Effect modification of long-term air pollution exposure on the association of physical activity with COPD hospitalization: a prospective cohort study of 0.5 million Chinese adults

Background: Little is known about how long-term ambient air pollution exposure modifies the potential benefits of physical activity against chronic obstructive pulmonary disease (COPD). We aimed to examine the interaction between ambient air pollution and physical activity in relation to risk of COPD hospitalization. Methods: In a prospective cohort study of 467,944 adults recruited from 10 areas of China and without spirometry-defined airflow obstruction at baseline in 2004–2008, we examined the exposure-response relationships of total physical activity levels (metabolic-equivalent of task-hour/day; MET-h/d) with COPD hospitalization, stratified by long-term exposure to ambient PM_2.5 (median: 58.7 μg/m³), NO₂ (33.3 μg/m³), and O₃ (87.8 μg/m³), respectively, using multivariable Cox regression. Findings: In 467,944 adults (mean [SD] age = 51.8 [10.6] years; 280,590 females, 187,354 males), 14,688 incidents of COPD were recorded during 5.9 million person-years of follow-up. Physical activity was inversely associated with COPD in participants exposed to low air pollution but showed weak or no association in those exposed to high air pollution (all p-interaction < 0.001). Comparing participants at the highest (≥29.8 MET-h/d) versus lowest quartile (<10.7 MET-h/d) of physical activity, there were 18–23% statistically significant lower risks among those with low air pollution exposure, but marginally significant elevated risks among those with high air pollution exposure (for PM_2.5 and NO₂). The effect modification strengthened when stratifying participants by higher cut-offs of air pollution exposure. Interpretation: Long-term exposure to ambient air pollution is associated with attenuation of the potential benefits of physical activity against COPD in Chinese adults. Funding: Sino-British Fellowship Trust, National Key Research and Development Program of China, Kadoorie Charitable Foundation, Wellcome, Noncommunicable Chronic Diseases-National Science and Technology Major Project, National Natural Science Foundation of China, UK Medical Research Council, Cancer Research UK, British Heart Foundation.