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It's a common occurrence to see and smell a black cloud of smoke rising from behind a diesel school bus. We expect that inhaling these fumes outside the bus would be dangerous for our health -- and it is. But does that same diesel exhaust pose a risk to children sitting inside the bus on their way to and from school? We initiated this study of diesel exhaust levels inside school buses to answer this question in light of the overwhelming evidence that diesel exhaust causes cancer and premature death and exacerbates asthma and other respiratory illnesses. In fact, government regulators estimate, based on lifetime risks, that diesel exhaust is responsible for a surprising 125,000 cancers nationwide.[1] Studies in California reveal that more than 70 percent of the risk of cancer from air pollution comes from diesel exhaust alone.[2]

We designed and performed this study to measure the level of diesel exhaust to which children are typically being exposed as they ride on buses to and from school each day, and to determine whether years of such exposure poses a health risk to a young child. The results were startling: A child riding inside of a diesel school bus may be exposed to as much as 4 times the level of toxic diesel exhaust as someone standing or riding beside it. Under federal law, these exposures translate into a significant risk of cancer to children. In fact, these exposures pose from 23 to 46 times the cancer risk level considered significant under federal law. What's more, these troubling results suggest that diesel exhaust on school buses could contribute to respiratory problems among sensitive children, such as asthmatics. Importantly, most of the buses we tested did not emit a significant amount of visible black smoke, as one would usually expect from a dirty diesel bus. The message is clear: Even a "smokeless" diesel school bus may be exposing children to potentially dangerous levels of diesel exhaust.

The harmful health effects of diesel exhaust have been studied and well documented for decades. In recent years, an increasing number of health authorities have recognized the cancer-causing effects of diesel exhaust, including the U.S. Environmental Protection Agency (EPA) and the state of California. [3 ] Aside from its cancer-causing properties, diesel exhaust is also known to be a major source of fine particles, which can lodge deep in the lungs and exacerbate asthma, a condition most prevalent among children.[4 ] In addition, smog-forming oxides of nitrogen, or "NOx," which are also emitted from diesel engines in mass quantities, have recently been linked to decreased lung function growth in children.[5 ] Indeed, children are generally more susceptible than adults to the negative health effects of air pollution. Among other reasons, a child's organs are still developing and are far less capable of defending the body from airborne toxics and pollutants.

Unfortunately, the vast majority of the nation's school bus fleets run on diesel fuel. Moreover, many of the these fleets contain a significant percentage of buses that are 15 years of age or older and that are much more polluting than even the diesel buses manufactured today. In fact, some fleets contain buses manufactured prior to 1977, before federal highway safety standards were even adopted. Ironically, this means that our children -- among the most vulnerable members of our society -- are riding on some of the highest polluting vehicles on the road today.

There is, however, some good news. Cleaner alternatives to diesel buses, such as those that run on natural gas and propane, are widely available and are being used by an increasing number of school districts across the country. In addition, federal, state and local governments have set aside funds earmarked exclusively to help public and private school fleet operators cover the incremental costs of purchasing these cleaner alternatives. These funding sources are still relatively limited, however, and parents, educators, and school administrators across the country need to pressure their elected officials to make replacement of old, dirty diesel school buses a top budgetary priority.

There are also interim solutions to help clean up existing diesel school buses prior to when they can be replaced. Initial testing shows that diesel aftertreatment technologies, such as particulate traps, can be a cost-effective means of reducing emissions from existing diesel vehicles like school buses. Because they have not yet been fully proven to reduce the risks posed to children, in our view, they should not be considered the ultimate solution. They can, however, be a valuable short-term measure. One current barrier to the widespread retrofit of school buses with particle traps is that the traps require the use of low-sulfur diesel fuel (and work best when the fuel has a sulfur content of 15 part per million or less), which is available so far in limited supplies only in Southern California, the San Francisco Bay Area of California, Houston, and New York City. Low-sulfur diesel fuel will be available more widely in the future in order to comply with the rule EPA adopted in December 2000 mandating the sale of lower-sulfur diesel fuel nationally in 2006. This rule is essential to an effective retrofit strategy and must not be weakened or repealed by EPA in the future.

Finally, there are immediate measures available to school districts and bus drivers which may be appropriate in individual situations. Our study reveals that diesel exhaust levels are highest in the rear of the bus and when the windows of the bus are closed. To the extent practicable, therefore, keeping the windows on the bus open and seating children in the front of the bus before filling the rear seats are additional options available to reduce exposures to children riding on diesel school buses.

This report is intended to inform parents, educators, school administrators, and federal, state and local policy makers of the hazards children face from exposure to diesel exhaust inside school buses and the cleaner alternatives which are readily available. Our major recommendations to address the risks posed by diesel school buses are:

  1. School districts should immediately modify their purchasing practices to replace aging diesel school buses with cleaner alternative fuel school buses such as natural gas;

  2. Federal, state, and local agencies and legislative bodies should make additional funding available for the purchase of cleaner alternative fuel school buses;

  3. Local air quality management districts should adopt rules similar to the rule scheduled for adoption by the South Coast Air Quality Management District (SCAQMD) in Southern California in March 2001, that require local school districts to purchase only alternative fuel school buses.

  4. Where low-sulfur diesel fuel is available, school districts should retrofit existing diesel school buses not scheduled for short-term retirement with particulate traps to reduce exposures. We also urge EPA to require that low-sulfur diesel fuel be made available in advance of 2006 so that school buses -- and other heavy-duty vehicles -- can be retrofitted with traps to reduce their hazardous emissions. Alternatively, we urge the California Air Resources Board and other states, where possible, to require the sale of low-sulfur diesel fuel in their states before 2006.

  5. School officials should ensure that bus drivers, to the extent feasible, keep windows open on school buses and seat students toward the front of the bus before filling the rear seats.

  6. If a child has asthma or another respiratory illness, a parent may wish to check whether the child's breathing symptoms worsen after riding on a diesel school bus and, if so, consult with the child's physician.

We urge parents and educators troubled by the findings in this report to contact their school board officials, elected officials, and federal, state, and local air quality regulators to take these actions. We have included sample letters in Appendix G of this report.


  • This monitoring study was designed to measure the level of diesel exhaust to which children are typically being exposed as they ride on buses to and from school each day, and to determine whether years of such exposure poses a health risk to a young child.

  • A child riding inside of a diesel school bus may be exposed to as much as 4 times the level of toxic diesel exhaust as someone riding in a car immediately in front of that same bus.

  • Diesel exhaust levels are higher in the back of the bus as compared to the front of the bus and are highest when the windows on the bus are closed.

  • Diesel exhaust levels inside the bus increase while driving uphill and sometimes while driving downhill.

  • We estimate that for every one million children riding the school bus for 1 or 2 hours each day during the school year, 23 to 46 children may eventually develop cancer from the excess diesel exhaust they inhale on their way to and from school. This means a child riding a school bus is being exposed to as much as 46 times the cancer risk considered "significant" by EPA and under federal law.

  • The National Institute for Occupational Safety and Health, the International Agency for Research on Cancer, United States Environmental Protection Agency, and the National Toxicology Program have all consistently agreed that there is a relationship between diesel exhaust exposure and lung cancer.[6]

  • The state of California listed diesel exhaust as a known carcinogen in 1990, and in 1998 the California Air Resources Board (CARB) listed diesel particulate as a Toxic Air Contaminant (TAC), also based on its carcinogenicity.[7]

  • Over 40 individual chemical compounds in diesel exhaust have separately been listed as TACs.[8 ] These chemicals are also identified by the EPA as compounds that cause cancer.

  • According to California studies, approximately 70 percent of the cancer risk from air pollution in the state comes from diesel-particle pollution.[9]

  • Two national associations of regulators have estimated, based on lifetime risk, that diesel exhaust is responsible for 125,000 cancer cases in the United States.[10]

  • According to a recent study published by the Health Effects Institute, more than 98 percent of the particles emitted from diesel engines are fine particles, less than 1 micron in diameter.[11] Numerous studies have found that fine particles impair lung function, aggravate respiratory illnesses such as bronchitis and emphysema, and are associated with premature deaths.[12]

  • Children are among those most susceptible to the health effects of diesel exhaust exposure as a result of the child's developing body and lungs, narrower airways, faster metabolism, and faster breathing rate than adults.[13]

  • A recent study by the USC Keck School of Medicine linked both NOx and PM pollutants to a potentially significant decrease in lung function growth among children living in Southern California.[14]

  • While children make up only 25 percent of the population, they represent 40 percent of all asthma cases.[15 ] Research indicates that diesel exhaust may increase the frequency and severity of asthma attacks and may lead to inflammation of the airways that can cause or worsen asthma.[16]

  • Cleaner alternatives to diesel, such as natural gas and propane, are already widely available for school bus applications. In addition, more advanced technologies, including hybrid-electric buses that run on natural gas instead of diesel, battery-electric buses, and fuel cells, will be available in the future.

  • A recent study of commercial buses in Boulder, Colorado demonstrated a 97 percent reduction in PM and a 58 percent reduction in NOx when the same buses were run using compressed natural gas (CNG) instead of diesel.[17]

  • Although a CNG school bus costs more than a diesel school bus (roughly $30-$40,000 more), operational and maintenance costs tend to be lower than those for a similar fleet operating on diesel, enabling fleets to recoup their initial investment.[18]

  • Approximately 130 school districts and other school bus fleet operators throughout the country currently operate a total of over 2,600 natural gas and propane school buses in their combined fleets, which have resulted in lower emissions and noise.[19]

  • Current government estimates show that new diesel school buses emit 51 times more air toxics than a new natural gas school bus.[20]

  • Even the cleanest diesel engine certified for school bus applications in the 2001 model year will emit about 2 times more NOx and 3.5 times more PM than the equivalent year 2001 natural gas engine.[21]

  • International Trucking Company's proposed "green" diesel engine, once certified, will emit 1.6 to 1.7 times more NOx than its natural gas counterparts and has not been tested to determine whether the new engine design has sufficiently reduced the toxicity of the exhaust.[22]

  • To reduce diesel emissions from school buses in the short-term, school districts can retrofit their existing diesel school buses with particulate traps at an estimated cost of $6,000 per bus if they can purchase low-sulfur diesel fuel, which is necessary for the traps to function properly.[23]

  • A growing number of states have created funds earmarked for cleaner school bus purchases, including California; Governor Davis set aside $50 million in the state's 2000-2001 budget for the purchase of "lower-emitting" school buses.

  • The South Coast Air Quality Management District in Southern California has proposed its sixth in a series of rules requiring public fleets to purchase only alternative fuel vehicles; this rule would require the purchase of only alternative fuel school buses.[24]

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1. State and Territorial Air Pollution Program Administrators and the Association of Local Air Pollution Control Officials, "Cancer Risk from Diesel Particulate: National and Metropolitan Area Estimates for the United States," March 15, 2000.

2. South Coast Air Quality Management District, "Multiple Air Toxics Exposure Study in the South Coast Air Basin (MATES-II)," March 2000, p. ES-2. California Air Resources Board, "Risk Reduction Plan to Reduce Particulate Matter Emissions From Diesel-Fueled Engines and Vehicles," September 2000, p. 15.

3. Cal EPA, "Chemical Known to the State to Cause Cancer or Reproductive Toxicity," Revised May 1, 1997; Discussed in U.S. EPA Office of Research and Development, Health Assessment Document for Diesel Emissions, Review Draft, EPA/600/8-90/057C, February 1998. Chapter 5.

4. Cackette, Tom, "Importance of Reducing Emissions From Heavy-Duty Vehicles," California Air Resources Board, October 5, 1999, p. 15.

5. Gauderman, W. James, et al., "Association between Air Pollution and Lung Function Growth in Southern California Children," May 2, 2000.

6. Dawson, et al., "Proposed Identification of Diesel Exhaust as a Toxic Air Contaminant, Part B: Health Risk Assessment for Diesel Exhaust." Public and Scientific Review Panel Review Draft, February 1998, pp. 1-8, 1-9

7. State of California listing under the Safe Drinking Water and Toxic Enforcement Act of 1986 (Proposition 65); CARB, Listing of Toxic Air Contaminants.

8. Natural Resources Defense Council, "Exhausted by Diesel: How America's Dependence on Diesel Engines Threatens Our Health," 1998, p. 6.

9. South Coast Air Quality Management District, "Multiple Air Toxics Exposure Study in the South Coast Air Basin (MATES-II)," March 2000, p. ES-2. California Air Resources Board, "Risk Reduction Plan to Reduce Particulate Matter Emissions From Diesel-Fueled Engines and Vehicles," draft (July 13, 2000), p. 15.

10. State and Territorial Air Pollution Program Administrators and the Association of Local Air Pollution Control Officials, "Cancer Risk from Diesel Particulate: National and Metropolitan Area Estimates for the United States," March 15, 2000.

11. Bagley, Susan T., et al., Health Effects Institute, "Characterization of Fuel and Aftertreatment Device Effects of Diesel Emissions," Research Report,76, September 1996.

12. Dockery, D.W., et al., "An Association Between Air Pollution and Mortality in Six U.S. Cities," New England Journal of Medicine, 329: 1753-59, 1993. Pope III, C.A., et al., "Particulate Air Pollution as a Predictor of Mortality in a Prospective Study of U.S. Adults," Am. J. Resp. Crit. Care Med., 151: 669 -674, 1995; Shprentz, D., "Breathtaking: Premature Mortality Due to Particulate Air Pollution in 239 American Cities," New York, NRDC, May 1996, pp. 13-32.

13. Lipsett, "The Hazards of Air Pollution to Children," in S. Brooks, et al. (eds), Environmental Medicine, St. Louis: Mosby, 1995; Wiley, J., J. Robinson, T. Piazza, L. Stork and K. Pladsen, "Final Report Study of Children's Activity Pattern," California State Senate, 1993; California State Senate (Ref. 24,25,26), "SB 25 Environmental Health Protection: Children," December 7, 1998, Amended 3/3/99; Natural Resources Defense Council, "Our Children at Risk: The 5 Worst Environmental Threats to Their Health," NRDC: New York, 1997.

14. Gauderman, W. James, et al., "Association between Air Pollution and Lung Function Growth in Southern California Children," May 2, 2000.

15. U.S. Environmental Protection Agency, Office of Air and Radiation, Office of Planning and Standards Fact Sheet, "Health and Environmental Effects of Particulate Matter," July 17, 1997; Kreutzer, Richard, M. Lipsett, J. Von Behren and E. Yamada., "Asthma in California: Laying the Foundation for Statewide Strategy," California Policy Seminar. Brief series, August 1998.

16. Miyamoto, T., "Epidemiology of Pollution-Induced Airway Disease in Japan," Allergy, 52: 30-34, 1997; Albright, J.F. and R.A. Goldstein, "Airborne Pollutants and the Immune System," Otolaryngol Head Neck Surg, 114(2): 232-8, 1996; Sagai, M., A. Furuyama and T. Ichinose, "Biological Effects of Diesel Exhaust Particles (DEP): Pathogenesis of Asthma-Like Symptoms in Mice," Free Radic. Biol. Med., 21(2):199- 209, 1996.

17. Hiura, T.S., et al., "Chemicals in Diesel Exhaust Particles Generate Reactive Oxygen Radicals and Induce Apoptosis in Macrophages," J. Immunol, 163(10): 5582-91, 1999; O. Fahy, et al., "Effects of Diesel Organic Extracts on Chemokine Production by Peripheral Blood Mononuclear Cells," J. Allergy Clin. Immunol., 103(6):1, 1999.

18. SunLine Transit Agency, "Three Year Comparison of Natural Gas and Diesel Transit Buses," 1999.

19. See Chapter 4 of this report .

20. South Coast Air Quality Management District, "Planning Rule Development and Area Sources," April 28, 1999.

21. California Air Resources Board, "2000 Model Year Heavy-Duty On-Road Engine Certification Listing Update," El Monte, California. p. 1-2.

22. See Chapter 5 of this report.

23. See Chapter 6 of this report.

24. See Chapter 8 of this report.

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