Future heatstroke mortality in Japan: Impacts of climate, demographic changes, and long-term heat adaptation

The International Classification of Diseases (ICD)-10: X30, an indicator that reflects the direct health impacts of excessive natural heat, has rarely been applied in Japan. This study projects heatstroke mortality for all 47 prefectures in Japan using ICD-10: X30, accounting for demographic changes and long-term heat adaptation (HA)-two key factors influencing heatstroke risk. Projections were made for two age groups (<65 and ≥ 65 years) using five climate models under three greenhouse gas (GHG) emission scenarios aligned with shared socio-economic pathways (SSP1-RCP2.6, SSP2-RCP4.5, and SSP5-RCP8.5) across three time periods: base year (1995-2014), mid-21st century (2031-2050), and end of the 21st century (2081-2100). By the end of the 21st century, in the absence of HA, the average heatstroke mortality rate (HMR; cases per population) in Japan is projected to increase 4.26- and 3.63-fold for the <65-year and ≥65-year groups, respectively, relative to the base year, under all climate models and GHG scenarios. With HA, these increases were reduced to 1.76- and 1.49-fold for the <65-year and ≥65-year groups, respectively. Without HA, the average number of heatstroke mortality cases (HMC) is projected to increase 2.07- and 3.86-fold for the <65-year and ≥65-year groups, respectively, relative to the base year, by the end of the 21st century. With HA, these values were reduced to 0.88- and 1.58-fold, respectively. Both HMR and HMC decreased when HA was considered. These findings suggest that incorporating long-term adaptation measures into heatstroke risk management would enhance the effectiveness of public health planning.