WASHINGTON — A study grading states on their school bus fleets' diesel exhaust gave Hawai'i a D for its soot pollution and ranked its cleanup program as "incomplete."

No state received an A grade in the Union of Concerned Scientists' National School Bus Report Card, although it noted that many are working to cut school bus emissions, which can contribute to asthma and other respiratory ailments.

The group found that the average Hawai'i school bus is 13 years old and emits 21.9 pounds of soot annually. As of 2005, there were no projects in Hawai'i to reduce school bus pollution.

The District of Columbia and Delaware, which received B grades, had the lowest rates of soot pollution: just more than 9 pounds of pollution per bus. Fourteen other states also received B's.

The worst polluter was South Carolina, closely followed by South Dakota. Both earned D grades, as did 11 other states.