In 2010, researchers at Purdue University and the University of New South Wales began an investigation into a macabre question: What's the upper temperature limit at which humans can survive? The answer, according to their calculations, was 95 degrees Fahrenheit (35 degrees Celsius) at 100% humidity — a "wet bulb" temperature that sets an absolute "unsurvivable" threshold for the human body.
Beyond this temperature, a cascade of irreversible physiological processes begins, sending organs into shock, causing blood to clot and ending in inevitable death.
As the climate warms and heat waves multiply, more places across the globe are creeping towards this threshold, with climate models projecting the first 35 C wet bulb temperatures by the middle of the century.
But conditions don't have to be this hot or humid to be unlivable in a practical sense for wide swaths of the population. The threshold is lower for those who are older, for women, and those with certain medical conditions. Any kind of physical activity also raises the risk of heat stress.
In some parts of Asia, South America and Aust...
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