Older Adults Now Face 900 Hours a Year When Heat Makes Daily Activity Unsafe

In Qatar, an older adult now spends roughly one-third of the year in conditions where the combination of heat and humidity makes it unsafe to do anything more strenuous than sweeping a floor in the shade. That is more than 2,820 hours annually, up from about 2,300 hours in the mid-20th century.

Qatar is an extreme case, but the trend it illustrates is global. A study published March 10 in Environmental Research: Health documents how climate change since the 1950s has expanded the periods when heat constrains what the human body can safely do. The analysis, led by Luke Parsons of The Nature Conservancy with co-author Jennifer Vanos of Arizona State University, covers 75 years of hourly temperature and humidity data overlaid on global population distributions.

Measuring what the body can do, not just how hot it feels

Most heat studies use indices like wet-bulb globe temperature or heat index to describe how dangerous conditions are. This study took a different approach. The researchers used a physiological model to estimate how much physical activity people of different ages could safely perform at various combinations of temperature and humidity without their core body temperature rising uncontrollably.

They defined "severe livability limitations" as conditions where any activity more strenuous than light housework in the shade would push the body toward heat illness. That threshold is lower than it sounds. Walking at a moderate pace, gardening, and most manual labor all exceed it.

The numbers, globally

For healthy young adults (ages 18 to 40), average global exposure to severe livability limitations rose from about 25 hours per year during 1950 to 1979 to about 50 hours per year during 1995 to 2024. That is a doubling.

For adults 65 and older, whose bodies regulate heat less efficiently, the shift is more dramatic in absolute terms: from about 600 hours per year in the earlier period to about 900 hours in the later one. A 50% increase, representing an additional 300 hours annually when basic outdoor activity becomes physiologically unsafe.

In 2024, the hottest year on record, more than 43% of young adults and nearly 80% of older adults worldwide experienced at least some periods of severely heat-limited conditions.

Where the heat hits hardest

South and Southwest Asia bear the greatest burden. In Cambodia, older adults now experience an additional 686 hours of severe limitations annually compared to the 1950s. In Thailand, the increase is 568 hours. In Bangladesh, 390 hours. In Qatar, the increase for young adults alone was 484 hours, bringing their total to 866 hours per year.

In the United States, older adults now face about 270 hours of severely heat-limited conditions annually, up from about 200 hours in the 1950s. Several areas across the South and Southwest show hundreds of hours of severe limitations. Southwestern and eastern North America rank among the regions with the largest absolute increases.

Southern South America, the eastern Sahara, much of Europe, and southern Australia also show significant increases.

One degree of warming, widespread consequences

The researchers noted that these widespread livability limitations have emerged with just over 1 degree Celsius of global warming driven by human activity. Current trajectories suggest warming of 2 to 3 degrees by century's end. If the relationship between warming and livability limitations scales roughly linearly, the future burden will be substantially larger than what has already been observed.

Regions already hot enough to impose severe limitations, like sub-Saharan Africa and southern Asia, are also expected to experience rapid population growth. The collision of increasing heat exposure and growing population means the number of person-hours spent in unsafe conditions will rise from both directions.

Adaptation is possible but uneven

Access to air conditioning, infrastructure, workplace protections, and medical care can reduce heat exposure substantially. But access is far from universal, even in wealthy nations. In the United States, low-income households and outdoor workers remain disproportionately exposed. In countries like Bangladesh and Cambodia, where many people in these regions have limited economic resources to cope with heat, the gap between physiological risk and adaptive capacity is particularly wide.

What the study cannot capture

The analysis uses hourly temperature and humidity data from weather stations and reanalysis products, not direct measurements of human heat exposure. People who spend time indoors with cooling, in vehicles, or in shade experience conditions different from what the ambient data describes. The results represent a physiological potential for harm rather than actual harm experienced.

The physiological model assumes healthy individuals. People with cardiovascular disease, diabetes, obesity, or other conditions are more vulnerable to heat than the model predicts. The actual population experiencing dangerous heat stress is likely larger than the estimates suggest.

The study focuses on temperature and humidity but does not incorporate solar radiation, wind, or behavioral adaptations like changing work schedules, all of which influence real-world heat exposure.

Despite these limitations, the core message is direct: the livable hours in a year are shrinking for hundreds of millions of people, and the primary driver is fossil fuel combustion.