What NASA and Other Organizations Have Said About Unsurvivable Temperatures for Humans

Researchers have successfully determined the highest combination of heat and humidity that a human being can endure.

Even a healthy young person will die after enduring six hours of 35 degrees (95 Fahrenheit) heat combined with 100 percent humidity, but new research suggests the threshold may be significantly lower.

At this point, sweat – the body’s main tool for lowering core temperature – no longer evaporates from the skin, eventually leading to heat stroke, organ failure and death.

That critical limit, which occurs at 35 degrees above the so-called “wet temperature,” has been exceeded only about a dozen times, mostly in South Asia and the Persian Gulf, Colin Raymond of NASA’s Jet Propulsion Laboratory told AFP.

None of these incidents lasted longer than two hours, meaning there have never been “mass mortality events” associated with this limit of human survival, said Raymond, who led a major study on the subject.

But extreme heat doesn’t have to be anywhere near that level to kill people, and everyone has a different threshold depending on age, health and other social and economic factors, experts say.

For example, more than 61,000 people died of heatstroke last summer in Europe, which rarely has enough moisture to cause dangerously wet temperatures.

But as global temperatures rise — last month was confirmed Tuesday as the hottest on record — scientists warn that dangerous wet-bulb incidents are also becoming more common.

The frequency of such events has at least doubled in the past 40 years, Raymond said, calling the increase a serious risk of human-induced climate change.

According to Raymond’s research projects, wet-weather temperatures will “regularly” exceed 35 degrees in many places around the world in the coming decades if the world warms 2.5 degrees above pre-industrial levels.

– “Really, really dangerous” –

Although now mostly calculated using heat and humidity readings, wet bulb temperature was originally measured by placing a wet cloth over the thermometer and exposing it to air.

This allowed it to measure how quickly water evaporated from the fabric, representing sweat from the skin.

The theoretical human survival limit of 35 degrees wet temperature represents 35 degrees dry heat and 100 percent humidity – or 46 degrees with 50 percent humidity.

To test this limit, researchers from the Pennsylvania State University in the US measured the core temperatures of young healthy people in a thermal chamber.

They found that participants reached their “critical environmental limit” – when their bodies were unable to stop their core temperature from continuing to rise – at a wet temperature of 30.6 degrees, well below the previously theorized 35 degrees.

The research team estimated that it would take five to seven hours for such conditions to reach a “really, really dangerous indoor temperature,” Daniel Vecellio, who worked on the study, told AFP.

– The most vulnerable –

Joy Monteiro, an Indian researcher who published a study last month in the journal Nature on temperatures in South Asia’s humid climates, said most of the deadly heatwaves in the region were well below the wet-bulb threshold of 35 degrees.

All such limits of human endurance are “wildly different for different people,” he told AFP.

“We don’t live in a vacuum – especially children,” said Ayesha Kadir, a pediatrician from the United Kingdom and a health adviser for Save the Children.

Young children are less able to regulate their body temperature, putting them at greater risk, he said.

Older people, who have fewer sweat glands, are the most vulnerable. Almost 90 percent of heat-related deaths in Europe last summer were among people over 65.

People who have to work outdoors in high temperatures are also at greater risk.

Whether people can occasionally cool their bodies – for example in air-conditioned rooms – is also an important factor.

Monteiro noted that people without access to toilets often drink less water, leading to dehydration.

“Like many impacts of climate change, the people who are least able to insulate themselves from these extreme impacts will suffer the most,” Raymond said.

His research has shown that El Nino weather events have increased wet bulb temperatures in the past. The first El Nino event in four years is expected to peak towards the end of this year.

Wet bulb temperatures are also closely related to ocean surface temperatures, Raymond said.

The world’s oceans reached their highest temperature ever last month, surpassing the previous record set in 2016, according to the European Union’s Climate Observatory.