If you drive behind a diesel truck, you might not be surprised when it spews a big plume of smoke from the tailpipe. That can also happen behind a school bus. Pollution from school buses is bad for the kids on board or anywhere nearby. But state and federal laws are starting to make some difference.

It’s fifteen minutes before school lets out for the day, and buses are lined up on the street by a Pittsburgh school building. Some of them are idling, sitting there letting the engine run.

Rachel Filippini of GASP, the Group Against Smog and Pollution, says even it is below freezing, state law  says buses aren’t allowed to idle.

“Regardless of the temperature, the law says you have to turn off your engine within five minutes.”

LISTEN: “Cleaning the Air, One School Bus at a Time”

Filippini says GASP has been monitoring buses in the Pittsburgh School District to see if they’re following the state’s no idling law. They’re worried about kids’ health—on board the bus and sitting in nearby classrooms, where diesel fumes can waft in. Filippini says the particles in diesel fumes are so tiny, they can make their way deep into the lungs and the blood.

“They’re kind of sticky, and irregularly shaped, and they sometimes have other toxics or heavy metals that are stuck on to them. And those things which you do not want to be inhaling, are getting a free ride into your body, where they can cause a lot of problems.”

The federal government says diesel exhaust is a likely cause of lung cancer, asthma attacks, chronic bronchitis, and heart disease.

Sue Roenig thinks buses are cleaner than they used to be.

“Actually, I think being around school buses my entire life, I can tell the difference just walking outside here,” Roenig says.

Roenig’s family owns a bus company that contracts with Pittsburgh and other school districts in the region. At their garage in rural Sarver, a half hour north of Pittsburgh, the country air can fill up with exhaust fumes.

“Like in the morning, when the buses are lined up, ready to go to pick up students, my eyes don’t water anymore when I walk through the parking lot. So that has to be something. I think there is a big difference actually.”

Beginning in 2007, the U.S. Environmental Protection Agency started limiting the pollution—particulate matter and nitrogen oxides—from heavy-duty diesel vehicles. Companies that run school buses in the Pittsburgh Public Schools are supposed to retrofit their exhaust systems or buy new vehicles with pollution controls built in.

The Heinz Endowments offered a half million dollars in 2007 to area bus companies to retrofit their buses with the new filters. But Roenig’s was one of the only companies to take them up on the offer.

“I think it was probably just a lot of people at the time holding back just to wait to see how it actually worked out. Because sometimes you do things, and it sounds really good on paper, but it doesn’t turn out that way.”

Roenig’s mechanic, Bill Ross, thought “it was going to be a disaster.” But he says it wasn’t. “This really worked good. It really did.”

The Allegheny County Health Department says they don’t know of a central database tracking how many school buses have the cleaner technology. But it estimates that around three-fourths should have lower emissions since the law went into effect.

Sue Roenig says some of the new buses won’t let drivers idle—not even to warm up on a winter morning.

“They just shut off. Every 10 minutes, you have to go restart them trying to get the motors warmed up. It makes for a long morning sometimes.”

Beginning in 2007, the EPA started limiting the pollution from heavy-duty diesel vehicles. Companies that run school buses in the Pittsburgh Public Schools are supposed to retrofit their exhaust systems or buy new vehicles with pollution controls built in.

Research shows when everyone in an area uses cleaner vehicles, it can make a real difference in air quality. Albert Presto studies air pollution at Carnegie Mellon University in Pittsburgh.

“When these technologies are working well, they can reduce over 90 percent of the particulate matter emissions from diesel vehicles.”

Presto gives the example of the seaport in Oakland, California. The state forced old trucks running from the port to storage facilities to reduce their emissions. Presto says it worked. They significantly reduced diesel pollution. “And that helped improve air quality at the port  and in the neighborhoods near the port.”

Pennsylvania is moving toward the same diesel control standards as California. But so far, Presto says it’s been difficult to parse out the impact of cleaner school buses in a city like Pittsburgh. There are so many older diesel buses and trucks still on the road without pollution controls.

Back at that Pittsburgh school, clean air advocate Rachel Filippini says construction sites—and even river boats—cause more diesel pollution than school buses. But buses have such a direct impact on children, especially when they idle. Filippini’s group GASP has given schools signs that say “No Idling” to post at pickup zones. Some drivers idled anyway, but many did shut down their engines.

“This is a vast improvement from what I saw last fall.”

Filippini says when they monitored buses that time, more than one-quarter of the buses surveyed were idling— and that’s too many. The state Department of Environmental Protection says it’s gone after some drivers for idling too long. But Filippini says better enforcement could go a long way to keeping the air cleaner for kids.