The average Utah school bus releases 19.6 pounds of soot into the air each year — 150 percent more pollution per mile than a tractor-trailer truck — potentially putting children's health at risk, according to a national report grading states on hazardous emissions from their school bus fleets.

For that, Utah earned a D-grade — the lowest offered — in the School Bus Pollution report Card 2006, researched by the Union of Concerned Scientists and endorsed by the American Lung Association, according to the group that advocates for environmental solutions. Utah also earned poor rankings for its "fledgling cleanup program" and bus smog pollution in the report issued Wednesday.

The gunk school buses release into the air contribute to increased asthma attacks, and subsequently, emergency room visits and hospitalizations, said Paul F. Detjen, member of the American Lung Association's board and co-founder of Mobile C.A.R.E., which delivers free medical care to children in underserved Chicago communities.

But halting the nation's 505,000 school bus fleet isn't the solution. Nor is pulling children off buses, said Patricia Monahan, senior vehicles analysts in the union's clean vehicles program in Berkeley, Calif.

"What we need is stronger commitment," she said, "to protect our kids from harmful pollution."

The report card gave no states an A. Its rankings range from good to poor; incomplete rankings went to Delaware, Hawaii, Idaho, Kansas, Louisiana, Oklahoma, South Dakota, Vermont, Wyoming and the District of Columbia for apparently taking no cleanup action in 2005. Only Alaska, Connecticut, Maine, Nevada and New York scored above national averages in all three categories.

Nationally, the average school bus is 9 years old; Utah's is 12. The Beehive State's 2,300 school buses use both diesel fuel and gasoline, said Larry Newton, director of school finance and statistics at the State Office of Education.

Bus fleets are a local district matter, he said.

Some are addressing pollution concerns.

Jordan School District has 30 buses — 10 percent of its fleet — powered by natural gas, spokeswoman Melinda Colton said. It's the only one in the state to adopt the alternative fuel, which prevents nearly 250 tons of emissions a year and eliminates the need for 33,000 gallons of diesel fuel, according to the Environmental and Energy Study Institute, which recognized the district's efforts in 2004.