Assessing the impact of climate and population changes on heatstroke incidence in Saitama City, Japan

There is growing concern about heat waves caused by climate change. In Japan, the population is rapidly aging, and protecting older people from heatstroke is attracting public attention. Formulating evidence-based adaptation strategies is urgent for local governments, but quantitative information on future heatstroke risk is limited. Here, we assess the impact of climate and population changes on heatstroke incidence in Saitama City, Japan. We obtain anonymized emergency call records from the local government and classify heatstroke cases into 12 groups by location (indoor and outdoor), age group (0–14, 15–64, and 65+), and sex (female and male). Using this dataset, we develop statistical models of heatstroke risk and project changes in heatstroke incidence from 2010 to 2100. Key findings are as follows: Climate change in SSP5-8.5 increases heatstroke risk (cases per 1000 people per year), but the impact varies by group. Projections for the 2090s suggest that males have a higher heatstroke risk than females, regardless of the combination of location and age group. Males aged 65+ have a higher heatstroke risk than other groups both indoors and outdoors. Females are more tolerant of temperature increases than males, but the indoor heatstroke risk for females aged 65+ rapidly increases. Assuming a standard population scenario, the total heatstroke incidence (cases per year) for Saitama City increases by 6.00 times between the 2010s and 2090s. In the same period, the indoor heatstroke incidence increases by 5.47 times, and the outdoor heatstroke incidence increases by 6.48 times. The increase in heatstroke incidence is mainly due to climate change rather than population change. The impact of climate change is much smaller in SSP1-2.6 than in SSP5-8.5. Our results highlight the need for adaptation strategies that take into account the diversity of heatstroke risk functions.