Daily exceedance concentration hours: A novel indicator to measure acute cardiovascular effects of PM_2.5 in six Chinese subtropical cities

Background Daily mean concentration cannot fully address the hourly variations of air pollution within one day. As such, we proposed a new indicator, daily exceedance concentration hours (DECH), to explore the acute cardiovascular effects of ambient PM_2.5 (particles with aerodynamic diameters less than 2.5 μm). The DECH in PM_2.5 was defined as daily total concentration-hours > 25 μg/m³. Methods A generalized additive model with a quasi-Poisson link was applied to estimate the associations between day-to-day variation in PM_2.5 DECH and day-to-day variation in cardiovascular mortality in six subtropical cities in Guangdong Province, China. Results The analysis revealed significant associations between PM_2.5 DECHs and cardiovascular mortality. A 500 μg/m³ ∗ h increase in PM_2.5 DECHs at lag₀₃ was associated with an increase of 4.55% (95% confidence interval (CI): 3.59%, 5.52%) in cardiovascular mortality, 4.45% (95% CI: 2.81%, 6.12%) in ischemic cardiovascular mortality, 5.02% (95% CI: 3.41%, 6.65%) in cerebrovascular mortality, and 3.00% (95% CI: 1.13%, 4.90%) in acute myocardial infarction mortality. We further observed a greater mortality burden using PM_2.5 DECHs than daily mean PM_2.5 (6478 (95% CI: 5071, 7917) VS 5136 (95% CI: 3990, 6305)). Conclusion This study reveals that PM_2.5 DECH is one important exposure indicator of ambient PM_2.5 to measure its cardiovascular mortality effects in Pearl River Delta region; and that using daily mean concentration could under-estimate the mortality burden compared with this new indicator.