How the body adapts to extreme heat

It was only after the temperature dropped a degree that she immersed herself, shoulder-deep, for the next two hours. Then she did it again — and again and again — every day for a week. Li-Maloney is trying to figure out whether women in their 60s, 70s and 80s, a group particularly at risk of death during heat waves, could train themselves to cope better with hotter temperatures.

If they could, it would be a convincing demonstration of heat acclimation in older individuals — and it could pave the way for protecting vulnerable groups in heat waves. When a person’s core body temperature rises above 37° for prolonged periods, heat exhaustion often sets in; hotter temperatures can eventually lead to delirium, seizures, organ failure and death. But repeated exposure to high temperatures in young, healthy individuals has been shown to trigger a host of protective physical changes that help cool the body, including modifications to sweat responses and blood vessels.

Now, researchers are trying to work out whether these changes hold for the rest of us — and whether we can train ourselves to better handle the heat that is becoming increasingly severe with climate change. As heat wave–related deaths mount globally — more than 10,000 Europeans are estimated to have died during a heat wave in June alone — harnessing the body’s natural responses could be one way to survive in an increasingly hot world. He’s hoping that the answer is “yes.” Studies, primarily in athletes, soldiers and outdoor workers, have long shown that repeated exposures to heat trigger physiological responses that help the body better cope with subsequent bouts of heat.

Sweating, which uses moisture to wick away heat, starts earlier and is more intense in people who have recently been exposed to hot temperatures, compared with naïve individuals. Tiny vessels in the skin also dilate more quickly in heat-exposed individuals, allowing more blood to move from the body’s toasty interior to its surface, where heat can radiate outward. These changes initially force the heart to beat faster to keep blood pressure steady; over time, the body adapts by expanding blood volume, reducing the heart rate.

Controlled exposures to heat can induce protective changes across the body — from the brain to the heart and even within individual cells — that help it better handle subsequent hot spells, studies suggest. While scientists are still working out the details, they’re finding that intentionally inducing these changes could increase the odds of surviving heat waves. The hypothalamus, a brain region that acts like a thermostat, becomes more sensitive to temperature and acts more quickly to trigger sweating and blood-vessel widening.

Sweating, which cools the body when the fluid evaporates, starts sooner upon heat exposure and is more intense. Cells in the blood (and probably elsewhere in the body) produce more heat shock proteins, which help protect cells from heat-related damage. The heart initially has to beat faster to accommodate the redistribution of fluid, but then blood volume expands, reducing the heart rate.