Children's Exposures to Diesel Exhaust on School Buses
A report by Environment and Human Health, Inc.

School Bus Emissions

A pdf of the report is available here.

The Facts: More than 99% of U.S. school buses are powered by diesel fuel. Diesel exhaust is comprised of very fine particles of carbon and a mixture of toxic gases. Most federal agencies have designated diesel exhaust to be a probable human carcinogen. Benzene, an important component of the fuel and exhaust, is designated to be a known human carcinogen. Components of diesel exhaust are genotoxic, mutagenic, and can produce symptoms of allergy, including inflammation and irritation of airways. There is no known safe level of exposure to diesel exhaust for children, especially those with respiratory illness.

The Centers for Disease Control estimates that 4.5 million U.S. children have asthma. This figure includes nearly 44,500 school-aged children in Connecticut diagnosed with the illness. Diesel exhaust can adversely affect children with underlying respiratory illnesses such as asthma, bronchitis, and infections. Diesel emissions may enhance the effects of some allergens among sensitive individuals. Children's airways are not yet fully developed and have a smaller diameter than those of adults. If airways are inflamed or constricted by asthma, allergies or infections, exposure to diesel exhaust may make breathing more difficult.

Fine particulate concentrations (PM2.5) measured on buses in this study were often 5-10 times higher than average levels measured at the 13 fixed-site PM2.5 monitoring stations in Connecticut. Levels of fine particles were often higher under certain circumstances: when buses were idling with windows opened, when buses ran through their routes with windows closed, when buses moved through intense traffic, and especially when buses were queued to load or unload students while idling.

The Conclusions: This study concludes that the laws intended to control air pollution in the U.S. and Connecticut must be strengthened to protect the health of children in several important respects. First, fixed monitoring facilities do not capture the variability in air pollution experienced by children. Second, air quality indoors and within vehicles is not regulated by EPA or the State of Connecticut, while Americans spend on average between 80-90% of their time indoors.

Third, tougher diesel regulations adopted by EPA last year are insufficient to protect health. Under the new provisions, they will be phased in between 2006-2010. This delay means that children may be exposed to increasing levels of diesel exhaust for nearly a decade, as truck and bus traffic are likely to continue their steady rate of increase. Fourth, ozone levels in Connecticut are already beyond federal limits, and if current trends continue, some urban areas are likely to be near or beyond the newly adopted PM2.5 limit. Given the limited monitoring facilities and extended averaging periods allowed by current law, state "compliance" with federal standards offers little assurance of sufficient health protection. Fifth, routine emissions testing for school buses is not required by federal law, and school buses are specifically exempted from testing in Connecticut. Sixth, Connecticut adopted idling regulations, limiting idling time to 3 minutes, however, few know of the restriction, and it is neither monitored nor enforced.

Actions to Reduce Children's Exposures Immediately: It is possible to reduce children's exposure to diesel emissions immediately. We suggest prohibition of bus idling, especially while loading and unloading students. Exposures could also be reduced by limiting the amount of time students spend on buses. The dirtiest buses should be identified by testing emissions and air quality within passenger compartments. The cleanest buses could then be assigned to the longest routes.

These interventions would provide some relief, but additional steps are needed to protect the respiratory health of children, and provide the "adequate margin of safety" required by the Clean Air Act. The current fleet of diesel-powered buses should soon be retrofitted with interior air filters, particle traps, catalytic converters, capable of trapping ultra fine particles and be powered by ultra-low sulfur fuels.

These strategies, if adopted together, would substantially reduce pollution levels in the air students breathe on their daily journeys to and from schools.

The Sierra Club of Minnesota recently released a brochure concerning harmful school bus diesel emissions. You can download the brochure as a PDF file at