US days above 105 degrees could triple as 'unlivable' heat edges closer to human limits

Researchers have spent years trying to define the point at which heat overwhelms the human body completely. Fresh analysis suggests some parts of the United States are moving closer to that limit as extreme heat becomes much more common.

Here's what to know

2010 research by scientists at Purdue University and the University of New South Wales found that a wet-bulb temperature of 95 degrees Fahrenheit (35 degrees Celsius) at full humidity represents an absolute "unsurvivable" limit for people. Because wet-bulb temperature accounts for both heat and moisture in the air, it offers a more realistic gauge of how punishing conditions are for the body.

Studies cited by Live Science indicate that in parts of the Southwest and Southeast, the hottest days qualify as "unlivable," meaning even a healthy adult resting in the shade with a slight breeze can still be in danger. The outlet also reported projections showing that U.S. days with a heat index above 100 degrees Fahrenheit may double, while days above 105 degrees could triple.

Heat is appearing more often in official mortality data. In 2023, 2,325 U.S. death certificates identified heat as the main cause or a major contributing factor, compared with 1,069 in 1999. After accounting for age, that was a 63% rise in heat-related deaths.

More background

Serious health risks arrive well before any theoretical survival boundary. Older adults, children and teens, outdoor workers, women, and people with certain medical conditions can face greater risk at lower temperatures, and physical exertion can cause heat stress to develop much faster.

Heat does not harm people only through heat stroke. It can also intensify existing illnesses, especially cardiovascular disease. Research highlighted by Live Science says extreme-heat days more than double the risk of heart attack, and the hottest days nearly double stroke risk. Under a high-warming scenario, U.S. deaths from all causes tied to heat could climb to nearly 20,000 annually by mid-century.

Heat can also do damage in less obvious ways. Hot nights reduce both sleep quality and quantity, and studies suggest that thinking and reasoning begin to worsen at about 89.6 degrees Fahrenheit. Kim Meidenbauer, a neuroscientist at Washington State University who studies heat and behavior, told Live Science, "We see worsened emotional state. We see increases in impulsivity."

What's being done?

Cities are increasingly opening cooling centers during dangerous heat waves, though researchers say it is still unclear which interventions save the most lives. To help sort that out, Stanford environmental policy professor Marshall Burke and his colleagues created the Environmental Hazard Adaptation Atlas, which brings together data on temperature, illness, deaths, and possible responses.

For households, the most effective protections are to follow heat alerts, stay hydrated, reduce strenuous outdoor activity during the hottest part of the day, and spend time in cooler indoor spaces whenever possible. People taking medications such as diuretics, beta blockers, or SSRIs may also want to talk with a doctor about additional heat-related risks, since those drugs can make it harder for the body to dissipate heat, as noted by Live Science.

Cutting the human-caused pollution driving rising temperatures remains essential to avoid more "unlivable" days, more overwhelmed hospitals, and more dangerous nights without relief.

Researchers say the next step is figuring out what truly protects people as heat intensifies. As Burke told Live Science: "We need good case studies of what things have worked. We need good systematic evidence."

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