Mortality risk associated with heatwave exposure based on daily maximum, minimum, and combined temperature thresholds.

Most prior studies assessed heatwave-related mortality using daily mean temperature as an indicator, limiting the ability to differentiate between daytime and nighttime heat effects. We collected individual mortality records with corresponding residential exposure data on daily temperature, relative humidity and ozone during warm seasons from 2016 to 2022 in Jiulongpo district, Chongqing, China. Heatwaves were categorized into three types: those defined by daily maximum temperature, daily minimum temperature, and a combination of both. A time-stratified case-crossover design was applied to assess the associations between heatwaves and mortality. During the study period, 17 552 deaths were recorded. We observed that heatwaves defined by combined temperature thresholds were associated with the highest mortality risks, with odds ratios (ORs) ranging from 1.08 (95% CI: 1.01-1.15) to 1.32 (95% CI: 1.17-1.48) under different heatwave definitions. For heatwaves defined by daily maximum temperature, ORs ranged from 1.06 (95% CI: 0.98-1.14) to 1.13 (95% CI: 1.03-1.24), while heatwaves defined by daily minimum temperature showed ORs ranging from 1.04 (95% CI: 0.99-1.10) to 1.27 (95% CI: 1.13-1.43). Exposure to heatwaves was consistently associated with increased risks of all-cause and cardiovascular mortality, but not respiratory mortality. The associations were stronger among men and under higher ozone conditions compared to their counterparts. Exposure to heatwaves significantly increased mortality risks, with the highest risks observed for compound heatwaves involving both daytime and nighttime heat. These findings underscore that the health risks associated with nighttime heat exposure should not be overlooked.