For 12 years, scientists thought they knew how much extreme heat human bodies could cope with. New research shows how wrong they were.
In the summer of 2023, a dozen people willingly walked into a steel chamber at the University of Ottawa designed to test the limits of human survival. Outfitted with heart rate monitors and temperature probes, they waited in temperatures of 42 degrees Celsius, or 107 degrees Fahrenheit, while the humidity steadily climbed, coating their bodies in sweat and condensation. After several hours, their internal body temperatures began ratcheting upward, as the heat cooked them from the outside in. "Few people on the planet have actually experienced temperatures like this," said Robert Meade, a postdoctoral researcher in epidemiology at the Harvard School of Public Health who led the study. "Imagine moisture condensing on the skin like a glass of water on a hot day. That's how hot it was, compared to skin temperature." Their experiment tested the body's ability to cope with extreme heat by exposing participants to temperatures at which they could no longer cool themselves. Their study, published last week in the science journal PNAS, confirmed that this dangerous threshold is much lower than scientists had previously thought: a so-called wet bulb temperature, which accounts for heat and humidity, of 26 to 31 degrees C. Scientists call this limit the point of "uncompensable" heat stress, "because the body cannot compensate for the heat load placed upon it," Meade said. "With climate change driving heat waves, there's been a lot of interest in defining these upper limits." When studying the health risks of heat, scientists often refer to wet bulb temperatures because moisture in the air can make heat waves much deadlier by blocking the body's ability to sweat out heat effectively. For over a decade, it was widely believed that the maximum wet bulb temperature that bodies could handle was 35 C — unlikely to become a common occurrence until global warming had reached a staggering 7 C over preindustrial temperatures. To support our nonprofit environmental journalism,…