Nevada's school buses emit fewer harmful pollutants linked to asthma and other upper respiratory infections than at least 45 other states, according to a report released Wednesday by the Union of Concerned Scientists and the American Lung Association.

However, there's still work to be done, said Patricia Monahan, author of the School Bus Pollution Report Card 2006, which evaluated states' school bus soot emissions as reported by state officials.

"No one should rest on their laurels,'' she said. "School buses still emit roughly 12 pounds of soot annually in Nevada. That's more soot per mile than the average tractor trailer.''

Nevada was one of five states to receive a B on the report card. No state received an A, Monahan said.

Nevada also received an "above average" grade in terms of its school bus cleanup program. Monahan said the Clark County School District has a newer fleet of school buses and uses biodiesel fuel a lot more than other states.

According to the American Lung Association, soot can cause shortness of breath, throat irritation, chest pains and respiratory tract infections.

Children are especially vulnerable, because their lungs are still developing and they breathe at a much higher rate than adults, Monahan said.

Ronald Despenza, transportation director for the Clark County School District, though appreciative that the state received above average grades, said he was disappointed it didn't get an A.

"We do everything we can to make our buses cleaner and safer,'' he said. "Above average is not the highest. We want the highest grade.''

An example of the Clark County School District's effort to keep bus emissions low includes the use of biodiesel in the majority of the district's 1,286 buses. Biodiesel is an alternative fuel that is biodegradable, burns cleanly and is made from a renewable resource, such as french fry oil.

Despenza said the district is also experimenting with two soot traps on separate buses to see which one is best at keeping emissions down.

Soot traps are filters that collect particles emitted from mobile and stationary engines that use diesel fuel.

"We are waiting for federal funding of about $1 million to get more of those soot traps. They actually cut down the majority of the particles in exhaust. In addition, we have about 150 new school buses coming in the next week or two. They have state-of-the-art fuel technology.''