Heat Exposure and Dementia-Related Mortality in China.

Importance: Although existing research has found daily heat to be associated with dementia-related outcomes, there is still a gap in understanding the differing associations of nighttime and daytime heat with dementia-related deaths. Objectives: To quantitatively assess the risk and burden of dementia-related deaths associated with short-term nighttime and daytime heat exposure and identify potential effect modifications. Design, Setting, and Participants: This case-crossover study analyzed individual death records for dementia across all mainland China counties from January 1, 2013, to December 31, 2019, using a time-stratified case-crossover approach. Statistical analysis was conducted from January 1, 2013, to December 31, 2019. Exposures: Two novel heat metrics: hot night excess (HNE) and hot day excess (HDE), representing nighttime and daytime heat intensity, respectively. Main Outcomes and Measures: Main outcomes were the relative risk and burden of dementia-related deaths associated with HNE and HDE under different definitions. Analysis was conducted with conditional logistic regression integrated with the distributed lag nonlinear model. Results: The study involved 132 573 dementia-related deaths (mean [SD] age, 82.5 [22.5] years; 73 086 women [55.1%]). For a 95% threshold, the median hot night threshold was 24.5 °C (IQR, 20.1 °C-26.2 °C) with an HNE of 3.7 °C (IQR, 3.1 °C-4.3 °C), and the median hot day threshold was 33.3 °C (IQR, 29.9 °C-34.7 °C) with an HDE of 0.6 °C (IQR, 0.5 °C-0.8 °C). Both nighttime and daytime heat were associated with increased risk of dementia-related deaths. Hot nights' associations with risk of dementia-related deaths persisted for 6 days, while hot days' associations with risk of dementia-related deaths extended over 10 days. Extreme HDE had a higher relative risk of dementia-related deaths, with a greater burden associated with extreme HNE at more stringent thresholds. At a 97.5% threshold, the odds ratio for dementia-related deaths was 1.38 (95% CI, 1.22-1.55) for extreme HNE and 1.46 (95% CI, 1.27-1.68) for extreme HDE, with an attributable fraction of 1.45% (95% empirical confidence interval [95% eCI], 1.43%-1.47%) for extreme HNE and 1.10% (95% eCI, 1.08%-1.11%) for extreme HDE. Subgroup analyses suggested heightened susceptibility among females, individuals older than 75 years of age, and those with lower educational levels. Regional disparities were observed, with individuals in the south exhibiting greater sensitivity to nighttime heat and those in the north to daytime heat. Conclusions and Relevance: Results of this nationwide case-crossover study suggest that both nighttime and daytime heat are associated with increased risk of dementia-related deaths, with a greater burden associated with nighttime heat. These findings underscore the necessity of time-specific interventions to mitigate extreme heat risk.