Extreme Heat Risks for Older Adults Were Underestimated, Stanford Study Finds

A Stanford-linked modeling study published in The Lancet Planetary Health suggests that the danger extreme heat poses to older adults has been substantially underestimated. The central issue is physiological: heat limits based mainly on healthy young adults do not reflect how ageing can reduce the body’s ability to cool itself. When researchers incorporated age-specific heat tolerance, projected repeated exposure to “uncompensable” heat was two to eight times higher than earlier estimates.

The study’s central finding

The study compared the heat conditions that different age groups can tolerate with climate and population projections extending to 2100. Its most striking comparison is that older adults under 1.5°C of global warming could face a greater heat risk than young adults would under roughly 4°C of warming.

At 1.5°C above pre-industrial temperatures, an estimated 22% of people aged 60 and over—about 290 million people—could encounter unsafe heat for at least one month each year. The researchers describe each additional degree of warming as potentially exposing roughly another billion people to at least 180 hours of heat and humidity beyond the body’s ability to cool itself.

These are model projections, not a forecast that every older person in those areas will experience the same health outcome. Heat risk varies with humidity, duration, exertion, hydration, health status, access to shelter and cooling, and other individual and local conditions.

Why older adults face greater heat risk

The body normally sheds heat through mechanisms including sweating and increased blood flow to the skin. With age, heat tolerance can decline. Older adults may also be more vulnerable because cardiovascular function is weaker, sweating responses are reduced, and chronic cardiovascular, respiratory, or kidney conditions can make prolonged heat more dangerous.

Earlier global heat studies often used thresholds measured in healthy young adults or relied on broad environmental cutoffs. That approach can miss differences between age groups. It can also make a dangerous combination of high temperature and humidity appear less threatening than it is for people whose cooling capacity is already impaired.

What “uncompensable heat” means

“Uncompensable heat stress” occurs when the body cannot shed heat as quickly as it gains it. Sweating and other cooling responses are no longer sufficient, so core body temperature continues to rise. Under conditions just above a person’s heat-tolerance limit, the body can reach heatstroke range in approximately three to 10 hours, according to the supplied study reporting.

The relevant danger is therefore not just the thermometer reading. A lower air temperature can still be hazardous when humidity is high enough to prevent sweat from evaporating. Conversely, the same air temperature can pose different risks depending on humidity, wind, clothing, activity, hydration, and a person’s age and health.

How the researchers modeled risk through 2100

The analysis combined laboratory-tested heat limits for younger, middle-aged, and older adults with projected climate conditions and population distributions. The age groups represented in the underlying experiments were younger adults aged 15–39, middle-aged adults aged 40–59, and older adults aged 60 and over.

The researchers used wet-bulb-temperature-based heat-tolerance experiments to estimate when heat and humidity would exceed the body’s ability to regulate temperature. They then mapped exposure across warming scenarios from about 1°C to 4°C above pre-industrial levels, using population projections through 2100.

This approach differs from applying one universal threshold to the entire population. It is intended to show not only where heat becomes dangerous, but also how the burden changes as populations age and warming increases.

South Asia and the Gulf are projected hotspots

At 3°C of warming, India, China, Pakistan, and Bangladesh together could contain about 1.5 billion people exposed to repeated uncompensable heat—nearly three-quarters of the projected global total.

Some countries could face especially high proportional exposure. The study reporting estimates that Bahrain and Qatar could see exposure across their entire populations, the United Arab Emirates could approach 95%, and Pakistan could have nearly 295 million people—more than 80% of its population—affected.

In Bangladesh, Cambodia, and several Gulf states, all older residents could be exposed under some scenarios. Above 3°C of warming, older adults in the most exposed locations could face dangerous heat during both day and night for consecutive months, leaving little opportunity for the body to recover.

Why displacement and migration could become part of the risk

The projected exposure is concentrated in many tropical and subtropical regions where poverty, limited electricity infrastructure, restricted access to air conditioning, and other adaptation barriers may make it harder to protect vulnerable residents. The researchers warn that this combination could contribute to internal displacement and, in some circumstances, cross-border climate migration.

That conclusion is a risk implication rather than a quantified migration forecast. The supplied evidence identifies the possibility of displacement but does not provide a precise number of future migrants attributable to this heat exposure.

What the findings imply for heat protection

The clearest way to reduce projected exposure is to limit additional warming through rapid emissions reductions. The estimates rise sharply with each additional degree, so mitigation changes the scale of the adaptation challenge.

The study also points to the need for heat-action plans designed around age-specific risk. Useful measures include:

• early-warning systems that identify dangerous heat and humidity;
• emergency plans that check on isolated older adults;
• health-system preparation for heat-related illness and chronic-disease complications;
• reliable electricity and equitable access to cooling;
• cooling centres, building shading, insulation, and urban trees;
• hydration support and clear public guidance during prolonged heat.

Air conditioning can be an important protective measure where it is available and affordable, but it is not a complete solution. The supplied evidence does not establish that air conditioning alone can safely offset prolonged, highly humid heat, especially where power reliability or access is limited.

What the study does—and does not—show

The findings strengthen the case for age-specific climate-health planning, but they should be read as modeled exposure estimates rather than individual medical predictions. They focus on combinations of heat and humidity that exceed experimentally derived heat-tolerance limits, not on every heatwave or every heat-related death.

They also do not provide a Europe-specific assessment. Although increasingly severe European heatwaves make the broader issue relevant to European health systems, the supplied study reporting emphasizes global exposure, particularly in South Asia, China, and the Gulf.

The main takeaway is straightforward: a global heat-risk map based on young, healthy adults can understate the danger faced by an ageing population. Planning for future heat therefore requires both faster emissions cuts and local protection strategies that account for how age changes the human ability to cool down.