School buses release particulate matter (soot), toxic air contaminants, and smog-forming pollu-tion from the tailpipe and leaky crankcases. While all of today’s school buses pollute, conventional diesel buses—particularly older models—release anywhere from 10 to more than 100 times as much soot as cleaner alternatives available today. Fine soot particles can evade the body’s normal defense mechanisms and lodge deep within the lungs. These particles have been shown to cause or exacerbate serious respiratory and cardiovascu-lar illnesses, even leading to premature death in adults. Diesel exhaust can also contain more than 40 toxic air contaminants, including many known or suspected cancer-causing substances.

Along with increased cancer risk, these toxic air contam-inants are linked with immune system disorders and reproductive problems. And because particu-late matter and toxic air contaminants can remain in the general vicinity of the emission source, children in or near high-emitting school buses are exposed to more of these pollutants.Children may be more vulnerable than adults to the harmful effects of air pollution. They breathe more rapidly, taking in more air (and pollution) per unit of body weight, and their developing bodies do not have the full range of defense mech-anisms that can protect against harmful exposures. Our polluted air has unfortunately provided researchers with ample evidence that children’s health is harmed by exposure to air pollution; recent studies have linked current levels of air pollution with deficits in lung growth, asthma exacerbations and hospitalizations, and even the possible development of asthma in healthy children.


Across the country, the pollution performance of state school buses varies widely depending on fleet age, fuel choice, and investments in retrofits and cleaner fuels. This report analyzes the amount of pollution released from the average state school bus. Each state received a letter grade (A B, C, or D) for estimated tailpipe emissions of soot, which warrants the most concern because of its potential to cause toxic “hot spots”—areas of higher expo-sure for children in or near buses. The emission performance of a diesel bus equipped with a diesel particulate filter (DPF, or “soot trap”) established the baseline for our highest grade (A), which no states came close to achieving. We distributed the remaining grades on a curve.

We also evaluated state performance in two secondary categories: school bus cleanup programs and tailpipe emissions of smog-forming pollution. In comparing cleanup programs, we calculated the percent of school bus soot reduced through pollution control retrofits and use of cleaner fuels such as natural gas and biodiesel, and assigned each state a rank of Good, Above Average, Aver-age, or Poor. States that failed to conduct any cleanup activities received a score of Incomplete. We also calculated smog-forming tailpipe emis-sions from the average state school bus and used a curve to assign each state a rank of Above Average, Average, or Poor. See Table 1 (p. 6) for state scores in each category.

potential annual reduction

Our key findings are:

  • School buses are some of the oldest vehicles on the road.The average school bus is nine years old and emits nearly twice as much pollution per mile as a tractor-trailer truck (or “big rig”).1 Thirty-seven percent of U.S. school buses are more than a decade old, and 1 in 12 do not have to meet any soot pollution standards.
  • Pollution performance varies widely across the country.The average school buses in the states with the dirtiest fleets, South Carolina and South Dakota, emit nearly three times more soot than the average bus in Delaware, which has the cleanest fleet. Only Alaska, Connecticut, Maine, Nevada, and New York scored above the national average in all three categories we evaluated.
  • Clean school bus programs have made significant strides. Nationally, soot pollution from school buses has been reduced more than two percent through local, state, and federal actions. Most of these cleanup actions have occurred in the last three years. California and Washington State lead the country in cleanup programs, with school bus soot reduced more than seven percent through retrofits and cleaner fuels. Thirteen other states scored above the national average, with active cleanup programs reducing school bus soot between 2.5 and 7 percent.
  • Many states are ignoring the problem of school bus pollution.Nine states and the District of Columbia did not appear to have taken any action to clean up school buses in 2005. Thirteen states have small programs achieving less than a one percent reduction in school bus soot.
  • All states need to increase investments in cleaner buses.The average bus in the cleanest state fleet emitted 20 percent more soot per mile than the average big rig, and emissions could be reduced by a factor of 10 using technologies and fuels available today. Even the states receiving our highest marks for school bus cleanup programs continue to have high-emitting buses, with Washington receiv-ing a D and California a C for soot pollution.
Leave a Reply

Leave a Reply

Your email address will not be published. Required fields are marked *