Predicting the reduction in indoor heatstroke mortality under urban modification scenarios using an urban climate model and epidemiological data

In Japan, heatstroke-related mortalities have exceeded one thousand people during hot years. Particularly, older people are more vulnerable to heat exposure and are at a higher risk of developing or dying from heat-related diseases. Therefore, population aging in megacities is a serious problem. In the 23 wards of Tokyo, the most populated metropolitan area in Japan, the population aged 65 and older increased from 2,120,427 (21.8% of the total population in Tokyo) in 2025, and is expected to reach 2,403,126 (24.2%) in 2035.

Future climate changes are anticipated to amplify the health concerns of concentrated and aging populations in urban societies. Indeed, climate and epidemiological studies have highlighted that mortality from extreme heat will eventually become commonplace if no mitigation and adaptation measures are undertaken in urban areas. More than half of all heatstroke events in Japan occur in people aged 65 and older, and most of them occur indoor at home. Elderly people often do not use air-conditioning when sleeping at night, which is known to trigger heatstroke occurrence.

This study challenged a novel method of the exposure-response analysis predicting the decrease in heatstroke risk from the indoor temperature and WBGT decreases under urban modification scenarios obtained by mesoscale meteorological simulations. The scenarios evaluated in this study were ground surface greening, no anthropogenic heat from buildings to the atmosphere, rooftop photovoltaics, and cool roofs. Daily heatstroke deaths in the 23 wards of Tokyo during the extremely hot summer of 2018 were analyzed here. The urban climate simulations showed that nighttime indoor temperatures dropped by nearly 1 deg.C as the ward average for cool roof case, in the most effective wards. This effect could decrease more than 30% of heatstroke mortality risk. Our findings suggested a potential of significantly reducing a heatstroke risk when sleeping at night, by modifying urban building façade.