Heatwaves, Cold Spells, and Renal Disease Mortality in Japan: Vulnerable Populations and Temporal Trends (1979-2019).

This study aims to investigate the short-term associations between heatwaves, cold spells, and renal disease mortality in Japan, focusing on temporal variations and identifying vulnerable populations. We obtained daily mortality counts for all renal diseases, acute renal failure (ARF), and chronic kidney diseases (CKD) from 47 prefectures in Japan from 1979 to 2019. Heatwaves and cold spells were defined based on duration (2, 3, and 4 consecutive days) and intensity (95th, 97.5th, and 99th percentile thresholds for heatwaves and 5th and 2.5th percentile thresholds for cold spells) using prefecture-specific daily mean temperature percentiles. A two-stage time-stratified case-crossover analysis was conducted. Prefecture-specific estimates were derived in the first stage by comparing the risk on extreme event days to nonevent days, and the national-level estimate was obtained from a univariate random-effect meta-analytical model in the second stage. Added effects of heatwaves and cold spells were estimated by adjusting for mean temperatures in the first-stage models. Subgroup analyses by age (0-64, 65-84, and >85) and sex and temporal analyses across four subperiods were also conducted. The statistical significance of changes over time was checked by conducting pairwise comparisons between the risk estimates from the first (1979-1988) with the subsequent subperiods. Our study included 300,506 renal deaths during summer and 367,663 during winter. Heatwaves were associated with increased renal disease-related mortality where the highest relative risks (RRs) for renal mortality were observed at the 99th percentile threshold lasting for ≥4 days (99th, 4 days): RR = 1.07 (95% confidence interval: 1.05-1.17) for all renal causes, 1.17 (1.07-1.28) for ARF, and 1.11 (1.05-1.17) for CKD. Cold spells showed stronger effects, with the highest RRs at the 2.5th percentile threshold and 2-day duration: RR = 1.18 (1.11-1.25) for all renal mortality, 1.26 (1.01-1.58) for ARF, and 1.12 (1.01-1.25) for CKD. Added effects of heatwaves were observed, but not in the case of cold spells. Older adults aged 65 years and over are particularly vulnerable. Temporal analysis revealed a significant reduction in heatwave-associated mortality risk over time, suggesting possible adaptation. In conclusion, heatwaves and cold spells affect renal disease-related mortality, underscoring the necessity for comprehensive strategies to mitigate the health impacts of changing climate on renal health.