R Lakshmanan has been making steel frames in the southern Indian city of Chennai for 20 years. His job involves standing for long hours outdoors at construction sites, pounding screws with careful precision onto steel rods. Each day he makes nearly 600 frames, which end up becoming the skeleton of a home. Often he works 12-hour shifts, beginning at 6 am. He always feels fortunate when he gets to work under a shady tree.But this year, that protection hasn’t been enough. Ever since temperatures in March hit a sizzling 38 degrees Celsius—4 degrees above normal for Chennai—the conditions have been stifling. The metal frames Lakshmanan works with have been too hot to touch, the steel burning his fingertips and leaving behind painful sores. He has seen construction workers, especially women, collapse around him, and has had to take breaks during the workday to cope with fits of dizziness and nausea. “On some days, there’s so much heat, it feels like you’re living in a fireball,” he says.When faced with these conditions, our bodies call upon a well-known mechanism to keep us from overheating: sweating. As perspiration evaporates from the skin, it cools the body’s temperature. But if the air is not only hot but also already filled with moisture, less sweat can evaporate, and this safety feature fails. In India, high temperatures and humidity are increasingly combining to pose a deadly threat—one the country isn’t prepared for.This danger to human life is measured using “wet-bulb temperature”—the lowest temperature that air can be cooled to via evaporation. It’s determined by wrapping the bulb of a thermometer in a wet cloth and seeing what temperature is recorded. Essentially the bulb is you—or me, or Lakshmanan—the wet cloth is our sweating skin, and the temperature recorded is the coolest we can hope to get by sweating.When heat and humidity combine to push wet-bulb temperatures past 32 degrees Celsius, physical exertion becomes dangerous. Consistent exposure to high wet-bulb temperatures—35 degrees Celsius and above—can be fatal. At this point the sweating mechanism shuts down, leading to death in six hours. On May 1, 2022, the wet-bulb temperature in Lakshmanan’s home city of Chennai hit 31 degrees Celsius. The same day, the district of Ernakulam in the Indian state of Kerala recorded a wet-bulb temperature of 34.6 degrees Celsius—a record high for the area.“Without the mechanism to rid the body of that excessive heat, there are many physiological changes that happen in quick succession,” says Vidhya Venugopal, a researcher in public health at the Sri Ramachandra Institute of Higher Education and Research in Chennai.
Raise your internal temperature by 3 to 4 degrees Celsius, and you’ll start to struggle. “As the body tries hard to restore your core temperature, all other processes slowly grind to a halt,” Venugopal says. Blood vessels dilate and circulation slows, particularly to the extremities. Not enough blood will flow to the brain, affecting its functioning. You lose alertness, become drowsy, and don’t feel thirst anymore. Soon organs shut down, one by one. “When the brain stops giving messages to the heart, the pulse slows and the person goes into a coma,” she says.