Fuming buses.

In addition to hauling you and your friends, your school bus may be carting around pollution.

School buses are some of the top polluters on the road, says Arthur Winer, an environmental-health scientist at the University of California at Los Angeles. That's because most bus engines burn diesel fuel for power. As the bus runs, it releases exhaust that contains pollutants. These gases and small particles can aggravate health problems such as asthma.

Because buses aren't as tightly sealed as most vehicles, exhaust can seep inside. To measure pollution inside and outside the bus, Winer fitted detectors to six buses. As a driver steered each otherwise-empty vehicle along its normal route, the sensors sniffed out pollutants.

One finding: Older buses trapped more pollution inside than newer ones.

To combat the problem, many schools are purchasing newer, cleaner buses. Plus, most schools now require drivers to turn off the engines while parked on school grounds. This should help you breathe easier.