If you've ever driven behind an old school bus, you've seen the dirty exhaust billowing from the tail pipe. What might surprise you is how much of that pollution never leaves the bus.
School children inside diesel buses inhale at least as much exhaust, and often more, than the rest of a city's residents.
"For every metric 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," Julian Marshall, a Ph.D. student at the University of California, Berkeley's Energy and Resources Group and lead author of the study.
"That the values were even close was shocking," Marshall said.
The study involved putting tracer gases into the engines of empty buses in Los Angeles, then driving the buses around and measuring the tracer gases, which revealed the difference between ambient pollution and pollution coming from a bus itself.
"We determined that concentrations of key air pollutants were higher inside the bus cabins than outside the cabins," said Eduardo Behrentz, a UCLA post-doctoral researcher. "While the conditions inside the cabin were affected by the emissions of other vehicles on the road, our tracer gas measurements revealed that a significant amount of the pollutants found inside the buses originated from the buses' own exhaust systems, especially when the windows were closed."
Older buses were the worst, especially one built in 1975. Ten percent of California's school buses were built before 1977, while 35 states have none that old.
But even some of the newer buses, including one outfitted with a particle trap in its exhaust system, leaked pollution into the cabin.
"The broader message from this study is that there are many exposures to air pollution that are flying below the radar because they are not being picked up by our current air monitoring system," said William Nazaroff, a UC Berkeley professor of civil and environmental engineering who was not part of the study.
The research will be detailed in the April 15 issue of the journal Environmental Science and Technology.
The researchers point out that children are particularly vulnerable to pollution. Compared with adults, their immune systems are less mature and, per body weight, they inhale more air per day.
The researchers say riding school buses is still safer than being driven to school by parents, so while policy makers ought to consider the findings, parents should not yank children from buses.
"School buses are built like a tank, and the chances of children getting killed or seriously injured from a traffic accident in a private passenger vehicle are significantly greater than if they are on a bus," Marshall said.
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