Identifying the threshold of outdoor PM2.5 reversing the beneficial association between physical activity and lung function: A national longitudinal study in China

Despite cumulative evidence reports the interaction effects of physical activity (PA) and air pollution on lung function, the findings have been inconsistent. We aimed to identify the threshold values that reverse the beneficial effects of PA on lung function.This multistage probability sampling study examined 13,032 individuals aged ≥45 years across China from 2011 to 2015. City-level particulate matter 2.5 μm or less in diameter (PM2.5) were estimated based on a two-stage machine learning model, with a spatial resolution of 0.1° × 0.1°. We assessed PA and a range of covariates using standardized self-reported questionnaires. The peak expiratory flow (PEF) was measured using a peak flow meter. We used mixed-effects linear regression models to examine the associations between PA and PM2.5, and their interactions with PEF.Participants were 60.4 ± 9.4 years old [mean ± standard deviation (SD)]. The mean ± SD of PM2.5 and PEF was 54.4 ± 23.0 μg/m3 and 273 ± 116 L/min, respectively. Each 10 μg/m3 increase in PM2.5 was associated with a 1.27 L/min decrease in PEF. The PEF increased by 2.48 (95% confidence interval, CI: 0.40, 4.55) L/min, 0.74 (95% CI: -1.17, 2.66) L/min, and 1.99 (95% CI: 0.001, 3.99) L/min following a 10 h/week increment of walking, moderate intensity PA, and vigorous intensity PA, respectively. Detrimental associations between PM2.5 and PEF outweighed PA benefits for approximate PM2.5 concentrations >81 μg/m3 (95% CI: 58.9, 111) and >77 μg/m3 (95% CI: 39.7, 102) for walking and vigorous intensity PA, respectively.We identified the threshold of ambient PM2.5 above which the beneficial association of PA with lung function may be reversed to an adverse one. Although the threshold may vary across populations and places, the findings suggested that reducing air pollution could enhance the benefits of PA on lung function.