Heat wave, fine particulate matter, and cardiovascular disease mortality: A time-stratified case-crossover study in Shenzhen, China

Background: In the context of global warming, the frequency of heat wave and the concentration of fine particulate matter (PM2.5) have increased, and more people are co-exposed to air pollution and extreme heat. However, the interaction between heat wave and PM2.5 on cardiovascular disease (CVD) mortality remained largely unknown. Methods: We conducted a time-stratified case-crossover study of 40,169 CVD deaths in Shenzhen, China between 2013 and 2022. Meteorological data and air pollutants information were obtained based on the residential addresses from the validated grid datasets. A total of 21 heat wave definitions were constructed using various relative temperature thresholds and durations. Conditional logistic regression was used to evaluate the independent and interactive effects of exposure to heat wave and PM2.5 on CVD mortality. Results: The odds ratios (ORs) and 95 % confidence intervals (CIs) for CVD mortality associated with heat waves ranged from 1.17 (95 % CI: 1.001,1.36) to 1.91 (95 % CI: 1.42, 2.56). For every increase of 10 mu g/m3 in PM2.5 exposure, the ORs (95 % CI) for CVD mortality ranged from 1.0283 (95 % CI: 1.0162, 1.0406) to 1.029 (95 % CI: 1.0169, 1.0413). There was a synergistic effect between heat wave and PM2.5 exposures on CVD mortality. It was estimated that up to 2.03 % of CVD deaths were attributable to heat wave and PM2.5 levels exceeding the interim target 4 in the World Health Organization air quality guidelines (>= 25 mu g/m3), resulting in 816 premature deaths. Females and individuals over 75 years old were vulnerable populations. Conclusions: Heat wave and PM2.5 exposures individually and synergistically contributed to increased risks of CVD mortality. Our findings indicate that reducing exposure to both heat wave and PM2.5 may yield significant health benefits and prevent a significant portion of premature deaths from CVDs.