It has generally been assumed that other vehicles on the road are the source of elevated particle levels. But a study of school buses in the Los Angeles area shows that much of the pollution inside a school bus comes from the bus itself, and children on board may be inhaling more diesel particles than previously believed.
Diesel particles are extremely small and can deposit deep in the lungs, whereas larger particles are filtered out by the nose, mouth and throat. A number of studies have linked diesel particles to adverse health effects. For example, a large assessment of air pollution in the Los Angeles area found that diesel particles are responsible for most of the cancer risk from outdoor air pollution.
Children are especially susceptible to air pollution because they have high inhalation rates and large lung surface area per body weight, as well as narrow airways and immature immune systems, the researchers say.
They analyzed results from a UCLA school bus experiment where the researchers took out the seats and essentially turned the buses into mobile chemistry labs, driving them along actual school bus routes in the greater Los Angeles area. Six buses were involved in the study: two older high-emitting diesel buses from 1975 and 1985; two diesel buses that are more representative of current fleets; one diesel bus outfitted with a particle trap; and one bus powered by compressed natural gas.
The researchers released a tracer gas into the engine exhaust and measured concentrations of that gas inside the buses. They then calculated the "intake fraction" — the fraction of the bus's exhaust that is inhaled by students on that bus, assuming an average population of 40 people on each bus.
The levels turned out to be substantial for all six buses, but older buses and close-windowed buses were higher, according to the researchers. The average value for intake fraction across all bus runs was 27 grams inhaled per million grams emitted, with the highest value at around 100 per million.
"This may not sound like a lot," says Julian Marshall, a doctoral candidate in the Energy and Resources Group at UC Berkeley and lead author of the study. "But intake fraction values for vehicle emissions are 5-15 per million in a typical U.S. urban area, and about 50 per million in a large urban area like Los Angeles."
The fact that these values are comparable is "shocking," according to Marshall. "This means that for every ton of pollution emitted by a school bus, the cumulative mass of pollution inhaled by the 40 or so kids on that bus is comparable to — or in many cases larger than — the cumulative mass inhaled by all the other people in an urban area."
The California Air Resources Board, which sponsored the original study by UCLA, recommends minimizing commute times, using the cleanest buses for the longest commutes, accelerating the retirement of older buses, and decreasing bus caravanning and idling time to reduce children's exposure to bus-related air pollutants.
"Based on our work, if a policymaker wants to reduce health effects from diesel for the population as a whole, then school buses are a good source to target," Marshall says.
The American Chemical Society is a nonprofit organization, chartered by the U.S. Congress, with an interdisciplinary membership of more than 158,000 chemists and chemical engineers. It publishes numerous scientific journals and databases, convenes major research conferences and provides educational, science policy and career programs in chemistry. Its main offices are in Washington, D.C., and Columbus, Ohio.
NOTE TO EDITORS: An image depicting differences in "exposure potential" from common diesel emission sources is available upon request.
— Jason Gorss
The online version of the research paper cited above was initially published March 16, 2005 on the journal's Web site. Journalists can arrange access to this site by sending an e-mail to newsroom@acs.org or calling the contact person for this release.
Journal
Environmental Science & Technology