A research group from the University of Tokyo conducted one of the largest global studies on the effects of humid heat environments on human health and released the findings. The study revealed that in regions like Japan and coastal areas of the United States, humidity significantly impacts mortality risk.
Humid Heat Index Could Become a Key Metric in the Era of Global Warming
This study was conducted in 739 cities across 43 countries and regions worldwide. The goal was to evaluate how the humid heat index, which considers both temperature and humidity, correlates with mortality risk. Notably, regions such as Japan, South Korea, coastal areas of the United States, and Peru were found to have a strong correlation between the humid heat index and mortality risk. These findings are expected to contribute to the improvement of heatstroke warning systems in the future.
The study used various humid heat indices, such as wet-bulb temperature (Tw), wet-bulb globe temperature (WBGT), and heat index (HI), to analyze the relationship between each city’s mortality risk. It became clear that the humid heat index could predict mortality risk more accurately in specific regions compared to when only temperature was considered.
In particular, the WBGT was found to have the strongest correlation with mortality risk in Japan, South Korea, coastal regions of the U.S., and areas around the Great Lakes. In these regions, fluctuations in temperature and humidity do not necessarily align, so neglecting humidity can lead to underestimating actual heat stress.
In recent years, the frequency and intensity of heatwaves have been increasing due to climate change, and it is expected to worsen in the future. While many countries and regions issue heatstroke warnings based on temperature alone, there is a growing emphasis on humidity-inclusive indices. This research highlights the risks of underestimating danger by ignoring hThe study also revealed that local climate characteristics and socio-economic factors influence the effectiveness of the humid heat index. In areas like Japan, where high humidity is typical during summer, the WBGT can serve as an effective index. These findings provide scientific support for Japan’s national heatstroke alerts.
The University of Tokyo’s research group plans to continue with more detailed analyses by region based on these findings. It is expected that they will propose specific strategies to mitigate health risks in different areas by considering appropriate responses to humid heat environments.
The research team was led by Professor Taikan Oki from the Graduate School of Engineering and Professor Masahiro Hashizume from the Graduate School of Medicine at the University of Tokyo. The team included specially appointed researcher Dr. Qiang Guo and members of an international collaborative research network. The findings have been published in PNAS Nexus and deepen global understanding of humid heat environments.
Press release available here