Clean diesel technology fleet transition programs

Evidence Rating  
Scientifically Supported
Evidence rating: Scientifically Supported

Strategies with this rating are most likely to make a difference. These strategies have been tested in many robust studies with consistently positive results.

Health Factors  
Decision Makers

Clean diesel technology transition programs replace or retrofit government or privately operated diesel vehicles (e.g., buses, garbage trucks, delivery service vans, etc.) until the entire fleet operates with clean diesel technology. Vehicles are often replaced towards the end of their useful life. Diesel powered engines can be retrofitted with control devices such as diesel particulate filters (DPF), diesel oxidation catalysts (DOC), and closed crankcase ventilation systems (CCV). Although several technologies reduce the concentrations of multiple dangerous pollutants in diesel emissions, there are no diesel technologies that stop all hazardous emissions in diesel exhaust1. Diesel exhaust is carcinogenic and contains numerous pollutants that negatively affect cardiovascular and respiratory health; estimates suggest exposure contributes to 3,700 heart attacks, 8,800 deaths, and $100 billion in health damages each year in the U.S.2. Investments in electric or alternative fuel vehicles can reduce air pollution, greenhouse gas emissions, and improve urban air quality and health outcomes more than any diesel fuel technology retrofits or replacements.

What could this strategy improve?

Expected Benefits

Our evidence rating is based on the likelihood of achieving these outcomes:

  • Reduced emissions

  • Improved air quality

Potential Benefits

Our evidence rating is not based on these outcomes, but these benefits may also be possible:

  • Improved health outcomes

What does the research say about effectiveness? This strategy is rated scientifically supported.

There is strong evidence that adopting clean diesel technology through diesel transition programs decreases environmental exhaust emissions3, 4, 5, 6. Retrofitting diesel buses has also been shown to improve in-cabin air quality by decreasing particulate matter, organic carbon, and elemental carbon in some circumstances4, 7, 8. Vehicle replacement, especially with electric or alternative fuel vehicles, is suggested instead of retrofitting for vehicles nearing the end of their useful life; replacement improves vehicle safety, reliability, and fuel efficiency, and reduces maintenance costs and emissions2, 9.

Decreases in emissions associated with retrofitted diesel buses appear to have positive effects on riders’ health. A Washington-based study, for example, indicates reductions in bronchitis, asthma, and pneumonia among children in school districts with retrofitted buses10, as well as improvements in lung function and reductions in absenteeism, especially among children with asthma11. Retrofitting may also decrease the costs associated with respiratory disease-related hospitalizations and treatment10.

Costs for diesel retrofit devices vary based on the manufacturer, labor requirements, and type of technology12. A Washington-based cost benefit analysis indicates that school bus retrofitting has positive net benefits10. The U.S. EPA estimates that every federal dollar invested in clean diesel retrofits or replacements generates between $11 and $30 in savings from improvements in public health2.

How could this strategy advance health equity? This strategy is rated potential to decrease disparities: suggested by expert opinion.

Efforts to reduce diesel exhaust emissions – such as through fleet transitions to clean diesel technology or other alternative clean energy – have the potential to decrease existing disparities in air pollution exposure rates by race, ethnicity, and income20, 21, 22. Available evidence suggests that racial disparities in air pollution exposure have declined, although racial disparities persist21. Experts suggest programs should identify neighborhoods where emissions reductions can reduce average exposure, exposure inequality, and exposure injustice as well as account for meteorology and the movement of air pollution to reduce disparities in exposure22.

Diesel vehicles, especially heavy-duty diesel vehicles, are a primary source of toxic air pollution and disparities in pollutant exposure20. In a study of 52 cities across the country, air quality monitors recorded higher concentrations of nitrogen dioxide (NO2) exposure among people of color and people with lower incomes than among white people and those with higher incomes20. Combining race and income data reveals even larger disparities in air pollution exposure levels between people of color with incomes near or below the poverty line compared to white people with incomes above the poverty line20.

Higher exposure to air pollution and toxic emissions has been associated with people of color and people with low incomes experiencing poorer health outcomes than white people and people with higher incomes20. Air pollution exposure and living in neighborhoods with heavy traffic is associated with many adverse health effects, including asthma, respiratory disease, cardiovascular disease, cancers, adverse birth outcomes, cognitive decline, dementia, and more21, 23. Children living near heavy traffic are more likely to have asthma than children living in neighborhoods without traffic; children of color are also more likely to have or develop asthma than white children23. Other studies suggest more Black individuals are diagnosed with and hospitalized for asthma, compared with white individuals24.

What is the relevant historical background?

Throughout U.S. history, discriminatory housing, lending, and exclusionary zoning policies entrenched racial residential segregation and concentrated poverty25, 26. Many urban areas in the U.S. experienced unrestrained industrialization without environmental regulations or land use controls creating environmental problems, including water and air pollution, waste production, overconsumption of natural resources, and loss of green space27. Early U.S. environmental movements focused on conservation and nature preservation and did not consider urban environmental inequities or public health28. The built environment in under-resourced communities is a significant contributor to health inequities for people of color with low incomes29, 30, 31. Formerly redlined neighborhoods remain more likely to include vacant lots and blighted properties, older homes in poor condition, coal-fired power plants, hazardous waste disposal sites, and other health risks32. Communities with low incomes and communities of color still have fewer places to engage in outdoor activities, have less access to cooling shade, experience more extreme heat, and experience poorer air quality33, 34, 35.

The Air Pollution Control Act of 1955 was the first federal legislation to identify and provide funds for air pollution research. The Clean Air Act of 1963 began the process of air pollution control, and many iterations of the legislation followed. Notably, the Clean Air Act of 1970 authorized the U.S. Environmental Protection Agency (EPA) to establish, regulate, monitor, and enforce National Ambient Air Quality Standards to protect public health from widespread hazardous air pollutants, especially particulate matter, ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, and lead. This legislation regulates and enforces reductions of many types of air pollution from multiple pollution sources, including motor vehicles. The 1990 Clean Air Act Amendments established updated goals and technology standards to reduce hazardous air pollution36, 37, 38. The federal Clean Air Act has reduced disparities in hazardous pollutant exposure, although racial disparities continue to persist39. EPA standards for cleaner vehicles and fuel aim to reduce pollutants which harm human health and the environment, as emissions from personal and commercial vehicles, and their fuel, continue to be a large source of pollution exposure40.

The Energy Policy Act of 2005 established the Diesel Emissions Reduction Act (DERA) Program and authorized the EPA to fund efforts to reduce hazardous diesel emissions, including fleet transition programs for school buses and heavy-duty vehicles41. Many grant and funding efforts are shifting away from diesel retrofits or replacements to cleaner alternatives such as electric vehicles with zero tailpipe emissions; for example, increased funding for the Clean School Bus program9.

Equity Considerations
  • What neighborhoods in your community experience heavy traffic, especially from diesel vehicles? Which areas in your community suffer from poor air quality and high levels of pollutant exposures? Who is assessing the impacts of traffic on human and animal health and the environment, for example, near schools, public green spaces, and areas with tourism?
  • How does your community engage neighborhood residents, especially residents of color and those with low incomes, in the planning and development of neighborhood spaces that influence traffic patterns? Do those making planning decisions about urban planning and traffic routes represent the local community?
  • How are you considering local climate, wind patterns, and efforts to mitigate climate change, as part of city fleet transition programs?
  • How are funds raised and allocated for fleet transitions in your community? Who is exploring grant opportunities for purchasing electric, alternative fuel, or cleaner diesel technology vehicles?
Implementation Examples

Many states have programs to support diesel retrofit efforts, as in Maryland and Oregon13, 14. New Jersey had a mandatory diesel retrofit program from 2007-2017; the program regulated school buses, solid waste vehicles, commercial buses, and publicly-owned vehicles15. Many states are developing emissions reduction programs that support alternative fuel, electric, or hybrid fleets instead of diesel, as in Texas16.

The U.S. Department of Transportation Federal Highway Administration’s Congestion Mitigation and Air Quality Improvement (CMAQ) Program funds efforts to improve air quality, including diesel engine retrofits17. The U.S. Environmental Protection Agency manages funds appropriated for the Diesel Emission Reduction Act (DERA) to retrofit older diesel vehicles18. The proposed federal budget for FY 2022 also includes grant funding for the DERA program, as well as additional funds to retrofit, replace or repower diesel equipment, especially school buses19.

Implementation Resources

Resources with a focus on equity.

US EPA-DERA - U.S. Environmental Protection Agency (U.S. EPA). Diesel Emissions Reduction Act (DERA) Funding: Reducing emissions that impact our health.

US EPA-CSBP - U.S. Environmental Protection Agency (U.S. EPA). Clean School Bus Program (CSBP).

US EPA-Clean diesel - U.S. Environmental Protection Agency (U.S. EPA). Learn about clean diesel: Impacts of diesel emissions, Diesel Emissions Reduction Act (DERA), and other federal funding.

World Emissions Clock - World Data Lab. World Emissions Clock.


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1 US EPA-Verified tech - U.S. Environmental Protection Agency (U.S. EPA). Verified technologies list for clean diesel.

2 CDC-Cleaner fleets - Centers for Disease Control and Prevention (CDC), Office of the Associate Director for Policy and Strategy. Cleaner and alternative bus fleets. Health impact in 5 years (HI-5): Transitioning bus fleets, cleaner air for a healthier community.

3 Zhang 2011 - Zhang Q, Zhu Y. Performance of school bus retrofit systems: Ultrafine particles and other vehicular pollutants. Environmental Science & Technology. 2011;45(15):6475-6482.

4 Barone 2010 - Barone TL, Storey JME, Domingo N. An analysis of field-aged diesel particulate filter performance: Particle emissions before, during, and after regeneration. Journal of the Air & Waste Management Association. 2010;60(8):968-976.

5 Sandhu 2015 - Sandhu GS, Frey HC, Bartelt-Hunt S, Jones E. In-use activity, fuel use, and emissions of heavy-duty diesel roll-off refuse trucks. Journal of the Air & Waste Management Association. 2015;65(3):306-323.

6 US EPA-Transportation 2006 - U.S. Environmental Protection Agency (U.S. EPA). Diesel retrofit technology: An analysis of the cost-effectiveness of reducing particulate matter emissions from heavy-duty diesel engines through retrofits. 2006.

7 Borak 2007 - Borak J, Srianni G. Studies of self-pollution in diesel school buses: Methodological issues. Journal of Occupational and Environmental Hygiene. 2007;4(9):660-668.

8 Trenbath 2009 - Trenbath K, Hannigan MP, Milford JB. Evaluation of retrofit crankcase ventilation controls and diesel oxidation catalysts for reducing air pollution in school buses. Atmospheric Environment. 2009;43(37):5916-5922.

9 US EPA-CSBP - U.S. Environmental Protection Agency (U.S. EPA). Clean School Bus Program (CSBP).

10 Beatty 2011 - Beatty TKM, Shimshack JP. School buses, diesel emissions, and respiratory health. Journal of Health Economics. 2011;30(5):987-999.

11 Adar 2015 - Adar SD, D’Souza J, Sheppard L, et al. Adopting clean fuels and technologies on school buses: Pollution and health impacts in children. American Journal of Respiratory and Critical Care Medicine. 2015;191(12):1413–1421.

12 US EPA-Diesel technologies - U.S. Environmental Protection Agency (U.S. EPA). Verified technologies for SmartWay and Clean Diesel.

13 MD DOE-Diesel - Maryland Department of the Environment (MD DOE). Diesel retrofit projects.

14 OR DEQ-Diesel - Oregon Department of Environmental Quality (OR DEQ). Air quality: Clean diesel success stories including Oregon's Clean Diesel Initiative.

15 NJ DOEP-Diesel - New Jersey Department of Environmental Protection (NJ DOEP). Mandatory Diesel Retrofit Program.

16 TCEQ-TERP - Texas Commission on Environmental Quality (TCEQ). Texas Emissions Reduction Plan (TERP) program grants.

17 US DOT-CMAQ - U.S. Department of Transportation (U.S. DOT). Congestion mitigation and air quality improvement (CMAQ) program.

18 US EPA-DERA - U.S. Environmental Protection Agency (U.S. EPA). Diesel Emissions Reduction Act (DERA) Funding: Reducing emissions that impact our health.

19 NACAA-Appropriations - National Association of Clean Air Agencies (NACAA). FY 2021 budget and appropriations information.

20 Demetillo 2021 - Demetillo MAG, Harkins C, McDonald BC, et al. Space-based observational constraints on NO2 air pollution inequality from diesel traffic in major U.S. cities. Geophysical Research Letters. 2021;48(17).

21 Liu 2021 - Liu J, Clark LP, Bechle MJ, et al. Disparities in air pollution exposure in the United States by race/ethnicity and income, 1990-2010. Environmental Health Perspectives. 2021;129(12):1-14.

22 Nguyen 2018 - Nguyen NP, Marshall JD. Impact, efficiency, inequality, and injustice of urban air pollution: Variability by emission location. Environmental Research Letters. 2018;13.

23 Commodore 2021 - Commodore S, Ferguson PL, Neelon B, et al. Reported neighborhood traffic and the odds of asthma/asthma-like symptoms: A cross-sectional analysis of a multi-racial cohort of children. International Journal of Environmental Research and Public Health. 2021;18(1):1-25.

24 McKane 2018 - McKane RG, Satcher LA, Houston SL, Hess DJ. Race, class, and space: An intersectional approach to environmental justice in New York City. Environmental Sociology. 2018;4(1):79-92.

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26 Kaplan 2007 - Kaplan J, Valls A. Housing discrimination as a basis for Black reparations. Public Affairs Quarterly. 2007;21(3):255-273.

27 Lampert 2021 - Lampert T, Costa J, Santos O, et al. Evidence on the contribution of community gardens to promote physical and mental health and well-being of non-institutionalized individuals: A systematic review. PLOS ONE. 2021;16(8):e0255621.

28 NEJAC 2006 - The National Environmental Justice Advisory Council (NEJAC). Unintended impacts of redevelopment and revitalization efforts in five environmental justice communities. 2006.

29 Prochnow 2022 - Prochnow T, Valdez D, Curran LS, et al. Multifaceted scoping review of Black/African American transportation and land use expert recommendations on activity-friendly routes to everyday destinations. Health Promotion Practice. 2022.

30 McAndrews 2022 - McAndrews C, Schneider RJ, Yang Y, et al. Toward a gender-inclusive Complete Streets movement. Journal of Planning Literature. 2022;38(1):3-18.

31 Brookings-Semmelroth 2020 - Semmelroth L. How Wilmington, Del. is revitalizing vacant land to rebuild community trust. Washington, D.C.: Brookings Institution; 2020.

32 Braveman 2022 - Braveman PA, Arkin E, Proctor D, Kauh T, Holm N. Systemic and structural racism: Definitions, examples, health damages, and approaches to dismantling. Health Affairs. 2022;41(2):171-178.

33 KFF-Ndugga 2022 - Ndugga N, Artiga S. Climate change and health equity: Key questions and answers. KFF. May 24, 2022.

34 CAP-Rowland-Shea 2020 - Rowland-Shea J, Doshi S, Edberg S, Fanger R. The nature gap: Confronting racial and economic disparities in the destruction and protection of nature in America. Washington, DC: Center for American Progress (CAP); 2020.

35 TPL-Chapman 2021 - Chapman R, Foderaro L, Hwang L, et al. Parks and an equitable recovery. San Francisco, CA: The Trust for Public Land (TPL); 2021.

36 US EPA-Evolving federal legislation - U.S. Environmental Protection Agency (U.S. EPA). Evolution of the Clean Air Act.

37 US EPA-CAA History - U.S. Environmental Protection Agency (U.S. EPA). Clean Air Act (CAA) requirements and history.

38 US EPA-CAA - U.S. Environmental Protection Agency (U.S. EPA). Summary of the Clean Air Act.

39 NBER-Currie 2021 - Currie J, Voorheis J, Walker R. What caused racial disparities in particulate exposure to fall? New evidence from the Clean Air Act and satellite-based measures of air quality. National Bureau of Economic Research (NBER). 2021: Working Paper 26659.

40 US EPA-Air pollution challenges - U.S. Environmental Protection Agency (U.S. EPA). Air pollution: Current and future challenges.

41 US EPA-DERA Impacts - U.S. Environmental Protection Agency (U.S. EPA). Learn about impacts of diesel exhaust and the Diesel Emissions Reduction Act (DERA).

Date Last Updated