Filed at 12:01 p.m. ET

LOS ANGELES (AP) -- In California's smoggiest communities, the most athletic children are three times more likely than their couch-potato peers to get asthma, a new study concludes.

In places with cleaner air there is no such link between exercise and asthma, according to the study of more than 3,500 children by the University of Southern California.

The most athletic children in polluted areas apparently get asthma at higher rates because they breathe in more air. Many studies have concluded that smog aggravates asthma, but the USC study is one of the few to find that ozone -- smog's principal ingredient -- can actually cause the disease.

Lead researcher Dr. Rob McConnell of USC's Keck School of Medicine said young athletes, coaches and parents should heed the findings by avoiding strenuous outdoor activity on the smoggiest days. But he stressed that exercise remains much healthier than a sedentary lifestyle.

``It would be doing a real disservice to public health if children stopped exercising or stopped playing sports because of this study. Exercise is real healthy for children for a whole lot of reasons,'' McConnell said.

Ozone is produced when volatile organic compounds and nitrogen oxides mix with sunlight. Motor vehicles are a primary source of those building blocks of smog, which also come from sources ranging from paint to power plants.

Ozone is likely just one of a multitude of things that can cause asthma, McConnell said.

Alan C. Lloyd, chairman of the California Air Resources Board, which funded the study, said it re-emphasizes the need for the state to reduce smog to meet federal standards, and signals that the current standards ``are not sufficiently protective of children's health.''

The state has cut deep into the high ozone levels of decades past, but much of California still falls short of air quality standards set by the federal Environmental Protection Agency.

Rudy Fortiz, an assistant coach at Huntington Park High School just southeast of Los Angeles, said the study's findings are in line with the health problems his athletes face. His school's playing fields abut busy Slauson Avenue and are close to industrial facilities.

Fortiz, in his third year at the school, said he sees more breathing problems than he did as a coach at another high school with fewer direct pollution sources.

The ranks of Huntington Park's track team are decreasing because ``the kids can't run,'' Fortiz said, adding that coaches send their teams into the weight room on the smoggiest days.

The study was published in the Feb. 2 edition of the British medical journal The Lancet.

The researchers tracked 3,535 children ages 9 to 16 with no history of asthma from 1993 to 1998. The children lived in six California communities with high ozone pollution, and six with low ozone pollution.

Eight percent of the children played three or more sports. Of those children, the ones who played in areas with high ozone pollution were three times more likely than other children to be diagnosed with asthma, the study found.

Exercise-induced asthma does not account for the difference, McConnell said, because three-sport athletes in low-pollution areas did not have higher asthma rates.

The study adjusted its figures to account for risk factors, such as secondhand smoke at home and low family incomes, that are associated with asthma in children.

The type of sport being played also appears to have a role in asthma risk. Children who played at least one ``high-intensity'' sport such as football, basketball, soccer, swimming or tennis had an asthma risk 60 percent higher than children who played no sports, McConnell said.

The risk was only 20 percent higher for young athletes who played only baseball, softball or volleyball, he said.

The Lancet report is one element of an ongoing 10-year study of children's health and air pollution sponsored by the state air board. Other findings so far connect air pollution to slower lung development in children, increased absenteeism at school and bronchitis.