The record-breaking heat that baked parts of the United States and Europe in June and July, sending hundreds to the hospital, highlights a pressing question: How hot is too hot for the human body?
“There’s been a lot of interest in defining [the] upper limits for what humans can tolerate for prolonged periods,” says thermophysiologist Robert Meade, a postdoctoral research fellow in epidemiology at the T. H. Chan School of Public Health, who studies how the body regulates its internal temperature—and why those systems can fail.
In comfortable conditions—cool temperatures, modest humidity—thermoregulation is usually simple. If a person exercises or is exposed to the sun, sensors in the skin and the central nervous system register an elevated core temperature and send signals to the brain, which directs the process of shedding heat. Blood vessels dilate to bring heat to the skin’s surface, where it can dissipate into the air. Or the body sweats, “and the evaporation of sweat from the skin helps draw out the heat,” Meade says.
But in extreme temperatures, he says, those mechanisms are not always enough. As heat builds in the body, it can trigger heat illness, accompanied by symptoms such as irritability, an itchy skin rash, and dizziness. A core temperature that rises above 100 degrees Fahrenheit can develop into heat stroke, which causes confusion and extreme lethargy, and can escalate to organ failure and death.
Meade compares the challenge of extreme heat to encountering someone who is treading water and “handing them a brick. There’s only so long they can keep it up.”
Some scientists tie the human heat limit to the concept of “wet bulb” temperature, a measure that includes both air temperature and humidity. One seminal 2010 paper proposed that the human heat limit is a wet bulb temperature of 95 degrees Fahrenheit, or 35 degrees Celsius; Meade says that’s “equivalent to 35 degrees air temperature with 100 percent humidity,” a level of moisture in the air that prevents sweat from evaporating to cool the body.
Yet subsequent experiments found—alarmingly—that the body’s thermoregulatory system can fail at wet bulb temperatures that are as low as 79 degrees Fahrenheit.
Meade recently conducted a study to confirm the reliability of those findings. He brought 12 volunteers, ages 28 to 32, into a stainless steel chamber roughly the size of a two-car garage, specially outfitted so that researchers could precisely control the temperature and humidity inside. Participants wore thermometers that monitored their internal temperature.
The room was initially warmed to 107 degrees Fahrenheit, with just 28 percent humidity. “We didn’t want them to get hit with this wall of humidity,” he says. After an hour, the researchers started to increase the humidity. Meade, who also served as one of the study subjects, reports that the conditions grew very uncomfortable. “As you start to get too hot, your brain is just screaming at you that you should leave that environment,” he says. The data showed that people’s core temperatures were stable for a while, then hit a sudden spike. “There’s just no ability for the body to offset the level of heat gain,” Meade says.
Based on the data they collected, Meade and his colleagues projected that at 107 degrees Fahrenheit and 70 percent humidity, participants would develop heat stroke after around 10 hours of exposure on average, “which is very, very rapid.”
More research is needed, particularly on older adults, he says, because the body grows less able to thermoregulate with age, and the elderly are more likely to have other conditions that extreme heat can exacerbate. Contrary to conventional wisdom, “heat stroke kills relatively few people,” Meade says. “Most heat related deaths actually occur due to cardiovascular conditions. Your heart has to work harder to maintain regular function when you’re hot,” increasing the risk of heart attacks and strokes.
Meade also wants to pursue future studies on the variable responses to high heat. The longer a person has lived in a hot climate, the higher their threshold for heat-related illness and death, he notes—possibly a result of physiological adaptations or infrastructure, including air conditioning, which is one of the most reliable ways to protect the body from the ravages of heat.
But less expensive options are needed around the globe, particularly in places where the electric grid is unreliable. Previous research by Meade and colleagues show that low-cost cooling options, such as fans and wet towels placed on the neck, make only small improvements to core temperatures.
Meade has been working with a team at Harvard’s Salata Institute to understand how extreme heat affects poor women in Gujarat, India, who work in very hot conditions and have faced multiple years of severe summer heat. Their data shows that even before temperatures reach the level at which the thermoregulatory system fails, there’s a period of negative impact the researchers call “the zone of misery.”
Heat alters sleep and mood. “You might not be at huge risk of heat-related health events, but you are uncomfortable, and you can’t work as much,” he says. “It can, in certain circumstances, have negative effects on how much you’re earning.”
Meade notes that the human ability to adapt is remarkable. People can cope with higher temperatures through changes in physiology or behavior—working at night, for example. But more research is urgently needed, Meade adds, because a rapidly warming climate means that, by 2050, many more people will be living with extreme heat.
