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) 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). Diesel exhaust is carcinogenic, and contains numerous pollutants that negatively affect cardiovascular and respiratory health; estimates suggest exposure contributes to 27,000 heart attacks, 14,500 hospitalizations, and 2.4 million lost work days each year1.

Expected Beneficial Outcomes (Rated)

  • Reduced emissions

  • Improved air quality

Other Potential Beneficial Outcomes

  • Improved health outcomes

Evidence of Effectiveness

There is strong evidence that adopting clean diesel technology through diesel transition programs decreases environmental exhaust emissions2, 3, 4, 5. 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, 6, 7.

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 buses8, as well as improvements in lung function and reductions in absenteeism, especially among children with asthma9. Retrofitting may also decrease the costs associated with respiratory disease-related hospitalizations and treatment8.

Costs for diesel retrofit devices vary based on the manufacturer, labor requirements, and type of technology. For example, a diesel particulate filter typically costs $5,000-$15,000, but can cost up to $50,000 for some large equipment. A diesel oxidation catalyst typically costs $600-$2,000 or more10. A Washington-based cost benefit analysis indicates that school bus retrofitting has positive net benefits8. The US EPA estimates that every federal dollar invested in clean diesel retrofits or replacements generates between $5 and $21 in savings from improvements in public health1.

Vehicle replacement 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 emissions11.

Impact on Disparities

No impact on disparities likely

Implementation Examples

Many states have mandatory or voluntary programs to support diesel retrofit efforts, as in California, Illinois, New Jersey, Oregon, and Texas12, 13, 14. Chicago and New York City are examples of municipalities that provide incentives and support to retrofit or replace diesel vehicles14, 15.

The US Department of Transportation Federal Highway Administration’s Congestion Mitigation and Air Quality Improvement (CMAQ) Program funds efforts to improve air quality, including diesel engine retrofits16. The US Environmental Protection Agency (US EPA) manages funds appropriated for the Diesel Emission Reduction Act (DERA) to retrofit older diesel vehicles14, 17. The proposed 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 buses18.

Implementation Resources

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

US EPA-Clean school bus program - US Environmental Protection Agency (US EPA). Clean school bus program.

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

Diesel Forum-Retrofit - Diesel Technology Forum. Retrofit: Why retrofit?


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

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

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):6475-82.

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):968-76.

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 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):660-8.

7 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):5916-22.

8 Beatty 2011 - Beatty TKM, Shimshack JP. School buses, diesel emissions, and respiratory health. Journal of Health Economics. 2011;30(5):987-99.

9 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.

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

11 US EPA-Clean school bus program - US Environmental Protection Agency (US EPA). Clean school bus program.

12 IL Green fleets - Illinois Green Fleets. Green environment, green energy, and green economics for a green Illinois.

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

14 Diesel Forum-Retrofit - Diesel Technology Forum. Retrofit: Why retrofit?

15 CDOT-DCC - Chicago Department of Transportation (CDOT). Drive Clean Chicago (DCC) program and the Drive Clean Truck-Voucher program.

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

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

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

Date Last Updated