School Bus Fumes

School Bus Fumes

When we think about pollution from cars, trucks, and buses, we usually imagine the effects on people outside the vehicles. But a recent study of school buses found that the passengers might get the worst of it.


Self-polluting school buses. I’m Bob Hirshon and this is Science Update.

A surprising fraction of the exhaust from school buses ends up back inside the bus. That’s according to a study led by Julian Marshall, a Ph.D. student in the Energy and Resources Group at the University of California at Berkeley. He says the impact of a single bus is much greater on its passengers than on the passers-by.


If you look at the total amount of pollution inhaled by students on a bus, say 40 or so students, those students inhale about the same amount of pollution, or more, as all the other residents in an urban area.

The worst offenders are older, diesel-powered buses with the windows closed. Marshall says the findings could spark improvements in emissions and insulation standards. But he stresses that parents shouldn’t pull their kids off the bus, since the traffic safety advantages far outweigh any potential health risks. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

First of all, listen carefully to the last line of the report: the school bus is still by far the safest way to get to school! Unlike cars, school buses rarely get into accidents. When they do, serious injuries are less likely. Deaths on school buses are nearly unheard of. And besides, mass transportation generally produces less air pollution per passenger than private cars.

That said, this study is interesting because it’s among the first to look at the effects of exhaust inside a vehicle, or “self-pollution.” School buses are more vulnerable to self-pollution than regular cars, largely because many of them still use diesel fuel, and because they seem to be built in a way that allows more of the exhaust to escape into the vehicle.

The researchers measured self-pollution by adding a harmless marker gas to the exhaust that could be easily measured. Then they measured the concentrations of that gas inside several different school buses as they ran their regular routes. Using these figures, they were able to calculate the average concentration of exhaust inhaled by each passenger.

These numbers varied widely from bus to bus and from trip to trip, but the researchers were able to draw some general conclusions. First, all the buses in the study self-polluted. Second, the amount of self-pollution wasn’t necessarily related to the amount of exhaust a bus emitted. In fact, a school bus equipped with a pollution-reducing “particle trap” didn’t self-pollute any less than similar buses without this feature.

Third, they found that generally speaking, older buses self-polluted more than newer buses and that buses with closed windows self-polluted much more than buses with open windows. The oldest buses in the study were built in 1975, which would be very old for a car. Because school buses tend to stay in use much longer than cars do, however, buses this old or older are not uncommon.

The researchers hope that the study will help lawmakers and bus designers think about air pollution in a different way. They also suggest that specific, cost-effective steps could be taken to make sure new school buses don’t self-pollute so easily. In order to do that, scientists will have to study the buses more closely to find out exactly how the exhaust is leaking into the passenger cabin.

Now try and answer these questions:

  1. How does this study differ from most studies of car and bus emissions?
  2. What kinds of buses are most likely to self-pollute?
  3. Aside from building better buses, what other steps can be taken to curtail this problem?
  4. Suppose the study found that the amount of self-pollution on a bus was strictly related to the amount of pollution emitted. (In other words, the more that comes out of the tailpipe, the more goes into the bus cabin.) How would this change the solution to the problem?

For Educators

The Urban Measurement of Aerosols and Asthma Project, from NASA’s Goddard Space Flight Center, encourages high-school students to measure the relationship between asthma and air pollution at their school.

The World of Child 6 Billion: Air looks at the effects of global air pollution on a contemporary child’s lungs and heart.

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