Nationwide high-resolution heat risk projections and intervention cost analysis for the elderly in Japan under climate and demographic changes.

Global warming and population aging are amplifying heat-related health risks in Japan. The country records approximately 1000 heatstroke deaths annually, over 80 % of which involve elderly people. This study aimed to project future elderly populations at risk of heat exposure and to evaluate the costs of potential interventions under climate and demographic changes. Using high-resolution (1 km) wet-bulb globe temperature (WBGT) projections combined with population data, we estimated the distribution and total of the at-risk elderly population (AREP), defined as the elderly population residing in grid cells where WBGT reach heatstroke risk thresholds, and their potential cumulative exposure (PCE, person-hours) under four shared socioeconomic pathways. We also projected intervention costs for residential air-conditioner installation and electricity subsidies. For the 2060s to the 2080s, using thresholds that reflect the regional vulnerability of the elderly, AREP was widely distributed across the mainland and southern areas, totaling 30-32 million. In most municipalities, AREP exceeded 40 % of the local population. PCE ranged from 4.8 billion person-hours under the lowest emission scenario to 10.0 billion under the highest, more than a twofold difference. Annual intervention costs were estimated at 164 to 177 million USD using 4 % discount rate. These costs were outweighed by benefits only when a low discount rate (0.1 %) was applied to future health impacts, underscoring the ethical implications of discounting in intergenerational health risk assessments. Our findings provide spatially detailed insights to support the design of effective, targeted interventions for managing future heat-health risks.