LITTLE ROCK, Ark. – Arkansas’ school buses are among the worst in the country and Maine’s are among the best in terms of pollution, according to a study released Wednesday by the Union of Concerned Scientists.

Arkansas school buses received a “D” rating, the lowest possible, for its soot pollution. The study also gave Arkansas poor rankings on smog pollution and its school bus cleanup programs. The study found that the average bus in the state is 13 years old and emits 22.3 pounds of soot annually.

Arizona, Colorado, Hawaii, Louisiana, Minnesota, Montana, Nebraska, Oklahoma, South Carolina, South Dakota, Utah and Washington all received “D” rankings. Alaska, Connecticut, Maine, Nevada and New York received the highest marks, a “B” ranking.

The study recommended “Five Rs” to lessen harmful emissions: retrofitting, refueling, replacement, repair and reduced idling.