How much can the body actually adapt?
While you can train your body to respond better in the heat, it has limits. Race organizers often rely on guidelines based on the wet-bulb globe temperature (WBGT) index, which accounts for temperature, sun angle, cloud cover, wind speed, and humidity, to determine when conditions become unsafe. In fact, Périard and fellow researchers found that over one-quarter of all endurance running events were held in moderate, high, or extreme heat.
One of the biggest factors that influences humans’ ability to regulate is humidity. As it rises, the risks of heat-related illnesses increase. When the air is already saturated with water vapor, sweat can’t evaporate as effectively, limiting the body’s ability to cool itself. That’s why a 90-degree day with high humidity can be more dangerous than a much hotter, drier environment. One ongoing study by researchers at Penn State found that the safety threshold for young, healthy adults (without heat acclimation) is around 87 degrees Fahrenheit at 100 percent humidity.
But not everyone reacts the same to the heat, either. Jessica Mee, an environmental physiology researcher at the University of Worcester, studies how men and women respond differently to heat training. Some of her research suggests women have a lower heat tolerance than men and may benefit from longer periods of acclimation, but the evidence is still evolving.
(Can you really sweat out toxins?)
Despite the limitations, the broader takeaway is clear: “Heat acclimation works,” Périard says. “And with climate change, it’s only getting warmer and athletes are competing more often in hotter conditions.” Because of the potential benefits of heat training, he is now researching the public health implications of heat therapy not only for athletes but also for older adults and vulnerable populations who may not be able to exercise.