Aging school buses rattling along U.S. highways are spewing harmful diesel fumes that pollute the countryside and place children riding the buses at risk, according to an analysis of federal and state data.

The Union of Concerned Scientists reports that at least 30 percent of the nation's school buses, more than 500,000 vehicles, have been in use for more than a decade. One aging school bus can produce between twice and 10 times as much diesel soot as an 18-wheel rig, according to the report.

The authors as well as other experts warn that large amounts of soot can accumulate inside the buses from open crankshafts.

About 95 percent of the nation's school bus fleet is powered by diesel and high levels of diesel exhaust and soot expose children to higher risk of asthma, cancer and other significant health problems, according to the report.

The worst polluter was South Carolina, closely followed by South Dakota. Both states earned "D" grades from the Union of Concerned Scientists as did 11 other states.

Several states are using alternative-fuel buses, replacing older buses with cleaner-burning models or retrofitting buses with devices that trap emissions.

A considerably more low-tech method also can reduce children's exposure to bus pollution, especially as they wait in the parking lot for a ride home: The driver can turn the engine off whenever possible.

The problem will cost a lot to fix. Some experts estimate $16 billion will be needed to retrofit or replace more than half a million buses across the United States.