Bus Exhaust Pollution Higher Inside Than Out

Within the last few decades, the impact of vehicle pollution on the environment has gradually garnered more attention in the public eye. However as researchers at UC Berkeley, UCLA and UC Riverside have discovered, within children's school buses, the real damage is done from within, as the exhaust leaks back into the vehicle and pollutes the air the school children are breathing.

In the first study of its kind to measure the amount of exhaust breathed in from school buses, researchers primarily focused on self-pollution, the exhaust from the vehicle that seeps back into its cabin.

"What we were looking to estimate is what fraction of the emissions from a bus are inhaled by the students of that same bus," said Julian Marshall, a graduate student from the Energy and Resources Group at UC Berkeley and lead author a recent study.

The researchers found that for every million grams of exhaust emitted from a bus, students inhale 30 grams. Though that may not sound like much, the amount inhaled by students on the bus is comparable to or greater than the total amount of school bus exhaust inhaled by everyone else in the surrounding urban areas. Since children are more vulnerable to exhaust pollution due to less-developed immune systems, this find was particularly shocking, said Marshall.

Researchers used a tracer gas experiment to measure school bus self-pollution. Sulfur hexafluoride, a gas not normally found in the air, was released at a known rate into the engine exhaust manifold, the area near the end of the tailpipe. The school buses were then driven on actual school bus routes and the amount of sulfur hexafluoride gas present inside the bus was measured. Knowing the rate the gas was being released and knowing the concentration of the gas inside the bus gave researchers information about the extent to which exhaust from school bus gets inside the bus.

"The biggest significance of these results is that it sends a message to regulators of air pollution-reducing emissions from school buses offers more health benefit than reducing other diesel sources on average, so it should be a higher priority," Marshall said. "You don't have to look too hard to see the cracks and leaks in the shells of some buses; maybe it shouldn't be so surprising that there is so much self-pollution."

School buses, which rely mainly on diesel to operate, are a source of diesel particle pollution, which is a significant contributor to outdoor pollution. Health effects connected to inhaling diesel particles include bronchitis, asthma attacks and increased lung cancer risk. The California Air Resources Board, a division of the California Environmental Protection Agency, estimates that diesel particles cause a majority of the outdoor air pollution lung cancer risk. However it also looked at traffic safety in terms of numbers of vehicular accidents, and found that while student's exposure to traffic emissions while riding on a school bus is important, school buses are actually quite safe in terms of traffic safety.

"The policy recommendation is that, considering traffic safety, children are still relatively safer riding the bus. We shouldn't just abandon school buses. Buses can and should be cleaned up, but there are other issues out there besides the pollution," Marshall said.

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