When it’s hot, you can often still exercise safely—check out our recommendations here—but our bodies have their limits. There really are temperatures and conditions that make it dangerous and difficult to exercise for very long. Here are some guidelines for figuring out when to call off your workout.
Most importantly, it’s not just about the temperature. Humidity matters, too. We’ll see below how to use the wet bulb globe temperature to tell when it is truly “too hot” to work out. But to give a few examples, if you’re trying to prevent wet bulb temperatures above 82 (a common recommendation), these scenarios would all be “too hot” for a long or intense run:
80 degrees would be too hot at 70% humidity
90 degrees would be too hot at 30% humidity
How to use the wet bulb globe temperature
Instead of just checking the temperature that is forecast for the day, the best way to figure out how the heat will affect your body is to look at a metric called wet bulb globe temperature, or WBGT. The wet bulb globe temperature comes from the idea that if you wrap the bulb of an old-fashioned thermometer in wet fabric, the thermometer will then give a reading that tells you how much the air can cool you down.
Adjust your training based on the temperature
Now that you have the WBGT, you need to decide what to do about it. U.S. Soccer has a map that divides the country into three regions, with different cutoffs for canceling practice in different areas. (If you live in Texas, they assume you can deal with a bit more heat than if you live in Minnesota.) Similarly, here are guidelines meant for organizers of road races.
For a general set of recommendations, here’s what the American College of Sports Medicine recommends for people who are not acclimated to working out in the heat:
If the WBGT is above 72, take extra rests and limit the duration of your exercise.
If the WBGT is above 78, take extra rests, limit duration, and also tone down the intensity (for example, don’t try to run as fast.)
If the WBGT is above 82, you should be resting as much as you’re working, doing all of the above and keeping a sharp eye out for signs of heat illness, because the risk here is high.
If the WBGT is above 86, pack it in and go home.
For people who are acclimatized to the heat, meaning that they have been safely exercising in the heat for at least several weeks, the safety guidelines can be relaxed a bit since your body has learned how to cool itself a little better. In those cases, the cutoffs look like this:
If the WBGT is above 72, you can still exercise as normal, but pay extra attention to your hydration.
If the WBGT is above 82, “plan intense or prolonged exercise with discretion” and keep an eye out for signs of heat illness in people who are at high risk.
If the WBGT is above 86, limit intense exercise and limit your exposure to the hot weather. Watch out for signs of heat illness.
If the WBGT is above 90, the risk is too great for even acclimatized athletes.
All of this advice goes alongside the usual precautions. Bring extra fluids and electrolytes for your hydration breaks, and make sure to find places in the shade to cool off. (A kiddie pool with a bag of ice is one of the suggestions in the U.S. Soccer guidelines.)
It’s also essential to know the signs and symptoms of heat exhaustion and heatstroke, and what to do if you observe them in yourself, or your teammate or workout buddy.
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When it’s hot, you can often still exercise safely—check out our recommendations here—but our bodies have their limits. There really are temperatures and conditions that make it dangerous and difficult to exercise for very long. Here are some guidelines for figuring out when to call off your workout.
