"When buses leave a school in a train, so to speak, one behind the other, the buses in the rear will load up with the exhaust from the ones in the front."

John Wargo is a Yale University professor studying children's exposure to diesel exhaust. Wargo says the fumes can pose a health risk to children, especially those with respiratory problems, like asthma.

"The chemicals that are coming out of the exhaust pipe, there are three that are known human carcinogens, there are a number of others that are known animal carcinogens. Then there are perhaps a hundred more that are hazardous substances that are classified and listed under the clean air act."

Wargo says shutting off the buses while waiting for students is the simplest way to reduce pollution.

But the Wissahickon School District in Montgomery County is going a few steps further. District official Perry Baer says the schools are retrofitting many of their buses with pollution control devices.

"These are similar to catalytic converters that are used on the automobile exhaust systems. And we're going to be using a new ultra low-sulfer diesel fuel on all of our diesel vehicles, and even the vehicles that may not be retrofitted at this time, so they'll all be burning cleaner."

Baer says emissions will be cut by 20 to 90 percent. Clean buses come at a cost, but the school district has a 250-thousand dollar state grant to help pay for the program over the next five years.

More information's available on the web at GreenWorks.tv. I'm Brad Linder.