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° Celsius, physical exertion becomes dangerous. Consistent exposure to high wet-bulb temperatures—35° Celsius and above—can be fatal. At this point the sweating mechanism shuts down, leading to death in six hours. //

At night the body should recover from the daytime assault of heat, but because nights are getting hotter, that recovery is hampered, says Dutta. Whenever people talk about the effects of heat, they usually refer to its direct effects—such as heat exhaustion and stroke, which can be fatal or debilitating—but these are only the tip of the iceberg, he says. “If heat stays high in the night, it affects the body’s homeostasis, its ability to regulate and maintain its internal body temperature.” Upset this and your cellular and metabolic activities become disrupted, which can be a driver of disease, and can even be fatal itself.