As heat records fall, how hot is too hot for the human body?
Extreme heat has been breaking records across Europe, Asia and North America, with millions of people sweltering in heat and humidity well above “normal” for days on end.
Death Valley hit a temperature of 128 degrees Fahrenheit (53.3 degrees Celsius) on July 16, 2023 – not quite the world’s hottest day on record, but close. Phoenix broke a record heat streak with 19 straight days with temperatures above 110 F (43.3 C), and had more in the forecast, accompanied by several nights that never got below 90 F (32.2 C). Globally, Earth likely had its hottest week on modern record in early July.
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Heat waves are becoming supercharged as the climate changes – lasting longer, becoming more frequent and getting just plain hotter.
One question a lot of people are asking is: “When will it get too hot for normal daily activity as we know it, even for young, healthy adults?”
The answer goes beyond the temperature you see on the thermometer. It’s also about humidity. Ourresearch is designed to come up with the combination of the two, measured as “wet-bulb temperature.” Together, heat and humidity put people at greatly increased risk, and the combination gets dangerous at lower levels than scientists previously believed.
To answer the question of “how hot is too hot?” we brought young, healthy men and women into the Noll Laboratory at Penn State University to experience heat stress in a controlled environmental chamber.
These experiments provide insight into which combinations of temperature and humidity begin to become harmful for even the healthiest humans.
Each participant swallowed a small telemetry pill that continuously monitored their deep body or core temperature. They then sat in an environmental chamber, moving just enough to simulate the minimal activities of daily living, such as showering, cooking and eating. Researchers slowly increased either the temperature in the chamber or the humidity in hundreds of separate experiments and monitored when the subject’s core temperature started to rise.
That combination of temperature and humidity at which the person’s core temperature starts to continuously rise is called the “critical environmental limit.”
Below those limits, the body is able to maintain a relatively stable core temperature over long periods of time. Above those limits, core temperature rises continuously and the risk of heat-related illnesses with prolonged exposures is increased.
When the body overheats, the heart has to work harder to pump blood flow to the skin to dissipate the heat, and when you’re also sweating, that decreases body fluids. In the direst case, prolonged exposure can result in heat stroke, a life-threatening problem that requires immediate and rapid cooling and medical treatment.
Our studies on young healthy men and women show that this upper environmental limit is even lower than the theorized 95 F. It occurs at a wet-bulb temperature of about 87 F (31 C) across a range of environments above 50% relative humidity. That would equal 87 F at 100% humidity or 100 F (38 C) at 60% humidity.
