Record high temperatures prompt research into the limits of human heat tolerance

Jul 10, 2021 | 8 comments

By Audrey Carleton

Last week’s historic heatwave saw portions of the U.S. Northwest breaking all-time temperature records and gearing up for wildfire risk. The temperatures are now being attributed to an excess of 100 deaths across the region as it gears up for another week of extreme highs.

The heat can feel apocalyptic, and scientists are increasingly studying the heat and humidity conditions at which some humans suddenly die, a phenomenon that is becoming increasingly common as a result of extreme weather driven by climate change. This is perhaps best illustrated in a study published last year in Science Advances, alarmingly titled “The emergence of heat and humidity too severe for human tolerance.”

Originally, conditions like this weren’t expected until the mid 21st century, according to climate models. But they are actually already here. In that study, Radley Horton, Lamont Research Professor at Columbia University’s Lamont-Doherty Earth Observatory and co-authors surveyed weather station data from across the world, collected between 1979 and 2017, and found over 7,000 instances of so-called “wet bulb” conditions, which can lead to human deaths. Wet bulb temperature is the point at which humidity and heat hit a point where evaporation due to sweat no longer works to cool a person. Most of these wet bulb conditions were concentrated in South Asia, the coastal Middle East, and southwest North America (areas denoted in red and orange on a map Horton and his colleagues created, below).

The conditions aren’t actually that hard to imagine: Wet bulb conditions occur when relative humidity is above 95 percent and temperatures are at least 88 degrees F, according to the study. The human body, Horton’s study found, is essentially unable to withstand wet bulb conditions at all once temperatures hit 95 degrees F. Under these conditions, it’s possible for otherwise healthy people to die.

“Even if they’re in perfect health, even if they’re sitting in the shade, even if they’re wearing clothes that make it easy in principle to sweat, even if they have an endless supply of water,” Horton said. “If there’s enough moisture in the air, it’s thermodynamically impossible to prevent the body from overheating.” 

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Horton’s research was supported by the National Oceanic and Atmospheric Administration, meaning that the U.S. government has been actively studying the weather conditions at which otherwise healthy humans spontaneously die: “Some locations have already reported combined heat and humidity extremes above humans’ survivability limit,” a NOAA press release stated. NOAA is also supporting a few projects that further study wet bulb conditions.

Sweating is a necessary function for coping with hot days. Once on the surface of the skin, sweat droplets can get hot enough that they turn into a gas and dissolve, eliminating heat and keeping the body’s internal temperature down. 

The problem is, the atmosphere’s ability to take up moisture is finite; water can only transform into a gaseous state if the air surrounding the body is dry enough to accept it. In overly humid conditions, sweat is less likely to evaporate into the atmosphere than in dry conditions, Horton said. Dry heat, like that of the desert, is typically more comfortable than humid heat for this reason. 

“We need a differential between the human body and the environment, and if the air is already holding as much moisture as it can, you don’t have that gradient,” Horton said. “Your body’s not able to get the atmosphere to take that moisture from it.” 

So, on humid days, the water our bodies emit just stays there, getting hotter and hotter without ever evaporating into a gas. (Think of what happens in a steam room, when your body collects sweat instead of eliminating it, until the heat becomes so overwhelming that you need to step out.)  

A growing number of other regions are nearing this point: The Southeast US, the Gulf of Mexico and Northern Australia, all denoted by green on the map, are seeing higher daily maximum wet bulb temperatures. 

Horton believes that, in the short term, reducing exposure to wet bulb temperature will be a matter of behavioral adaptation—simply avoiding these conditions by taking respite in air conditioning, for example. But as severe heat straddles the country and energy grids in Texas, New York and beyond show signs of buckling under the pressure of extreme use, the reliability of AC becomes increasingly uncertain. 

Access to AC is also not guaranteed, Horton notes. Migrant and farm laborers and those living in energy poverty will have a harder time withstanding these conditions by cooling off indoors, he says. 

Matthew Lewis, director of communications at housing advocacy group California YIMBY, noted in a recent Twitter thread that wet bulb temperatures could soon be a factor at the helm of climate migration.

“Many of the places humans currently live on the planet are on their way to being functionally uninhabitable by humans,” he said. “They will have to move.”

Lewis urges states and municipalities to prepare for this eventuality: “Depose the NIMBYs in your city government. Defeat the car-stans who deny that all of this is happening,” Lewis wrote. 

He also urges weather broadcasts to include wet bulb indices in temperature announcements “as a matter of public service,” the way some do air quality and humidity metrics. Horton says “feels like” measurements are the closest many forecasts come to this—but units aren’t standardized across weather stations, and this can create confusion.  

“The very fact that there isn’t one standard that everyone uses, and that most people couldn’t explain exactly what these mean suggests that we could do more,” he said.
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Credit: Vice

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