With climate change causing temperatures to rise across the globe, extreme heat is becoming more common — and thus a bigger health threat. The human body is resilient, but it can only handle so much. So what is the highest temperature people can endure?
The answer is straightforward: a wet-bulb temperature of 95 degrees Fahrenheit (35 degrees Celsius), according to a 2020 study in the journal Science Advances. Wet-bulb temperature is not the same as the air temperature you might see reported by your local meteorologist or favorite weather app. Rather, a wet-bulb temperature is measured using a thermometer covered in a water-soaked cloth, and the resulting metric takes into account both heat and humidity. The latter is important because with more water in the air, it's harder for sweat to evaporate off the body and cool a person down.
If the humidity is low but the temperature is high, or vice versa, the wet-bulb temperature probably won't near the human body's tipping point, said Colin Raymond, a postdoctoral researcher at NASA's Jet Propulsion Laboratory who studies extreme heat. But when both the humidity and the temperature are very high, the wet-bulb temperature can creep toward dangerous levels.
For example, when the air temperature is 115 F (46.1 C) and the relative humidity is 30%, the wet-bulb temperature is only about 87 F (30.5 C). But when the air temperature is 102 F (38.9 C) and the relative humidity is 77%, the wet-bulb temperature is about 95 F (35 C), the "danger zone" for humans.
The reason people can't survive at high heat and humidity is that they can no longer regulate their internal temperature. "If the wet-bulb temperature rises above the human body temperature" — around 98.6 F (37 C) — "you can still sweat, but you're not going to be able to cool your body to the temperature that it needs to operate at physiologically," Raymond told Live Science.
At this point, the body becomes hyperthermic, experiencing heat stroke at an internal temperature above 104 F (40 C). This can lead to symptoms such as a rapid pulse, confusion, delirium, itchy skin, faintness and coma, according to Cleveland Clinic.
A wet-bulb temperature of 95 F won't cause immediate death, however; it probably takes about 3 hours for that heat to kill you, Raymond said. There's no way to know for sure the exact amount of time, he said — some studies have tried to estimate it, but it's not ethical to actually subject humans to such high temperatures for hours.
There also isn't a way to confirm that 95 F is the exact wet-bulb temperature that's unsurvivable. Raymond estimated that the true number is somewhere in the range of 93.2 F to 97.7 F (34 C to 36.5 C).
Who's at risk of unlivable temperatures?
Air conditioning can save people from unlivable heat — but not all people have access to it, and even in places where many people have air conditioning, the electrical grid may be unreliable, Raymond said.
A few locations have hit a wet-bulb temperature of 95 F in recorded history, according to the Science Advances study, though they've only done so for short periods of time. Since the late 1980s and 1990s, hotspots have been the Indus River Valley of central and northern Pakistan and the southern shore of the Persian Gulf.
Jacobabad in Pakistan, sometimes called the hottest city in the world, has surpassed a 95 F (35 C) wet-bulb temperature at least four times, and cities including La Paz, Mexico; Port Hedland, Australia; and Abu Dhabi, UAE, have recorded temperatures higher than 90 F (32 C) wet-bulb.
"There are places that are already starting to experience these conditions for an hour or two," Raymond said. "And with global warming, that's only going to become more frequent." Regions at risk of these temperatures in the next 30 to 50 years include northwest Mexico, northern India, Southeast Asia and West Africa, he added.
"Unfortunately, with the climate change that's already locked in, we'll continue to warm up a fair bit, even if we stopped emitting greenhouse gases today," Raymond said. "I think it's inevitable that those places I mentioned will be grappling with this issue for the foreseeable future, and I hope more places don't get added to that list."
The "force multiplier" of disease
Some researchers argue that the 95 F wet-bulb threshold actually underestimates the dangers of high temperatures on human health.
Pope Moseley, a researcher at Arizona State University who studies heat-related illnesses, cautions that the number focuses on the maximum temperature a person can handle under optimum conditions. In reality, factors including age, physical fitness, certain medications, and health conditions can all limit a person's ability to cope with the heat even at lower temperatures.
For instance, exercise and exposure to direct sunlight make it easier to overheat at lower temperatures. Older adults; people with certain health conditions, such as hormonal disorders or obesity; and people who take antipsychotics can't regulate their temperature as well, so they're more vulnerable to heat-related death.
Moseley gives the example of a young, fit person versus an older person with heart failure and dementia who forgets to drink water. "The upper limit of survival for those two people looks very, very, different," he cautioned.
What's more, heat-related deaths and illnesses aren't always straightforward. Mosely said that during heatwaves, many deaths may not be recorded as primarily heat-related, because the majority of deaths occur due to the exacerbation of pre-existing conditions.
High temperatures can worsen many medical issues; heat can aggravate high blood pressure and other cardiac issues, for example, because it affects the constriction of blood vessels. People with kidney disease are at risk in the heat because it impacts fluid levels, which people with kidney disease need to strictly regulate. And some severe psychiatric illnesses can impact a person's ability to recognize early signs of heat stress, putting them in danger in heat waves. Other medical conditions that can be worsened by heat include diabetes, asthma, and dementia.
Because of these connections, Moseley calls heat a "force multiplier of disease" — a factor that makes many other conditions worse. So the real impact of heat is actually much more far-reaching, and many people could face deadly health problems at wet-bulb temperatures far lower than 95 F.
"There are a lot of people who would suffer catastrophic illness before that," Moseley said.
This article is for informational purposes only and is not meant to offer medical advice.
Editor's note: This article was originally published on July 31, 2021. It was updated on Sept. 25, 2025 to include more information about high temperatures around the world and how wet bulb heat can be a "force multiplier" of disease.
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