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Diesel School Buses: Health Effects and Opportunities for Change

Exhaust from diesel-powered school buses is a source of serious health risks for the millions of children who ride these buses every day. Alternatives such as natural gas-powered school buses can significantly reduce those health risks.

School Buses in the US

Every school day in the US, our nation’s fleet of nearly 600,000 school buses transport 24 million students to school. The vast majority of these buses (at least 99%) are powered by diesel fuel. Over the past decade, concern has been growing among government, environmental, health, and community leaders about the price we are paying for our reliance on diesel-fueled buses. Emissions from these vehicles are polluting our environment and damaging the health of our children. Recently, the effects of the many toxic chemicals contained in diesel exhaust have also come into clearer focus-about half are known or suspected of being carcinogenic. However, INFORM’s research shows that many promising alternative fuels and vehicle technologies are now available to bus operators nationwide. The challenge is making the shift to these cleaner options as quickly as possible.

Emissions from Diesel Exhaust

Over 133 million Americans live in areas that violate federal clean air standards for one or more of the six “criteria pollutants” deemed by the US Environmental Protection Agency to be harmful to human health. The source of most of this pollution is vehicle exhaust. When fossil fuels such as gasoline and diesel are burned in cars, trucks, and buses, it produces hydrocarbons and nitrogen oxides (NOx), which mix to form ozone, as well as carbon monoxide and other pollutants. These emissions are responsible for most of the pollution blanketing US cities.

Diesel Particulate Matter. One of the most problematic pollutants generated by diesel fuel combustion is particulate matter. Large particulates, known as soot, create visible clouds of black smoke that dirty the air. Fine particulates (less than 2.5 micron) are too small to be seen by the human eye but can be inhaled deeply and evade the human lung’s natural defenses. Fine particulates make up nearly 94% of the particulate matter contained in diesel exhaust. Numerous studies have found a causal relationship between fine particulates and serious health effects, including aggravation of asthma, respiratory problems, and cancer.

Air Toxics. Diesel exhaust also contains dozens of toxic chemicals and chemical compounds that, according to the California Air Resources Board, may cause or contribute to an increase in mortality or serious illness. These include a class of highly persistent and toxic chemicals called polychlorinated dioxins, as well as formaldehyde, acetaldehyde, benzene, and 1,3-butadiene, all known carcinogens.

Health and Environmental Studies Outlining the Health Impacts of Diesel-Fueled School Buses

  1. Failing the Grade: How Diesel Buses Threaten Our Children’s Health. Coalition for Clean Air, 1999
  2. No Breathing in the Aisles, Natural Resources Defense Council, 2001
  3. Children's Exposure to Diesel Exhaust on School Buses, Environment and Human Health, Inc., 2002
  4. Pollution Report Card: Grading America's School Bus Fleets, Union of Concerned Scientists, 2002

Health Effects of Diesel Exhaust

Numerous studies have found a causal relationship between diesel exhaust and serious health problems, including increased mortality among people with cardiopulmonary disease; exacerbation of the symptoms of asthma, bronchitis, and pneumonia; decreased lung function; and retarded lung development. It has also been correlated with increased hospital admissions and emergency room visits for respiratory illness.

More recently, exposure to diesel exhaust has been associated with elevated cancer rates. At New York University and the Harvard School of Public Health, scientists have found a link between fine particulates and elevated lung cancer death rates and premature death. In May 2002, the Journal of the American Medical Association reported that people living in the most heavily polluted areas have a 12% higher risk of dying from lung cancer than people in the least polluted areas. According to the Harvard School of Public Health, up to 11 percent of infant deaths in the US - about 3,000 per year - may be caused by inhaling fine particulates. Diesel exhaust is classified as a “probable” human carcinogen by many governmental authorities, including the International Agency for Research on Cancer, the US National Toxicology Program, and the US Environmental Protection Agency. California classifies diesel exhaust as a “known” carcinogen.

Effects on Children’s Health

Children may be especially susceptible to the health risks associated with exposure to diesel exhaust. Fine particulates can penetrate children’s narrow airways and lodge deep within the lung, where they are more likely to be retained. Children also have higher respiration rates than adults, which can increase their exposure per unit of body weight.

Asthma Facts

  • Of the estimated 17 million asthma sufferers in the US, more than 4.8 million are children. It is estimated that 7% to 10% of US children now have the disease.
  • Asthma is the most common childhood chronic illness and ranks second only to injuries as the leading cause of hospitalizations among children.
  • Between 1980 and 1994, the incidence of asthma increased 74% among US children between the ages of 5 and 14. Every year, over 160,000 children under the age of 15 are hospitalized for the disease.
  • Total costs related to asthma in the US were estimated at $12.7 billion in 2000, with direct costs amounting to $8.1 billion and lost earnings due to illness and death totaling $4.6 billion.

Center for Children's Health and the Environment, "Childhood Asthma," fact sheet, updated June 2002,
American Lung Association, "Asthma Attack! A Campaign to Support the Asthma Research Initiative,"

A major health challenge in this country is pediatric asthma, and diesel exhaust is a key aggravating factor in soaring rates of asthma throughout the US-especially among children in poor and minority urban neighborhoods. Nationwide, an estimated 4.8 million children now suffer from asthma. The cost of care for a child with asthma is, on average, $500 per year. That figure covers the price of medications, physician care, and hospital treatment, but does not account for such costs as school absenteeism, loss of parental work time, psychological effects, and abnormal social development.

Opportunities for Change

INFORM’s research shows that many currently available alternative fuels and technologies can significantly reduce the impacts of diesel exhaust on children’s health. These alternatives are fully commercial, clean, healthy, and safe, making it easy for school bus operators to leave their conventional diesel buses behind.

Cleaner Natural Gas. One alternative, the conventional natural gas bus, is today’s best choice. Natural gas is a high-quality fuel that provides an excellent substitute for gasoline and diesel. Often found in underground reservoirs, it is produced from wells drilled at the surface but unlike oil does not require extensive refining prior to marketing. Nearly 90% of the natural gas consumed in the US is from domestic sources, compared to less than 50% of the oil. Historically, natural gas has been less costly than gasoline and diesel fuel on a per gallon equivalent basis nationwide. In August 2002, the US Department of Energy reported that compressed natural gas was available in the Northeast at about the same cost as diesel.

Regional Fuel Prices, July 2002

Region CNG ($/gge) Diesel ($/gal) B-20 ($/gal)
New England 1.40 1.40 1.58
Central Atlantic 1.04 1.39 1.39
Nationwide 1.31

Source: US Dept. of Energy, “The Alternative Fuel Price Report,” August 8, 2002, http://www.afdc.doe.gov/pdfs/afpr_8_8.pdf

Natural gas buses demonstrate diesel-like performance with a 90% reduction in noise. They are virtually toxic-free and emit significantly fewer pollutants than diesel buses: 40% to 86% less particulate matter and 38% to 58% less NOx. Moreover, production of natural gas avoids the pollution risks associated with the manufacture of diesel, such as crude oil spills, releases of toxic pollutants from refineries, and leaks from underground tanks into groundwater.

The major obstacles to the expanded use of natural gas buses are their higher cost compared to conventional diesel buses and the costs involved in establishing the infrastructure needed for refueling. Training and garage modifications to accommodate methane detection and ventilation systems may also be needed. Although these costs can be significant -- the incremental cost of a natural gas bus is approximately $25,000 to $40,000 more than a conventional diesel bus -- fleets can make a cost-effective transition to natural gas by taking advantage of funding sources for alternative-fuel vehicle programs, such as Congestion Mitigation and Air Quality (CMAQ) grants, the US DOE State Energy Program funds distributed through the national Clean Cities program, and federal tax incentives.

Available Natural Gas Buses

Make Model Features
Bluebird All American RE School bus for 66 or 84 passengers powered by John Deere CNG engine
Bluebird MicroBird Student transport shuttle for 30 passengers on Ford E450 chassis with CNG engine
Omnitrans GM/Isuzu School Bus Chassis School bus with GM CNG engine
Thomas Saf-T Liner School bus with Cummins 5.9L or John Deere CNG engine

Other Fuel Options

Biodiesel and ultra-low-sulfur diesel are receiving attention from school bus operators because of the low capital costs they involve compared to natural gas and other alternative fuels.

Biodiesel. Biodiesel is a cleaner-burning version of diesel fuel made from natural, renewable sources such as vegetable oils rather than petroleum. Biodiesel may be used as a blend fuel (as low as 5% to 20% biodiesel) or as a single neat fuel (100% biodiesel). While biodiesel and synthetic diesel are now being tested in heavy-duty applications, the emissions and toxicity reductions they achieve are still unclear. Preliminary studies indicate that biodiesel may generate slightly fewer hydrocarbons and particulates but up to 14% more nitrogen oxides than 100% petroleum diesel.1 (Because biodiesel contains no sulfur, however, vehicles powered by this fuel can use advanced NOx emission control devices.)

According to US EPA estimates, 100% biodiesel can be purchased for approximately $2.00 to $3.00 per gallon, depending on the feedstock and supplier, while 20% biodiesel costs $.30 to $.40 more per gallon than conventional diesel. Biodiesel blend fuels are increasingly popular because they can be used in conventional engines with few or no modifications.

Ultra-Low-Sulfur Diesel. Because it is made from the further refining of conventional diesel, ultra-low-sulfur diesel is not considered an alternative fuel. However, many bus operators around the country are testing its effectiveness, in combination with the latest generation of emission control technologies, in reducing emissions from their vehicles.

The sulfur contained in diesel fuel has two effects: it contaminates a truck’s emission control system and reacts to form sulfur dioxide during combustion. New EPA regulations require a 97% reduction in the sulfur content of diesel fuel by 2006, which will permit new emission control devices to be used to meet heavy-duty emission standards that take effect in 2007. These technologies are still in the demonstration stage in the US, and at this point it is still unclear how they and the low-sulfur fuel they require will perform with different types of vehicle, including school buses, and what their costs and durability will be.

Recent tests conducted by the New York City transit fleet showed that the use of new technologies such as particulate filters and oxidation catalysts, in conjunction with low-sulfur diesel, can bring significant reductions in particulate emissions from transit buses.2 However, a recent California Air Resources Board study (August 2002) shows that heavy-duty natural gas vehicles continue to be cleaner for all criteria pollutants (except carbon monoxide) than vehicles powered by low-sulfur diesel and equipped with the most advanced after-market treatments, although the emissions of both natural gas and diesel vehicles could be improved with further technological advances. US EPA estimates that its new low-sulfur diesel fuel standards will increase the cost of producing and distributing diesel fuel by 4.5 to 5 cents per gallon when the program is fully implemented. In the Northeast, ultra-low-sulfur diesel fuel has cost approximately 5 cents to 17 cents more per gallon than conventional diesel fuel.

School boards and fleet operators can:

  1. Utilize currently available grants and other economic incentives to cover the incremental costs of new natural gas buses and refueling facilities.
  2. Develop partnerships with fuel suppliers, refueling infrastructure builders, and vehicle providers to help reduce the costs of implementation.
  3. Encourage or require the use of clean fuels in school bus operator contract negotiations and awards.

Federal, state, and local governments can:

  1. Provide economic incentives for vehicle procurement, infrastructure development, and alternative-fuel use that reward investment in clean-fuel vehicles.
  2. Implement stronger regulatory measures to ensure the shift away from diesel buses to cleaner fuels.

Community groups can:

  1. Develop public/private partnerships specifically designed to implement alternative-fuel school bus programs.
  2. Disseminate information about opportunities for expanding the use of clean-fuel school buses.
  3. Raise awareness in their communities about the harmful effects of diesel exhaust on children’s health.


1 W.G. Wang et al., "Emissions from Nine Heavy Trucks Fueled by Diesel and Biodiesel Blend without Engine Modification," Environmental Science and Technology, February 2000, http://pubs.acs.org/hotartcl/est/2000/research/es981329b_rev.html.

2 "Interim Report: Emissions Results from Clean Diesel Demonstration Program with CRT Particulate Filter at New York City Transit," New York State DEC, MTA NYCT, Johnson Matthey, Equilon, Corning, Environment Canada, RAD Energy, February 2001, http://www.epa.gov/otaq/retrofit/documents/ny_crt_presentation.pdf.

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