A Global Assessment of Humid Heat Limitations on Livability

Humid heat exposure presents an increasing problem for human health and well-being, particularly for vulnerable populations with limited adaptive capacity. Much previous work has shown where ambient heat exposure could present the most severe threat to human health using heat stress metrics, such as wet-bulb temperature or heat index. Yet, estimating human health impacts using heat stress metrics typically involves assumptions about certain levels of physical activity (rest or minimal activity) and age/health status (healthy, younger adults). Here we employ a human heat balance model (Python Human Heat Balance model, or PyHHB; Vanos et al., 2023) to estimate globally where and when ambient heat and humidity limit ‘liveability’, or the level of physical activity that a person can sustain without increasing internal heat storage. Specifically, we use hourly ERA5-Land reanalysis data (1950-2023) to assess where ambient temperature and humidity limit physical activity in the shade after 3 hours of exposure (assuming air movement of 1m/s, as would be generated by an electric fan) for acclimatized healthy, younger adults (age 18-40 years) as well as acclimatized healthy older adults (age >65 years). We also examine the number of hours per year that are hot and humid enough to limit physical activity to light to moderate rates. We find that for younger adults, heat and humidity limit light to moderate activity for the most hours per year in low-lying coastal countries in South and Southwest Asia. Furthermore, over the last 70 years, these areas in South and Southwest Asia have also experienced the largest increases in numbers of hours per year that are too hot and humid for light to moderate physical activity. For older adults, heat and humidity limit light to moderate physical activity for the most hours per year in low-lying coastal countries in South and Southeast Asia as well as Sub-Saharan Africa. Finally, we stratify country-level average livability limitations by Human Development Index (HDI) to assess where heat hazard spatially intersects with potentially more and less vulnerable populations. We show that for younger adults, although heat and humidity limit activity in many vulnerable (low-HDI) countries, physical activity is limited for the most hours per year in several high-HDI countries in Southwest Asia. By contrast, for older adults, heat and humidity limit physical activity for the most hours per year in several low-HDI and middle-HDI countries in Southeast Asia, South Asia, and tropical Africa.