With climate change causing temperatures to rise across the globe, extreme heat is becoming more and more of a 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 forecaster or favorite weather app. Rather, a wet-bulb temperature is measured by a thermometer covered in a water-soaked cloth, and it 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).
Related: Why is humidity so uncomfortable?
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, 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 — above 104 F (40 C). This can lead to symptoms such as a rapid pulse, a change in mental status, a lack of sweating, faintness and coma, according to the National Institutes of Health.
A wet-bulb temperature of 95 F won't cause immediate death, however; it probably takes about 3 hours for that heat to be unsurvivable, Raymond said. There's no way to know for sure the exact amount of time, he said, but studies have tried to estimate it by immersing human participants in hot water tanks and removing them when their body temperatures began to rise uncontrollably. 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 in the range of 93.2 F to 97.7 F (34 C to 36.5 C).
Although no one can live at a wet-bulb temperature higher than about 95 F, lower temperatures can also be deadly. Exercise and exposure to direct sunlight make it easier to overheat. Older people; people with certain health conditions, such as obesity; and people who take antipsychotics can't regulate their temperature as well, so it's easier for heat to kill them. This is why people sometimes die in heat that does not reach a wet-bulb temperature of 95 F.
Luckily, air conditioning can save people from unlivable heat. But, of course, 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.
Few locations have hit a wet-bulb temperature of 95 F in recorded history, according to the Science Advances study. 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. "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." Locations that are 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."
Originally published on Live Science.