Day in and day out, children across the United States are riding to school on aging buses, breathing what some activists say is a dangerous brew of pollutants up to five times dirtier than the air outside.

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 initiatives for clean school buses. 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. Research has also found that 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 United States, according to the EPA. Most newer buses were manufactured to meet stricter emissions guidelines and do not need filters. But more than 100,000 buses were manufactured before 1990 and are big polluters, according to the EPA.

Researchers say older buses also let lots of emissions enter through doors and windows. The longer the ride, the more harmful to children.

"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.

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.

Advocates hail California as a leader on the issue. Voters there approved $200 million last year to clean up the fleet there.

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. The Bush administration proposed modest funding in its last two budget requests. But Congress has not acted and states have gotten nothing.