As the global climate changes and the world grows hotter, more people are suffering from heat-related diseases such as heat exhaustion and heatstroke. To protect citizens, many governments publish national heat-health warnings. However, as Assistant Professor Guo points out, such warnings are often based on air temperature alone, despite a growing body of research—including two recent studies Guo has led—showing that additional factors, especially humidity, can also play a critical role.  

Why Humidity Increases Heat Stress 

The effect of global warming on human health has been a subject of scientific research for several decades. Mounting evidence shows that in addition to elevated air temperatures, factors like solar radiation, geographic location, and wind—and in particular high humidity in combination with high temperatures—play important roles in diseases caused by heat-induced stress. 

Our human bodies rely on perspiration (sweating) to maintain a safe temperature. If the air is both humid and hot, sweat evaporates less efficiently. This reduces the body’s ability to cool itself, resulting in physiological stress. Because many government heat-health warnings do not incorporate humidity, we decided to examine the association between multiple heat stress indicators (HSIs) and mortality and morbidity in countries around the world, and to see if these findings were consistent across different environments. 

Global Survey Reveals Regional Variation

We worked with government agencies, health authorities, and related institutions in 739 cities in 43 countries and gathered epidemiological and climatological data such as air and humidity conditions, wind speeds, solar radiation, and mortality rates. Then we applied statistical modeling and machine learning techniques to evaluate how different heat-stress indicators incorporating humidity and those based on air temperature (Tair) alone were associated with heat-related deaths.   

Best Heat Measures for Mortality (Based on qAIC Model-Fit Score) 

Figure 1. Heat measures that best explain daily heat-related mortality across 739 cities.
Panel (a) maps each city’s Best-Fit Indicator (BFI)—the heat metric (either simple air temperature or a temperature–humidity index) that gave the lowest qAIC, a statistical score used to determine which heat measure best predicts mortality, when modeling deaths. The color bar under the world map shows how much each heat stress indicator (HSI) incorporates humidity, from low (yellow) to high (red). Panel (b) charts the number of cities where each indicator was the best predictor of mortality. In many coastal or humid regions, humidity-sensitive measures outperform plain air temperature, but in some areas air temperature alone works best. 
Modified from Guo Q. et al., 2024: Regional Variation in the Role of Humidity on City-level Heat-Related Mortality, PNAS Nexus, 3(8), pgae290, DOI: 10.1093/pnasnexus/pgae290. Copyright 2024 Guo Q. et al. 

In 70 percent of the cities—including many in Japan, South Korea, Peru, and the U.S.—heat measures that incorporate humidity showed stronger associations with mortality. Notably, these locations were often near the coast or large lakes and rivers where high temperatures and high humidity frequently occur together, compounding heat stress. By comparison, in roughly 30 percent of the cities, air temperature alone performed as well—or better than—heat-stress indicators incorporating humidity. Typically, in these locations (Thailand and South Africa are two examples) when temperatures rise, relative humidity tends to fall, which reduces the additional heat stress from the moisture in the air. 

These findings suggest that governments in humid regions relying on air temperature alone for heat-health warnings should consider incorporating additional variables, especially humidity.  

Japan’s Heat-Health Warning System 

Numerous Japanese cities experience severely hot, humid summers and fall into the 70 percent category noted above. In 2021, the government revised its national heat-health warning system, changing it from a method based on air temperature to one based on the wet-bulb globe temperature (WBGT) index. WBGT is a measure of heat as it affects humans and includes humidity, solar heat, and wind, in addition to air temperature. 

To investigate the rationality of the government change for all of Japan’s 47 prefectures, we tested the predictive power of different HSIs in modeling heat-related mortality and morbidity. Regarding mortality, heat–health indicators incorporating humidity performed similarly to air temperature alone at predicting deaths for 2015-2019. However, for heat-related illnesses like heatstroke, WBGT regularly outperformed air temperature in identifying increases in such illnesses.  

Figure 2. Comparison of heat metrics in identifying extreme heat days across Japan.
This figure shows the overlap rate (%) in the timing of the hottest ten days of the warm season (June–September, averaged for 2015–2019) as detected by different heat stress indicators and air temperature (Tair).  
Panel (a) shows the overlap rate in the timing of the ten hottest days detected by wet-bulb globe temperature (WBGT) and Tair across Japan’s 47 prefectures. Panel (b) shows the overlap rate between each heat stress indicator (HSI) and Tair  across the 47 prefectures, ordered from north to south; the legend below it identifies the HSIs used. 
Guo Q. et al., 2024: Evaluating Japan’s revised heat-health warning system in the face of recent escalating heat stress, Environmental Research Letters, 19, 054002, DOI: 10.1088/1748-9326/ad3a81. Copyright 2024 Guo Q. et al. 

This is an especially important distinction where Japan is concerned. The country is experiencing a rapidly aging population, a segment of society particularly vulnerable to heat stress. Consequently, heat-related morbidity can burden the healthcare system, and in cases of heatstroke, can be fatal for the aged. A warning system that more accurately predicts peaks of morbidity will not only enable more appropriate heat-health warnings, but will also better prepare health authorities, ambulance services, hospitals, and cooling centers for an influx of patients. 

Implications for a Warming Planet and Further Research 

The findings of our research support the Japanese government’s change of policy in adopting the wet-bulb globe temperature index. The results also contribute to the ongoing scientific discussion concerning which environmental factors are more important in determining heat morbidity—however, caution must be exercised in extrapolating the findings to other geographies given the diversity of climate conditions around the world. Even in Japan, we found significant differences in how the various HSIs identified heat–stress events, particularly in southern parts of Japan. This caveat notwithstanding, our study indicates that, provided a government is careful in its choice of a HSI for a particular location, systems incorporating humidity will outperform systems based on air temperature alone in identifying heat-related morbidity, particularly in hot, humid environments like Japan. 

We are now aiming to contribute to the development of climate-smart cities that can both adapt to and mitigate the negative impact of anthropogenic climate change. Areas of focus include urban heat–island effects, optimizing urban surface and radiative properties to regulate urban temperatures, and assisting urban energy transition and decarbonization. Looking ahead, we aim to explore how urban environments can be modified and designed to achieve healthier and more comfortable living conditions.  

(Date of interview: February 12, 2026)