Intensifying global heat threatens livability for younger and older adults

Heat exposure presents a growing threat to human health and well-being, particularly for vulnerable populations. Here, we employ a human heat balance model - specifically the human/environmental adaptation and threshold limit model (HEAT-Lim) - to estimate, globally, where ambient temperature and humidity already limit ‘livability’, or the level of physical activity that a person can safely sustain without experiencing an uncontrolled rise in body temperature. Specifically, we use hourly ERA5-Land reanalysis data to assess historical (1950–2024) livability limitations for partially acclimated healthy, younger (age 18–40 years) and older (age >65 years) adults in the shade. We also examine the number of hours/year in which physical activity should be limited to light-to-moderate intensity (e.g., sitting, walking, light housework) to avoid uncontrollable rises in core body temperature. We find, globally, heat-associated livability limitations are on average greatest in high-vulnerability areas. Furthermore, there have been significant increases in livability limitations for both younger and older adults over the last 75 years, with noticeable spikes in El Niño years and in 2024. For younger adults, restrictions to light-to-moderate activity for the highest number of hours are geographically concentrated in moderate- to low-vulnerability countries in South and Southwest Asia. For older adults, restrictions on light-to-moderate activity are widespread in tropical Southwest, South, and Southeast Asia, as well as Sub-Saharan Africa. In the hottest hours of the year, some locations have already experienced ‘unlivable’ conditions (i.e., when no activity is possible to compensate for environmental heat loads). Results highlight that with just over 1 °C of historical global warming, livability limitations are already widespread and growing, particularly for older adults. If warming is not stopped and adaptation measures are not more widely implemented, livability constraints will only expand, particularly as the global population ages.