A new study out of Stanford University has upended long-standing assumptions about who is most at risk when temperatures spike, finding that older adults reach the point where their bodies can no longer shed heat effectively at temperatures far lower than previously modeled. The research, which for the first time projects global uncompensable heat stress broken down by age group, found that people aged 60 and older have a heat-tolerance threshold roughly 8.5 to 13.5 degrees lower than younger adults, depending on humidity.
The findings, published by The Mercury News, describe uncompensable heat stress as a state in which heat exceeds the body's ability to maintain a stable core temperature, causing it to climb even after that threshold is crossed. Researchers combined climate projections, population estimates and laboratory heat-tolerance limits to build the model, according to the same report. Previous global heat projections relied on healthy adults in their 20s as the baseline, a methodological choice the new study argues dramatically understated risk for older populations.
Why Aging Bodies Struggle to Cool Down
The physiological reasons are straightforward but significant. Aging can weaken the thirst response, reduces blood-vessel dilation that moves warm blood toward the skin, and diminishes sweat-gland effectiveness — all mechanisms the body relies on to release heat. Chronic health conditions and the medications used to treat them can further interfere with sweating, circulation and other heat-tolerance mechanisms, according to the Mercury News report.
Researchers monitored participants' core temperatures in controlled laboratory experiments involving progressively increasing heat and humidity in environmental chambers to identify the exact point at which thermal balance is lost. Those heat-tolerance model thresholds come largely from controlled experiments conducted in the United States, though people living routinely in hotter climates may be somewhat more acclimatized to heat than the study's baseline populations. As a result, older adults are more likely to experience dehydration, and heat can be difficult to recognize before it progresses to heat exhaustion.
Warming Scenarios and Who Bears the Burden
Under a world warmed nearly three times as much as today, older adults are projected to experience heat their bodies cannot tolerate far more often than young adults would face in that same scenario, according to the Stanford study. People aged 60 and older worldwide represent about 22%, or roughly 290 million people, projected to face at least 180 hours per year of heat exceeding their age-specific tolerance under 2.7 degrees of global warming. Each additional degree of warming beyond that could expose roughly another billion people aged 15 and older worldwide to at least 180 hours of such heat annually.
The gap between generations widens further under more extreme warming. Younger adults would not experience comparable exposure until global warming approaches 7.2 degrees above preindustrial levels, and older people in parts of South Asia and the Persian Gulf could face uncompensable heat day and night for three consecutive months annually under 5.4 degrees of warming. South Asia and the Arabian Peninsula are projected to have some of the most intense convergence of aging populations and uncompensable heat, and children born now could experience severe climate effects as older adults toward the end of the century.
What Stanford Researchers Are Saying
Eran Bendavid, a Stanford professor of medicine and study co-author, said the study's central contribution is showing how much dangerous heat exposure will be borne by older people. Higher heat-tolerance limits used in warmer-climate scenarios reduced the estimated exposure figures somewhat but did not change the study's overall conclusions, per the Mercury News account.
Lara Cushing, an associate professor of environmental health sciences at UCLA, said the analysis illustrates a fundamental limit to society's natural ability to respond to rising temperatures. She added that resources are central to mitigating heat risk, noting that low-income households, unhoused people and outdoor workers may have fewer resources to escape dangerous temperatures. Air conditioning, cooling centers and other interventions can protect people from heat, Cushing and the study's authors note, but they do not eliminate the underlying physical limits of heat tolerance itself.
The California Picture
The stakes are not abstract for California, where residents aged 65 and older already suffer a heat-related mortality rate of 0.43 per 100,000, 2.5 times higher than the statewide average, according to state tracking data compiled by Tracking California and the state's environmental health hazard office. In the San Joaquin Valley, adults 65 and older account for 28% of all heat-related fatalities recorded since 2020 while making up just 12% of the region's population, per Public Health Watch. Inland Alameda, Contra Costa and Santa Clara counties experience much higher temperatures during heat waves than their coastal neighbors, while Bay Area coastal communities are somewhat buffered by the Pacific Ocean — though that buffer is thin, since San Francisco sees the largest percentage increase in emergency hospitalizations during extreme heat of any California city, with heat warnings triggered at just 85°F, according to the San Francisco Department of Public Health.
California has moved on several fronts to address the risk. In 2022, the state enacted Assembly Bill 2238, becoming the first in the nation to build a statewide extreme heat ranking and early warning system based on projected health impacts rather than raw temperature, according to the California Department of Insurance. Earlier this month, the Governor's Office of Land Use and Climate Innovation released the state's updated 2026 Extreme Heat Action Plan, directing agencies to coordinate outreach, cooling center expansions and protective measures for seniors and other vulnerable groups.
Gaps in Indoor Heat Protection Remain
Workplace rules have also tightened: Cal/OSHA regulations that took effect in 2024 require employers to monitor heat indices, provide water and guarantee cool-down areas whenever indoor work site temperatures reach 82°F. But no equivalent standard exists for rental homes. The California Department of Housing and Community Development submitted a report in February 2025, urging lawmakers to establish a maximum safe indoor residential temperature threshold of 82°F for the state's 14.6 million rental units — a gap that leaves many older tenants without any legal guarantee of a cool home, even as outdoor heat records fall.
Qinqin Kong, another voice in the research, said exposure to uncompensable heat stress does not mean immediate death, but warned that heat action plans should recognize that heat risk is not one-size-fits-all. Kong said communities may need accessible cooling spaces and systems designed to reach isolated people, along with earlier heat warnings tailored specifically for older adults. Access to air conditioning, transportation and safe housing can determine whether people facing identical weather conditions end up facing very different levels of risk, underscoring that continued warming will increasingly put people into conditions their bodies simply cannot tolerate.
