Assessing PM_2.5-associated risk of hospitalization for COPD: an application of daily excessive concentration hours

Existing PM_2.5-morbidity studies using daily mean concentration as exposure metric may fail to capture intra-day variations of PM_2.5 concentrations, resulting in underestimated health impacts to some extent. This study introduced a novel indicator, daily excessive concentration hours (DECH), defined as sums of per-hourly excessive concentrations of PM_2.5 against a specific threshold within a day. PM_2.5 DECHs were separately calculated as daily concentration-hours >8, 10, 15, 20, and 25 μg/m³ (abbreviations: DECH-8, DECH-10, DECH-15, DECH-20, and DECH-25). We adopted a time-stratified case-crossover design with conditional logistic regression models to compare risks of hospitalizations for chronic obstructive pulmonary disease (COPD) associated with PM_2.5 mean and DECHs in Shenzhen, China. We observed highly comparable PM_2.5-COPD associations using exposure metrics of daily mean and DECHs with above-defined thresholds. For instance, PM_2.5 mean and DECHs showed similar increases in risks of COPD hospitalization for an interquartile range rise in exposure, with odds ratio estimates of 1.26 (95% confidence interval: 1.06-1.50) for PM_2.5 mean, 1.24 (1.05-1.46) for DECH-10 and 1.21 (1.06-1.39) for DECH-25, respectively. Findings remained robust after further adjusting for gaseous pollutants (e.g., SO₂, NO₂, CO, and O₃) and meteorologic factors (e.g., wind speed and air pressure). Our study strengthened the evidence that DECHs could come be as a novel exposure metric in health risk assessments associated with short-term exposure to ambient PM_2.5.