Estimating the impact of acclimatization on heat-related illnesses under various representative concentration pathways among three Japanese cities in 2030s and 2050s

As more cities adopt resilient net-zero policies, local-level assessments of the current and future health impacts of warmer temperatures are becoming increasingly important. These assessments may be even more useful if they demonstrate how health impacts of different predicted temperatures vary across acclimatized/non-acclimatized groups. This study therefore assesses the health impacts associated with different climate scenarios and temperature increase for current and future population projections in three cities of varying sizes and capacities in Japan, namely Kawasaki city, Niigata city, and Hachinohe city. In the analysis, we used summer-specific (May to June), daily heatstroke data for each city that spanned from 2015 to 2019 and developed a temperature-heatstroke function that was further utilized as a basis for future acclimatized and non-acclimatized impact assessment. We found higher future heatstroke cases per 1000 (> 65-year-old) population among the theoretically non-acclimatized populations than acclimatized populations. Older individuals exhibit greater susceptibility under both climate scenarios, with risk further elevated under conditions of limited acclimatization. This result was consistent across the three cities. Initiatives supporting resilient net-zero transition, accounting or focusing on population acclimatization to increasing temperature, may lead to reduced heatstroke cases in the future.