It is a situation that Congress and many states have sought to fix in recent years. In fact, in 2005 federal lawmakers passed a measure to replace or retrofit the dirtiest diesel engines across the nation.
But little has been done.
Around the country, state officials are struggling to find the money to carry out clean school bus initiatives. And Congress has yet to deliver on the $1 billion it promised over five years to help states clean up diesel fleets, including school buses.
"I think at one time or another all our kids are going to be on a bus breathing that harmful air, and that should bother everybody," said Karen Slay, a Lubbock, Texas, mother of four boys who have ridden buses. "In the big scheme of things, it doesn't seem to be that expensive, to me, to retrofit these."
Breathing high concentrations of diesel emissions - known as particulates - can cause minor ailments such as headaches, wheezing and dizziness. But studies have also found the contaminants can do more serious damage. Recent studies by the Environmental Protection Agency and other groups link the emissions to asthma and lung cancer.
Two types of filters are available to reduce the most dangerous emissions on older buses. Diesel particulate filters - which are installed in place of mufflers at an estimated cost of $700 each - can reduce tailpipe emissions by at least 85 percent. Closed crankcase filtration systems, which go under the hood and cost $7,500, can reduce engine soot by about 90 percent. A bus can be fitted with one or both filters.
An estimated 390,000 diesel school buses are on the road in the U.S., according to the EPA. Most newer buses were manufactured to meet stricter emissions guidelines and do not need filters. But about one-third of the nation's diesel school-bus fleet, or more than 100,000 buses, were manufactured before 1990 and are big polluters, according to EPA.
Researchers say older buses also let lots of emissions enter through doors and windows. The longer the ride, the more harmful to children, they say.
"The exhaust that swirls around the bus gets into the bus and can stay elevated throughout the ride," said Betin Santos, an air quality specialist for the group Environmental Defense.
In Texas, lawmakers two years ago created a grant program to help schools pay for the filters. But they never funded the effort. The money was supposed to have come from an emissions reduction fund supported by fees on vehicle sales, registrations and inspections. But lawmakers have diverted much of that revenue to pay for other things.
Many other states also are struggling to pay for cleaner school bus fleets, said Conrad Schneider of the Boston-based Clean Air Task Force, which fights air pollution.
"I think that once people start to understand that there is a simple economic fix to the problem, we could go from a situation where kids are being exposed to a high level of pollution to one where their exposure is virtually eliminated," Schneider said. "State governments will dig down and try to find the money to get this accomplished."
Clean bus advocates hail California as the leader on the issue. Voters there approved $200 million last year to clean up its school bus fleet.
"There were studies done about the health risks for children riding in diesel-powered buses," said Patricia Rey, a spokeswoman for the California Air Resources Board. "The governor and Legislature found it to be a priority. And Californians agreed with that."
But in Pennsylvania, Louisiana and elsewhere, state money to help schools retrofit buses has been nonexistent.
Congress passed the Diesel Emissions Reduction Act, a bipartisan initiative that authorized $1 billion to help states clean up diesel fleets. But states have seen none of that money. The Bush administration proposed modest funding for DERA in its last two budget requests, but Congress has not acted.
Illinois Sen. Barack Obama, one of the bill's co-sponsors, is continuing to push for funding. The Democratic presidential hopeful has accused his colleagues of "foot-dragging."
The Clean Air Task Force in Boston put electronic monitors in students' backpacks to test air quality inside school buses. The organization said it found that the diesel exhaust levels were on average five times greater than they were outside. Experts say children are particularly vulnerable because soot particles can disrupt development of their respiratory systems. Also, children breathe more quickly than adults and take in more air per pound. EPA spokesman Dave Ryan said the agency has no independent measurements of diesel soot inside school buses, and he would not comment on the other studies. But the EPA acknowledges diesel emissions can be hazardous to children. Not everyone is convinced that the air in school buses is a threat. Texas state Rep. Warren Chisum, chairman of the House Appropriations Committee, said it doesn't make sense to spend huge sums on a cleanup, because the "science is not very good." But Larry Soward, commissioner of the Texas Commission on Environmental Quality, said the dirtiest diesel buses are likely to last another 20 years or more, and "I, for one, don't think we can wait two decades." "It's going to cost money," Soward said, "but the health of our children demands that we commit the funds." --- On the Net: Environmental Protection Agency: http://www.epa.gov/cleanschoolbus Group Against Smog and Pollution: http://www.gasp-pgh.org Clean Air Council: http://www.cleanair.org AP-CS-05-04-07 1412EDT