Idling cars and buses aren't just noisy and wasteful; their emissions pose a threat to students, USA Today reported.

New research headed by the Cincinnati Children's Hospital Medical Center and other community organizations proved idling increases the concentration of asthma-causing air particles. The study also demonstrated how particle levels could be decreased by simply decreasing the frequency of idling.

Researchers measured the air quality at four Cincinnati public schools before and after instituting "anti-idling" campaigns, according to USA Today. All four schools had asthma rates above 10 percent, according to self-reported surveys by parents. Following two years of signs warning against idling, as well as, teachers and administrators warning parents and bus drivers, air quality improved at three of the four schools.

"The concentration of air pollutants near schools often significantly exceeds background levels in the community, particularly when idling school buses are present," said Patrick Ryan, the lead author of the study. "Anti-idling campaigns are frequently attempted to improve air quality, but until now, no one has evaluated how effective they are.

"We have to keep reinforcing the habits," Ryan said.

The particles produced by idling are actually the biggest contributors to asthma, USA Today reported. They linger at playgrounds and school parking lots for hours. At 2.5 millimeters and smaller, they are inhaled deeply by the lungs.

Children are more susceptible to the microscopic particles than teachers and parents because they spend more time outside, they inhale 50 percent more air per pound of body weight, and they're shorter so they're closer to the bad air, according to USA Today.

Ryan and his team haven't yet looked at the health effects following the anti-idling campaign, but they expect to find fewer cases of asthma. He's also currently involved in a study that samples indoor air quality in schools, according to USA Today.

Ryan said her research could be particularly useful in larger schools with more frequent commuting.

"Where (anti-idling) could really make an impact is at suburban schools with campuses" that draw in dozens of buses and have few students who walk, Ryan said. "Most districts are aware (of this research) and are trying to do something."