As a result of recent scientific studies, the International Agency for Research on Cancer has upgraded its classification of diesel engine emissions to “carcinogenic to humans.” Added to longstanding evidence of diesel-related noncancer respiratory illnesses and the large numbers of individuals exposed at work and in communities, diesel exhaust emissions represent a major public health hazard. The Occupational Safety and Health Administration (OSHA) has no standard for diesel emissions, nor does it have one on its future rule-making agenda. The US Department of Transportation (DOT), responsible for the health and safety of interstate truck and bus drivers, has neither a standard for diesel emissions nor other health standards with explicit exposure limits. This policy calls on OSHA to begin rule-making toward a diesel emissions standard and on DOT to adopt health standards, including a standard for diesel emissions. The Environmental Protection Agency (EPA) has enacted many important diesel regulations since 2001 that are also technology neutral, that is, compatible with retrofitting existing engines and equipment. With diesel lifetimes of 20 to 30 years and millions of older engines still in use, air pollution from these older engines remains a major public health problem. However, EPA’s proactive program for retrofitting and replacing diesel engines has been cut back dramatically by Congress, which explicitly intends to end its federal funding. This policy calls on Congress to restore and enhance this funding. A hopeful development in light of these cutbacks is the effort to reduce diesel pollution by states such as California.
Relationship to Existing APHA Policy Statements
• APHA Policy Statement 7111: The Right to a Healthful Work Environment
• APHA Policy Statement 7415: Prevention of Occupational Cancer
• APHA Policy Statement 8912: Public Health Control of Hazardous Air Pollutants
• APHA Policy Statement 9606: The Precautionary Principle and Chemical Exposure Standards for the Workplace
• APHA Policy Statement 9912: Preventing Environmental and Occupational Health Effects of Diesel Exhaust
• APHA Policy Statement 200011: The Precautionary Principle and Children’s Health
• APHA Policy Statement 200017: Confirming Need for Protective National Health Based Air Quality Standards
• APHA Policy Statement 20099: Improving Health Through Transportation and Land-Use Policies
In June 2012 the International Agency for Research on Cancer (IARC), a division of the World Health Organization, upgraded its classification of diesel engine exhaust from probably carcinogenic to humans (Group 2A) to carcinogenic to humans (Group 1), its highest level of determination of a material’s carcinogenicity. This decision was based on “sufficient evidence that exposure is associated with an increased risk for lung cancer.”1 A report documenting the reclassification was published the following year.2
IARC’s decision was influenced by two major US studies (one a cohort study and the other a nested case-control study) on the effects of diesel exhaust among nonmetal and noncoal miners, begun in 1992 as a joint project of the National Institute for Occupational Safety and Health (NIOSH) and the National Cancer Institute (NCI). These investigations, which faced 17 years of industry lawsuits, were finally completed and published in 2012.3–7 The studies were designed to assess the relationship between respirable elemental carbon particulates in diesel exhaust and cancers. They examined populations across eight different mines, none of which used gasoline engines. The cohort study detected significantly elevated levels of lung cancer among all of the participating miners, with some evidence of an exposure-response effect, while the case-control study showed clear, consistent evidence of such an exposure-response effect. In its report, IARC identified 35 different chemicals and metals in diesel exhaust emissions that are currently judged by the agency as known, probable, or possible carcinogens (Groups 1, 2A, and 2B), including arsenic, cadmium, benzene, formaldehyde, and polycyclic aromatic hydrocarbons.2
These results come after three decades of epidemiological studies observing significant excesses of respiratory and other cancers among railroad and trucking industry workers, often confounded by co-exposures with gasoline engine exhaust, as well as experimental, animal, and cellular studies suggesting possible carcinogenicity caused by diesel exhaust emissions.2,8 Traffic exhaust studies conducted during this period, again involving mixed diesel and gasoline engine exhaust exposures, also provide evidence of possible diesel engine contributions to noncancer respiratory conditions and diseases.9–13
Protection of workers from diesel exhaust hazards: According to the Occupational Safety and Health Administration (OSHA), individuals with potential exposures to diesel engine exhaust as a result of their occupation include "miners, construction workers, heavy equipment operators, bridge and tunnel workers, railroad workers, oil and gas workers, loading dock workers, truck drivers, material handling operators, farmworkers, long-shoring workers, and auto, truck and bus maintenance garage workers."14 OSHA and its approved state and territorial plans have sole jurisdiction to protect worker health and safety for many of these occupations, including the manufacturing, construction, agriculture, and intrastate transportation industries. The Mine Safety and Health Administration (MSHA) has primary jurisdiction over health and safety in the mining industry, although it shares some responsibilities with OSHA through an interagency agreement.15 The US Department of Transportation (DOT) has primary health and safety jurisdiction in the case of workers engaged in interstate truck transportation, although OSHA covers these truck workers when they are at their workplace (e.g., loading and unloading).16
The total US worker population regularly exposed to diesel emissions on the job numbers in the millions. As early as 1983, NIOSH estimated that roughly 1.35 million workers were exposed to diesel engine emissions in approximately 80,000 workplaces in the United States.17 In addition, according to the 2002 federal Vehicle Inventory and Use Survey, approximately 3.5 million trucks in the United States were powered by diesel fuel (excluding pickups, minivans, other light vehicles, and sport utilities) during that year, more than 80% of which were used for business and industry.18 Also, OSHA and its associated state plans have jurisdiction over employees engaged in intrastate transportation, such as the 38,000 members of Transport Workers Union, Local 100; many of these individuals, who are primarily employees of the New York City Transit Authority, are exposed to diesel exhaust in their work as bus drivers and maintainers, forklift operators, and subway employees.19 Large numbers of the 5.5 million construction employees covered by OSHA operate and work near heavy diesel equipment.20
Three federal agencies — OSHA, MSHA, and DOT — are responsible for protection of diesel-exposed workers. Their policies and regulations are described below.
There is no OSHA standard specifically for diesel exhaust despite the large numbers of US workers exposed to diesel emissions on the job and the known respiratory hazards (even before the recent NCI/NIOSH studies), as well as earlier calls for such a standard by APHA (Policy Statement 9912) and other groups.14
Only a small proportion of the “thousands of chemical components present in the gas and particulate phases” of diesel emissions2 are covered by OSHA standards, and most of these standards are specification standards whose sole requirement is that specific upper exposure limits not be typically exceeded over an 8-hour work shift. Components in the gas phase rarely exceed their limits; their greatest potential threat comes from their adsorption onto diesel engine particulates (DEPs), bringing them deep into the lungs.14
In view of its carcinogenicity and the great number of US workers exposed, a comprehensive standard for diesel exhaust is needed. Such a standard, as with other comprehensive OSHA standards, would be expected to include DEP exhaust limits, safe work practices, medical examinations, appropriate respiratory protections, and worker training and education.
New-technology diesel engines (those introduced since 2007) have resulted in important reductions in emissions and decreased exposure rates among workers and the general public.2 However, older diesel engines, typically elements of major capital equipment, are costly and difficult to replace with these new engines. According to the Environmental Protection Agency (EPA), there were approximately 11 million older diesel engines (installed before 2007) in use in the United States in 2010, many in the land and marine transportation, agriculture, and construction industries.21 EPA estimates that the life span of diesel engines and equipment used in such industries is 20 to 30 years.22–24 Thus, many workers will be exposed to significant levels of carcinogenic diesel exhaust for decades to come unless and until OSHA exercises its worker protection mandate.
Unfortunately, the latest US Department of Labor (DOL) agency rule list (fall 2013) does not contain any reference to rule-making regarding diesel exhaust, indicating that OSHA does not intend to commence such rule-making in the near future.25 To its credit, OSHA promptly updated its website in 2012 and issued a new hazard warning after the IARC cancer determination.14,26 This does not substitute, however, for the more substantive protections of a comprehensive standard.
MSHA issued two standards in 2001, one focusing on underground coal mines and the other on underground metal and nonmetal mines, with the expressed purpose of reducing DEP exposures.27,28 After publication of the 2012 NIOSH/NCI studies and the subsequent IARC upgrade, MSHA updated its web page on diesel emissions and issued a joint hazard alert with OSHA.26 However, like OSHA, MSHA did not add diesel engine emissions to its list of intended rule-making for 2014.25 Since MSHA most recently updated its diesel standards in 2001, the agency should review and reconsider its current standards in light of new scientific developments.
The DOT division responsible for truck and bus regulations and driver safety is the Federal Motor Carrier Safety Administration (FMCSA).29 Although FMCSA has a long, detailed list of driving safety standards, the list contains only a single entry under "employee safety and health standards," namely a safety standard for entering and exiting commercial motor vehicles.30,31 There are no entries on the list requiring exposure limits in the driver’s cabin for gases or particulates such as DEPs and carbon monoxide.
The only health protection provided to interstate truck drivers is a 1988 DOT regulation requiring that exhaust from trucks (both diesel and gasoline) "discharge to the atmosphere at a location to the rear of the cab or, if the exhaust projects above the cab, at a location near the rear of the cab"; the regulation also requires that no part of the exhaust system "leak or discharge at a point forward of or directly below the driver/sleeper compartment."32 This DOT regulation provides a basic level of protection for interstate drivers but again is no substitute for specific exposure standards. As the only federal agency responsible for the health and safety of interstate truck drivers, the DOT has an obligation to formally adopt health standards for these drivers, including a DEP standard.
That said, federal clean air regulations mandate that the DOT work closely with EPA to reduce environmental exposures from interstate trucks and motor carriers, as well as from other DOT-regulated diesel engines.33,34 This helps reduce exposures among operators of diesel vehicles as well.
Protection of the general public from DEP hazards: In addition to those exposed to diesel exhaust on their jobs, vast populations both in the United States and abroad are exposed to ambient air DEPs from trucks, buses, and some automobiles, as well as railroad, aviation, and ship exhaust and exhaust from construction and industrial diesel equipment. According to IARC’s press release classifying diesel exhaust as carcinogenic, “The main studies that led to this conclusion were in highly exposed workers. However, we have learned from other carcinogens, such as radon, that initial studies showing a risk in heavily exposed occupational groups were followed by positive findings for the general population. Therefore actions to reduce exposures should encompass workers and the general population."1
Ionizing radiation and other less studied carcinogens remain subjects of scientific debates about the linearity versus nonlinearity of dose-response curves (toxicology) and exposure-response curves (occupational epidemiology).35,36 Nevertheless, many in science and public health appropriately adopt a precautionary approach that carcinogens should be treated as if no such threshold exists and that any dose of these agents may cause cancers, albeit at lower rates as the dose decreases. Such precautionary approaches were endorsed by APHA in 1996 (Policy Statement 9606) and reaffirmed in 2000 (Policy Statement 200011). It should further be noted that significant sources of diesel exhaust such as bus depots and industrial facilities are often concentrated near low-income communities of color, as noted by APHA in 1999 (Policy Statement 9912). While efforts have been made since then to better assess and alleviate racial/ethnic disparities, these disparities persist (albeit at diminished levels).37
In 2001, EPA announced new standards for heavy-duty trucks and buses to take effect in model year 2007, requiring low-sulfur diesel fuel and high-efficiency catalytic exhaust emission control devices.38 EPA estimates that particulates will be reduced by 90% and nitrogen oxides by 95%, thereby lowering emissions to “gasoline-like” levels. EPA issued a tiered series of similar regulations between 1994 and 2004 covering nonroad diesel engines in construction, agriculture, and other covered industries (e.g., backhoes, tractors, emergency power generators, and airport ground service vehicles).39 In 2011, as part of federal climate change initiatives, EPA enacted additional regulations (to take effect in model years 2014–2018) reducing greenhouse gas emissions and improving the fuel efficiency of medium- and heavy-duty vehicles.40
However, as important as these regulations are in reducing DEP emissions, they are all technology neutral, that is, compatible with retrofitting existing diesel vehicles and equipment. With diesel engine and equipment lifetimes of 20 to 30 years and millions of older (pre-2007) diesel engines still in use, the air pollution from these older engines remains a major public health problem.22–24
Meanwhile, EPA’s proactive program for retrofitting and replacing diesel engines, funded under the 2005 Diesel Emissions Reduction Act (DERA),21 has been cut back dramatically by Congress. EPA received initial DERA funding of a mere $49 million in fiscal year 2008, followed by a sixfold increase to $300 million in fiscal year 2009 under the American Recovery and Reinvestment Act.21 Since then Congress has steadily reduced funding for this national program, virtually wiping it out with an appropriation of only $6 million in fiscal year 2014.41 In its fiscal year 2014 budget message, Congress explicitly announced its intention to end federal funding for the program: “Requested resources support a new approach, initiated in FY2013, designed to transition the program away from ongoing Federal support.” Since EPA has the authority to establish regulations for diesel exhaust emissions and has already done so for entire classes of diesel engines and equipment, this congressional action already largely amounts to an unfunded federal mandate for states and will become entirely so if Congress’ intent to end all funding is realized.
In its first 3 years, this underfunded program was able to retrofit, replace, or repower "more than 50,000 vehicles and equipment."21 This represents only 0.5% of the 11 million older diesel engines reported to be in vehicular use by EPA in 2010.21 The funding for this important program needs to be not only restored to fiscal year 2009 levels but greatly increased to assist in retrofitting a greater proportion of these millions of old, polluting diesel engines and support the purchase of new-technology diesel engines by hard-pressed states and communities.
In the absence of robust federal support and funding for reducing DEP pollution, various US states, led by California, have taken a lead in this public health effort. In 1998, the California Air Resources Board declared DEPs a toxic air contaminant, and since then the board has developed an array of pollution-reduction programs for on-road and off-road vehicles and equipment, railroad locomotives, agricultural equipment, marine vessels (including recreational watercraft), and stationary engines (including irrigation pumps and emergency and portable generators), among others.42 Currently all US states have some or many diesel pollution remediation programs, as reflected in the DERA appropriations for these programs in 45 states in 2013, albeit at woefully inadequate levels varying from $72,000 (South Dakota) to $205,000 (California).43 Many of these programs focus on funding for school bus, public, and marine transportation.44 DERA funds in 2013 were also allocated to the District of Columbia and five US territories.
In addition to its various DERA-funded programs under the umbrella of the National Clean Diesel Campaign, EPA has played an important role in the establishment of seven regional clean diesel collaboratives across the United States.45 This initiative links state and local governments, nonprofit organizations, and private industry in helping to solve problems in a given region. EPA has also helped encourage diesel pollution control programs in US coastal ports such as the port of New York/New Jersey and California’s Bay area.46,47
There are several different types of questions or counterarguments that might be raised, including those offered below.
Would APHA be acting prematurely by passing the proposed resolution? After all, only the IARC has reclassified diesel exhaust emissions as carcinogenic to humans. Other reputable agencies have not changed their classifications from suspected, possible, or probably carcinogenic. The response to this question is that the carcinogenicity of diesel exhaust has been suspected for decades, and in the meantime much indirect evidence of its harmful effects has accumulated from the results of traffic studies, studies of miners and railroad workers, and animal and cellular studies. The two NIOSH/NCI studies, carefully researched for more than a decade, were the first to definitively examine diesel exhaust alone, absent possible confounding from gasoline engine exposures. In 1999, in Policy Statement 9912, APHA noted that there was already sufficient evidence to take public health action. With important new research now confirming its carcinogenicity, further actions to limit diesel exhaust exposure among workers and the general public are certainly long overdue.
With respect to the general public, EPA is doing a good job reducing DEP exposures and, if EPA’s retrofit program is refunded, eventually exposures among the general public will be greatly reduced. Why not let this process take its course before APHA rushes in? Without any OSHA or DOT diesel standards to protect them, millions of US workers will be significantly exposed to a carcinogen on the job. Also, with diesel engine and equipment lifetimes of 20 to 30 years, EPA’s efforts along the lines it is now taking would leave millions in the general public significantly exposed for the next few decades (i.e., another generation of Americans). We have a responsibility as public health professionals to reduce exposures to known carcinogens to the extent feasible. Allowing delays in protection of worker and community heath when more protective technology exists and could be rapidly implemented is unacceptable.
Diesel exhaust standard setting would present difficulties for OSHA and DOT because of the complex composition of diesel exhaust, its variability under different operational and climatological conditions, and the many different types of work situations covered. It should be noted that MSHA has had a diesel exhaust standard for underground miners since 2006, and industry and labor have been able to work since then in accordance with its provisions, including worker training. Also, to set standards, OSHA and DOT are obligated under their mandates only to demonstrate that the risk to their covered workers is significant, which it is according to existing scientific evidence (and this is a determination that has already been upheld in court in response to MSHA’s DEP rule), and that there are feasible controls to significantly reduce exposures (which there most certainly are, according to MSHA’s and EPA’s current regulatory actions).
Evidence-based Strategies to Address the Problem
The proposed resolution calls for three government agencies — OSHA, MSHA, and DOT — to exercise their statutory powers to mandate changes that would protect the health of workers and communities. It also calls upon Congress to restore and enhance funds to strengthen EPA programs to retrofit older model diesel engines currently in use and assist state and local groups in the purchase of new-technology diesel engines. If, as requested, these agencies exercise their powers and Congress restores funding, the public health hazards posed by diesel emissions will be greatly reduced.
This policy is an update of Policy Statement 9912 (Preventing Environmental and Occupational Health Effects of Diesel Exhaust), adopted by APHA in 1999. As part of that policy, APHA made six recommendations: two primarily addressing health and safety protections for diesel-exposed workers, three focusing on environmental protections for the general public (which also improve health and safety protections for many workers), and one requesting that state and local authorities consider diesel health effects in siting and permitting facilities in their states and communities.
Occupational safety and health protections: During the past 15 years, little progress has been made in protecting the health and safety of workers exposed to DEPs on the job. In particular, OSHA has not established a permissible exposure limit for DEP exposures as requested in Policy Statement 9912, nor does it even have such a standard on its current rule-making agenda 15 years later. MSHA has promulgated a diesel exposure standard to protect underground coal miners, as requested in that policy statement, as well as another standard to protect underground metal and nonmetal miners. Not noted in Policy Statement 9912 but noted in this updated policy, the Department of Transportation, which has sole responsibility for on-the-road health and safety protection of interstate truck and bus drivers through its Federal Motor Carrier Safety Administration, not only has no diesel standard but has no health standards that specifically limit these workers’ exposures on the job to any chemicals or particulates, including carbon monoxide and DEPs.
Environmental protections: In contrast to the lack of progress in health and safety protections for workers exposed to diesel engine exhaust on the job, there has been much progress in reducing exposures to this exhaust among members of the general public, which of course benefits diesel-exposed workers both on and off their jobs. This important progress has been led by EPA at the federal level and by state governments across the country, nowhere more prominently than in California. During the past 15 years, EPA has promulgated regulations that reduce exhaust exposures from mobile and stationary diesel engines in industries including transportation, construction, agriculture, and ports and marine. It has also promulgated regulations reducing the sulfur content in diesel fuels so that more advanced exhaust devices can be used by all individuals operating older diesel engines; these regulations also allow the operation of new-technology diesel engines with their dramatic reductions in exhaust emissions. These two interactive sets of regulations more than fulfill two of the environmental recommendations made by APHA in Policy Statement 9912.
The single environmental recommendation from Policy Statement 9912 that has not been acted upon positively is that asking Congress to preferentially support the purchase of transportation equipment and development of technologies that either do not use diesel fuel or do not release harmful levels of particulates. Indeed, Congress in recent years has moved in the opposite direction by sharply cutting state and local funds for retrofitting and replacing diesel engines under the 2005 Diesel Emissions Reduction Act.
Public health education: With the recent scientific advances in research on diesel exhaust carcinogenicity, many even in the public health community may not yet be aware of how significantly these results enhance our appreciation of the dangers diesel exhaust presents to public health. This means going beyond e-mail and other social media messages to reach populations in the United States who do not regularly communicate via computers; information dissemination efforts must include written articles in newspapers and magazines and video presentations on the subject. It also means that relevant agencies or consortia thereof should be encouraged to conduct public awareness sessions in their communities and with their public health agencies in alliance with state and local APHA chapters, National Council for Occupational Safety and Health groups, environmental organizations, labor unions, and industry councils. In many areas, state and local legislators will be happy to speak at and participate in these conferences, thereby raising the public profile of such efforts.
This policy calls upon:
1. OSHA to add diesel exhaust to its proposed rule-making list and begin the process of setting a new standard to protect the large number of affected US workers under its jurisdiction from the toxic effects of DEP exposures.
2. MSHA to review and reconsider its current diesel emission standards in light of recent scientific developments regarding the carcinogenicity of diesel engine exhaust.
3. The DOT to formally adopt health standards for interstate truck and bus drivers, including a standard for diesel exhaust emissions.
4. The US Congress to restore and enhance federal funding for EPA programs under the Diesel Emissions Reduction Act to prevent threats to children’s health posed by exposures to emissions from older model (pre-2007) engines.
5. The US Congress to support the purchase of transportation equipment and the development of technologies that do not use diesel fuel or release levels of particulates or substances that are harmful to health.
6. State and local authorities to take into account the health effects of facilities that are significant sources of emissions in deciding whether to renew permits for existing facilities or site new facilities near vulnerable populations, including low-income communities and communities of color.
7. Federal agencies with primary responsibilities for public health, including DOL/OSHA, the Centers for Disease Control and Prevention, and the US Department of Health and Human Services, as well as similarly responsible state and local agencies to conduct educational campaigns informing the public of the dangers of diesel exhaust exposures. These campaigns should include newspaper and magazine articles, video presentations, and community public awareness sessions.
1. International Agency for Research on Cancer. Diesel engine exhaust carcinogenic. Available at: http://www.iarc.fr/en/media-centre/pr/2012/pdfs/pr213_E.pdf. Accessed January 22, 2015.
2. International Agency for Research on Cancer. Diesel and gasoline engine exhausts and some nitroarenes. Available at: http://monographs.iarc.fr/ENG/Monographs/vol105/mono105.pdf. Accessed January 22, 2015.
3. Attfield MD, Schleiff PL, Lubin JH, et al. The Diesel Exhaust in Miners Study: a cohort mortality study with emphasis on lung cancer. J Natl Cancer Inst. 2012;104:869–883.
4. Silverman DT, Sarmanic CM, Lubin JH, et al. The Diesel Exhaust in Miners Study: a nested case-control study of lung cancer and diesel exhaust. J Natl Cancer Inst. 2012;104:855–868.
5. National Cancer Institute. Diesel Exhaust in Miners Study. Available at: http://dceg.cancer.gov/research/what-we-study/environment/diesel-exhaust-miners-study-dems. Accessed January 22, 2015.
6. Tollefson J. Embattled scientists publish study linking (surprise) diesel exhaust and cancer. Available at: http://blogs.nature.com/news/2012/03/embattled-scientists-publish-miner-cancer-study.html. Accessed January 22, 2015.
7. Monforton C. Weight of the evidence or wait for the evidence? Protecting underground miners from diesel particulate matter. Am J Public Health. 2006;96:271–276.
8. International Agency for Research on Cancer. Diesel and gasoline engine exhausts and some nitroarenes: monograph 46. Available at: http://monographs.iarc.fr/ENG/Monographs/vol46/index.php. Accessed January 22, 2015.
9. Braback L, Forsberg B. Does traffic exhaust contribute to the development of asthma and allergic sensitization in children: findings from recent cohort studies. Environ Health. 2009;8:17.
10. Hesterberg TW, Long CM, Bunn WB, Sax SN, Lapin CA, Valberg PA. Non-cancer health effects of diesel exhaust: a critical assessment of recent human and animal toxicological literature. Crit Rev Toxicol. 2009;39:195–227.
11. Boehmer TK, Foster SL, Henry JR, Woghiren-Akinnifesi EL, Yip FY. Residential proximity to major highways—United States, 2010. MMWR Surveill Summ. 2013;22(suppl 3):46–50.
12. Gowers AM, Cullinan P, Ayres JG, et al. Does outdoor air pollution induce new cases of asthma? Biological plausibility and evidence; a review. Respirology. 2012;17:887–898.
13. Ghio AJ, Smith CB, Madden MC. Diesel exhaust particles and airway inflammation. Curr Opin Pulm Med. 2012;18:144–150.
14. Occupational Safety and Health Administration. Diesel exhaust. Available at: https://www.osha.gov/SLTC/dieselexhaust/. Accessed January 22, 2015.
15. Occupational Safety and Health Administration. Interagency agreement between the Mine Safety and Health Administration, US Department of Labor, and the Occupational Safety and Health Administration, US Department of Labor. Available at: https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=MOU&p_id=222. Accessed January 22, 2015.
16. Occupational Safety and Health Administration. Industry hazards: highway driving—interstate and intrastate Available at: https://www.osha.gov/SLTC/trucking_industry/highwaydriving.html. Accessed January 22, 2015.
17. National Institute for Occupational Safety and Health. Carcinogenic effects of exposure to diesel exhaust. Available at: http://www.cdc.gov/niosh/docs/88-116/. Accessed January 22, 2015.
18. US Census Bureau. Vehicle Inventory and Use Survey: 2002 Economic Census. Available at: http://www.census.gov/prod/ec02/ec02tv-us.pdf. Accessed January 22, 2015.
19. Transport Workers Union. TWU homepage. Available at: http://www.twulocal100.org/new-yorks-public-transit-union. Accessed January 22, 2015.
20. Bureau of Labor Statistics. Workplace injuries and illnesses, 2011. Available at: http://www.bls.gov/news.release/archives/osh_10252012.pdf. Accessed January 22, 2014.
21. Environmental Protection Agency. Second report to Congress: highlights of the Diesel Emissions Reduction Program. Available at: http://www.epa.gov/cleandiesel/documents/420r12031.pdf. Accessed January 22, 2015.
22. Environmental Protection Agency. National Clean Diesel Campaign: clean agriculture. Available at: http://www.epa.gov/cleandiesel/sector-programs/ag-overview.htm. Accessed January 22, 2015.
23. Environmental Protection Agency. National Clean Diesel Campaign: ports and marine. Available at: http://www.epa.gov/cleandiesel/sector-programs/ports-overview.htm. Accessed January 22, 2015.
24. Environmental Protection Agency. National Clean Diesel Campaign: clean construction. Available at: http://www.epa.gov/cleandiesel/sector-programs/construct-overview.htm. Accessed January 22, 2015.
25. US Department of Labor. Agency rules list, fall 2013. Available at: http://www.reginfo.gov/public/do/eAgendaMain;jsessionid=CEB7E1F0B63C7B60A0F0CF07FB23DBB8?operation=OPERATION_GET_AGENCY_RULE_LIST¤tPub=true&agencyCode=&showStage=active&agencyCd=1200&Image58=Submit. Accessed January 22, 2015.
26. Occupational Safety and Health Administration. OSHA/MSHA hazard alert: diesel exhaust/diesel particulate matter. Available at: https://www.osha.gov/dts/hazardalerts/diesel_exhaust_hazard_alert.html. Accessed January 22, 2015.
27. Mine Safety and Health Administration. Diesel particulate matter exposure of underground coal miners: final rule. Available at: http://www.gpo.gov/fdsys/pkg/FR-2001-01-19/pdf/01-995.pdf. Accessed January 22, 2015.
28. Mine Safety and Health Administration. Diesel particulate matter exposure of underground metal and nonmetal miners: final rule. Available at: http://www.gpo.gov/fdsys/pkg/FR-2001-01-19/pdf/01-996.pdf. Accessed January 22, 2015.
29. Federal Motor Carrier Safety Administration. FMCSA homepage. Available at: http://www.fmcsa.dot.gov/. Accessed January 22, 2015.
30. US Department of Transportation. Federal Motor Carrier Safety Administration rules and regulations. Available at: http://www.fmcsa.dot.gov/rules-regulations/rules-regulations.htm. Accessed January 22, 2015.
31. US Department of Transportation. Federal Motor Carrier Safety Administration employee safety and health standards. Available at: http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/FmcsrGuideDetails.aspx?menukey=399. Accessed January 22, 2015.
32. US Department of Transportation. Miscellaneous parts and accessories: exhaust systems. Available at: http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?reg=393.83. Accessed January 22, 2015.
33. US Department of Transportation. Federal Highway Administration Congestion Mitigation and Air Quality Improvement Program. Available at: http://www.fhwa.dot.gov/environment/air_quality/cmaq/. Accessed January 22, 2015.
34. US Department of Transportation. Federal Highway Administration air quality laws and regulations. Available at: http://www.fhwa.dot.gov/environment/air_quality/air_toxics/laws_and_regs/. Accessed January 22, 2015.
35. National Research Council. Health risks from exposure to low levels of ionizing radiation. Available at: http://www.nap.edu/openbook.php?isbn=030909156X. Accessed January 22, 2105.
36. Neumann HG. Risk assessment of chemical carcinogens and thresholds. Crit Rev Toxicol. 2009;39:449–461.
37. Post ES, Bellova A, Huang J. Distributional benefit analysis of a national air quality rule. Int J Environ Res Public Health. 2011;8:1872–1892.
38. Environmental Protection Agency. Control of air pollution from new motor vehicles: heavy-duty engine and vehicle standards and highway diesel fuel sulfur control requirements: final rule. Available at: http://www.gpo.gov/fdsys/pkg/FR-2001-01-18/pdf/01-2.pdf. Accessed January 22, 2015.
39. Environmental Protection Agency. Control of emissions of air pollution from nonroad diesel engines and fuel: final rule. Available at: http://www.gpo.gov/fdsys/pkg/FR-2004-06-29/pdf/04-11293.pdf. Accessed January 22, 2015.
40. Environmental Protection Agency. Greenhouse gas emissions standards and fuel efficiency standards for medium- and heavy-duty engines and vehicles: final rule. Available at: http://www.gpo.gov/fdsys/pkg/FR-2011-09-15/pdf/2011-20740.pdf. Accessed January 22, 2015.
41. Environmental Protection Agency. FY2014 budget in brief. Available at: http://nepis.epa.gov/Exe/ZyNET.exe/P100GCS2.txt?ZyActionD=ZyDocument&Client=EPA&Index=2011%20Thru%202015&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C11THRU15%5CTXT%5C00000007%5CP100GCS2.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=21. Accessed January 22, 2015.
42. California Air Resources Board. Diesel programs and activities. Available at: http://www.arb.ca.gov/diesel/diesel.htm. Accessed January 22, 2015.
43. Environmental Protection Agency. National Clean Diesel Campaign state allocations. Available at: http://www.epa.gov/cleandiesel/prgstate-alloc.htm. Accessed January 22, 2015.
44. Environmental Protection Agency. National Clean Diesel Campaign national projects. Available at: http://www.epa.gov/cleandiesel/projects-national.htm. Accessed January 22, 2015.
45. Environmental Protection Agency. National Clean Diesel Campaign clean diesel collaboratives. Available at: http://www.epa.gov/cleandiesel/ncdc-cmt.htm#whereyoulive. Accessed January 22, 2015.
46. Port Authority of New York and New Jersey. Environmental initiatives at the port of New York and New Jersey. Available at: http://www.panynj.gov/about/port-initiatives.html. Accessed January 22, 2015.
47. California Air Resources Board. Ports activities. Available at: http://www.arb.ca.gov/ports/ports.htm. Accessed January 22, 2015.