LEILA FADEL, HOST:
There's a question on many scientists' minds as climate change heats up the planet.
ROB MEADE: Basically, how hot is too hot for humans to survive for long periods?
FADEL: That's Harvard scientist Rob Meade, and his new study is getting scientists closer to an answer. Alejandra Borunda with NPR's climate desk reports.
ALEJANDRA BORUNDA, BYLINE: Rob Meade looks at how human bodies handle heat. But in his new study, Meade wasn't just a researcher. He was a guinea pig. He put himself and 11 other volunteers into a heat chamber.
MEADE: We exposed them to 107 Fahrenheit, so 42 degrees Celsius.
BORUNDA: So, you know, hot.
MEADE: We let them equilibrate for an hour, and then we slowly cranked the humidity up.
BORUNDA: Then he and other researchers watched what happened to people's core body temperatures. They would heat up pretty steadily, and then they would cross a threshold.
MEADE: I've been referring to them as you're screwed limits.
BORUNDA: Beyond that threshold, their core temperatures skyrocketed. Left unchecked, their bodies would've succumbed to heat stroke and eventual death. But the researchers pulled them out of the heat chamber right after they passed that threshold. Meade lasted six hours in the heat chamber.
MEADE: Your body's screaming at you let's get out of the heat, let's get out of the heat.
BORUNDA: But millions of people around the world can't get out of the heat, especially as climate change makes it more intense. And that growing threat opened a scientific question - are there places on Earth that might become too hot to live? Scientists assumed there was a physical threshold beyond which people couldn't survive for more than a few hours, and for years they thought it was 35 degrees Celsius measured on a scale called the wet bulb temperature. That's equivalent to 95 degrees Fahrenheit at 100% humidity. But this new study and a few others suggest that...
JENNI VANOS: The limits to survivability, or these critical environmental limits to compensability, are lower than we thought.
BORUNDA: That's Jenni Vanos. She's a heat expert at Arizona State University who did a similar analysis last year. And Vanos says scientists also know that people die from heat well below these extreme limits because heat puts so much stress on people's bodies. For example, the participants in Meade's study...
MEADE: Their body temperature was elevated by a degree to a degree and a half. Their hearts were beating 40 beats a minute faster than normal.
BORUNDA: Stress like that can trigger issues like heart attacks and strokes. There are places in the world where people already deal with heat at that limit. King's College London climate scientist Tom Matthews mapped those places out a few years ago.
TOM MATTHEWS: We found that wet bulb temperature of 35 degrees had already been reached around the Persian Gulf, in the Indus Valley.
BORUNDA: And further climate change will push more places toward those heat limits, especially if the limits are actually lower than 35 C on the wet bulb temperature. But Matthews says the next step is to really understand how people who already live in those places are surviving the most extreme extremes because...
MATTHEWS: We're not simply products of the - or not totally at the mercy of the temperature and humidity that's outside.
BORUNDA: People find ways to survive that super intense heat and learning how they do so could help millions of others.
Alejandra Borunda, NPR News.
(SOUNDBITE OF CHANCLA AND OATY.'S "AZOIA") Transcript provided by NPR, Copyright NPR.
NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.