Cleaner and Alternative Fuel Bus Fleets

Diesel emissions can adversely affect health by contributing to poor air quality.  School buses and public transit buses that use diesel fuel are major producers of air pollution.^[1] Transitioning bus fleets to alternative fuels (such as, but not limited to, battery-electric),^[2] or to clean diesel technology, can reduce these emissions.^[3]  Learn more about how implementing cleaner bus fleets can reduce air pollution and improve health in communities.^[2,4]

What are clean diesel bus fleets?

School buses and public transit buses are major producers of air pollution.^[1] Transitioning bus fleets to alternative fuels (such as, but not limited to, battery-electric),^[2] or to clean diesel technology, can reduce these emissions.^[3]  Diesel bus emissions can be reduced by retrofitting or modifying older diesel engines to run more cleanly.^[3] Retrofitting can include installing emission control devices^[5] that help to reduce emissions^[4] or remove diesel soot (particulate matter) from the exhaust.^[6-8] The primary purpose of transitioning bus fleets to cleaner or alternative fuels is to reduce the amount of air pollution produced by bus fleets.^[3,9]

Transitioning bus fleets to produce less air pollution can be implemented at the state or local level through federal grant programs, voluntary incentives, and mandatory regulations.^[9] For example, the Texas Commission on Environmental Quality administers the Texas Emissions Reduction Plan (TERP), which provides financial incentives to local governments, businesses, and individuals to reduce emissions.^[10] New Jersey requires owners of regulated vehicles, including school and commercial buses, to comply with a mandatory program enacted in 2005 under the Diesel Retrofit Law.^[11] Additional federal funding for the transition to clean diesel technology is available in the form of grants and rebates from the U.S. Environmental Protection Agency (EPA).^[12] The EPA Clean School Bus Program is one EPA program that is providing $5 billion of funding through fiscal year 2026 to replace fleets with low and no emissions models.^[2]  The EPA Clean School Bus Program funds replacements that are battery-electric or are fueled by compressed natural gas, or propane.^[13]  The Federal Government’s Low or No Emission Vehicle Program provides funds for cities and states to purchase or lease zero emission and low emission transit buses.^[14] For example, the City and Borough of Juneau, Alaska, received funds to replace aging diesel buses with new electric buses; and the City of Tucson, Arizona, invested in electric buses and infrastructure to support the charging of buses.^[15] Sixteen states and the District of Columbia signed a Memorandum of Understanding in July 2020 to support sales of electric medium- and heavy-duty vehicles, which can include school buses and transit buses, with the goal of 100% of these vehicles being zero emission by 2050.^[16]

CDC’s EXHALE package includes evidence-based strategies to improve asthma control and reduce healthcare costs, including policy information about clean diesel bus fleets.

What is the public health issue?

According to the EPA, exposure to diesel pollution from transportation contributes to 3,700 heart attacks, 8,800 deaths, and $100 billion in health damages each year.^[17] Newer diesel engines operate more cleanly than in the past, but many older diesel engines that emit far more air pollution are still on the road.^[12] Diesel emissions contain numerous pollutants, including soot, nitrogen oxides, and carbon monoxide, that adversely affect cardiovascular and respiratory health.^[12] Evidence also shows that diesel exhaust causes cancer in humans, and it was classified as a Group 1 carcinogen by the International Agency for Research on Cancer in June 2012.^[18,19] Diesel soot from school buses has also been associated with reduced lung function and increased incidences of pneumonia in children. Exposure to diesel emissions can be especially harmful for children with asthma.^[1,4]

What is the evidence of health impact and cost effectiveness?

Alternatives to diesel engines include battery-electric and compressed natural gas fleets. Retrofitting existing buses or replacing buses with those that use cleaner or alternative fuels reduces air pollution, including soot. Retrofitting diesel buses can reduce diesel emissions of particulate matter by up to 90 percent^[3] whereas battery-electric buses have zero tailpipe emissions.^[20-22] Evidence shows that reducing people’s exposure to soot can decrease the risk of mortality, heart attacks, and hospitalizations for heart disease and cancer.^[23-26]  Using zero emission buses can further decrease exposure to air pollutants.^[27] For example, a single zero emission bus can eliminate 1,690 tons of carbon dioxide over its 12-year lifespan, the equivalent of taking 27 cars off the road.^[28]

Clean diesel school bus fleets have been shown to reduce the amount of air pollution that school children are exposed to, improve lung function, and reduce hospitalizations due to pneumonia.^[1,4,29] One study in the Puget Sound region of Washington showed that school districts that installed any type of clean diesel retrofit technology in their school buses experienced a 23 percent decrease in the number of pediatric bronchitis and asthma hospitalizations and a 37 percent decrease pediatric pneumonia hospitalizations per month. Districts that specifically adopted crankcase ventilation filter installations experienced a 33 percent drop in pediatric bronchitis and asthma hospitalizations.^[1]

According to the EPA, each federal dollar invested in clean diesel projects has led to $11 to $30 in savings from public health benefits, and more than $2 in savings on fuel.^[30]  Once implemented, zero emission buses emit no harmful-to-health tailpipe emissions, which can lead to improved air quality.^[22]

Evidence of economic impact of switching to a zero emissions bus fleet, compared to diesel engines, is still emerging. For electric buses, the initial cost of the bus can be higher than for diesel buses, but fuel and maintenance costs can be lower.^[31] Once implemented, electric buses can potentially result in cost savings when calculations include purchase price and operating costs such as fuel and maintenance over a span of 12 years.^[31] When considering the financial implications of implementing a zero emissions bus fleet, there are many contextual factors to consider. Some factors could include: technology type, route characteristics, and transit agency resources available.^[20,31]