A research group from the University of Tokyo Graduate School has conducted the world's largest study to investigate the health effects of a humid and hot environment and found that the humid and hot index is more highly associated with mortality risk than temperature alone in coastal areas and the Great Lakes region of the United States, Peru, South Korea, and Japan.
The research group conducted the world's largest study, covering 43 cities in 739 countries and regions, and used daily mortality data and meteorological reanalysis data to examine the association between multiple humid heat indices (wet bulb temperature, wet bulb globe temperature, and heat index) and daily summer mortality risk in each city.
The 10 days with the highest annual humidity and heat index were evaluated over the past 40 years, and the period of high heat stress differed depending on the humidity and heat index. This suggests that selecting an appropriate humidity and heat index is important for improving the accuracy and effectiveness of heatstroke warning information.
In addition, among the heat and humidity indices, the wet bulb globe temperature (an index that takes into account humidity, thermal environment, and temperature) showed a higher correlation with mortality risk than temperature alone, particularly in coastal areas of the United States, areas around the Great Lakes, Peru, South Korea, and Japan. This is because, unlike areas where heat stress can be expressed by temperature alone due to the strong correlation between temperature and humidity, such as when temperature is high and humidity is low, in these areas the correlation between temperature and humidity is weak.
The results of this study provide strong evidence that the wet bulb globe temperature used in heatstroke alerts is a useful indicator in Japan, where the risk of heatstroke is higher on days with high temperatures and humidity.
Paper information:[PNAS Nexus] Regional Variation in the Role of Humidity on City-level Heat-Related Mortality
