Association between long-term extreme high temperature and mortality: a 9-million prospective cohort study

Climate change has led to the increasing frequency of extreme high temperature events. However, few attentions have been paid to effective protective measures to mitigate the negative impacts of extreme high temperature. This study aimed to explore the relationship between long-term exposure to extreme high temperature and mortality, as well as effective protective measures.

We established a large cohort using medical and general health checks data from nearly 10 million people in Northwestern China between 2019 and 2023. We defined three distinct long-term extreme high temperature indicators based on the 99th percentile threshold of individual temperatures over the past 10 years: extreme high temperature durations (EHTD), extreme high temperature frequency (EHTF), and extreme high temperature severity (EHTS). A proportional hazards model was employed to explore the relationship between extreme high temperature and mortality, as well as to investigate the protective role of general health checks.

For each unit increase in the EHTD, EHTF, and EHTS, the hazard ratio for mortality risk was 1.0022 (95% CI, 1.0020-1.0025), 1.0148 (95% CI, 1.0137-1.0159), and 1.0036 (95% CI, 1.0036, 1.0024-1.0048), respectively. However, when extreme high temperature was defined based on regional thresholds (county or city level) rather than individual thresholds (street level), there were no significant associations. Finally, we observed that general health checks' participants had a lower mortality risk from extreme high temperature exposure compared to non-participants.

Our study provided new evidence for assessing the adverse health effects of extreme high temperature. Moreover, this study highlighted that general health checks can effectively mitigate the negative health impacts of extreme high temperature.