Breathe Carefully During an Ozone Alert : Health: Experts recommend exercising inside when summer heat and pollution mix, degrading the air supply.

Bad air is bad news for exercisers. When summer heat cooks smog into an ozone stew, athletes should think about cutting back and maybe staying inside, experts warn.

Ozone is a form of oxygen that develops when hydrocarbons from car and industry emissions mix in summer heat and sunlight. It is chemically similar to the atmospheric ozone layer that filters skin-damaging ultraviolet radiation from sunlight. But there’s a difference--that layer is 15 miles up, safely out of breathing range.

The pollution-caused ozone gets created close to home, within breathing range. And as an irritant to airways and lungs, it can make breathing more difficult, said Dr. Henry Fishman, a clinical assistant professor of allergy and immunology at Georgetown and George Washington universities.

High summer humidity can make things even worse, because moisture in the air traps ozone, according to Dr. William Eschenbacher, an associate professor of medicine at Baylor College of Medicine in Houston.

Ozone typically peaks in late afternoon, after cars and businesses have had time to spew pollution and the sun has had opportunity to cook it, Eschenbacher said.

The federal government considers ozone to be unhealthful when an hour’s readings average 120 parts per billion. That’s 120 molecules of ozone in 1 billion molecules of air in general.

But unhealthful is a relative term. Exercisers with lung problems such as asthma are at special risk and could wind up in an emergency room if they exercise outdoors when the ozone level is high, said Eschenbacher, medical director of Respiratory Care Services at the Methodist Hospital in Houston.

Because ozone irritates the nerve fibers along the respiratory tract, people may notice a sore throat, coughing or chest discomfort, Eschenbacher said.

Some athletes with lung problems may have trouble breathing. Asthmatics may need to increase their medication, Fishman said.

Others may not notice the effects until they discover that it is taking them longer to go a set distance, or they can’t go as far as they normally do, Eschenbacher said.

Studies indicate that 10% to 20% of people are ozone responders, Eschenbacher said. These people have an “accentuated dramatic response, with a change in lung function,” he said.

The other 80% to 90% would also suffer some limits to their lung function when ozone levels are unhealthful, Eschenbacher said. But they would not be affected so dramatically, and many might not even notice, he said.

Why some normally healthy people are affected more than others is still a mystery, Eschenbacher said, adding: “We have yet to define what it is.” Also unclear is whether the deterioration progresses gradually with increased ozone or shoots up sharply after some point, he said.

The problems of ozone don’t give athletes an excuse to sack out on bad ozone days; they just need to adjust. They might, for instance, decide to reduce their exercise goals so their workouts fit inside their personal comfort zones.

Or they could do their outdoor exercise in the morning, when pollution is lower, Fishman said.

They might also switch to exercising indoors, Eschenbacher said. Because air moves more slowly through a building, buildings act as ozone buffers, mixing high-dose afternoon air with a plentiful stored supply of air from the rest of the day, he said.

This gives indoor exercisers a lower ozone reading than they’d get outdoors in the afternoon, Eschenbacher said.