Heatstroke mortality under climate and demographic transitions: 28-year spatio-temporal analysis in Japan

The escalation of heat-related health risks due to climate change and population ageing is a growing global concern. However, their independent 
contributions to medically certified heatstroke mortality remain insufficiently quantified.

We analysed 28 years (1995–2022) of municipality-level records of medically certified heatstroke deaths (ICD-codes) across 1,831 municipalities and wards in Japan. We fitted hierarchical Bayesian spatiotemporal models incorporating summer temperature anomalies and the municipal share of residents aged ≥65 years.

Heatstroke mortality was higher in warmer summers and in municipalities with older population structures: 42% (RR=1.42, 95% CrI 1.36–1.49) per 0.65°C (1 SD) increase in summer temperature anomalies, and 24% (RR=1.24, 95% CrI 1.18–1.31)  per 10-percentage-point increase in the ≥65-year population share. Decomposition of national trends indicated that population ageing progressively increased baseline risk, while large positive temperature anomalies produced additional mortality peaks on this elevated baseline. A sustained upward shift in risk was observed from around 2007 onward. Residual spatial heterogeneity remained after adjustment, with clusters of elevated risk in both urban and rural municipalities.

These findings highlight the need for targeted, place-specific heat-health adaptation as climatic warming and population ageing continue to progress. As a super-aged nation, Japan offers valuable insights into mitigating future heat-related health burdens under ongoing climatic and demographic transitions.