Bad grades for school buses. (Children's Health).

When parents put their children on a school bus, their chief concerns may be the safety of the driving or the traffic on the streets. A recent study suggests, however, that another real threat to children's health may come from the air inside the bus.

The study, conducted by researchers from the Natural Resources Defense Council (NRDC), the University of California at Berkeley School of Public Health, and the California-based advocacy group Coalition for Clean Air, suggests that riding diesel-fueled school buses may increase children's risk for cancer and aggravate respiratory problems. Most of the 442,000 school buses carrying some 23 million children today use diesel fuel.

The researchers rode four empty school buses for a total of 20 hours along actual elementary school bus routes in Los Angeles. Using equipment that continuously sampled the air in the buses, they found interior exhaust concentrations that were more than eight times the average concentrations found in the ambient air in California. The average concentrations inside the buses also were as much as four times higher than those inside cars driven by other team members traveling in front of each bus. "In many of these buses, the engine is in front, and the tail pipe is in the back, and the exhaust runs underneath the entire inside of the bus," says Gina Solomon, a senior scientist at the NRDC. "If there are small cracks or holes in the exhaust system, diesel exhaust may come right in from underneath the bus."

The researchers estimate that for a child riding a school bus one hour each day, the cancer risk would be roughly 23-46 cases per million children, says Solomon. The U.S. Environmental Protection Agency considers a risk of 1 per million to be significant.

"We wanted to give a sense of what the risk may be compared to other risks," Solomon says. "These numbers are close to the magnitude of risk associated with secondhand cigarette smoke. We think that in some cases school buses may be a significant risk to kids. And it's an avoidable risk because there are ways to clean buses up."

Stephen Rappaport, a professor of environmental sciences and engineering at the University of North Carolina at Chapel Hill, says the study is worth attention but should not cause undue alarm. "It's a preliminary study based on a small number of observations," he says. "But there's enough concern there to motivate a larger study to find out what the exposures really are."

Solomon also points out limitations of the small pilot study. "We expect that there are a lot of buses out there that are probably cleaner than the ones we looked at, and I'll bet there are a lot of buses out there that are dirtier too," she says. The buses used in the study were manufactured in the 1980s. Buses made before 1993 are fairly common across the country and cannot be retrofitted with optional particle traps that help them run more cleanly.

Though the researchers do not suggest that parents pull their children off buses, they do recommend that children ride as close to the front of the bus as they can, with the windows open when possible. In the long term, the NRDC recommends that parents urge schools to switch as soon as possible to buses that use alternative fuels such as natural gas so that children are not exposed to excessive diesel exhaust for years. School buses are good candidates for using natural gas because, unlike heavy-duty trucks, they are usually fueled each day at a central location. Schools can also reduce emissions greatly by regularly inspecting and repairing buses.