"Fifteen years ago, scientists proposed an environmental threshold at which no person would be able to survive for six hours.
But these conditions have never been tested on humans.
Until now.
This study is all about human survivability,” Dr Cheng says."

The test used a Marathon runner of course

"a wet-bulb temperature of 35C — the critical threshold at which no human can survive for more than six hours, according to the original theory."

"Halfway into the three-hour experiment, Owen’s core temperature is starting to climb — currently at 38.4C, up from his starting temperature of 37.13C.From a core temperature of 39C, mild heat exhaustion, such as headaches and faintness, can begin to occur.
At 40C the risk of severe heat exhaustion, including vomiting and disorientation, becomes increasingly likely.
At more than 40.5C, your risk of heat stroke escalates rapidly."

Scientists testing deadly heat limits on humans show thresholds may be much lower than first thought - ABC News
Trivia - Out Running last night in a windy 12 degrees I was sweating like I was in this test

"A 2020 study, published in Science Advances, found there have been a handful of instances, all in the past decade, where places have briefly reached that threshold — in Saudi Arabia and Pakistan."

Most of us would have been in many 40 degree plus days. Knowing how hot is too hot is important.