A study of 271 children with asthma living in Connecticut and Massachusetts showed those with the worst asthma started to suffer from shortness of breath, coughing and chest tightness at "good" air quality levels, as designated by the Environmental Protection Agency.

"It should be of concern to parents of children with active asthma," said Janneane Gent, an epidemiologist at Yale University who led the study.

Her team followed the 271 asthmatic children, all under age 12, over the summer of 2001. It was a moderate year for air pollution in the region, she said in a telephone interview.

Yet on at least half the days, the children with severe asthma -- defined as needing daily use of medication such as pills or an inhaler -- suffered increased symptoms. These days were linked with higher ozone levels.

"Studies of children with asthma living in regions with levels of pollution within or near compliance with EPA air quality standards (120 parts per billion over a one-hour average ... for ozone) significantly enhances the risk of respiratory symptoms, asthma medication use and reduced lung function," they wrote.

CHEST TIGHTNESS

"An ozone concentration of 63.3 ppb or higher, measured as the maximum eight-hour average on the same day as the reported symptom, was associated with a 30 percent increase in chest tightness."

If ozone levels reached 52 ppb on the day before, more kids reported a tight feeling in the chest, cough and trouble breathing.

"We found an immediate, same-day effect of ozone on wheeze, chest tightness and shortness of breath," they wrote.

Gent said that on half the days that summer, the ozone levels were above 55 parts per billion.

"When the air quality index says that the air is good, that means levels of ozone in the air are anywhere between 0 and 60 parts per billion," she said.

"When it is 'moderate' it is between 60 and 120 parts per billion. It really is at that point that parents of asthmatic children would be well advised to keep their children indoors and make sure they have a low level of activity."

Gent said she was not in a position to advise the EPA to tighten pollution standards, but added, "We've provided some more information that those who are in the business of setting policy can refer to."

In a commentary, George Thurston of the New York University School of Medicine and Dr. David Bates of the University of British Columbia in Vancouver agreed.

"Of the many triggers of asthma in the environment, air pollution is one of the few that can be legislated and regulated," they wrote.

"Patients and parents of children with asthma should be aware of the ozone alert forecast, which is widely publicized in news reports, and listed in the United States on the Internet (available at: http://www.epa.gov/airnow). Patients with asthma should stay indoors on high-pollution days," they added.