Synergistic effects of daytime, nighttime, and compound extreme temperature events and ambient air pollution on cardiovascular mortality

Extreme temperature events (ETEs) and air pollution are established cardiovascular mortality risks, yet their synergistic effects under diurnal-specific joint exposure remain understudied. We aimed to examine whether joint exposure to diurnal-specific ETEs and air pollution exert a synergistic effect on cardiovascular mortality. This time-stratified case-crossover study included 148,081 cardiovascular deaths in Bozhou, China, from 2013 to 2023. Daytime, nighttime, and compound heatwave and cold spell exposures were assigned based on residential addresses. Air pollutant concentrations (PM2.5, PM₁₀, ozone) were averaged over lag0-3 days. Conditional logistic regression was employed to estimate independent effects and additive interactions of these exposures on cardiovascular mortality, with attributable deaths and economic losses calculated to quantify corresponding disease burden. Both air pollution and ETEs were associated with elevated cardiovascular mortality risk. For each 10 μg/m³ increment in pollutant concentration, the odds ratios (ORs) were 1.019 for PM2.5, 1.006 for PM₁₀, and 1.005 for ozone. Among ETEs, daytime heatwaves and nighttime cold spells emerged as the most threatening in warm and cold seasons, with calculated ORs of 1.254 and 1.801, respectively. Significant additive interactions were observed between particulate matter and compound heatwaves, with a relative excess risk due to interaction of 0.231 for PM2.5 and 0.200 for PM₁₀. Attributable burden analysis showed PM₁₀, nighttime cold spells and compound heatwaves accounted for the highest excess cardiovascular deaths, with corresponding economic losses representing 0.856-2.751% of local gross domestic product. Our findings reveal that ambient air pollutants, diurnal-specific heatwaves and cold spells increase the risk of cardiovascular mortality, with particulate matter exhibiting significant positive additive interactions with compound heatwaves.