On an April day in 1905, the scientist J.S. Haldane descended hundreds of feet into a Cornish tin mine to find out if he could cook himself to death.
Amateur researchers had long known that humans have an extraordinary ability to withstand dry heat. One 18th century experimenter found he could tolerate temperatures up to 115 degrees Celsius (240 Fahrenheit), hot enough to cook steaks. But the moist, saturated air in the Dolcoath mine, dug through hot rock deep below the water table, seemed to change things. Though the temperature never climbed above 31.5C, Haldane’s body temperature and pulse rose with each minute, hitting feverish levels before he ascended after three hours. “It becomes impracticable for ordinary persons to stay for long periods” when the humid temperature rises above 31C, he wrote.
That finding hasn’t significantly changed over the years since — but our atmosphere has. As the climate warms, conditions once experienced only in saunas and deep mineshafts are rapidly becoming the open-air reality for hundreds of millions of people, who have no escape to air conditioning or cooler climes. After a few hours with humid heat above 35C — a measure known as the wet-bulb temperature — even healthy people with unlimited shade and water will die of heatstroke. For those carrying out physical labor, the threshold is closer to Haldane’s 31C, or even lower.
Brajabandhu Sahu knows the physical signs all too well. A street vendor selling foods like dosa, idli and uttapam on the corner of two busy roads in Bhubaneswar, the capital of the eastern Indian state of Odisha, he’s surrounded at times by what feels like a wall of fire from which he cannot escape. When the day is at its hottest, his head spins, his heart races, his skin blisters and the waves of nausea are constant. The moisture-laden winds that blow in from the Bay of Bengal put citizens in this region at particular risk.
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