WITH CLIMATE CHANGE causing temperatures to rise worldwide, extreme heat is becoming more and more of a health threat. The human body is resilient, but it can only handle so much. The big question: What is the highest temperature people can endure?

The answer is a wet-bulb temperature of 95 degrees Fahrenheit (35 degrees Celsius), according to a study. 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. But when both the humidity and the temperature are very high, the wet-bulb temperature can move 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 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.

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 health experts.

A wet-bulb temperature of 95 F won’t cause immediate death – it probably takes about 3 hours for that heat to be survivable. There’s no way to know for sure the exact amount of time, but studies have tried to estimate it by immersing human participants in hot water tanks and removing them when their body temperatures begin to rise uncontrollably. There also isn’t a way to confirm that 95 F is the exact wet-bulb temperature that’s survivable. It is 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.

Few locations have hit a wet-bulb temperature of 95 F in recorded history. There are places that are already starting to experience these conditions for an hour or two. 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 (Philippines?) and West Africa. By Manny Palomar, PhD (EV Mail July 10-16, 2023 issue)