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.

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.

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.

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.

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.

Although no one can live at a wet-bulb temperature higher than about 95 F, lower temperatures can also be deadly?

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, according to the Science Advances study.