We conducted a prospective study of a cohort of 3091 nonsmokers, ages 27 to 87 years, to evaluate the association between long-term ambient ozone exposure and development of adult-onset asthma.

Over a 15-year period, 3.2% of males and 4.3% of females reported new doctor diagnoses of asthma.

For males, we observed a significant relationship between report of doctor diagnosis of asthma and 20-year mean 8-h average ambient ozone concentration (relative risk (RR)=2.09 for a 27 ppb increase in ozone concentration, 95% CI=1.03 to 4.16).

We observed no such relationship for females.

Other variables significantly related to development of asthma were a history of ever-smoking for males (RR=2.37,95% CI=1.13 to 4.81), and for females, number of years worked with a smoker (RR=1.21 for a 7-year increment, 95% CI=1.04 to 1.39), age (RR=0.61 for a 16-year increment, 95% CI=0.44 to 0.84), and a history of childhood pneumonia or bronchitis (RR=2.96,95% CI=1.68 to 5.03).

Addition of other pollutants (PM10, SO4, NO2, and SO2) to the models did not diminish the relationship between ozone and asthma for males.

These data suggest that long-term exposure to ambient ozone is associated with development of asthma in adult males.