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This study evaluated the acute effects of ambient ozone (O3) fine particulate matter (PM2.5). and strong aerosol acidity on the pulmonary function of exercising adults. During the summers of 1991 and 1992, volunteers (18--64 years of age) were solicitedfrom hikers on Mt. Washington, New Hampshire. Volunteer nonsmokers with complete covariates (n = 530) had pulmonary function measured before and after their hikes. We calculated each hiker's posthike percentage change in forced expiratory volume in 1 sec (FEV1), forced vital capacity (FVC), the ratio of these two (FEV1/FVC), forced expiratory flow between 25 and 75% of FVC (FEF25--75%), and peak expiratory flow rate (PEFR). Average O3exposures ranged from 21 to 74 ppb. After adjustment for age, sex, smoking status (former versus never), history of asthma or wheeze, hours hiked, ambient temperature, and other covariates, there was a 2.6%decline in FEV1 [95% confidence interval (CI), 0.4--4.7; p = 0.02] and a 2.2% decline in FVC (CI, 0.8--3.5; p = 0.003) for each 50 ppb increment in mean O3. There were consistent associations of decrements in both FVC (0.4% decline; CI 0.2--0.6, p = 0.001) and PEFR (0.8% decline; CI, 0.01--1.6; p = 0.05) with PM2.5 and of decrements in PEFR (0.4% decline; CI, 0.1--0.7; p = 0.02) with strong aerosol acidity across the interquartile range of theseexposures. Hikers with asthma or a history of wheeze (n = 40) had fourfold greater responsiveness to ozone than others. With prolonged outdoor exercise, low-level exposures to O3, PM2.5 and strong aerosol acidity were associated with significant effectson pulmonary function among adults. Hikers with a history of asthma or wheeze had significantly greater air pollution-related changes in pulmonary function. [ABSTRACT FROM AUTHOR]