LOS ANGELES—Although elevations in ambient ozone levels are known to trigger asthma exacerbations, the role of chronic ozone exposure in asthma etiology has remained unclear. A new study shows that children engaging in outdoor team sports in high-ozone areas are at an increased risk for developing asthma.[1]

“It is well known that air pollution causes short-term exacerbations of asthma, but there has been only limited evidence that ozone or other air pollution actually causes new-onset asthma,” remarked Rob McConnell, MD, Associate Professor of Preventive Medicine at the University of Southern California in Los Angeles. “Our study provides evidence that ozone is involved in the development of new-onset asthma in children who exercise heavily,” thereby presumably increasing exposure of their lungs to ozone, he explained.

Dr. McConnell and colleagues investigated 12 southern California communities with a broad range of ambient concentrations of ozone, particles less than 10 µm in diameter, and nitrogen dioxide. Of these areas, six were categorized as high-pollution and six as low-pollution sites. With the help of their parents, 3,535 children ages 9 to 16 years having no prior diagnosis of asthma completed questionnaires regarding participation in sports and time spent outside. Of these children, 2,752 (78%) had no history of wheezing. Participants were interviewed yearly thereafter for up to five years to assess onset of asthma symptoms.

Across all communities, the risk of receiving a new diagnosis of asthma was 1.8 times higher among the children who had played three or more team sports in the previous year than among those who had participated in no sports. However, this risk was 3.3 times higher among the children from high-ozone (mean maximum concentration, 75.4 ppb) communities who played three or more sports. By contrast, playing multiple team sports had no impact on the asthma risk in low-ozone (mean maximum, 50.1 ppb) communities.

Further, in high-ozone communities but not in low-ozone areas, children who played at least one high-activity sport, such as tennis, swimming, or soccer, had a 1.6-fold elevation in the risk for developing asthma, and their risk increased with each additional sport played. Time spent outdoors was also associated with higher asthma risk in high-ozone communities but not in low-ozone areas.

EXERCISE MAY MAXIMIZE OZONE EXPOSURE

In analyzing their results, the researchers made a key observation: In the high-ozone communities, exercise elevated asthma risk regardless of wheeze history. This suggests that the increased asthma risk might be explained both by cases of new-onset asthma and by exacerbations of latent asthma. And, given that exercise did not affect the asthma risk in low-ozone communities, an increase in exercise-induced asthma alone is unlikely to explain the increased asthma incidence among sports participants.

Rather, the association of high-activity sports and increased time spent outdoors with new-onset asthma implicates increased ozone exposure as a likely cause. During heavy exercise, the authors note, ventilation rates may increase 17-fold, and increased ventilation and oral breathing may shift ozone uptake deeper into the lung, where it probably inflicts the most damage.

OZONE PROBLEM WIDESPREAD

Acute ozone exposure causes chest pain, cough, and pulmonary congestion, and long-term chronic exposure has also been linked with decreased flow rates in adults. Experimental work demonstrates that ozone can damage lung tissue, reduce lung function, and sensitize the lungs to other irritants. Dr. McConnell noted that there is “strong evidence that air pollution is a potent cause of asthma-like conditions in experimental animals when air pollution is combined with exposure to allergens that are common in our everyday environment.”

Formed through chemical reactions between volatile organic compounds and nitrogen oxides in the presence of sunlight, ground-level ozone is a frequent health problem in areas with an abundance of vehicular or industrial exhaust and hot, sunny, stagnant weather. Ground-level ozone levels can exceed an hourly value of 0.241 ppm in southern California, well above the Environmental Protection Agency’s 0.12-ppm maximum one-hour average compliance limit.

IMPLICATIONS FOR PRACTICE

“No one should stop exercising because of our study,” emphasized Dr. McConnell. “The bottom line is this: Exercise is really healthy for children, for many reasons, and children should be encouraged to play team sports. But on days when air pollution levels are expected to be high, children should limit prolonged outdoor exertion.”

Some may view having to limit children’s physical activity as unacceptable, however. Dr. McConnell voiced the view of many residents of high-ozone areas: “If ozone is causing asthma, then, ultimately, the solution is to reduce the levels of ozone.” He added, “[W]e and our government must do more to control pollution … not just because of [its] role … in childhood asthma” but also because of numerous other adverse health effects in children, the elderly, and those with respiratory disease.

—Mimi Zucker, PhD