How hot is too hot for humans?

Record-setting heat waves have gripped the U.S. only weeks into summer, and at least 38 people are suspected to have died from heat-related issues so far this summer.

Climate change is fueling new heat extremes, and researchers who are racing to better understand the limits of human survival are increasingly focused on determining at what temperature thresholds people can survive without air conditioning and how soon some parts of the world could reach deadly levels.

Some scientists worry that a widespread heat wave could strike a region without reliable air conditioning and cause a mass death event. And recent research suggests that the threshold for surviving heat is far lower than what some scientists once thought, particularly for older people and in dry climates.

“We can only sweat so much, and older people even less,” said Jennifer Vanos, an associate professor in the School of Sustainability at Arizona State University.

“Where we live in the U.S., heat deaths should be 100% percent preventable. There should be air conditioning in some location. But, in some parts of the world, that doesn’t exist.”

Here’s what to know:

Keep the core cool

The human body can’t tolerate its temperature reaching 43 degrees C (about 109.4 degrees F).

“Anyone who reaches that core temperature — 99.9% would die,” Vanos said.

This is the upper limit of survival. Heat often kills in more subtle ways — by worsening pre-existing issues, like cardiovascular or renal disease.

Humidity can be key

Humans keep their core cool by sweating and shedding heat produced by their bodies.

“If it’s too humid, your sweat doesn’t evaporate, and then the outside of your skin can’t get cooler than the inside of your skin,” said Scott Denning, a climate scientist and professor of atmospheric science at Colorado State University. “Metabolic heat builds up inside your body and leads to a very high fever and heat stroke and death.”

Scientists often use a measure called the wet-bulb temperature to measure heat and humidity together.

It’s the measure of temperature if a thermometer was covered with a wet cloth to evaporate heat and approximates the cooling of skin by sweat.

“The wet-bulb temperature has to be a few degrees cooler than the inside of your body,” Denning said. Otherwise, heat won’t flow away from your body.

An influential limit

After an influential study in 2010, climate researchers often use a wet bulb temperature of 35 degrees C — roughly equivalent to 95 degrees F at 100% humidity — as an upper limit for human survival and adaptability without cooling.

But this maximum doesn’t factor things like sun exposure, clothing and movement.

Nearing that limit, briefly

In a 2020 study, researchers found that extremely humid heat had doubled in frequency since 1979 and that several parts of the world, including coastal India, Pakistan, the Persian Gulf and the Gulf of Mexico, had neared that wet-bulb threshold. Two locations briefly exceeded it.

Denning said researchers are concerned about a mass mortality event in those regions, as global temperatures continue to rise.

“In large urban areas in the Arabian sea and the Indian Ocean region, there are hundreds of millions of people who don’t have air conditioning.” Underestimated risks

But new research suggests the upper limits are not realistic.

A 2022 study in the Journal of Applied Physiology, which used data from heat chamber tests, estimated that the threshold for wet-bulb temperatures is probably closer to 30-31 degrees Celsius (86 to 87.8 degrees F) for “young, healthy” subjects performing basic life tasks.

Dry heat worries, particularly for older people

Vanos said everyone’s survival limit will be different and factors like age are critical.

Her research, which was published in Nature Communications last winter, suggests the physiological thresholds for heat stroke are far lower than previously understood, particularly in dry heat.

The research suggests most older adults, who can’t sweat as effectively, are at grave risk if exposed for six hours to temperatures above 115 degrees F, even when the humidity is low and they are shaded. “There will never be one threshold to survivability, and thinking that way is dangerous,” Vanos said.

This article was originally published on NBCNews.com