RATIONALE: Physical activity enhances uptake of air pollutants in the lung, possibly augmenting their harmful effects on chronic lung disease during exercise.

OBJECTIVES: To examine whether benefits of physical activity with respect to the risk of asthma and chronic obstructive pulmonary disease (COPD) are moderated by exposure to high air pollution levels in an urban setting.

METHODS: A total of 53,113 subjects (50-65 yr) from the Danish Diet, Cancer, and Health cohort reported physical activity at recruitment (1993-1997) and were followed until 2013 in the National Patient Register for incident hospitalizations for asthma and COPD. Levels of nitrogen dioxide (NO2) were estimated at subject residences at the time of recruitment. We used Cox regression to associate physical activities and NO2 (high/medium/low) with asthma and COPD, and then introduced an interaction term between each physical activity and NO2.

MEASUREMENTS AND MAIN RESULTS: A total of 1,151 subjects were hospitalized for asthma and 3,225 for COPD during 16 years. We found inverse associations of participation in sports (hazard ratio [95% confidence interval]: 0.85 [0.75-0.96]) and cycling (0.85 [0.75-0.96]) with incident asthma, and of participation in sports (0.82 [0.77-0.89]), cycling (0.81 [0.76-0.87]), gardening (0.88 [0.81-0.94]), and walking (0.85 [0.75-0.95]) with incident COPD admissions. We found positive associations between NO2 and incident asthma (1.23 [1.04-1.47]) and COPD (1.15 [1.03-1.27]) hospitalizations (comparing ≥21.0 μg/m(3) to <14.3 μg/m(3)). We found no interaction between associations of any physical activity and NO2 on incident asthma or COPD hospitalizations.

CONCLUSIONS: Increased exposure to air pollution during exercise does not outweigh beneficial effects of physical activity on the risk of asthma and COPD.