. When such a
relationship is observed, it is given serious consideration in the
process of determining causality. However, the absence of a dose-
response relationship does not necessarily indicate the absence of a
causal relationship.
OSHA has been criticized for its conclusion that the epidemiologic
data suggest a dose-response relationship. (Ex. 113) The IISRP offers a
different interpretation of the data. In their opinion, the data
provide a ``consistent finding of an inverse relationship between
duration of employment and cancer mortality.'' (Ex. 113, A-34) This
observation is further described by John F. Acquavella, Ph.D., Senior
Epidemiology Consultant, Monsanto Company, as ``the paradox of
butadiene epidemiology.'' (Ex. 34-4, Vol. I, Appendix A) This
interpretation assumes that cumulative occupational exposure to BD will
increase with duration of employment, and, thus, cancer mortality will
increase with increasing duration of employment. (Ex. 113, A-35-39)
In OSHA's opinion, this is an erroneous assumption; the
epidemiologic data for BD tell a different story. For the workers in
these epidemiologic studies, it is unlikely that occupational exposure
to BD was constant over the duration of employment. According to
Landrigan, BD exposures were most likely higher during the war years
than they were in subsequent years. (Tr. 1/15/91, p.146) It is logical
that exposures would be especially intense during this time period
because of wartime production pressures, the process of production
start-up in a new industry, and the general lack of industrial hygiene
controls during that phase of industrial history. Unfortunately,
without quantitative industrial hygiene monitoring data, the true
levels of BD exposure for wartime workers cannot be ascertained. In the
absence of such data, however, OSHA believes it is reasonable to
consider wartime workers as a highly exposed occupational subgroup.
(Tr. 1/15/91, p. 121; Tr. 1/16/91, pp. 225-227) Thus, the excess
mortality seen among these workers provides another piece of the
evidence to support a dose-response relationship between occupational
exposure to BD and LH cancers.
Additional support that excess mortality, among workers exposed to
BD, is dose-related can be found in the analyses of the work area
exposure groups. The studies by Divine, Matanoski, and Matanoski and
Santos-Burgoa all provide evidence that excess mortality is greatest
among production workers. (Ex. 34-4, Vol. III, H-1; 34-4, Vol. III, H-
6; 23-109, respectively) Production workers are typically the most
heavily exposed workers to potentially toxic substances. (Ex. 34-4)
The most compelling data that support the existence of a dose-
response relationship for occupational exposure to BD and LH cancers
are those in the study by Delzell et al. (Ex. 117-1) Analysis of the
cumulative time-weighted BD exposure in ppm-years indicates a relative
risk for all leukemias that increases positively with increasing
exposure. This relationship is present even with statistical adjustment
for age, years since hire, calendar period, race, and exposure to
styrene. It is OSHA's opinion that identification of a positive dose-
response in an epidemiologic study is a very powerful observation in
terms of causality.
(iv) Observation of Short Latency Periods. Short latency periods,
i.e., time from initial BD exposure to death, were seen in two
epidemiologic studies. In the NIOSH study, three of the six leukemia
cases had a latency period from three to four years. (Ex. 2-26)
Additionally, five of these six workers were employed prior to 1945.
(Ex. 2-26)
[[Page 56762]]
In the Texaco study update, a latency of less than 10 years was seen in
four of the nine non-Hodgkin's lymphoma (lymphosarcoma) cases, and
seven of these workers were also employed during the wartime years.
(Ex. 34-4, Vol. III, H-1)
According to OSHA's expert witness, Dr. Dennis D. Weisenburger,
these findings are contrary to the accepted belief that, if a
carcinogen is active in an environment, one should expect the * * *
SMRs to be higher for long-term workers than for short-term workers
(i.e., larger cumulative dose). (Ex. 39, p. 9)
Thus, it has been argued that these findings appear to lack coherence
with what is known of the natural history and biology of LH cancers.
(Ex. 113, A-40-42) Furthermore, these findings have been interpreted as
evidence against a causal association between BD and these LH cancers.
(Ex. 113, A-42)
In OSHA's opinion, there are other possible explanations for these
observations. First, as proffered by Dr. Weisenburger, a median latency
period of about seven years has been found for leukemia in studies of
atomic bomb victims, radiotherapy patients, and chemotherapy patients
who have received high-dose, short-term exposures. (Ex. 39) In
contrast, Dr. Weisenburger points out that low-dose exposure to an
environmental carcinogen, such as benzene, has a median latency period
for leukemia of about 15-20 years. (Ex. 39) He concludes that short-
term, high-dose exposures may be associated with a short latency
period, whereas long-term, low-dose exposures may be associated with a
long latency period.
Second, the occurrence of short latency periods for LH cancer
mortality in these two studies was concentrated in workers first
employed during the wartime years. As previously discussed, it is
possible that exposure to BD during the wartime years was greater than
in subsequent years. (Ex. 39; Tr. 1/15/91, p. 121) Dr. Weisenburger
suggests that the ``short latency periods for LH cancer in these
studies may be explained by intense exposures to BD over a relatively
short time period.'' (Ex. 39, p. 10)
In his testimony, Dr. Landrigan, another OSHA expert witness, makes
the point that ``duration of employment is really only a crude
surrogate for total cumulative exposures, not itself a measure of
exposure.'' (Tr. 1/15/91, p. 121) In other words, it is possible that
short-term workers employed during the wartime years may have actually
had heavier exposures to BD than long-term workers. (Tr. 1/15/91, pp.
115-205) On cross-examination, Dr. Landrigan cautioned against
``assuming that duration of exposure directly relates to total
cumulative exposure.'' (Tr. 1/15/91, p. 180) He also emphatically
stated that an increased cancer risk in short-term workers would not be
inconsistent with a causal association. (Tr. 1/15/91, p. 204)
(v) The Potential Role of Confounding Exposures and Observed
Results. In epidemiologic studies ``confounding'' may lead to invalid
results. Confounding occurs when there is a mixing of effects. More
specifically, confounding may produce a situation where a measure of
the effect of an exposure on risk, e.g., SMR, RR, is distorted because
of the association of the exposure with other factors that influence
the outcome under study.
For example, the IISRP has suggested that confounding exposures
from other employment were responsible for the LH cancers observed in
the studies of BD epidemiology. (Ex. 113, A-43) This argument is based
on the past practice of using petrochemical industry workers, who may
have also been exposed to benzene, to start up the SBR and BD
production plants. The IISRP finds support for this position in the
observation of elevated SMRs in short-term workers employed during the
wartime years, precisely those most likely to be cross-employed. (Ex.
113, A-43)
However, there are a number of research methods in occupational
epidemiology that are available to control potential confounding
factors. Research methods that eliminate the effect of confounding
variables include: Matching of cases and controls; adjustment of data;
and regression analyses. In the nested case-control study, for example,
cases and controls were matched on variables that otherwise might have
confounded the study results. In the testimony provided by Santos-
Burgoa, he states that the ``matching scheme allowed us to control for
potential confounders and concentrate only on exposure variations.''
(Ex. 40, p. 12)
On cross-examination, Landrigan also addressed the potential role
of confounding exposures and the observed study results. First, he
observed that Dr. Philip Cole, Professor, Department of Epidemiology,
School of Public Health, University of Alabama at Birmingham, one of
the outspoken critics of OSHA's proposed rule, found no evidence for
confounding in his review of the Matanoski study. (Tr. 1/15/91, p. 178)
Second, Dr. Landrigan dismissed the notion of previous exposure to
benzene as the causative agent for the observed results in the short-
term workers. (Tr. 1/15/91, p. 178-179)
In their analyses of mortality patterns by estimated monomer
exposure, Delzell et al. used Poisson regression to control for
potential confounding factors. (Ex. 117-1) As previously stated, the
analyses conducted to determine the association between BD ppm-years
and leukemia indicated a positive dose-response relationship, even
after controlling for styrene ppm-years, age, years since hire,
calendar period, and race. In the opinion of the investigators, benzene
exposure did not explain the excess of leukemia risk, and BD is the
most likely causal agent. (Ex. 117-1, p. 85)
(vi) The Biological Basis for Grouping Related LH Cancers. The
epidemiologic studies that have examined the association between
occupational exposure to BD and excess mortality have grouped related
LH cancers in their analyses. This approach has been criticized as
evidence of a lack of ``consistency with respect to cell type'' which
``argues against a common etiologic agent.'' (Ex. 113, A-45) In other
words, these critics suggest that the relationship between BD and
excess mortality does not meet the specificity of association
requirement for a causal relationship. This requirement states that the
likelihood of a causal relationship is strengthened when an exposure
leads to a single effect, not multiple effects, and this finding also
occurs in other studies.
More specifically, OSHA has been criticized for its position that
``broad categories such as `leukemia' or `all LHC' should be used to
evaluate the epidemiologic data.'' (Ex. 113, A-46) Dr. Cole, for
example, commented that:
It is a principle of epidemiology--and of disease investigation
in general--that entities should be divided as finely as possible in
order to maximize the prospect that one has delineated a homogeneous
etiologic entity. Entities may be grouped for investigative purposes
only when there is substantial evidence that they share a common
etiology. (Ex. 63, p. 11)
It is Dr. Cole's opinion that LH cancers are ``distinct diseases'' with
``heterogeneous and multifactorial'' etiologies. (Ex. 63, p. 47)
Dr. Weisenburger, OSHA's expert in hematopathology, provided
testimony to the contrary. (Ex. 39, pp. 7-8) According to Dr.
Weisenburger, ``LH (cancer) cannot be readily grouped into `etiologic'
categories, since the precise etiologies and pathogenesis of LH
(cancer) are not well understood.'' (Ex. 39, p. 7) In his opinion,
because LH cancers are ``closely related to one
[[Page 56763]]
another and arise from common stem cells and/or progenitor cells, it is
valid to group the various types of LH (cancer) into closely-related
categories for epidemiologic study.'' (Ex. 39, p.7)
The issue of grouping related LH cancers to observe a single effect
was also addressed by Dr. Landrigan in his testimony. (Tr. 1/15/91, pp.
131-133) The first point raised by Dr. Landrigan is that the
``diagnostic categories [for LH cancers] are imprecise and * * *
overlapping.'' (Tr. 1/15/91, p. 131) For example, he explained that in
clinical practice transitions of lymphomas and myelomas into leukemias
may be observed. In such a case, one physician may record the death as
due to lymphoma and another may list leukemia as the cause of death.
(Tr. 1/15/91, p. 131-132) Additionally, Dr. Landrigan testified that
``some patients with lymphomas or multiple myeloma may subsequently
develop leukemia as a result of their treatments with radiation or
cytotoxic drugs.'' (Tr. 1/15/91, p. 132)
These recordings of disease transition are further complicated by
the historical changes that have occurred in nomenclature and The
International Classification of Diseases (ICD) coding. According to Dr.
Landrigan,
certain lymphomas and * * * leukemias, such as chronic lymphatic
leukemia are now considered by some investigators * * * to represent
different clinical expressions of the same neoplastic process. There
have been recent immunologic and cytogenetic studies which indicate
that there are stem cells which appear to have the capacity to
develop variously into all the various sorts of hematopoietic cells
including T-lymphocytes, plasma cells, granulocytes, erythrocytes,
and monocytes. (Tr. 1/15/91, p. 132)
Dr. Landrigan summarized his testimony on this issue by stating that
``these different types of cells share a common ancestry * * * there is
good biologic reason to think that they would have etiologic factors in
common.'' (Tr. 1/15/91, pp. 132-133)
OSHA maintains the opinion, which is well supported by the record,
that there is a biological basis and a methodologic rationale for
grouping related LH cancers. Furthermore, OSHA rejects the criticism
that the observation of different subtypes of LH cancers argues against
the consistency and specificity of the epidemiologic findings.
(vii) Relevance of Worker Subgroup Analyses. OSHA has been
criticized for focusing on and emphasizing the ``few positive results''
seen in the results of worker subgroup analyses. (Ex. 113, A-48) It has
been pointed out, for example, that in the update of the Matanoski
cohort study ``there were hundreds of SMRs computed in that study and
it's not surprising that one or two or even more would be found to be
statistically significant even when there is in fact nothing going
on.'' (Tr. 1/22/91, p. 1444) Additionally, it has been suggested that
OSHA has ignored the ``clearly overall negative results'' of the
epidemiologic studies. (Ex. 113, A-48)
OSHA agrees with the observation that when many statistical
analyses are done on a database, it is possible that some positive
results may be due to chance. However, OSHA rejects criticism that the
Agency has inappropriately concentrated on the positive results and
disregarded the negative results. It is OSHA's opinion that there is a
compelling pattern of results in the epidemiologic studies.
Furthermore, a reasonable explanation for the elevated SMR for
black production workers in the update of the Matanoski cohort study is
that this subset of the population actually had heavy exposure to BD.
Support for this explanation can be found in the industrial hygiene
survey results of Fajen et al. (Ex. 34-4) In this case, then, the risk
for excess mortality would be concentrated in a small subset of
otherwise very healthy and unexposed workers that would be diluted when
analyses are based on the entire group being studied. The only way to
observe the risk in the most highly exposed subset would be to analyze
the data by subgroups of the population.
(viii) Appropriateness of Selected Reference Populations. OSHA also
has been criticized for ``ignor[ing] the fact that most of the
epidemiologic studies of butadiene-exposed workers only used U.S.
cancer mortality rates for comparison to worker mortality.'' (Ex. 113,
A-49) The significance of this criticism is based on the observation by
Downs that ``use of local (mortality) rates (for comparison) tended to
bring the SMRs closer to 100.'' (Ex. 17-33, p.14) This finding results
from cancer rates along the Texas Gulf coast that are higher than
national rates. (Ex. 17-33) In other words, it has been argued that
national comparison rates artificially inflate the SMRs, while local
rates provide a more accurate picture of the mortality experience of
workers with occupational exposure to BD. (Ex. 113, A-50)
Dr. Landrigan captured the essence of this issue in his testimony
on cross-examination,
This is a perennial debate in epidemiology of whether to use
local comparison rates or regional or national, and there's [sic]
arguments [to] go both ways. (Tr. 1/15/91, p. 154)
He presented several arguments for using national rates. First, U.S.
mortality rates are based on the entire population, so they are more
stable. Second, national rates are more commonly used, so it is easier
to compare results from different studies.
On the other hand, the argument in favor of using local rates
centers on the fact that people in a local area may truly be different
from the total population or a regional population(s). Thus, comparing
a local subpopulation with the entire local population may provide more
accurate results. However, the weakness in this argument was
highlighted by Dr. Landrigan when he said that,
* * * if there are factors acting in the local population, such as
environmental pollution that may elevate rates in the local area so
that they are closer to the rates in the occupationally exposed
population, then theoretically at least one could argue that the
local population is overmatched, too similar to the employee
population and that the use of the national comparison group
actually give [sic] a better reflection of reality. (Tr. 1/15/91, p.
155)
In fact, he went on to point out that the BD plants have been
identified by the Environmental Protection Agency (EPA) as ``major''
polluters of the local environment with BD. (Tr. 1/15/91, p. 155)
OSHA acknowledges that there are pros and cons to both approaches
of reference population selection. However, in the study by Delzell et
al. mortality data of the USA cohort subgroup were analyzed using both
state, i.e., local, general population rates and USA general population
rates. (Ex. 117-1) As previously stated, there was little difference in
the overall pattern of these analyses. (Ex. 117-1, p. 60) Additionally,
the Santos-Burgoa and Matanoski nested case control study used the most
appropriate comparison group of all: Those employed at the same
facilities. (Ex. 23-109 and 34-4, Vol. III, H-4) Thus, given the
available data in the record, OSHA is of the opinion that it cannot
ignore the findings of excess mortality that are based on national
comparison rates.
(ix) Summary and Conclusions. (a) Summary. Table V-4 lists the
criteria that can be used to judge the presence of a causal association
between occupational exposure to BD and cancer of the
lymphohematopoietic system. When the available epidemiologic study
results are examined in this way, there is strong evidence for
causality. The data fulfill all of the listed criteria: Temporal
relationship; consistency;
[[Page 56764]]
strength of association; dose-response relationship; specificity of
association; biological plausibility; and coherence.
In his testimony, OSHA's epidemiologist expert witness agreed that
there is ``definite evidence for the fact that occupational exposure to
1,3-Butadiene can cause human cancer of the hematopoietic and lymphatic
organs.'' (Tr. 1/15/91, p. 133) Dr. Weisenburger, OSHA's expert witness
in hematopathology, also concluded that ``it would be prudent to treat
BD as though it were a human carcinogen.'' (Ex. 39, p. 11)
Table V-4.--Evidence That 1,3-Butadiene Is a Human Carcinogen
------------------------------------------------------------------------
Criterion for causality Met by BD
------------------------------------------------------------------------
Temporal relationship....................... Yes.
Consistency................................. Yes.
Strength of association..................... Yes.
Dose-response relationship.................. Yes.
Specificity of association.................. Yes.
Biological plausibility..................... Yes.
Coherence................................... Yes.
------------------------------------------------------------------------
(b) Conclusion. On the basis of the foregoing analysis, OSHA
concludes that there is strong evidence that workplace exposure to BD
poses an increased risk of death from cancers of the
lymphohematopoietic system. The epidemiologic findings supplement the
findings from the animal studies that demonstrate a dose-response for
multiple tumors and particularly for lymphomas in mice exposed to BD.
C. Reproductive Effects
In addition to the established carcinogenic effects of BD exposure,
various reports have led to concern about the potential reproductive
and developmental effects of exposure to BD. The term reproductive
effects refers to those on the male and female reproductive systems and
the term developmental refers to effects on the developing fetus.
Male reproductive toxicity is generally defined as the occurrence
of adverse effects on the male reproductive system that may result from
exposure to chemical, biological, or physical agents. Toxicity may be
expressed as alterations to the male reproductive organs and/or related
endocrine system. For example, toxic exposures may interfere with
spermatogenesis (the production of sperm), resulting in adverse effects
on number, morphology, or function of sperm. These may adversely affect
fertility. Human males produce sperm from puberty throughout life and
thus the risk of disrupted spermatogenesis is of concern for the entire
adult life of a man.
Female reproductive toxicity is generally defined as the occurrence
of adverse effects on the female reproductive system that may result
from exposure to chemical, biological, or physical agents. This
includes adverse effects in sexual behavior, onset of puberty,
ovulation, menstrual cycling, fertility, gestation, parturition
(delivery of the fetus), lactation or premature reproductive senescence
(aging).
Developmental toxicity is defined as adverse effects on the
developing organism that may result from exposure prior to conception
(either parent), during prenatal development, or postnatally to the
time of sexual maturation. Developmental effects induced by exposures
prior to conception may occur, for example, when mutations are
chemically induced in sperm. If the mutated sperm fertilizes an egg,
adverse developmental effects may be manifested in developing fetuses.
Mutations may also be induced in the eggs. The major manifestations of
developmental toxicity include death of the developing fetus,
structural abnormality, altered growth and function deficiency.
To determine whether an exposure condition presents a developmental
or reproductive hazard, there are two categories of research studies on
which to rely: Epidemiologic, or studies of humans, and toxicologic, or
experimental studies of exposed animals or other biologic systems.
Many outcomes such as early embryonic loss or spontaneous abortion
are not easily detectable in human populations. Further, some adverse
effects may be quite rare and require very large study populations in
order to have adequate statistical power to detect an effect, if in
fact one is present. Often, these populations are not available for
study. In addition, there are fewer endpoints which may be feasibly
measured in humans as compared to laboratory animals. For example,
early embryonic loss is difficult to measure in the study of humans,
but can be measured easily in experimental animals. There are no human
studies available to address reproductive and developmental effects of
BD exposure to workers. Thus, evidence on the reproductive and
developmental toxicity of BD comes from toxicologic studies performed
using primarily mice.
Animal studies have proved useful for studying reproductive/
developmental outcomes to predict human risk. A very important
advantage to the toxicological approach is the ability of the
experimenter to fully quantitate the exposure concentration and
conditions of exposure. Although extrapolation of risk to humans on a
qualitative basis is accepted, quantitative extrapolation of study
results is more complex.
In his testimony, OSHA's witness, Dr. Marvin Legator, an
internationally recognized genetic toxicologist from the University of
Texas Medical Branch in Galveston, cautioned that in assessing risk
``humans in general have proven to be far more sensitive than animals *
* * to agents characterized as developmental toxicants.'' (Ex. 72) He
also noted that ``of the 21 agents considered to be direct human
developmental toxins, in 19 * * * the human has been shown to be more
sensitive than the animal * * *'' He also pointed to the possibility
that sub-groups of the human population may be even more highly
sensitive than the population average.
OSHA believes that the animal inhalation studies designed to
determine the effect of BD on the reproduction and development of these
animals indicate that BD causes adverse effects in both the male and
female reproductive systems and produces adverse developmental effects.
These studies are briefly summarized and discussed below.
Toxicity to Reproductive Organs
In the first NTP bioassay, an increased incidence of testicular
atrophy was observed in male mice exposed to BD atmospheric
concentrations of 625 ppm. (Ex. 23-1) In female mice, an increased
incidence of ovarian atrophy was observed at 625 and 1,250 ppm. These
adverse effects were confirmed in reports of the second NTP study,
which used lower exposure concentrations. The latter lifetime bioassay
exposed male and female B3C6F1 mice to 0, 6.25, 20, 62.5, 200, and 625
ppm BD. (Ex. 114, p 115) See Table V-5. Testicular atrophy in males was
significantly increased at the highest dose tested, 625 ppm, and
reduced testicular weight was observed from BD exposures of 200 ppm.
(Ex. 96) These latter data are not shown in the Table. In female mice
at terminal sacrifice, 103 weeks, ovarian atrophy was significantly
increased at all exposure levels including the lowest dose tested, 6.25
ppm, compared with controls. Evidence of ovarian toxicity was also seen
during interim sacrifices, but in these cases was the result of higher
exposure levels. After 65 weeks of exposure, 90% of the mice exposed to
62.5 ppm experienced ovarian atrophy.
[[Page 56765]]
Table V-5.--Ovarian and Testicular Atrophy in Mice Exposed to BD
--------------------------------------------------------------------------------------------------------------------------------------------------------
Exposure concentration (ppm)
Lesion Weeks of -------------------------------------------------------------------------------------------------------------
exposure 0 6.25 20 62.5 200 625
--------------------------------------------------------------------------------------------------------------------------------------------------------
(5) Incidence (%)
-------------------------------------------------------------------------------------------------------------
Testicular atrophy........... 40 0/10(0) NE NE NE 0/10(0) 6/10(60)
65 0/10(0) NE NE NE 0/10(0) 4/7(57)
103 1/50(2) 3/50(6) 4/50(8) 2/48(4) 6/49(12) 53/72(74)
Ovarian atrophy.............. 40 0/10(0) NE NE 0/10(0) 9/10(90) 8/8(100)
65 0/10(0) 0/10(0) 1/10(10) 9/10(90) 7/10(70) 2/2(100)
103 4/49(8) 19/49(39) 32/48(67) 42/50(84) 43/50(86) 69/79(87)
--------------------------------------------------------------------------------------------------------------------------------------------------------
NE, not examined microscopically.
Source: Ex. 114.
Extensive comments on the BD induced ovarian atrophy were received
from Dr. Mildred Christian, a toxicologist who offered testimony on
behalf of the Chemical Manufacturers Association. She questioned the
relevance of using the data from studies of mice to extrapolate risk of
ovarian atrophy to humans because most of the evidence was observed
among the animals who were sacrificed after the completion of the
species reproductive life and only after prolonged exposure to 6.25 ppm
and 20 ppm (Ex. 118-13, Att 3, p. 4) On the other hand, Drs. Melnick
and Huff, toxicologists from the National Institute of Environmental
Health Sciences stated that: ``Even though ovarian atrophy in the 6.25
ppm group was not observed until late in the study when reproductive
senescence likely pertains, the dose-response data clearly establish
the ovary as a target organ of 1,3-butadiene toxicity at concentrations
as low as 6.25 ppm, the lowest concentration studied.'' (Ex. 114, p.
116) In addition, it should be noted that an elevated incidence of
ovarian atrophy was observed at periods of interim sacrifice of female
mice exposed to 20 ppm that took place at the 65 week exposure period,
a time prior to the ages when senescence would be expected to have
occurred. NIOSH also accepted Dr. Melnick's view that mice exposed to
6.25 ppm BD demonstrated ovarian atrophy. (Ex. 32-35) OSHA remains
concerned about the ovarian atrophy demonstrated at low exposure levels
in the NTP study. Thus, OSHA concludes that exposure to relatively low
levels of BD resulted in the induction of ovarian atrophy in mice.
Sperm-Head Morphology Study
NTP/Battelle investigators also described sperm head morphology
findings using B6C3F1 mice exposed as described in the dominant
lethal study mentioned below, e.g., exposures to 200, 1000 and 5000 ppm
BD. The mice were sacrificed in the fifth week post-exposure and
examined for gross lesions of the reproductive system. (Ex. 23-75) The
study authors chose this interval as having the highest probability for
detecting sperm abnormalities. Epididymal sperm suspensions were
examined for morphology. The percentage of morphologically abnormal
sperm heads was significantly increased in the mice exposed at 1,000
ppm and 5,000 ppm, but not for those exposed to 200 ppm. The study
authors concluded that ``these significant differences in the
percentage of abnormalities between control mice and males exposed to
1000 and 5000 ppm [BD] indicated that their late spermatogonia or early
spermatocytes were sensitive to this chemical.'' (Ex. 23-75, p. 16)
In reviewing this study, Dr. Mildred Christian stated that these
results are not necessarily correlated with developmental abnormalities
or reduced fertility and are ``reversible in nature'' and that the
observed differences are ``biologically insignificant.'' (Ex. 76, p.
14) In its submission, the Department of Health Services of California
said: ``A conclusion as to the reproductive consequences of these
abnormalities cannot be made from this study.'' (Ex. 32-168) In
reviewing Dr. Christian's comments, OSHA is in agreement that the
observation of a significant excess of sperm head abnormalities as a
result of BD exposure is not necessarily correlated with the
development of abnormal fetuses or of reduced fertility; however, the
Anderson study, which did evaluate fetal abnormality and reduced
fertility, demonstrated a significant excess of both fetal abnormality
plus early and late fetal mortality as a result of male mice exposure
to BD. (Ex. 117-1, P. 171) These observations of fetal mortality could
only occur as a result of an adverse effect on the sperm. In response
to Dr. Christian's comment that the sperm head abnormality observed in
the study is reversible, the reversibility would be dependent upon
cessation of exposure. Since workers may be exposed to BD on a daily
basis, the significance of reversibility may be moot.
Developmental Toxicity
Dominant Lethal Studies
A dominant lethal study was conducted by Battelle/NTP to assess the
effects of a 5-day exposure of male CD-1 mice to BD atmospheric
concentrations of 0, 200, 1,000 and 5,000 ppm BD for 6 hours per day on
the reproductive capacity of the exposed males during an 8-week post-
exposure period. (Ex 23-74) If present, dominant lethal effects are
expressed as either a decrease in the number of implantations or as an
increase in the incidence of intrauterine death, or both, in females
mated to exposed males. Dominant lethality is thought to arise from
lethal mutations in the germ cell line that are dominantly expressed
through mortality to the offspring. In this study, the only evidence of
toxicity to the adult male mouse was transient and occurred over a 20
to 30 minute period following exposure at 5,000 ppm. Males were then
mated to a different female weekly for 8 weeks. After 12 days, females
were killed and examined for reproductive status. Uteri were examined
for number, position and status of implantation. Females mated to the
BD-exposed males during the first 2 weeks post-exposure were described
as more likely than control animals to have increased numbers of dead
implantations per pregnancy.
For week one, the percentage of dead implantations in litters sired
by males exposed to 1,000 ppm was significantly higher than controls.
There were smaller increases at 200 ppm and 1000 ppm that were not
statistically significant. The percentage of females with two or more
dead implantations was significantly higher than the control value for
all three exposure groups. For week two, the numbers of dead
implantations per
[[Page 56766]]
pregnancy in litters sired by males exposed to 200 ppm and 1000 ppm
were also significantly increased, but not for those exposed to 5000
ppm. No significant increases in the end points evaluated were observed
in weeks three to eight. These results suggested to the authors that
the more mature cells (spermatozoa and spermatids) may be adversely
altered by exposure to BD. (Ex. 23-74)
The State of California Department of Health Services concluded
that the above mentioned study showed no adverse effect from exposure
to BD, with the possible exception of the increase in intrauterine
death seen as a result of male exposures to 1000 ppm BD at the end of
one week post exposure. (Ex. 32-16) Since values for the 5000 ppm
exposure group were not significantly elevated for this same period of
follow up, the California Department of Health thought the biological
significance of the results of the 1000 ppm exposure was questionable.
(Ex. 32-16) On the other hand, Dr. Marvin Legator stressed the low
sensitivity of the dominant lethal assay which, he felt was due to the
endpoint-lethality. He expressed the opinion that the studies were
``consistent with an effect on mature germ cells.'' (Ex. 72) He felt
that since an effect was observable in this relatively insensitive
assay that only the ``tip of the iceberg'' was observed, and that
``[t]ransmissible genetic damage, displaying a spectrum of abnormal
outcomes can be anticipated at concentrations (of BD) below those
identified in the dominant lethal assay procedure.'' (Ex. 72, p. 17)
The dominant lethal effect of BD exposure was more recently
confirmed by Anderson et al. in 1993. (Ex. 117-1, p. 171) They studied
CD-1 mice using a somewhat modified study design. Two exposure regimens
were used. In the first, ``acute study,'' male mice were exposed to 0
(n=25), 1250 (n=25), or 6250 (n=50) ppm BD for 6 hours only. Five days
later they were caged with 2 untreated females. One female was allowed
to deliver her litter and the other was killed on day 17 of gestation
and examined for the number of live fetuses, number of early and late
post-implantation deaths and the number and type of any gross
malformation. The authors stated that sacrifice on day 17 (rather than
the standard days 12 through 15) allowed examination of near-term
embryos for survival and abnormalities. The mean number of implants per
female was reduced compared with controls at both concentrations of BD,
but was statistically significant only at 1250 ppm. Neither post-
implantation loss nor fetal abnormalities were significantly increased
at either concentration. The authors concluded that ``a single 6-hour
acute exposure to butadiene was insufficient to elicit a dominant
lethal effect.'' (Ex. 117-1, p. 171)
In the second phase of the study, the ``subchronic study,'' CD-1
mice were exposed to 0 (n=25), 12.5 (n=25), or 1250 (n=50) ppm BD for 6
hours per day, 5 days per week, for 10 weeks. They were then mated. The
higher 1250 ppm BD exposure resulted in significantly reduced numbers
of implantations and in significantly increased numbers of dominant
lethal mutations expressed as both early and late deaths. See Table V-
6. Non-lethal mutations expressed as birth abnormalities were also
observed in live fetuses (3/312; 1 hydrocephaly and 2 runts).
The lower exposure (12.5 ppm) did not result in decreases in the
total number of implants, nor in early deaths; however, the frequencies
of late deaths and fetal abnormalities (7/282; 3 exencephalies in 1
litter and one in another, two runts and one with blood in the amniotic
sac) were significantly increased.
The authors felt that their finding of increased late deaths and
fetal abnormalities at a subchronic, low exposure of 12.5 ppm was the
main new finding of the study. They noted that these adverse health
effects were increased 2-3 fold over historical controls. In evaluating
these latter two studies OSHA notes that while there was no
demonstrable effect on dominant lethality as a result of a single
exposure to 1250 ppm BD, subchronic exposure to 12.5 ppm, the lowest
dose tested, resulted in the induction of dominant lethal mutations and
perhaps non-lethal mutations. (Ex 117-1, p 171) OSHA has some
reservations about whether or not the fetal abnormalities observed in
the Anderson et al. ``subchronic'' study were actually caused by non-
lethal mutations or by some other mechanism because they were observed
in only a few of the litters produced by the mice. (Ex. 117-1, p. 171)
Table V-6.--Effect of BD on Reproductive Outcomes in CD-1 Mice
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Implantations Early deaths Late deaths Late deaths including dead Abnormal fetuses
------------------------------------------------------------------------------------------------ fetuses -------------------------------
--------------------------------
No. Mean No. Mean a No. Mean a No. Mean a No. Mean a
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Control......................... 278 12.091.276 13 0.0500.0597 0 ......................... 2 0.0070.0222 0 .........................
12.5............................ 306 12.752.507 16 0.0530.0581 7 0.23**0.038 8 0.0260.0424 b7 0.024*0.062
1250 ppm........................ 406 10.68**3.103 87 0.204***0161 6 0.014***0.032 7 0.0160.339 c3 0.011**0.043<
4 l
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
* Significantly different from control at: *p0.05; **p0.01; ***p0.001 (by analysis of variance and least significance test on arc-sine transform data).
a Per implantation.
b Four exencephalies (three in one litter), two runts (70% and 60% of mean body weight of others in litter; total litter sizes 7 and 9, respectively one fetus with blood in amniotic
sac but no obvious gross malformation (significance of difference not altered if this fetus is excluded).
c One hydrocephaly, two runts (71% and 75% of mean body weight of others in litter; total litter sizes; 2 and 11, respectively).
A dominant lethal test was also performed by Adler et al. (Ex. 126)
Male(102/E1XC3H/E1)F1 male mice were exposed to 0 and 1300 ppm BD.
They were mated 4 hours after the end of exposure with untreated virgin
females. Females were inspected for the presence of a vaginal plug
every morning. Plugged females were replaced by new females. The mating
continued for four consecutive weeks. At pregnancy day 14-16 the
females were killed and uterus contents were evaluated for live and
dead implants. Exposure of male mice to 1300 ppm BD caused an increase
of dead implants during the first to the third mating week after 5 days
of exposure. The dead implantation rate was significantly different
from the concurrent controls only during the second mating week. Adler
et al. concluded that dominant lethal mutations were induced by BD in
spermatozoa and late stage spermatids and that these findings confirmed
the results of the Battelle/NTP study which showed effects on the same
stages of
[[Page 56767]]
sperm development. (Ex. 23-74) The authors were of the opinion that BD
may induce heritable translocations in these germ cell stages.
The earliest reproductive study reported on BD was conducted by
Carpenter et al. in 1944. (Ex. 23-64) In this study, male and female
rats were exposed by inhalation to 600, 2,300 or 6,700 ppm BD, 7.5
hours per day, six days per week for an 8-month period. Although this
study was not specifically designed as a reproductive study, the
fertility and the number of progeny were recorded. No significant
effects due to BD exposure were noted for either the number of litters
per female animal or for the number of pups per litter.
In the Hazelton study, Sprague-Dawley (SD) rats were exposed by
inhalation to 0, 200, 1,000 or 8,000 ppm BD on days 6 though 15 of
gestation. (Ex. 2-32) There were dose-related effects on maternal body
weight gain, fetal mean weight and crown-to-rump length. Post-
implantation loss was slightly higher in all BD-exposed groups. In
addition, there were significant increases in hematoma in pups in the
200 and 1,000 ppm exposure groups. In the 8,000 ppm exposure group, a
significantly increased number of pups had lens opacities and there was
an increased number of opacities per animal. According to the authors,
the highest exposure groups also had a significantly increased number
of fetuses with skeletal variants, a higher incidence of bipartite
thoracic centra, elevated incidence of incomplete ossification of the
sternum, higher incidence of irregular ossification of the ribs, and
``other abnormalities of the skull, spine, long bones, and ribs.'' The
authors concluded that the fetal response was not indicative of a
teratogenic effect, but was the result of maternal toxicity.
In the Battelle/NTP study, pregnant Sprague-Dawley (SD) rats and
pregnant Swiss mice were exposed to 0, 40, 200, or 1,000 ppm BD for 6
hours per day from day 6 through day 15 of gestation. (Ex. 23-72)
Animals were sacrificed and examined one day before expected delivery.
In the rat, very little effect was noted; in the 1,000 ppm exposure
group only there was evidence of maternal toxicity, i.e., depressed
body weight gains during the first 5 days of exposure. No evidence of
developmental toxicity was observed in the SD rats evaluated in the
study, e.g., the number of live fetuses per litter and the number of
intrauterine deaths were within normal limits.
In the mouse, exposure to the above mentioned concentrations did
not result in significant maternal toxicity, with the exception of a
reduction in extra-gestational weight gain for the 200 ppm and 1000 ppm
BD exposed dams. In the female mice, there was a significant depression
of fetal body weight only at the 200 and 1,000 ppm exposure levels.
Fetal body weight for male pups was reduced at all exposure
concentrations, including the 40 ppm exposure level, even though
evidence of maternal toxicity was not observed at this exposure
concentration. No significant differences were noted in incidence of
malformations among the groups. However, the incidence of supernumerary
ribs and reduced ossification of sternebrae was significantly increased
in litters of mice exposed to 200 and 1,000 ppm BD.
In reviewing these data, Drs. Melnick and Huff noted that since
maternal body weight gain was reduced at the 200 and 1000 ppm exposure
levels and body weights of male fetuses were reduced at the 40, 200,
and 1000 exposure levels ``[t]he male fetus is more susceptible than
the dam to inhaled 1,3-butadiene.'' (Ex. 114, p. 116) They further
stated that ``the results of the study in mice reveal that a toxic
effect of 1,3-butadiene was manifested in the developing organism in
the absence of maternal toxicity.'' On the basis of this study, the
authors concluded that ``1,3-butadiene does not appear to be
teratogenic in either the rat or the mouse, but there is some
indication of fetotoxicity in the mouse.'' (Ex. 23-72)
On the other hand, Dr. Mildred Christian was of the opinion that
the significant decrease in male mouse fetal weight gain in the 40 ppm
exposure group was not a selective effect of BD on the conceptus, but
rather was a result of the statistical analysis used which she
considered inappropriate. (Ex. 118-13, Att. 3, p. 6) She was also of
the opinion that the larger litter sizes in the 40 ppm exposure group
as compared with the control group contributed to the statistical
finding. Dr. Christian, however, did not present any specific
information on the type of analysis used for statistical testing that
she thought made the results inappropriate. In general, one would
expect that the evaluation of data from larger litter sizes would give
one more confidence in the statistical findings.
In reviewing the same study, the State of California, Department of
Health Services was more cautious. It stated that ``The increased
incidence of reduced ossifications and the fetal weight reductions in
the absence of apparent maternal toxicity in the 40- and 200-ppm groups
is evidence of fetotoxicity * * * in the Swiss (CD-1) mouse.'' After
reviewing the study results and arguments about the study, OSHA
concluded that the NTP study provides evidence of fetotoxicity in the
mouse. (Ex. 23-72)
Mouse spot test
Adler et al. (1994) conducted a spot test in mice. (Ex. 126) The
spot test is an in vivo method for detecting somatic cell mutations. A
mutation in a melanoblast is detected as a coat color spot on the
otherwise black fur of the offspring. Pregnant females were exposed to
0 or 500 ppm BD for 6 hours per day on pregnancy days 8, 9, 10, 11 and
12. They were allowed to come to term and to wean their litters.
Offspring were inspected for coat color spots at ages 2 and 3 weeks.
Gross abnormalities were also recorded. Exposure to a concentration of
500 ppm did not cause any embryotoxicity, nor were gross abnormalities
observed. The BD exposure, however, significantly increased the
frequency of coat color spots in the offspring. This study demonstrates
that BD exposure is capable of causing transplacentally induced somatic
cell mutations that can result in a teratogenic effect in mice.
Summary of Reproductive and Developmental Effect
OSHA has limited its discussion on reproductive and developmental
hazards to a qualitative evaluation of the data. This approach was
chosen because no generally accepted mathematical model for estimating
reproductive/developmental risk on a quantitative basis was presented
during the rulemaking. For example, the CMA Butadiene panel disagreed
with OSHA's findings in the proposal regarding the potential
reproductive and developmental risks presented by BD exposure using an
uncertainty factor approach. (See Ex. 112) They cited Dr. Christian's
conclusion that the mouse possessed a ``special sensitivity'' to BD and
should not be used as a model on which to base risk estimates.
The agency has determined, however, that animal studies, taken as a
whole, offer persuasive qualitative evidence that BD exposure can
adversely effect reproduction in both male and female rodents. The
Agency also notes that BD is mutagenic in both somatic and germ cells.
(Ex. 23-71; Ex. 114; Ex. 126)
Some evidence of maternal and developmental toxicity was seen in
rats exposed to BD, but the concentrations used were much higher than
those that elicited a response in mice. (Ex. 118-13, Att. 3, p. 2) In
mice, evidence of fetotoxicity was observed in either the presence or
absence of maternal toxicity, the latter evidence being
[[Page 56768]]
provided by decreased fetal body weight in male mice whose dams were
exposed to 40 ppm BD, the lowest dose tested in the study. In addition,
a teratogenic effect was observed in mice (coat color spot test) as a
result of transplacentally induced somatic cell mutation.
OSHA is also concerned about the observation of a significant
excess of sperm head abnormalities as a result of BD exposure, even
though this expression of toxicity is not necessarily correlated with
the development of abnormal fetuses or of reduced fertility. The
Anderson study, which did evaluate reduced fertility and fetal
abnormality, demonstrated a significant excess of both early and late
fetal mortality and perhaps fetal abnormality as a result of male mice
exposure to BD. (Ex. 117-1, P. 171) This observation could only occur
as a result of an adverse effect on the sperm. Two additional studies
also provide evidence of dominant lethality as a result of male
exposure to BD. (Ex. 23-74; Ex. 126) The observation of germ cell
effects is supported by additional evidence of genotoxicity in somatic
cells, as demonstrated by positive results in the micronucleus test and
in the mouse spot test. (Ex. 126)
Some of the adverse effects related to reproductive and
developmental toxicity in the mouse, e.g., ovarian atrophy, testicular
atrophy, reduced testicular weight, abnormal sperm heads, dominant
lethal effects, were acknowledged by Dr. Christian, but she urged the
Agency not to rely on these findings because of negative study results
in other species, or because positive findings in other species
required much higher exposure levels. (Ex. 118-13, Att. 3, p. 1)
For example, a CMA witness has argued that the diepoxide is
responsible for the ovarian atrophy observed in relation to low level
BD exposure (6.25 ppm). (Ex. 118-13, Att. 3) However, the monoepoxide
could also play a role in the ovarian atrophy and evidence indicates
that humans can form the monoepoxide of BD and that humans have the
enzymes present that could cause conversion to the diepoxide. Therefore
on a qualitative basis, the observation of ovarian atrophy in the mouse
is meaningful in OSHA's view. In addition, the metabolic factors
related to testicular atrophy, malformed sperm and dominant lethal
mutations in the mouse are not known. (See section on in vitro
metabolic studies.) These observations further support the findings in
mice as being meaningful for humans on a qualitative basis. The mouse
spot test which demonstrates a somatic cell mutation leading to a
teratogenic effect inconsistent with data showing the ability of BD to
cause adverse effects on chromosomes and hprt mutations in humans
exposed to BD.
OSHA also notes that studies of workers exposed to low
concentrations of BD demonstrated a significant excess of chromosomal
breakage and an inability to repair DNA damage. Thus, BD exposure seems
capable of inducing genetic damage in humans as a result of low level
exposure. Therefore, the mouse studies which demonstrate genetic damage
(mutations) in both somatic and germinal cells seem to be a better
model on a qualitative basis than the rat for predicting these adverse
effects in humans.
D. Other Relevant Studies
1. Acute Hazards
At very high concentrations, BD produces narcosis with central
nervous system depression and respiratory paralysis. (Ex. 2-11)
LC50 values (the concentration that produces death in 50 percent
of the animals exposed) were reported to be 122,170 ppm (12.2% v/v)
in mice exposed for 2 hours and 129,000 ppm (12.9% v/v) in rats exposed
for 4 hours. (Ex. 2-11, 23-91) These concentrations would present an
explosion hazard, thus limiting the likelihood that humans would risk
any such exposure except in extreme emergency situations. Oral
LD50 values (oral dose that results in death of 50 percent of the
animals) of 5.5 g/kg body weight for rats and 3.2 g/kg body weight for
mice have been reported. (Ex. 23-31) These lethal effects occur at such
high doses that BD would not be considered ``toxic'' for purposes of
Appendix A of OSHA's Hazard Communication Standard (29 CFR 1910.1200),
which describes a classification scheme for acute toxicity based on
lethality data.
At concentrations somewhat above the previous permissible exposure
level of 1,000 ppm, BD is a sensory irritant. Concentrations of several
thousand ppm were reported to cause irritation to the skin, eyes, nose,
and throat. (Ex. 23-64, 23-94) Two human subjects exposed to BD for 8
hours at 8000 ppm reported eye irritation, blurred vision, coughing,
and drowsiness. (Ex. 23-64)
2. Systemic Effects
In the preamble to the proposal, OSHA reviewed the literature to
discern the systemic effects of BD exposure. (55 FR 32736 at 32755)
OSHA discussed an IARC review which briefly examined several studies
from the former Soviet Union. In these, various adverse effects, such
as hematologic disorders, liver enlargement and liver and bile-duct
diseases, kidney malfunctions, laryngotracheitis, upper respiratory
tract irritation, conjunctivitis, gastritis, various skin disorders and
a variety of neurasthenic symptoms, were ascribed to occupational
exposure to BD. (Ex. 23-31) OSHA and IARC have found these studies to
be of limited use primarily due to their lack of exposure information.
Except for sensory irritant effects and hematologic changes, evidence
from studies of other exposed groups have failed to confirm these
observations.
Melnick and Huff summarized the observed non-neoplastic effects of
BD exposure in the NTP I and NTP II mouse bioassays. They listed the
following effects associated with exposure of B6C3F \1\ mice to BD for
6 hours per day 5 days per week for up to 65 weeks:
* * * epithelial hyperplasia of the forestomach, endothelial
hyperplasia of the heart, alveolar epithelial hyperplasia,
hepatocellular necrosis, testicular atrophy, ovarian atrophy and
toxic lesions in nasal tissues (chronic inflammation, fibrosis,
osseous and cartilaginous metaplasia, and atrophy of the olfactory
epithelium.) (Ex. 114, p. 114)
They noted that the nasal lesions were seen only in the group of male
mice exposed to 1250 ppm BD and that no tumors were observed at this
site. Further, Melnick and Huff suggested that some of the
proliferative lesions observed in the bioassay might represent pre-
neoplastic changes.
The findings of testicular and ovarian atrophy are discussed more
fully in the Reproductive Effects section of this preamble,.
Nephropathy, or degeneration of the kidneys, was the most common
non-carcinogenic effect reported for male rats in the Hazelton
Laboratory Europe (HLE) study in which rats were exposed to 1000 or
8000 ppm BD for 6 hours per day, 5 days per week for up to 2 years.
Nephropathy was one of the main causes of death for the high dose
males. (Ex. 2-31, 23-84) The combined incidence of marked or severe
nephropathy was significantly elevated in the high dose group over
incidence in the low dose group and over incidence in the controls
(p<.001). HLE's analysis of ``certainly fatal'' nephropathy shows a
significant dose-related trend (p<.05), but when ``uncertainly fatal''
cases were included, the trend disappeared.
The HLE study authors concluded that the interpretation of the
nephropathy incidence data was equivocal. They stated that ``an
increase in the prevalence of the more severe grades of nephropathy, a
common age-
[[Page 56769]]
related change in the kidney, was considered more likely to be a
secondary effect associated with other unknown factors and not to
represent a direct cytotoxic effect of the test article on the
kidney.''
Upon reviewing the HLE rat study for the proposed rule, OSHA
expressed concern that only 75% of the low-dose male rats in the HLE
study exhibited nephropathy, while 87% of the control rats had some
degree of nephropathy, suggesting low-dose male rats were less
susceptible to kidney degeneration than control rats, thereby
decreasing the comparability between rats in the low-dose and control
groups. (55 FR 32736 at 32744) Dr. Robert K. Hinderer, in testifying
for the CMA BD Panel, countered that the NTP I mouse study also had
``selected instances where the response in the test group (was) lower
than that in the controls'' and that ``* * * (o)ne cannot look at
single or a few individual site responses to evaluate the health status
or overall effect of the chemical.'' (Ex. 51) OSHA agrees that there
may be some variability in background response rates for specific
outcomes. However, the Agency believes that it is important to assess
the impact of the variability in background response rates when drawing
conclusions about dose-related trends in the data. This was not done in
the HLE study nephropathy analysis.
Other non-carcinogenic effects observed in the HLE rat study were
elevated incidence of metaplasia in the lung of high dose male rats at
terminal sacrifice as compared with incidence in male controls at
terminal sacrifice, and a significant increase in high dose male rat
kidney, heart, lung, and spleen weights over the organ weights in
control male rats.
3. Bone Marrow Effects
There was a single study of BD-exposed humans discussed in the
proposal--a study by Checkoway and Williams that examined 163 hourly
production workers who were employed at the SBR facility studied by
McMichael et al.. (described more fully in the Epidemiology Section of
this Preamble.) (Ex. 23-4, 2-28).
Exposure to BD, styrene, benzene, and toluene was measured in all
areas of the plant. BD and styrene concentrations, 20 (0.5-65) ppm and
13.7 (0.14-53) ppm, respectively, were considerably higher in the Tank
Farm than in other departments. In contrast, benzene exposures,
averaging 0.03 ppm, and toluene concentrations, averaging 0.53 ppm,
were low in the Tank Farm. The authors compared the hematologic
profiles of Tank Farm workers (n=8) with those of the other workers
examined.
The investigation focused on two potential effects, bone marrow
depression and cellular immaturity. Bone marrow depression was
suspected if there were lower levels of erythrocytes, hemoglobin,
neutrophils, and platelets. Cellular immaturity was suggested by
increases in reticulocyte and neutrophil band form values.
Although the differences were small, adjusted for age and medical
status, hematologic parameters in the Tank Farm workers differed from
those of the other workers. Except for total leukocyte count, the
hematologic profiles of the Tank Farm workers were consistent with an
indication of bone marrow depression. The Tank Farm workers also had
increases in band neutrophils, a possible sign of cellular immaturity,
but no evidence that increased destruction of reticulocytes was the
cause.
While acknowledging the limitations of the cross-sectional design
of the study, the authors felt, nevertheless, that their results were
``suggestive of possible biological effects, the ultimate clinical
consequences of which are not readily apparent.'' OSHA finds any
evidence of hematological changes in workers exposed at BD levels well
below the existing permissible limit (1000 ppm) to be of concern since
such information suggests the inadequacy of the present exposure limit.
However, this cross-sectional study involved only 8 workers with
relatively high levels of exposure to BD and low levels of exposure to
benzene, so it is quite insensitive to minor changes in hematologic
parameters.
In a review of BD-related studies, published in 1986, an IARC
Working Group felt the study of Checkoway and Williams could not be
considered indicative of an effect of BD on the bone marrow (Ex. 2-28).
In 1992, IARC concluded that the ``changes cannot be interpreted as an
effect of 1,3-butadiene on the bone marrow particularly as alcohol
intake was not evaluated.'' (Ex. 125, p. 262)
In light of the more recent animal studies that were not available
to IARC, however, OSHA believes that the bone marrow is a target of BD
toxicity. Furthermore, the fact that changes in hematologic parameters
could be distinguished in workers exposed to BD at 20 ppm indicates
that such measurements may prove a sensitive indicator of excessive
exposure to BD.
In testimony for the CMA BD Panel, Dr. Michael Bird stated his
conclusion that the hematological differences between the 8 tank farm
workers and the lesser exposed group of workers was not ``statistically
significant by the usual conventional statistics.'' (Tr. 1/18/1991, p.
1078) He believed that although the raw data were not available, the
reported means were within the historical and expected range for these
parameters. (Tr. 1/18/1991, p., 1078) In contrast, OSHA concludes from
this study that the hematologic differences observed in BD-exposed
workers, although small, are suggestive of an effect of BD on human
bone marrow under occupational exposure conditions.
Thus OSHA considers the Checkoway and Williams study to be
suggestive of hematologic effects in humans, but does not regard it as
definitive. No other potential systemic effects of BD exposure on this
population were addressed in the Checkoway and Williams study.
In 1992, Melnick and Huff reviewed the toxicologic studies of BD
exposure in laboratory animals. (Ex. 114) Only slight to no systemic
effects were observed in an early study of rats, guinea pigs, rabbits
and a dog exposed to BD up to 6,700 ppm daily for 8 months. (Ex. 23-64)
The study of Sprague Dawley rats exposed to doses of BD up to 8,000 ppm
daily for 13 weeks also did not result in hematologic, biochemical,
neuromuscular, nor urinary effects. However, there were marked effects
seen in exposed mice.
Epidemiologic studies of the styrene-butadiene rubber (SBR)
industry suggest that workers exposed to BD are at increased risk of
developing leukemia or lymphoma, two forms of hematologic malignancy
(see preamble section on epidemiology). Consequently, investigators
have looked for evidence of hematopoietic toxicity resulting from BD
exposure in animals and in workers. For example, Irons and co-workers
at CIIT found that exposure of male B6C3F1 mice to 1,250 ppm of BD
for 6-24 weeks resulted in macrocytic-megaloblastic anemia, an increase
in erythrocyte micronuclei and leukopenia, principally due to
neutropenia. Bone marrow cell types overall were not altered, but there
was an increase in the number of cells in the bone marrow of exposed
mice due to an increase in DNA synthesis. (Ex. 23-12)
Melnick and Huff also reviewed the available information on bone
marrow toxicity. (Ex. 114, p. 114) Table V-7 represents the reported
findings of a study of 10 B6C3F1 mice sacrificed after 6.25-625
ppm exposure to BD for 40 weeks. The authors concluded that these data
demonstrated a concentration-dependent decrease in red blood cell
number, hemoglobin concentration, and packed red cell
[[Page 56770]]
volume at BD exposure levels from 62.5 to 625 ppm. The effects were not
observed at 6.25 and 20 ppm exposure levels. Melnick and Kohn also
noted the increase in mean corpuscular volume in mice exposed at 625
ppm, and suggested that this and other observations (such as those of
Tice (Ex. 32-38D)) who observed a decrease in the number of dividing
cells in mice and decreased rate of their division), suggested that BD
exposure led to a suppression of hematopoiesis in bone marrow. Melnick
and Huff concluded that this, in turn, led to release of large immature
cells from sites such as the spleen, which was considered indicative of
macrocytic megaloblastic anemia by Irons. They concluded that these
findings ``(establish) the bone marrow as a target of 1,3-butadiene
toxicity in mice.'' (Ex. 114, p. 115)
Table I.--Hematologic Changes in Male B6C3F1 Mice Exposed for 6 Hours/Day, 5 Days/Week for 40 Weeks
----------------------------------------------------------------------------------------------------------------
Red blood cell
BD exposure (ppm) count ( x 10 Hemoglobin Volume packed Mean corpus-
\6\/ul) conc. (g/dl) RBC (ml/dl) cular vol
----------------------------------------------------------------------------------------------------------------
0............................................... 10.4a 9.9a 15.9a 45.9a 9.6a 15.6a 45.4a 7.6a 13.5a 39.9a 53.2a Different from chamber control (0 ppm), P<0.05. Results of treated groups were compared to those of control
groups using Dunnett's t-test.
4. Mutagenicity and Other Genotoxic Effects
OSHA discussed the genotoxic effects of BD exposure in some detail
in the proposal. (55 FR 32736 at 32760) Briefly, BD is mutagenic to
Salmonella typhimurium strains TA 1530 and TA 1535 when activated with
S9 liver fraction of Wistar rats treated with phenobarbital or Arochlor
1254. These bacterial strains are sensitive to base-pair substitution
mutagens. Since the liver fraction is required to elicit the positive
mutagenic response, BD is not a direct-acting mutagen and likely must
be metabolized to an active form before becoming mutagenic in this test
system. IARC published an extensive list of ``genetic and related
effects of 1,3-butadiene.'' (Ex. 125) They noted in summarizing the
data that BD was negative in tests for somatic mutation and
recombination in Drosophila, and that neither mouse nor rat liver from
animals exposed to 10,000 ppm BD showed evidence of unscheduled DNA
synthesis.
As OSHA described in the proposed rule, and Tice et al. reported in
1987, BD is a potent in vivo genotoxic agent in mouse bone marrow cells
that induced chromosomal aberrations and sister chromatid exchange in
marrow cells and micronuclei in peripheral red blood cells. (55 FR
52736 at 52760) Some of these effects were evident at exposures as low
as 6.25 ppm (6 hours/day, 10 days). However, similar effects were not
observed in rat cells exposed to higher levels of BD (10,000 ppm for 2
days).
Sister chromatid exchange is a recombinational event in which
nucleic acid is exchanged between the two sister chromatids in each
chromosome. It is thought to result from breaks or nicks in the DNA.
Irons et al. described micronuclei as ``* * * chromosome fragments or
chromosomes remaining as the result of non-dysjunctional event. Their
presence in the circulation is frequently associated with megaloblastic
anemia.'' (Ex. 23-12).
In a subsequent study, Filser and Bolt exposed B6C3F1 mice to
the same 3 concentrations of BD, 6.25, 62.5 or 625 for 6 hours/day, 5
days/week, for 13 weeks. (Ex. 23-10) Peripheral blood samples were
taken from 10 animals per group and scored for polychromatic
erythrocytes (PCE) and micronucleated normochromatic erythrocytes (MN-
NCE). The MN-NCE response, which reflects an accumulated response, was
significantly increased in both sexes at all concentrations of BD,
including 6.25 ppm.
Certain metabolites of BD also produce genotoxic effects. These are
detailed in a number of reviews (see for example, Ex. 114, 125).
Briefly, epoxybutene (the monoepoxide) is mutagenic in bacterial
systems in the absence of exogenous metabolic activation. Epoxybutene
also reacts with DNA, producing two structurally identical adducts and
has been shown to induce sister chromatid exchanges in Chinese hamster
ovary cells and in mouse bone marrow in vivo.
IARC in its review concluded that the diepoxide, 1,2,:3,4-
diepoxybutane, induced DNA crosslinks in mouse hepatocytes and, like
epoxybutene, is mutagenic without metabolic activation. As discussed
below, BD diepoxide also induced SCE and chromosomal aberrations in
cultured cells.
A human cross-sectional study involving a limited number of workers
in a Texas BD plant indicated genotoxic effects. (Ex. 118-2D)
Peripheral lymphocytes were cultured from 10 non-smoking workers and
from age- and gender-matched controls who worked in an area of very low
BD exposure (0.03 ppm). Production areas in the plant had a mean
exposure of 3.5 ppm BD, with most exposed workers in this sample
experiencing exposure of approximately 1 ppm BD.
Standard assays for chromosomal aberrations and a gamma irradiation
challenge assay that was designed to detect DNA repair deficiencies
were performed. The results of the standard assay indicated that the
exposed group had a higher frequency of cells with chromosome
aberrations and higher chromatid breaks compared with the control
group. This difference was not statistically significant. In the
challenge assay, the exposed group had a statistically significant
increased frequency of aberrant cells, chromatid breaks, dicentrics
(chromosomes having 2 centromeres) and a marginally significant higher
frequency of chromosomal deletions than controls. Au and co-workers
concluded that cells exposed to BD are likely to have more difficulty
in repairing radiation induced damage. (Ex. 118-2D)
To determine the mutagenic potential of both BD and its three
metabolite epoxides, Cochrane and Skopek studied effects in human
lymphoblastoid cells (TK6) and in splenic T cells from exposed
B6C3F1 mice. (Ex. 117-2, p. 195) TK6 cells were exposed for 24
hours to epoxybutene (0-400 uM), 3,4-epoxy-1,2-butanediol (0-800 uM),
or diepoxybutane (0-6 uM). All
[[Page 56771]]
metabolites were mutagenic at both the hprt (hypoxanthine-guanine
phosphoribosyl transferase) and tk (thymidine kinase) loci, with
diepoxybutane being active at concentrations 100 times lower than
epoxybutane or epoxybutanediol.
They also studied mice exposed to 625 ppm BD for 2 weeks and found
a 3-fold increase in hprt mutation frequency in splenic T cells
compared with controls. They also intended to give daily IP doses of
epoxybutene (60, 80 or 100 mg/kg) or diepoxybutane (7, 14, or 21 mg/kg)
every other day for three days. However, only animals given the lowest
dose of the diepoxide received three doses because of lethality. After
two weeks of expression time, cells were isolated for determination of
mutation frequency. Both exposure regimens resulted in increased
mutation frequency. For example, at the highest exposure to
epoxybutene, the average mutation frequency was 8.6 x 10\6\, while the
diepoxide exposed group had a frequency of 13 x 10\6\, compared to a
control mutation frequency of 1.2 x 10\6\.
Cochrane and Skopek used denaturing gradient gel electrophoresis to
study the nature of the splenic T cell hprt mutants in the DNA. They
found about half were frameshift mutations. A potential ``hotspot'' was
also described in which a plus one (+1) frameshift mutation in a run of
six guanine bases was observed in four BD-exposed mice, in four
expoxybutene-exposed mice and in two mice exposed to the diepoxide.
They observed both G:C and A:T base pair substitutions in the epoxide
treated group; however, similar to the findings of Recio, et al.
(described below), A:T substitutions were observed only in the BD-
treated group. The authors offered no hypothesis for this observation.
These researchers also noted a significant correlation of dicentrics
with the presence of a BD metabolite, (1,2-dihydroxy-4-(N-acetyl-
cysteinyl-S)butane) in the urine of exposed workers. They further
concluded that:
This study indicates that the workers had exposure-induced
mutagenic effects. Together with the observation of gene mutation in
a subset of the population, this study indicates that the current
occupational exposure to butadiene may not be safe to workers. (Ex.
118-2D)
An abstract by Hallberg submitted to the Environmental Mutagenesis
Society describes a host-cell reaction assay in which lymphocytes
transfected with a plasmid with an inactive chloramphenicol acetyl
transferase (CAT) reporter gene were challenged to repair the damaged
plasmid and reactivate the CAT gene. No effect was noted among cells of
workers exposed to 0.3 ppm benzene; however, BD-exposed workers (mean
exposure 3 ppm) had significantly reduced DNA repair capacity
(p=0.001). The authors believed that this finding confirmed the DNA
repair defect due to BD exposure observed in the Au et al. study's
challenge assay. (Ex. 118-2D)
Ward and co-workers reported the results of a preliminary study to
determine whether a biomarker for BD exposure and a biomarker for the
genetic effect of BD exposure could be detected in BD-exposed workers.
(Ex. 118-12A) The biomarker for exposure was excretion of a urinary
metabolite of BD, (1,2-dihydroxy-4-(n- acetylcysteinyl-S)butane. The
genetic biomarker was the frequency of lymphocytes containing mutations
at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus.
Study subjects included 20 subjects from a BD production plant and 9
from the authors' university; all were verified non-smokers. Seven
workers were in areas or at jobs that were ``considered likely to
expose them to higher levels of butadiene than in other parts of the
plant.'' Ten worked in areas where the likelihood of BD exposure was
low. Three ``variable'' employees worked in both types of jobs or
areas. hprt assays of 6 of the 7 high exposure group and 5 of the 6
non-exposed groups were completed at the time of the report. Air
sampling was used to estimate exposure. In the production area, the
mean was approximately 3.5 ppm, with most samples below 1 ppm. In the
central control area (lower exposure) the mean was 0.03 ppm. The
frequency of mutant lymphocytes in the high-exposure group compared
with either the low- or no-exposure group was significantly increased.
The low- and non-exposed groups were not significantly different from
each other in mutant frequencies.
Similarly, the concentration of the BD metabolite in urine was
significantly greater in the high exposure group than in the lower- or
non-exposed groups. There was a strong correlation among exposed
subjects between the level of metabolite in urine and the frequency of
the hprt mutants (r=0.85). (Ex. 118-2A)
Another study of humans for potential cytogenetic effects of BD
exposure was reported recently by Sorsa et al. in which peripheral
blood was drawn from 40 BD production facility workers and from 30
controls chosen from other departments of the same plants, roughly
matched for age and smoking habits. (Ex. 124) Chromosome aberrations,
micronuclei and sister-chromatid exchanges were analyzed. No exposure
related effects were seen in any of the cytogenetic endpoints. The
typical exposure was reported as less than 3 ppm. (Ex. 124)
Among the limited number of human studies involving BD exposed
workers is that of Osterman-Golker who evaluated post-exposure adduct
formation in the hemoglobin of mice, rats, and a small number of
workers. (Ex. 117-2, p. 127) Mice and rats were exposed at 0, 2, 10, or
100 ppm for 6 hours per day, 5 days per week for 4 weeks and their
blood tested for the presence and quantity of the BD metabolite, 1,2-
epoxybutene, forming an adduct with the N-terminal valine of
hemoglobin. The result was a linear response for mice at 2, 10 and 100
ppm; and, for rats at 2 and 10 ppm, with the 100 ppm dose group
deviating from linearity. In addition, while the adduct level per gram
of globin in the 100 ppm rats was about 4 times lower than the level
observed in mice exposed to 100 ppm BD, at lower exposures, the adduct
levels were similar.
In the portion of the study dealing with effects on humans, blood
was taken from four workers in two areas of a chemical production plant
with known BD exposure, and five workers from two non-production areas
where BD concentrations were low. In the higher exposure area, the mean
BD exposure was about 3.5 ppm, as determined by environmental sampling.
The lower exposure areas had a mean BD level of about 0.03 ppm. On a
mole of adduct per gram of hemoglobin level, the adduct levels in the
higher BD exposed workers were 70 to 100 times lower than those of
either the rat or mice exposed at the 2 ppm level discussed above.
Production workers had adduct levels ranging from 1.1 to 2.6 pmol/g
globin. Most controls in the study were below the level of detection of
the assay (0.5 pmol adduct/ g globin). (Two heavy smokers reported from
a previous study had higher adduct levels than non-smokers; their
levels approached those observed in BD exposed workers and were
consistent with the amount of BD in mainstream smoke.)
Similar results for mice and rats exposed to BD were reported by
Albrecht et al. (Ex. 117-2, p. 135) In this study which exposed the
rodents to 0, 50, 200, 500 or 1300 ppm for 6 hours/day, for 5
consecutive days, BD monoepoxide adduct levels in the hemoglobin of
mice were about five times that of the rat at most BD exposure
concentrations. Humans were not studied in this report.
Another observation pertaining to human cytogenetics with
potentially important implications for BD-induced
[[Page 56772]]
human disease is contained in a report by Wiencke and Kelsey. (Ex. 117-
2, p. 265) These researchers studied the impact of the BD metabolite,
diepoxybutane, exposure on sister chromatid exchange (SCE) frequencies
in several groups of human blood cell cultures (n=173 healthy workers).
They discovered that the study populations were bimodally distributed
according to their sensitivity to induction of SCEs when cell cultures
were exposed to 6 uM diepoxybutane. Wiencke and Kelsey reported that
they had observed in earlier studies that ``genetic deficiency of
glutathione S-transferase type u leads to bimodal induction of SCEs by
epoxide substrates of the isozyme'' and that cells from individuals
with the deficiency had SCE induction scores that were significantly
higher than those observed in the general population. (Ex. 117-2, p.
271) Approximately 20% of the tested groups were sensitive to induction
of SCE and the remaining 80% were relatively insensitive.1
Subsequent testing indicated that the sensitive population was also
sensitive to induction of chromosomal aberrations by diepoxybutane with
significant increases in the frequencies of chromatid deletions,
isochromatid deletions, chromatid exchanges and total aberrations. The
relevance of these findings in not yet clear; however, they may
indicate that certain subsets of the population are more highly
susceptible to the effects of this mutagenic metabolite of BD.
---------------------------------------------------------------------------
1 For example, in the 58 newspaper workers tested, 24% had
greater than 95 SCE/cell, while the remaining 76% had fewer than 80
SCE/cell.
---------------------------------------------------------------------------
Recio et al. used transgenic mice containing a shuttle vector with
a recoverable lac 1 gene to study in vivo mutagenicity of BD and the
spectrum of mutations produced in various tissues. (Ex. 118-7D) Mice
were exposed to 62.5, 625 or 1250 ppm BD for 4 weeks (5 days/week, 6
hours/day). The investigators extracted DNA from bone marrow and
determined mutagenicity at the lac 1 transgene.
The mutant DNA was sequenced. Dose-dependent mutagenicity--up to a
3-fold increase over air controls--was observed among mice exposed at
625 or 1250 ppm. Although a number of differences in patterns were
noted, the most striking was that sequence analysis indicated an
increased frequency of in vivo point mutations induced by BD exposure
at adenine and thymine (A:T) base pairs following inhalation.
In further studies of BD-exposed transgenic mice, Sisk and co-
workers exposed male B6C3F1 mice to 0, 62.5, 625, or 1250 ppm, BD
for 4 weeks (6 hour/day, 6 days/week). (Ex. 118-7Q) Bone marrow cells
were isolated and mutation frequency and spectrum evaluated. Lac 1
mutation frequencies were significantly increased at all 3 exposure
levels and were dose-responsive in the 62.5 and 625 ppm BD-exposed
mice, compared to controls. A plateau in mutation frequencies was
observed at 1250 ppm BD-exposed mice, perhaps indicating saturation or
mutant loss due to the effects of high level exposure.
When the mutants were sequenced, several from the same animal were
found to have identical mutations. Although they might have arisen
independently, Sisk et al. felt that this was likely due to clonal
expansion of a bone marrow cell with a mutated lac 1 gene.
As had Recio et al., Sisk et al. observed a higher frequency of
mutations at A:T sites in the exposed mice DNA, compared with controls.
A:T to G:C transitions comprised only 2% of the background mutations,
but made up 15% of those in the exposed mice.
Sisk et al. concluded that their observation coupled with in vitro
studies `` * * * suggest that BD may mutate hematopoietic stem cells.''
(Ex. 118-7Q, p. 476)
As discussed in the animal carcinogenicity section in this
preamble, BD-induced mouse tumors have been found to have activated
proto-oncogenes. Specifically, the K-ras oncogene is activated and is
the most commonly detected oncogene in humans. (Ex. 129)
OSHA concludes that BD is mutagenic in a host of tests which show
point and frameshift mutations, hprt mutations, chromosome breakage,
and SCEs in both animals and humans. The data suggest that mice are
more susceptible than rats to these alterations. In addition, certain
subsets of the human population may be more susceptible to the effects
of BD exposure than others (based on the Wiencke and Kelsey study of
human blood cell cultures, Ex. 117-2, p. 265). OSHA further notes with
concern the fact that the data suggest that BD exposure at relatively
low levels adversely affects DNA repair mechanisms in humans and is
associated with mutational effects.
5. Metabolism
In vitro genotoxicity studies have shown that BD is mutagenic only
after it is metabolically activated. Biotransformation is probably also
important to the carcinogenicity of this gas. It is thought that the
formation of epoxides, specifically epoxybutene, also termed the
``monoepoxide'' and 1,2:3,4-diepoxybutane, termed the ``diepoxide,'' is
required for activity and that the reaction is cytochrome P450 mediated
2. Both the mono- and diepoxide are mutagenic in the Salmonella
assay, with the diepoxide being more active. The reactive epoxides can
bind to DNA, and formation of DNA adducts is hypothesized to initiate a
series of events leading to malignancy.
---------------------------------------------------------------------------
2 Cytochrome is defined as any of a class of hemoproteins
whose principal biologic function is electron transport by virtue of
a reversible valency change of its heme iron. Cytochromes are widely
distributed in animal and plant tissues.
---------------------------------------------------------------------------
As described earlier, for most cancer sites, mice are more
sensitive than rats to the carcinogenic effects of BD exposure. Studies
of the metabolism of BD have been undertaken in an attempt to elucidate
the contributions of dose-metric factors for the observed differences
in carcinogenicity between the species.
Much of the research in this area has been performed at the
Chemical Industry Institute of Toxicology and in German laboratories.
Work on metabolism of BD was described by OSHA in the 1990 proposal.
(55 FR 32736 at 32756) OSHA reviewed the current literature in the
record and concluded:
1. The rate of metabolism of BD in mice is approximately twice that
in rats;
2. Mice accumulate more radiolabelled BD equivalents in a 6 hour
exposure than do rats at the same concentration;
3. Mice have about twice the concentration of the metabolite (1,2-
epoxy-3-butene) (BMO) in blood as rats exposed at similar
concentrations;
4. Over a wide range of exposures, mice received a larger amount of
inhaled BD per unit body weight than rats, and had a higher
concentration of BMO in the blood than rats (As expected, because of
body size differences and breathing rates, and some enzymology);
5. BD is readily absorbed and widely distributed in tissues of both
mice and rats, with tissue concentrations per umole BD inhaled higher
in mice than in rats, by factors of 15-fold or more;
6. While there are species differences in the amount of BD
metabolism at various sites, both mice and rats metabolize BD to the
same reactive metabolites suspected of being ultimate carcinogens.
In comments on OSHA's proposal, Dr. Michael Bird of Exxon testified
on behalf of the CMA BD Task Group that the mouse ``will attain a
significantly higher amount of the epoxides over a longer period of
time than the rat. . . or primate when exposed to butadiene.''
[[Page 56773]]
(Ex. 52, p. 27) Dr. Bird concluded that the differences in metabolism
of BD in the species help ``explain the greater sensitivity of the
mouse to BD carcinogenic activity.'' He further concluded that the
differences in rates of enzyme mediated processes indicate non-human
primates have lower internal concentrations of BD or BMO, and ``man is
more similar to the primate with respect to 1,2-epoxy-3-butene
formation than the rat or mouse.'' (Ex. 52, p. 22) He argued that the
mouse may be ``uniquely sensitive `` to BD carcinogenicity due to its
greater uptake, faster BD metabolism and ``elimination of the epoxide
1,2-epoxy-3-butene is saturable in mice but not in rats.'' (Ex. 52, p.
21) He felt this observation correlated well with the observed
cytogenetic and bone marrow response (seen in mouse, but not rats.)
Others hold an opposing view, e.g., Melnick and Kohn argued that
``[b]ecause the rat appears to be exceptionally insensitive to
leukemia/lymphoma induction, the mouse must be considered as the more
appropriate model for assessing human risk for lymphatic and
hematopoietic cancers.'' (Ex. 130, p. 160)
Dr. Bird urged OSHA to use the monkey data of Dahl, et al. which
indicated that the retention rate for BD in primates is over 6 times
lower than that for the mouse, in ``drawing any firm conclusions about
the cancer risk to humans.'' (Ex. 52, p. 36) During the public hearing,
the work of Dahl was presented as a preliminary report. (Ex. 44) Dahl
exposed 3 cynomolgus monkeys to BD and measured uptake and metabolism.
Each animal was exposed to three concentrations of C14-labeled BD,
progressing from 10,300 to 8000 ppm with at least 3 months separating
the re-exposure of each monkey. Post-exposure blood was taken. Each
animal's breathing frequency and tidal volume was measured.
Dahl and co-workers found BD uptake to be lower in monkeys than in
rats. The reported blood levels of the epoxides were also lower in the
monkey than the levels reported by Bond et al. in rats and mice.
Dahl et al. attempted to quantitate total BD metabolites through
collection of feces, urine and exhaled material though use of cryogenic
traps. Measurement of residual labeled material retained in the animals
at the end of the 96 hour post exposure period was not determined. HPLC
(high-performance liquid chromatography) identification of the trapped
material (at 95 C) indicated that only 5 to 15% of the radioactivity
was present as monoepoxide.
Melnick and Huff, in reviewing this study, found its significance
``clouded'' because only three animals of unknown age were studied and
there was uncertainty about the ability of vacuum line cryogenic
distillation alone to identify and quantitate BD metabolites. (Ex. 114,
p. 133) In testimony at the public hearing, Dr. James Bond of CIIT
acknowledged the limitations of the use of vacuum-line cryogenic
distillation as follows:
* * * there will be some material no matter what kind of vacuum
you apply to it * * * simply will not move into the traps. That's
referred to as non-volatile material.
We don't know what that material is and I think that's an
important component of this study, because, in fact, in many cases
it can represent 70 to 80 percent of the material that actually
distills out. (Tr. 1/22/91, p. 1553)
Melnick and Huff were also concerned that only the monkeys, not the
mice or rats, were anesthetized during exposure and question what
impact that might have had on respiratory rates and cardiac output and
what the influence might be on inhalation pharmacokinetics of BD. (Ex.
114, p. 133) In their 1992 review, Melnick and Huff concluded that
studies to date have not revealed species pharmacokinetic differences
of sufficient magnitude ``to account for the reported different toxic
or carcinogenic responses in one strain of rats compared to two strains
of mice.'' (Ex. 114, p. 134) In post hearing comments Dr. David A.
Dankovic of NIOSH reviewed this topic and concluded ``* * * the most
prudent course is to base 1,3-butadiene risk assessments on the
external exposure concentration, unless substantial improvements are
made in the methodology used for obtaining `internal' dose estimates.''
(Ex. 101, Att. 2, p. 5)
Recent Studies
Recent studies have focused on the metabolism of BD to the
epoxides, epoxybutene and diepoxybutane, and their detoxification by
epoxide hydrolase and glutathione. Bond et al. recently reviewed BD
toxicologic data. (Ex. 118-7G) Epoxybutene and diepoxybutane were
reported to be carcinogenic to mice and rats via skin application and/
or subcutaneous injection, with the diepoxide having more carcinogenic
potency. Bond et al. also concluded that the diepoxide is more
mutagenic than the monoepoxide by a factor of nearly 100 on a molar
basis. The diepoxide also induces genetic damage in vitro mammalian
cells (Chinese hamster ovary cells and human peripheral blood
lymphocytes). These studies are summarized in this preamble discussion
of reproductive effects.
In vitro metabolic studies
In 1992 Csanady et al. reported use of microsomal and cytosolic
preparations from livers and lungs of Sprague-Dawley rats, B6C3F1
mice and humans to examine cytochrome P450-dependent metabolism of BD.
(Ex. 118-7AA) The preparations were placed in sealed vials and BD was
injected by use of a gas-tight syringe. Air samples were taken from the
head space at 5 minute intervals and analyzed by gas chromatography for
epoxybutene.
Cytochrome P450-dependent metabolism of the monoepoxide to the
diepoxide was examined. Enzyme mediated hydrolysis of BMO by epoxide
hydrolase was measured. (Non-enzyme mediated hydrolysis was determined
using heat-inactivated tissue and none was observed.) Second order rate
constants were determined using 100 mM monoepoxide and 10 mM GSH. The
human samples were quite variable, with rates ranging from 14 to 98
nmol/min/mg protein.
The maximum rates for BD oxidation to monoepoxide (Vmax) were
determined to be highest for mouse liver microsomes 3 (2.6 nmol/mg
protein/min); the Vmax values for humans were intermediate, at 1.2
nmol/mg protein/min; the Vmax values for rats was 0.6 nmol/mg protein/
min. For lung microsomes, the Vmax in the mouse was found to be similar
to the mouse liver rate, but over 10-fold greater than that of either
humans or rats.
---------------------------------------------------------------------------
\3\ A microsome is defined as one of the finely granular
elements of protoplasm, resulting from fragmentation
(homogenization) of the endoplasmic reticulum.
---------------------------------------------------------------------------
From these data Csanady et al. calculated a ratio of activation to
detoxification for each species tested. These values, expressed as mg
cytosolic protein/gm liver [glutathione-S-transferase is a cytosolic
enzyme], resulted in the determination of an overall
activation:detoxification ratio of 12.3 for the mouse, 1.3 for the rat,
and 4.4 for the human samples.
If these in vitro liver microsomal studies can be extrapolated to
the whole animal in vivo, then this implies, as pointed out by Kohn and
Melnick, that the mouse produces 2.8 times as much BMO per mol of BD as
the human and that the human activation:detoxification ratio is 3.4
times that of the rat. However, the Csanady et al. study demonstrated a
wide variability in BD metabolic activity among the 3 human liver
microsomes, and a 60-fold variation was found in 10 human liver
[[Page 56774]]
samples by Seaton et al. (Ex. 118-7N) Kohn and Melnick noted that this
human variability in CYP2E1, the P450 enzyme primarily responsible for
the activity, suggests that a ``* * * fraction of the human population
may be as sensitive to butadiene as mice are.'' (Ex. 131, p. 620).
A study similar to that of Csanady et al., reported by Duescher and
Elfarra in 1994, determined that the Vmax/Km ratios for BD metabolism
in human and mouse liver microsome were similar and were nearly 3 to
3.5 fold higher than the ratio obtained with rat liver microsomes. (Ex.
128) Duescher and Elfarra suggest that differences between their
results and those of Csanady et al. may have been due in part to
experimental methodology differences, such as incubation and assay
methods. Duescher and Elfarra found that two P450 isozymes, 2A6 and
2EI, were most active in forming BMO of the 7 isozymes tested. They
concluded that since human liver microsomes oxidized BD at least as
efficiently as mouse liver microsomes (and much more so than rat liver
microsomes), this ``suggests that if [BMO] formation rate is the
primary factor which leads to toxicity, humans may be at higher risk of
expressing BD toxicity than mice or rats, and that the mouse may be the
more appropriate animal model for assessing toxicity.'' Duescher and
Elfarra felt that since P450/2A6 appears to play a major role in BD
oxidation in human liver microsomes, and that it is more similar to
that of mouse P450/2A5 than to rat P450/2A1, the mouse may be a better
model to use in assessing human risk.
In 1994 Himmelstein et al. hypothesized that ``[S]pecies
differences in metabolic activation and detoxification most likely
contribute to the difference in carcinogenic potency of BD by
modulating the circulating blood levels of the epoxides.'' (Ex. 118-13,
Att 3) To address this, Himmelstein and colleagues looked at the levels
of BD, BMO, and BDE in blood of rats and mice exposed at 62.5, 625, or
1250 ppm BD. Samples were collected at 2, 3, 4, and 6 hours of exposure
for BD and BMO and at 3 and 6 h for the BDE. Blood was collected from
mice by cardiac puncture and from rats through an in-dwelling jugular
cannula. Melnick and Huff criticized earlier studies which failed to
use in-dwelling cannulae.
Because steady state levels of [monoepoxide] are lower in rats
than in mice and because the metabolic elimination rate for this
compound is 5 times faster in rats than in mice, any delay in
obtaining immediate blood samples would have a much greater effect
on analyses in blood samples obtained from rats than those obtained
from mice. (Ex. 114, p. 133)
Himmelstein et al. found that the concentration of BD in blood was
not directly proportional to the inhaled concentration of BD,
suggesting that the uptake of BD was saturable at the highest inhaled
concentration. In both rats and mice BD and the BMO blood levels were
at steady state at 2, 3, 4 and 6 hours of exposure and declined rapidly
when exposure ceased. This is consistent with exhalation being the
primary route of elimination of BD. (Ex. 118-7B)
Genter and Recio used Western blot and immunohistochemical analyses
to detect P450/2E1 in bone marrow of B6C3F1 mice. (Ex. 118-7T)
Although both methods detected the presence of the protein in livers of
both male and female mice, non was seen in the bone marrow. The limits
of detection were not stated in the report. The author hypothesized the
BD might be converted to the monoepoxide in the liver prior to uptake
by the bone marrow or that another pathway (e.g., myeloperoxidase) is
responsible for BD oxidation in the marrow. Recio and Genter suggest
that the greater sensitivity of mice to BD-induced carcinogenicity can
be explained in part by the higher levels of both epoxides in the blood
of mice compared with that of rats.
Himmelstein et al. furthered this work in 1995 in a report in which
they determined levels of the epoxides in livers and lungs of mice and
rats exposed to BD. (Ex. 118-7/O) Animals were exposed at 625 or 1250
ppm of BD for 3 or 6 hours. Himmelstein et al. found that in mice
exposed to this regimen, the monoepoxide levels were higher in lungs
than in livers. Rats at 625 and 1250 ppm had lower concentrations of
BMO in lungs and livers than mice. When rats were exposed to 8000 ppm
BD, the maximum concentration of BMO in the lung and liver was nearly
the same. The diepoxide levels in lungs of mice exposed at 625 and 1250
ppm were 0.71 and 1.5 nmol/g respectively. The diepoxide was not
detected in livers or lungs of rats exposed at any tested level.
Himmelstein et al. also observed depletion of glutathione in liver
and lung samples from both rodent species. Following 6 hours of
exposure, the lungs of mice exhibited greater depletion of GSH than
mouse liver, rat liver or rat lung at all concentrations of BD tested.
The conclusion reached by the study authors was that their data
indicate that GSH depletion is associated with tissue burden of the
epoxides and that this target organ dosimetry might help explain some
of the non-concordance of cancer sites observed between the species.
OSHA notes, however, that while % GSH depletion was highest in the
mouse lung, the major increase in depletion was at 1250 ppm BD, while
lung tumor incidence was increased in the female mice at 6.25 ppm and
in male mice at 62.5 ppm. Depletion of glutathione was dependent on
concentration and duration of BD exposure.
Himmelstein et al. stressed the importance of the fact that the
diepoxide was detected in the mouse lung but was not quantifiable in
the mouse liver, and stated that if the diepoxide was formed in the
liver, it is rapidly detoxified or otherwise moved out of the liver.
They also found that depletion of glutathione was greater in mouse than
rat tissues for similar inhaled concentrations of BD and concluded that
conjugation of the monoepoxide with glutathione by glutathione S-
transferase is an important detoxification step.
In contrast to rats and mice, lungs and livers from humans had much
faster rates of microsomal monoepoxide hydrolysis by epoxide hydrolase
compared to cytosolic conjugation with glutathione by the transferase.
(Ex. 118-7AA)
Thornton-Manning et al. in 1995 examined the production and
disposition of monoepoxide and diepoxide in tissues of rats and mice
exposed at 62.5 ppm BD. (Ex. 118-13, Att. 3) They found monoepoxide was
above background in blood, bone marrow, heart, lung, fat, spleen and
thymus tissues of mice after 2 or 4 hours of exposures to BD. In rats,
levels of monoepoxide were increased in blood, fat, spleen and thymus
tissues. No increase in monoepoxide in rat lung was observed. The more
mutagenic diepoxide was detected in all tissues of the mice examined
immediately following 4 hours of exposure. It was detected in heart,
lung, fat, spleen and thymus of rats, but at levels 40- to 160-fold
lower than those seen in mice.
In mice, the level of diepoxide exceeded the monoepoxide levels
immediately after exposure in such target organs as the heart and
lungs. Thornton-Manning et al. concluded that the high concentrations
of diepoxide in heart and lungs they observed suggested to them that
this compound may be particularly important in BD-induced
carcinogenesis.
The study authors noted that neither epoxide was detected in rats'
liver and was present only in quite low concentrations in the livers of
mice. Thornton-Manning et al. found this
[[Page 56775]]
surprising since epoxides present in blood in the liver should have
yielded values greater than those observed in the liver samples. They
hypothesized that it might be due to prior metabolism of the epoxides
before reaching the liver or it might be an artifact due to post-
exposure metabolism of the epoxides in the liver.
Thornton-Manning et al. did not detect the monoepoxide in rat
lungs, and found the diepoxide level to be quite low. In contrast, in
the mice they found both epoxides present in lung tissue, with the
monoepoxide level present at a concentration less than expected using
blood volume values, and the diepoxide level agreeing with that
expected as a function of blood volume. Thornton-Manning et al.
concluded that these results ``* * * suggest that the lung is capable
of metabolizing BDO, but perhaps is less active in metabolizing
BDO2. (Ex. 118-13, Att. 3) Moreover, Thornton-Manning et al.
believed that although BD is oxidatively metabolized by similar
metabolic pathways in the rats and mice, the quantitative differences
in tissue levels between species may be responsible for the increased
carcinogenicity of BD in mice.
Table V-8.--Tissue Levels [pmol/gm tissue, meanS.E.] of Epoxybutene and Diepoxybutane in Rats and
Mice Following a 4-Hour Exposure to 62.5 ppm BD by Inhalation
----------------------------------------------------------------------------------------------------------------
Epoxybutene Diepoxybutane
Tissue ---------------------------------------------------
Rats Mice Rats Mice
----------------------------------------------------------------------------------------------------------------
Blood....................................................... 361............................................... 0.2; n=3 or 4 for each determination.
Adapted from Ex. 118-13, Att. 3.
These data are shown in Table V-8.
Seaton et al. examined the activities of cDNA-expressed human
cytochrome P450 (CYP) isozymes for their ability to oxidize epoxybutene
to diepoxybutane. (Ex. 118-7N) They also determined the rate of
formation of the diepoxide by samples of human liver microsomes (n=10)
and in mice and rat liver microsomes. Seaton et al. found that two of
the cytochrome P450 isozymes, CYP2E1 and CYP3A4, catalyzed oxidation of
80 uM of monoepoxide to detectable levels of diepoxide, and that CYP2E1
catalyzed the reaction at higher levels of monoepoxide (5mM),
suggesting the predominance of 2E1 activity at low substrate
concentrations. Hepatic microsomes from all 3 species formed the
diepoxide when incubated with the monoepoxide. Seaton et al.
hypothesized that the difference between these results and those of
Csanady et al. (who did not detect the diepoxide when the monoepoxide
was substrate in a similar microsomal assay) was due to differences in
experimental methodology.
Seaton et al. noted a 25-fold variability in Vmax/Km among the 4
human livers. They reported that Vmax/Km for oxidation of the
monoepoxide to the diepoxide for the 4 human samples was 3.8, 1.2, 1.3
and 0.15, while that of the pooled rat samples was 2.8, and the mouse
ratio was 9.2.
The authors, using available data, calculated an overall
activation/detoxification ratio (Vmax/Km for oxidation of BD to the
monoepoxide) taking into account hydrolysis of the monoepoxide by
epoxide hydrolase and conjugation with glutathione. The activation/
detoxification ratio was estimated at 1295 for the mouse, 157 for rats
and 230 for humans. However, Melnick and Kohn point out that ``when
yields of microsomal and cytosolic protein content and liver size were
considered, the activation to detoxification ratio was only 2.8 times
greater in mice than in humans and 3.4 times greater in humans than in
rats. These ratios do not take into account inter-individual
variability in the activities of the enzymes involved.'' (Ex. 131)
Recently, Seaton et al. studied production of the monoepoxide in
whole airways isolated from mouse and rat lung. (Ex. 118-7C) They
explained the impetus to use fresh intact tissue by stating that lung
subcellular fractions, as employed in experiments by Csanady et al.,
described above, contained mixtures of cell type ``so that the
metabolizing capacities of certain cell populations may have been
masked.'' They anticipated that use of airway tissue would allow more
precise quantitation of differences in lung metabolism of BD.
Whole airways or bronchioles isolated from both male B6C3F1
mice and male Sprague-Dawley rats were incubated for 60 min with 34 um
BD. Levels of 10.45.6 nmol epoxybutene/mg protein were
detected in mouse lungs, while 2-3 nmol/mg protein was observed in rat
lung airway regions. Seaton et al. noted that while the species
differences ``are not dramatic,'' they may in part contribute to the
differences in carcinogenicity observed in mice and rats.
To characterize conjugation of BD metabolites with glutathione
(GSH), Boogard et al. prepared cytosol from lungs and livers of rats
and mice and from 6 human donor livers and incubated them with 0.1 to
100 mM diepoxide and labeled glutathione (GSH). (Ex. 118-7J) NMR
(nuclear mass resonance) and HPLC techniques were used to characterize
and quantitate conjugate formation.
Non-enzymatic reaction was concluded to be negligible. The
conjugation rates (Vmax) in mouse and rat livers were similar and 10-
fold greater than those observed in the human samples. The initial rate
of conjugation (Vmax) was much higher in mouse than rat lung. Both
rodent species exhibited higher initial rates of conjugation than
human. This led Boogard et al. to conclude that the higher diepoxide
levels observed in BD-exposed mice compared with rats ``are not due to
differences in hepatic or pulmonary GSH conjugation of BDE (the
diepoxide),'' and further that since humans oxidize BD to the epoxides
at a low rate, the low activity of GSH conjugation of the diepoxide in
human liver cytosol demonstrated in this study ``will not necessarily
lead to increased BDE (diepoxide) levels in humans
[[Page 56776]]
potentially exposed to BD.'' They also pointed out the need to
determine the rate of BDE detoxification by other means, specifically
by epoxide hydrolase in all three species.
Studies of Urinary Metabolites of BD
Two metabolites of BD have been identified in urine of exposed
animals by Sabourin et al. (Ex. 118-13 Att. 3) These are 1,2-dihydroxy-
4-N-acetylcysteinyl-S-)-butane, designated MI, and MII, which is 1-
hydroxy-2-N-acetylcysteinyl-S-)-3-butene. (Ex. 118-13-Att. 3)
These mercapturic acids are formed by addition of glutathione (GSH)
at either the double bond (MI) or the epoxide (MII). MI is thought to
form by conjugation of GSH with butenediol, the hydrolysis product of
the monoepoxide, while MII is thought to form from conjugation of the
monoepoxide with GSH.
Sabourin et al. measured MI and MII in urine from rats, mice,
hamster and monkeys. Mice were observed to excrete 3 to 4 times as much
MII as MI, while the hamsters and rats produced about 1.5 times as much
MII as MI. The monkeys produced primarily MI.
The ratio of formation of metabolite I to the total formation of
the two mercapturic acids, MI and MII, correlated well with the known
hepatic epoxide hydrolase activity in the different species, suggesting
that the monoepoxide undergoes more rapid conjugation with glutathione
in the mouse than in the hamster or rats, and that the least rapid
conjugation occurs in the monkey. The epoxide availability is inversely
related to the hepatic activity of epoxide hydrolase, which removed the
epoxide by hydrolysis.
In 1994, Bechtold et al. published a paper describing a comparison
of these metabolites between mice, rats, and humans.\4\ In workers
exposed to historical atmospheric concentrations of 3 to 4 ppm BD,
Bechtold measured urine levels of MI and MII by use of isotope-dilution
gas chromatography, and found MI, but not MII, to be readily
detectable. Bechtold et al. found that employees who worked in
production areas (having 3-4 ppm BD exposure) could be distinguished by
this assay from outside controls and that low level human exposure to
BD resulted in formation of epoxide.
---------------------------------------------------------------------------
\4\ A preliminary study on the human population of this study is
described in the section of this preamble dealing with the genetic
toxicology of BD exposure.
---------------------------------------------------------------------------
Bechtold et al. stated in their abstract that since monkeys
displayed a higher ratio of MI to MI + MII than mice did, and ``because
humans are known to have epoxide hydrolase activities more similar to
those of monkeys than mice, we postulated that after inhalation of
butadiene, humans would excrete predominantly MI and little MII.'' (Ex.
118-13 Att. 3) Their observations suggested that the predominant
pathway for clearance of the monoepoxide in humans is by hydrolysis
rather than conjugation with glutathione.
Bechtold et al. found when mice and rats were exposed to 11.7 ppm
BD for 4 hours and the ratio of the two metabolites was then measured,
for mice, the ratio of MI to MI MII (or the % of total
which is MI) was 20%, that of rats was 52%, while humans exhibited more
than 97% MI. These data also indicate the predominance of clearance by
hydrolysis pathways rather than GSH conjugation in the human.
Nauhaus et al. used NMR techniques to study urinary metabolites of
rats and mice exposed to ([(1,2,3,4)-13C]-butadiene). (Ex. 118-7I)
They characterized metabolites in mouse and rat urine following
exposure by inhalation to approximately 800 ppm BD for 5 hours. Urine
was collected over 20 hours from exposed and control animals,
centrifuged and frozen.
The findings of this study are quite extensive and are briefly
summarized as follows. Nine metabolites were detected and chemically
identified in mouse urine and 5 in that of rats. Five were similar in
the 2 species, though differing markedly in concentration. One was
unique to the rat and four to the mouse. Nauhaus et al. observed that
``when normalized to body weight (umol/kg body weight), the amount of
diepoxide-derived metabolites was four times greater in mouse urine
than in rat urine.'' They further hypothesized that ``the greater body
burden of (diepoxide) in the mouse and the ability of rats to detoxify
[it] though hydrolysis may be related to the greater toxicity of BD in
the mouse.'' Nauhaus et al. found that both mice and rats conjugated
the monoepoxide with glutathione, but the rat preferentially conjugated
at the two carbon, while the mouse preferentially conjugated at the one
carbon. Additionally, the finding of a metabolite of 3-butenal, a
proposed intermediate in the oxidation of BD to crotonaldehyde, an
animal carcinogen, is suggestive of an alternative carcinogenic pathway
for BD. In general, this study supports the in vitro findings of
Csanady et al. who reported similar rates for BMO conjugation with
glutathione between rats and mice. (Ex. 118-7AA)
Interaction of Butadiene With Other Chemicals
Bond et al. described use of available data to simulate the
potential interaction of BD with other workplace chemicals. (Ex. 118-
7V) Specifically they modeled potential interaction assuming
competitive inhibition of BD metabolism by styrene, benzene and
ethanol. The model predicted that co-exposure to styrene would reduce
the amount of BD metabolized, but that because of its relative
insolubility, BD would not effectively inhibit styrene metabolism.
Benzene, which, like BD, is metabolized by P450/2E1, was also predicted
to be a highly effective inhibitor of BD metabolism because of its
solubility in tissues. The models predicted that ethanol would have
only a marginal effect on BD metabolism at concentrations of BD
``relevant to human exposure.''
BD and styrene co-exposures often occur in the SBR industry and
both are metabolized by oxidation to active metabolites, in major part,
by cytochrome P450/2E1. To determine the metabolic effect of joint
exposure to BD and styrene, Levans and Bond developed and compared two
PBPK models, one with one oxidative pathway and competition between BD
and styrene and the other with two oxidation pathways for both BD and
styrene. (Ex. 118-7E) For model validation, Levans and Bond exposed
male mice to mixtures of BD and styrene of 100 or 1000 ppm BD and 50,
100 or 250 ppm styrene for 8 hours. They used chamber inlet and outlet
concentrations to calculate uptake and, when steady-state was reached,
calculated the rate of metabolism. They analyzed blood for styrene,
styrene oxide, epoxybutene and diepoxybutane by GC-MS.
Leavens and Bond found BD metabolism was inhibited when mice were
co-exposed to styrene. The inhibition approached maximum value at co-
exposure concentrations of styrene above 100 ppm.
The report also described the preliminary development of
pharmacokinetic models to simulate the observed rate of BD metabolism
in co-exposed mice. Their results supported the hypothesis that ``more
than one isozyme of P450 metabolized BD and styrene and competition
does not occur between BD and styrene for all isozymes.'' They were
unable to accurately predict blood concentrations of styrene following
exposure, and felt that `` perhaps the diepoxide may inhibit metabolism
of styrene by competing for the same P450 enzyme.''
[[Page 56777]]
Although preliminary in nature and reflecting effects of relatively
high exposures, these observations of interactions between styrene and
BD exposure may have implications for the observed pattern of BD-
induced effects in human populations jointly exposed. Specifically, the
cancer effects seen in SBR production workers may underestimate the
effects of BD with no styrene or benzene exposure.
Pharmacokinetic Modeling of BD Metabolism
In a recent publication, Bond et al. reviewed the results of
application of a number of physiologically-based pharmacokinetic (PBPK)
dosimetry models. (Ex. 118-7M) They noted that three of the models
which included monoepoxide disposition (Kohn and Melnick, Johanson and
Filser, Medinsky) predicted that, for any BD exposure concentration,
steady-state monoepoxide levels will be higher for mice than for rats.
Bond et al. further observed that ``while the three models accurately
predict BD uptake in rats and mice, they overestimate the circulating
blood concentrations of (monoepoxide) in these species compared to
those experimentally measured by Himmelstein.'' Their results also led
Bond et al. to conclude that the disagreement between model predictions
for the monoepoxide and experimental data suggests that the structure
and/or parameter values employed in these models are not accurate for
predicting blood levels of BD epoxides, and conclusions based on model
predictions of BD epoxide levels in blood or tissue may be wrong.''
(Ex. 118-7M, p. 168) OSHA agrees with these authors that BD epoxide
levels should not be used in assessing risk. In the discussion, the
authors pointed to the need for inclusion of diepoxide toxicokinetics
(as well as that of the monoepoxide) in future modeling exercises,
since they believe the diepoxide to be the ultimate carcinogenic
metabolite of BD.
Kohn and Melnick, in a recent publication, used available data and
attempted to apply a PBPK model to see whether it was consistent with
observed in vivo uptake and metabolism. (Ex. 131) The model included
compartments for rapidly and for slowly perfused tissues. Rate
equations for monoepoxide formation, its hydrolysis, and for
conjugation with glutathione were included.
Kohn and Melnick acknowledged numerous sources of uncertainty in
applying the model to the data (in which there are many gaps),
necessitating various assumptions. Their calculations led them to
conclude that the ``model reproduces whole-body observations for the
mouse and rat'' and that it predicts that ``inhalation uptake of
butadiene and formation and retention of epoxybutene are controlled to
a much greater extent by physiological parameters than by biochemical
parameters. . . `` (Ex. 131)
When Kohn and Melnick interchanged the biochemical parameters in
the mouse and human models to see if ``the differences in calculated
net uptake of butadiene among the three species were due to differences
in metabolic activity,'' they found that use of human parameters in the
mouse model decreased the level of absorption of BD, but not to a level
as low as that of the human. Kohn and Melnick noted that the model
predictions of epoxybutene levels in the heart and lung of mice and
rats failed to account for the observation that mice, but not rats,
develop tumors at these sites. Kohn and Melnick suggested that factors
other than epoxybutene levels, not accounted for in the model, are
probably crucial to induction of carcinogenesis.
Conclusions
Many metabolism studies have been conducted both in vitro and in
vivo, mostly in mice and rats, to determine the BD metabolic,
distribution, and elimination processes, and these studies have been
extended in attempts to explain, at least in part, the greater
carcinogenic potency of BD in the mouse, whether the mouse or the rat
is a better surrogate for human cancer and reproductive risk
assessment, and what is the proper dose-metric to use in dose-response
assessments. The question of whether the mouse or the rat is a better
model for the human on the basis of tumor response is partly addressed
in the risk assessment section of this preamble. This section more
specifically considers whether these metabolic studies in total can
explain the different cancer responses and potencies observed in the
mouse, rat, and human. What is clear throughout the record is that most
scientists who study the topic consider not BD itself, but the major
epoxide metabolites of BD, BMO and BDE and 1, 2-epoxybutane-3,4-diol,
to be the putative carcinogenic agents. Most of this research has
focused on the relative species production of BMO and BDE. Both BMO and
BDO have been reported in early studies to be carcinogenic to mice and
rats via skin application and/or subcutaneous injection, with BDO being
somewhat more potent. (Ex. 23-88, Ex. 125).
Metabolism of BD to BMO in both the liver and lung of mice, rats
and humans is by the P450 oxidation pathway, with CYP2E1 and CYP1A6
being the major enzymes. Based on the studies reviewed by OSHA, overall
the mouse metabolizes BD to the monoepoxide and the diepoxide in these
organs at a faster rate than do the rat and human. This is supported by
the following evidence: (1) The mouse has higher BMO and BDE levels in
blood, lung, and liver (i.e., see Ex. 118-7S, Ex. 118-7D, and Ex. 118-
13), which are the target organs for cancer in the mouse but not the
rat; (2) the mouse has higher in vitro lung and liver microsome Vmax/Km
ratios for both BD and BMO metabolism than do rats or humans (Ex. 118-
7AA); and (3) the mouse has higher hemoglobin-BMO adduct levels than
rats and much higher levels than humans. (Ex. 118-7Y) A major exception
to the findings of these studies is the study by Duescher and Elfarra,
who found the in vitro BD Vmax/km ratios to be the same in mice and
human liver microsomes and 3-4 times higher than they were in rats,
suggesting that mice and humans have similar BD metabolic potential, at
least in the liver. (Ex. 128) Large variations, about 60 fold, were
found among 10 human liver microsome BD metabolic activities. (Ex. 118-
7N) A recent BD in vitro metabolism study by Seaton et al. on whole rat
and mouse lung airway isolates found that the mouse produced about
twice the amount of BMO as the rat (this difference could not explain
the difference between mouse and rat tumor incidence). (Ex. 118-7C)
BMO and BDE were also measured in heart, spleen, thymus, and bone
marrow (target sites for mouse but not rat tumors) following 4 hour BD
inhalation exposure (62.5 ppm) to mice and rats. (Ex. 118-13) In these
tissues, mouse BMO and BDE levels were 3 to 55 fold higher than rat
levels for the same metabolites, although the mice organ levels of
these metabolites correlated poorly with the mouse target organ cancer
response at this exposure level. Only high BDE levels in the mouse lung
were consistent with the mortality adjusted cancer incidence (see
hazard identification--animal studies section, Ex. 114). This suggests
that BD metabolite tissue levels can, at best, only partly explain
differences in carcinogenic response. Differences in both species and
tissue sensitivity must also be accounted for.
The Thornton-Manning and other studies also provided information
about BD elimination. (Ex. 118-7I) With higher experimental exposure
levels, the major route of elimination of BD is via expiration.
Elimination of BMO occurs
[[Page 56778]]
by different pathways in different species and different organs. At
higher BD exposure concentrations, some BMO is expired. The mouse liver
and lung appear to eliminate BMO predominantly by direct conjugation
with GSH 5. For the rat there is approximately equal elimination
by the GSH and EH mediated pathways, while for the human and monkey
hydrolysis to butanediol is the major pathway for excretion. ( Ex. 118-
13 Att. 3) This species elimination pathway difference is a partial
explanation for the higher levels of both BMO and BDE seen in the
mouse, assuming that most of the BD metabolism takes place in the
liver. With respect to the bone marrow BD distribution and metabolism,
mouse levels of the BD metabolites in the bone marrow were lower than
at any of the other target organs studied. (Ex. 118-13) In vitro
studies by Gentler and Recio have found no detectable P4502E1 in the
bone marrow of B6C3F1 mice. (Ex. 118-7T) These authors conclude
that this ``suggests that BD is converted to BMO outside of bone marrow
and is subsequently concentrated in bone marrow, or that the conversion
of BD to BMO occurs by an alternate enzymatic pathway within the bone
marrow.'' The latter appears to be the more likely since Maniglier-
Poulet and co-workers showed that in vitro BD metabolism to BMO in both
B6C3F1 mouse and human bone marrow occur by a peroxidase-mediated
process and not via the P450 cytochrome system. (Ex. L-133) Since in
their system both human and mouse bone marrow generated about the same
amount of BMO/cell, this suggests that both BD distribution to bone
marrow and local metabolic reactions should be considered in species-
to-species extrapolations and in PBPK modeling.
---------------------------------------------------------------------------
\5\ One exception: Seaton et al. found evidence ``that in mouse
airways hydrolysis of BMO by epoxide hydrolase (EH) contributes to
BMO detoxification to a greater extent than does glutathione
conjugation.'' (Ex. 118-7C)
---------------------------------------------------------------------------
Inclusion of bone marrow local reactions becomes even more
important when considering the animal species to use for modeling human
cancer. BD is genotoxic in the bone marrow of mice, but not in rats.
(Tice et al. 1987; Cunningham et al. 1986, reported in Ex. 131) BD and
BMO have been implicated as affecting primitive hematopoietic bone
marrow stem and progenitor cells related to both T-cell leukemia and
anemia in the mouse. (Irons et al., 1993, in Ex. 117-2) BD causes
lymphoma in mice, but no lymphoma or leukemia in rats even at 8,000
ppm. Furthermore, the body of epidemiologic evidence strongly indicates
that BD exposure poses an increased risk of human leukemia (see the
epidemiologic section and especially Ex. 117-1).
Fat storage of BD during exposure, and release following cessation
of exposure, is also a major concern, both in estimating target organ
levels and in determining species differences. There is little in the
record on the effect of fat storage and release. In the Thornton-
Manning study discussed above, both mouse and rat fat levels of both
BMO and BDE declined rapidly following cessation of exposure,
suggesting little lingering effect. However, Kohn and Melnick present a
model in which post-exposure release of BD from the fat would result in
extended epoxide production in humans in contrast with the mouse. (Ex.
131)
Bond et al. suggest that the more rapid metabolism of BD to BMO in
the mouse, and the more rapid EH BMO elimination pathways in the rat
and human may be an explanation for lower, if any, BDE levels seen in
rat and human liver microsomes and why BD will not be carcinogenic to
humans at exposure levels seen in the environment or the workplace.
(Ex. 130) They also conclude that ``Since significant tumor induction
in male rats occurs only at 8000 ppm BD, BMO levels are probably not
predictive of a carcinogenic response.'' Thornton-Manning et al.
characterize the peak levels of BDE in the mouse lung and heart as
being either greater than or equivalent to peak levels of BMO, and
suggest ``that the formation of BDE may be more important than the
formation of BMO in the ultimate carcinogenicity of BD.'' (Ex. 118-13)
However, BMO levels in these organs were also quite high, and were
higher than BDE levels in blood and bone marrow, target organs for
hematopoietic system cancers. OSHA believes that the evidence is not
sufficient to dismiss the potential contribution of BMO to mouse, rat
or human carcinogenicity; to conclude that BDE should be considered
more actively carcinogenic than BMO; or to find that BDE levels are
sufficiently characterized in either mouse or human tissue to be used
as the dose metric for BD human risk assessment.
Thus, OSHA concludes, based on the body of metabolic and other
evidence presented, and the above discussion, that the mouse is a
suitable animal model for the human for BD cancer risk assessment
purposes, and that metabolism of BD to active metabolites is probably
necessary for carcinogenicity. However, while the uptake, distribution,
and metabolism of BD to active carcinogenic agents are important, local
BD metabolic reactions and specific species sensitivities appear to
have at least as large an impact on BD potency in the various species.
This is likely to be especially true in the human, whose metabolic
processes appear to be much more variable with respect to BD. Thus,
although the metabolism studies provide insight into BD's metabolic
processes in various species and organs (with the possible exception of
mouse lung tumorigenicity related to lung BDE levels and protein cross
linking), OSHA finds that too many questions remain unanswered, both
with PBPK modeling efforts and with actual in vivo measurements (and
the lack of such measurements in humans) to base a quantitative risk
assessment on BD metabolite level equivalence between mice and humans.
(Ex. L-132)
VI. Quantitative Risk Assessment
A. Introduction
In 1980, the United States Supreme Court ruled on the necessity of
a risk assessment in the case of Industrial Union Department, AFL-CIO
v. American Petroleum Institute, 448 U.S. (607), the ``Benzene
Decision.'' The United States Supreme Court concluded that the
Occupational Safety and Health (OSH) Act requires, prior to issuance of
a standard, that the new standard be based on substantial evidence in
the record considered as a whole, that there is a significant risk of
health impairment at existing permissible exposure limits (PELs) and
that issuance of the standard will significantly reduce or eliminate
that risk. The Court stated that, before the Secretary of Labor can
promulgate any permanent health or safety standard, he 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. (448 U.S. 642)
In 1981, the Court's ruling on the OSHA's Cotton Dust Standard
(American Textile Manufacturers Institute v. Donovan, 452 U.S. 490
(1981)) reaffirmed its previous position in the Benzene Decision, that
a risk assessment is not only appropriate, but that OSHA is required to
identify significant health risk to workers and to determine if a
proposed standard will achieve a reduction in that risk, and OSHA as a
matter of policy agrees that assessments should be put into
quantitative terms to the extent possible.
For this rulemaking, OSHA has conducted a quantitative risk
assessment to estimate the excess risk for cancer and consequently for
premature deaths associated with
[[Page 56779]]
exposure to an 8-hour time-weighted-average (TWA), 5 days/week, 50
weeks/year, 45-year exposure to BD at concentrations ranging from 0.1
to 5 ppm, the range of permissible exposure limits (PELs) considered by
OSHA in this rulemaking. The data used in the quantitative risk
assessment were from a National Toxicology Program (NTP) chronic
inhalation study in which B6C3F1 mice of both sexes were
exposed to either ambient air or BD exposure concentrations ranging
from 6.25 to 200 ppm, known as NTP II. (Ex. 90) For seven gender-tumor
site combinations, multistage Weibull time-to-tumor models were fit to
these NTP II data. The best fitting models were chosen via a log-
likelihood ratio test.
OSHA's maximum likelihood estimate (MLE) of the excess risk of
developing cancer and subsequent premature death as a result of an 8-
hour TWA occupational lifetime exposure to 2 ppm BD, the PEL proposed
by OSHA in 1990, was 16.2 per 1,000 workers, based on the most
sensitive gender-tumor site combination, female mouse lung tumors. If
the occupational lifetime 8-hour time-weighted-average (TWA) exposure
level is lowered to 1 ppm BD, based on female mouse lung tumors, the
estimate of excess cancer and premature death drops to 8.1 per 1,000
workers. In other words, an 8-hour TWA lifetime occupational exposure
reduction from 2 ppm to 1 ppm BD would be expected to prevent, on
average, 8 additional cases of cancer and probable premature deaths per
1,000 exposed workers. Based on the individual tumor site dose-response
data, which were best characterized by a 1-stage Weibull time-to-tumor
model, (male-lymphoma, male-lung, female-lymphoma and ovarian), on
average, one would expect there to be between 1 and 6 fewer excess
cases of cancer per 1,000 workers based on a 8-hour TWA occupational
lifetime exposure to BD at 1 ppm versus BD at 2 ppm. Estimates of
leukemia deaths at the former 8-hour TWA PEL of 1,000 ppm of BD, for an
occupational lifetime, are not presented because contemporary BD
exposures are generally far lower than this level.
B. Assessment of Carcinogenic Risk
1. Choice of Data Base for Quantitative Risk Assessment
The choice of data provides the platform for a quantitative risk
assessment (QRA). Either animal studies which evaluate the dose-
response relationship between BD exposure and tumorigenesis or
epidemiological dose-response data may be suitable sources of data.
Estimates of the quantitative risks to humans can be based on the
experience of animals from a chronic lifetime exposure study. Chronic
lifetime inhalation bioassays with rats and mice generally last 2 years
or two-thirds of the lifespan of the animal. (Ex. 114) These types of
studies provide insight into the nature of the relationship between
exposure concentration, duration and resulting carcinogenic response
under a controlled environment. Furthermore, some researchers have
estimated a variety of measures of dose of BD, including inhaled and
absorbed dose as well as BD metabolites, to estimate human risks based
on the observed dose-response relationship of animals in a bioassay;
the form of the dose used in a dose-response analyses is called the
dose-metric.
The carcinogenicity of lifetime inhalation of BD was studied in
Sprague-Dawley rats by the International Institute of Synthetic Rubber
Producers (IISRP) and in B6C3F1 mice by the National
Toxicology Program. The IISRP sponsored a two-year inhalation bioassay
of Sprague-Dawley rats performed at Hazelton Laboratories Europe (HLE).
(Ex. 2-31) Groups of 110 male and female Sprague-Dawley rats were
exposed for 6-hours per day, 5 days per week to 0, 1,000, or 8,000
parts per million (ppm) of BD. The males were exposed for 111 weeks and
the females for 105 weeks. Statistically significant increased rates of
tumors were found in both male and female rats. Among exposed male
rats, there were increased occurrences of pancreatic and testicular
tumors and among the exposed female rats there were higher incidence
rates of uterine, zymbal gland, mammary and thyroid tumors than in the
control groups.
The National Toxicology Program (NTP) has performed two chronic
inhalation bioassays using B6C3F1 mice. (Ex. 23-1; 90;
96) The first study, NTP I, was intended to be a two-year bioassay,
exposing groups of 50 male and female mice to 0, 625, or 1,250 ppm of
BD for a 6-hour day, 5 days/week. The study was prematurely curtailed
at 60 weeks for the males 61 weeks for the females caused by an
unusually high cancer mortality rate due to malignant neoplasms in
multiple organs. Despite some weaknesses in the way the study was
conducted, the results of this study show that BD is clearly
carcinogenic in these mice, with statistically significant increases in
malignant lymphomas, heart hemangiosarcomas, lung tumors, and
forestomach tumors in comparison to the controls for exposed male and
female mice. (Ex. 90)
The second NTP BD chronic inhalation bioassay, NTP II, had groups
of 70 (except for the group exposed to the highest concentration, which
contained 90) male and female mice exposed to concentrations of 0,
6.25, 20, 62.5, 200 and 625 ppm for 6 hours/day, 5 days/week for up to
104 weeks. The NTP II bioassay provided lower exposures, closer to
prevailing occupational exposure levels, than the NTP I and HLE chronic
inhalation studies. The NTP II supported the pattern of carcinogenic
response found in NTP I. Both male and female mice exposed to BD
developed tumors at multiple sites including: lymphomas, heart
hemangiosarcomas, and tumors of the lung, liver, forestomach, and
Harderian gland (an accessory lacrimal gland at the inner corner of the
eye in animals; they are rudimentary in man). Reproductive tissues were
also adversely affected. Among the exposed males there were significant
increases in tumors of the preputial gland; among females there were
significant increases in the incidence of ovarian and mammary tumors.
In 1996, a retrospective cohort study by Delzell and co-workers of
about 18,000 men who worked in North American synthetic rubber plants
was submitted to OSHA. (Ex. 117-1) In this study researchers derived
estimates of occupational exposure to BD using a variety of resources,
such as work histories, engineering data, production notes, and
employees' institutional memories. In their October 2, 1995 report Dr.
Delzell et al., characterized their effort as follows:
Retrospective quantitative exposure estimation was done to
increase the power of the study to detect associations and to assist
with the assessment of the impact of specific exposure levels on
mortality from leukemia and other lymphopoietic cancers. (Ex.
117-1)
In April 1996, Dr. Delzell expressed concern with possible
discrepancies between estimated cumulative exposures and actual
measurements. (Ex. 118-2) OSHA believes that in a well-conducted study,
retrospective exposure estimates can be reasonable surrogates for true
exposures; misclassifications or uncertainty can decrease the precision
of the risk estimates derived from such a study, but the problem must
be severe and widespread to invalidate the basic findings.
At the time of publication of the proposed standard on occupational
exposure to BD (August 1990), only the NTP I mouse and HLE rat
bioassays were available for quantitative risk assessments (QRA).
Presented in Table
[[Page 56780]]
V-9 is an overview of authorship and data sets used in the various QRAs
submitted to the OSHA docket. With one exception, the rest of the QRA's
in the BD Docket have relied on animal chronic exposure lifetime
bioassays. Each of the five risk assessments discussed in the proposal
based its quantitative risk assessment on one or both of the higher-
exposure chronic bioassays (exposure groups exposed to BD
concentrations ranging between 625-8,000 ppm). (Exs. 17-5; 17-21; 23-
19; 28-14; 29-3; 32-27) The three QRAs conducted using bioassay data
subsequent to the publication of the NTP II study used NTP II data with
exposures of 6.25-625 ppm BD, closer to actual occupational exposures,
for calculating their best estimates of risk. (Exs. 90; 118-1b; 32-16)
A summary of each of the ten QRA's follows:
Table V-9.--Summary Table of Quantitative Risk Assessments (QRAs) In Order of Their Review in the OSHA BD
Standard
----------------------------------------------------------------------------------------------------------------
Exhibit Author Data-set
----------------------------------------------------------------------------------------------------------------
90.......................... National Institute for Occupational NTP II a bioassay (preliminary).
Safety and Health (NIOSH) (Preliminary).
118-1b...................... NIOSH................................... NTP II bioassay.
118-1....................... NIOSH................................... Delzell et al. epidemiological study.
17-21....................... United States EPA Carcinogen Assessment NTP I b and HLE c bioassays;
Group (CAG). Epidemiological based on Fajen Exposure
Data.
32-27....................... California Occupational Health Program NTP I; HLE bioassays Epidemiological
(COHP) of the California Department of based on Fajen Exposure Data
Health services (CDHS).
32-16....................... Shell Oil Corporation................... NTP I, NTP II and HLE bioassays.
17-5........................ United States EPA Office of Toxic NTP I bioassay.
Substances (OTS).
23-19....................... ICF/Clement Inc......................... NTP I bioassay.
29-3........................ Center for Technology, Policy, and NTP I and HLE bioassays.
Industrial Development at the
Massachusetts Institute of Technology.
28-14....................... Environ Inc............................. HLE bioassay.
----------------------------------------------------------------------------------------------------------------
a NTP II, The National Toxicology Program, Technical Report 434, 2-year bioassay of B6C3F1 mice to 5 exposure
groups receiving between 6.25 and 625 parts per million (ppm) of BD
b NTP I, The National Toxicology Program, prematurely terminated longtime bioassay of B6C3F1 mice to 2 exposure
groups receiving either 625 or 1,200 ppm of BD
c HLE, Hazelton Laboratories Europe's, lifetime bioassay of Sprague Dawley rats, exposed groups received 1,000
ppm of BD or 8,000 ppm of BD
NIOSH-Quantitative Risk Assessments based on NTP II
In the early 1990's, two QRAs were conducted sequentially by the
National Institutes for Occupational Safety and Health (NIOSH). One was
a preliminary and the other a final, with the latter using final
pathology data for histiocytic sarcomas and one particular type of
lymphoma from NTP II. In 1991, NIOSH submitted a preliminary QRA using
the then preliminary NTP II tumor pathology data for various individual
organ sites (8 from the female mice and 6 from the male mice) to
estimate excess cancer risk at different BD exposures over an
occupational lifetime. (Ex. 90) For all gender-tumor site analyses,
NIOSH excluded the 625 ppm exposure group in its best estimate of risk
since the plethora of competing tumors 6 in this high exposure
group provide less information for a dose-response analysis of
individual tumor sites than do data from some of the lower exposure
groups. Another reason for the exclusion was that the dose-time-
response relationship in mice is saturated for exposures above 500 ppm
and the data would thus provide very little additional information for
low dose extrapolation. NIOSH's QRA relied on an allometric conversion
of body weight to the three-quarters power, (mg/kg)\3/4\, and equated a
900-day-old mouse to a 74-year old human. To avoid duplication of
risks, NIOSH presented only maximum likelihood estimates based on the
aggregate of all types of lymphomas even though dose-response data were
also available for the lymphocytic lymphoma subset.
---------------------------------------------------------------------------
\6\ Competing tumors refers to the lack of opportunity of a
later developing tumor to express itself due to the occurrence of
early developing lethal tumor; Among the 625 ppm exposure group
lymphocytic lymphomas were mortal early developing tumors which
prevented later developing disease such as heart hemangiosarcomas
from possibly developing.
---------------------------------------------------------------------------
Of the fourteen gender-tumor site data sets NIOSH modeled to
extrapolate animal data to humans, 12 (86%) yielded excess risks
greater than 2 cancer deaths per 1,000 workers, given an 8-hour TWA
lifetime occupational exposure of 1 ppm BD. Estimates of excess risks
to workers based on the best fitting models for each of the six dose-
time-response relationships for male tumor sites were between 0.4 and
15.0 per 1,000 workers assuming an 8-hour TWA, 45 year occupational
exposure to 1 ppm BD. Among estimates based on male mice's dose-
response data, the lowest and highest excess risk estimates were from
the heart hemangiosarcoma and Harderian gland dose-response
relationships, respectively. For estimates of excess risk based on
either gender's set of individual tumor dose-response relationships,
only the heart hemangiosarcoma data predicted a risk of less than 1 per
1,000 workers with an occupational lifetime exposure of 1 ppm: these
data predicted 0.4 and 3 x 10-3 excess cancer cases per 1,000
workers based on the best fitting models for male and female mice,
respectively.
Based on tissue sites in females, the excess risk estimates for 8-
hour TWA occupational lifetime exposure to 1 ppm BD range between 4 and
31 per 1,000 workers.
NIOSH presented its findings for lifetime exposure to 2 ppm as
follows:
Based on tumors at the most sensitive site, the female mouse
lung [assuming (mg/kg)\3/4\ conversion], our maximum likelihood
estimates of the projected human increased risk of cancer due to a
lifetime occupational exposure to BD at a TWA PEL of 2 ppm is
approximately 60 in 1,000 (workers). (Ex. 90)
For the linear models, if scaling were on a (mg/kg) basis rather
than the (mg/kg)\3/4\ used by NIOSH for allometric conversion, the
revised estimate of excess cancer risk for an 8-hour TWA occupational
lifetime exposure to 2 ppm BD would decrease approximately 6 fold to
9.2 per 1,000 workers based on the same female mouse lung tumor data.
In 1993, NIOSH finalized its estimates of excess risk caused by
occupational exposure based on the tumorigenesis
[[Page 56781]]
experience of mice in the NTP II study. (Ex. 118-1B) The rounded
maximum likelihood estimates (MLE) from the final QRA are presented in
Table V-10. NIOSH expanded the gender-tumor sites to include
histiocytic sarcoma for both male and female mice. NIOSH chose to
present only its risk estimate based on lymphocytic lymphoma, rather
than an assessment based on the aggregate of lymphomas. In the
preliminary and final NIOSH QRAs, 1-stage time-to-tumor models''
rounded estimates of risk associated with lifetime exposure to 1 ppm BD
ranged from 1 to 30 excess cancer cases per 1,000 workers, with
estimates based on the male-lymphocytic lymphoma and the female-lung
dose-response data providing the lower and upper ends of the range of
risk, respectively.
As part of its sensitivity analyses, NIOSH derived the estimates of
risk based on (1) equating a human lifespan to a mouse equivalent age
of 784 days, a figure OSHA has used, and (2) equating a human lifespan
to a mouse lifespan of 900 days (a figure more often used by NIOSH.)
The best estimates of risk equating human lifespan to a mouse lifespan
of 784 days were lower, by about one-third, than those assuming a human
lifespan equivalency to 900 days for the mouse, all else held constant.
Table V-10.--NIOSH's a Final Quantitative Risk Assessment's (QRA)
Maximum Likelihood Estimates (M.L.E.s) b per 1,000 Workers of Lifetime
Excess Risk Due to an Occupational c Exposure to 1 ppm of BD Using Best
Fitting Models, as Designated by Number of Stages of the Weibull Time-to-
Tumor Model
------------------------------------------------------------------------
MLE, Final QRA
Gender-tumor site (Stages)
------------------------------------------------------------------------
Male mouse: ................
Forestomach....................................... 0.03 (2)
Harderian gland................................... 10 (1)
Heart hemangiosarcoma............................. 0.5 (2)
Histiocytic sarcoma............................... 8 (1)
Liver............................................. 4 (1)
All Lymphoma...................................... NA
Lymphocytic lymphoma.............................. 0.9 (1)
Lung.............................................. 10 (1)
Female mouse: ................
Forestomach....................................... 5 (1)
Harderian Gland................................... 7 (1)
Heart hemangiosarcoma............................. 3 x 10-3 (3)
Histiocytic sarcoma............................... 10 (1)
Liver............................................. 7 (1)
All lymphoma...................................... NA
Lymphocytic lymphoma.............................. 9 (1)
Lung.............................................. 30 (1)
Mammary........................................... 4 (1)
Ovarian........................................... 9 (1)
------------------------------------------------------------------------
a Based on NTP II, excluding the 625 ppm exposure category, equating a
900-day-old mouse to a 74-year old human and assuming an allometric
conversion of (mg/kg)3/4.
b Rounded to one significant figure.
c Occupational lifetime is an 8-hour time-weighted-average, 40-hours per
week, 50-weeks per year, time-weighted-average (TWA) for 45-years.
The Carcinogen Assessment Group QRA
The Carcinogen Assessment Group (CAG) and the Reproductive Effects
Assessment Group of the Office of Health and Environmental Assessment
at the United States Environmental Protection Agency (EPA) also
conducted an assessment of the mutagenicity and carcinogenicity of BD.
(Ex. 17-21) In its quantitative risk assessment, CAG used both male and
female response data from the two chronic bioassays available at the
time, NTP I with B6C3F1 mice and the HLE Sprague Dawley
rat study. The CAG analysis is based on EPA's established procedures
for quantitative risk analyses, which fit the total number of animals
with significantly increased or highly unusual tumors with the
linearized multistage model and use the upper 95% confidence interval.
Mice dying before week 20 and rats dying during the first year of the
study (before the observation of the first tumor) were eliminated from
the analysis to adjust for non-tumor differential mortality.
The dose-metric was based on a preliminary report by the Lovelace
Inhalation Toxicology Research Institute of its six-hour exposure study
in B6C3F1 mice and Sprague Dawley rats at different
concentrations of BD, roughly corresponding to the concentrations used
in NTP I and HLE, with total internal BD equivalent dose expressed as a
function of inhalation exposure concentration. Then CAG estimated the
amount and percent of BD retained for various exposure concentrations
in these bioassays. These internal dose-estimates were then
extrapolated to humans based on animal-to-human ppm air concentration
equivalence.
CAG adjusted risk estimates from the mouse study by a factor of
(study duration/lifetime) \3\ to account for less-than-lifetime
observations, since the NTP I study was prematurely terminated at 60
weeks for males and 61 weeks for females due to predominating cancer
mortality. CAG extrapolated the short lifespan mouse data to an
expected mouse lifetime, 104 weeks, in order to estimate lifetime risk
to humans.
CAG estimated all risks based on continuous exposure to BD, 24
hours per day, 365 days per year, for a 70-year lifetime. The
incremental unit risk estimates for the female mouse were about eight
times as high as those for the female rat; for the males, the
incremental unit risk estimate for mice was about 200 times as high as
for rats. The CAG final incremental unit risk estimate of 0.64
(ppm)-1 is based on the geometric mean of the upper-limit slope
estimates for male and female mice and would predict an upper limit of
640 excess cancers per 1,000 people exposed to 1 ppm continuously
throughout their lifetime, 70 years. Extrapolating this same estimate
to an equivalent 45-year working lifetime of 240 work days per
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year at an 8-hour TWA exposure to 1 ppm BD would yield an upper-limit
risk estimate of 90 excess cancers per 1,000 workers. If the working
day is assumed to require one-half (10m \3\) the daily tidal volume,
the total amount of air inhale, the excess would be 135 cancers per
1,000 workers.
California Occupational Health Program (COHP) QRA
In 1990, five years after the CAG conducted its quantitative risk
assessment, the California Occupational Health Program (COHP) produced
its estimates of risk with a similar assessment of the carcinogenicity
of BD, using the same available bioassays, with more recent information
on BD risk in humans, pharmacokinetic (PK) modeling, and animal low
exposure absorption efficiency. (Ex. 32-16) Using three separate dose-
metrics for each bioassay and multistage models to characterize the
basic dose-response relationship, CAG presented several quantitative
estimates of incremental lifetime unit risks. Quantal lifetime response
multistage models were fit to the data. COHP, like NIOSH, used the
individual data with a multistage Weibull time-to-tumor model to
characterize the dose response relationship. COHP stated that it also
fit Mantel-Bryan and log-normal models to the data, and that the
multistage models gave a better fit; the results obtained with these
other models were not reported.
COHP performed calculations on each primary tumor site separately,
and also did calculations on the pool of primary tumors that showed
significantly increased tumor incidences. For their main dose-metric,
COHP refined the CAG approach, using a revised estimate of low-exposure
absorption via inhalation. COHP also included an estimate of the PK
model derived BD monoepoxide metabolites, but de-emphasized their use
by stating that these were ``presented for comparative purposes only.''
The third dose-metric was straight ppm for animal-to-human species
conversion (adjusting for duration of exposure). COHP stated:
(COHP) followed standard EPA practice and assumed that a certain
exposure concentration in ppm or mg/m 3 in experimental animals
was equivalent to the same exposure concentration in humans. (Ex.
32-16)
Like CAG, COHP also adjusted for less than lifetime survival in the NTP
I mouse study, by using a cubic power of time, (study duration/
lifetime) \3\. COHP's potency estimate adjustment for the male mouse
study with 60-week survival was 5.21; for the 61-week female mouse
survival the adjustment was 4.96.
With all the combinations of sites, species, sexes, models, and
dose-metrics, COHP presented over 60 potency estimates for the rat and
over 100 for the mouse. As with the CAG and other analyses, the
estimates based on NTP I were typically one to two orders of magnitude
greater than those based on the rat for similar dose-metrics, models
and total tumors. COHP chose the estimates based on the male mouse as
final indicators of human risk based on the ``superior quality of the
mouse study.'' From these estimates, using the quantal form of the
multistage model, COHP chose ``the upper bound for plausible excess
cancer risk to humans.'' COHP's final cancer potency estimate of 0.32
(ppm)-1 presented in units of continuous lifetime exposure, is
based on all significant tumors in the male mouse and uses the internal
BD equivalent dose conversion factor of 0.54 mg/kg-d/ppm for the mouse
and animal-to-human ppm equivalency. COHP's final potency estimate was
one-half the value of 0.64 (ppm)-1 calculated by the CAG; the
difference is due mainly to a low exposure absorption modification by
COHP. The continuous lifetime exposure potency factor converts to a
working lifetime risk of 45 to 67 excess cancers per 1,000 workers,
exposed to 1 ppm of BD at an 8-hour TWA over a 45 year working
lifetime.
COHP, like CAG, attempted to determine whether its animal-based
risk extrapolation could predict the leukemia mortality observed in
epidemiology studies. Following the approach employed by CAG in its
analyses of the Meinhardt (1982) study, the COHP compared its estimates
of risk from bioassays to the then most recent epidemiological studies
of Downs et al. (1987) and Matanoski and Schwartz (1987). Both COHP and
CAG used MLEs based on mouse lymphoma for comparing the animal-derived
potency estimates with the occupational response. In addition, neither
COHP nor CAG used the upward adjustment factor of approximately 5 to
correct for the less-than-lifetime duration of NTP I. Because neither
of these epidemiology studies (Downs et al. (1987) or Matanoski and
Schwartz (1987)) had recorded exposure estimates, the COHP relied on 8-
hr TWA estimates of 1 and 10 ppm taken at different but similar plants
reported by Fajen et al. (1986). For lifetime unit risk estimates, COHP
used the initial MLE of 0.0168 (ppm)