Short-term mediating effects of PM_2.5 on climate-associated COPD severity

The impact of short-term exposure to environmental factors such as temperature, relative humidity (RH), and fine particulate matter (PM_2.5) on chronic obstructive pulmonary disease (COPD) remains unclear. The objective of this study is to investigate PM_2.5 as a mediator in the relationship between short-term variations in RH and temperature and COPD severity. A cross-sectional study was conducted on 930 COPD patients in Taiwan from 2017 to 2022. Lung function, COPD Assessment Test (CAT) score, and modified Medical Research Council (mMRC) dyspnea scale were assessed. The mean and differences in 1-day, 7-day, and 30-day individual-level exposure to ambient RH, temperature, and PM_2.5 were estimated. The associations between these factors and clinical outcomes were analyzed using linear regression models and generalized additive mixed models, adjusting for age, sex, smoking, and body mass index. In the total season, increases in RH difference were associated with increases in forced expiratory volume in 1 s (FEV₁) / forced vital capacity (FVC), while increases in temperature difference were associated with decreases in FEV₁ and FEV₁/FVC. Increases in PM_2.5 mean were associated with declines in FEV₁. In the cold season, increases in temperature mean were associated with decreases in CAT and mMRC scores, while increases in PM_2.5 mean were associated with declines in FEV₁, FVC, and FEV₁/FVC. In the warm season, increases in temperature difference were associated with decreases in FEV₁ and FEV₁/FVC, while increases in RH difference and PM_2.5 mean were associated with decreases in CAT score. PM_2.5 fully mediated the associations of temperature mean with FEV₁/FVC in the cold season. In conclusion, PM_2.5 mediates the effects of temperature and RH on clinical outcomes. Monitoring patients during low RH, extreme temperature, and high PM_2.5 levels is crucial. Capsule of findings The significance of this study is that an increase in ambient RH and temperature, as well as PM_2.5 exposure, were significantly associated with changes in lung function, and clinical symptoms in these patients. The novelty of this study is that PM_2.5 plays a mediating role in the association of RH and temperature with COPD clinical outcomes in the short term.