How Do You Know If It’s Too Hot to Exercise Outside?
When it’s hot, you can often exercise safely – check out our recommendations here – but our bodies have their limits. Indeed, there are temperatures and conditions under which prolonged exercise is dangerous. Here are some tips for knowing when to stop exercising outdoors.
Use the wet bulb temperature of the globe
Instead of just checking the temperature forecast for the day, the best way to find out how the heat will affect your body is to look at a metric called the wet bulb temperature, or WBGT. This comes from the idea that if you wrap the bulb of an old-fashioned thermometer with a damp cloth, the thermometer will show you how much air can cool you. (I’m simplifying, the actual process is more complex and involves three thermometers .)
This is important because one of our body’s main cooling mechanisms – sweating – can’t work well in high humidity. Luckily, you don’t need a thermometer and a wet rag: you can approximate this by looking at the temperature and humidity on a graph, or you can use a weather app like WeatherFX ( iPhone , Android ) that reports it directly.
Adjust your workout based on temperature
Now that you have the WBGT, you need to decide what to do with it. US Soccer has a map that divides the country into three regions with different restrictions on practice cancellations in different areas. (If you live in Texas, they suggest that you can handle a little more heat than if you lived in Minnesota.) Similarly, there are recommendations for road race organizers (such as marathons). If the WBGT exceeds 82 degrees, the race should be cancelled. Above 73, “extreme caution and a slower pace” are highly recommended.
To give you an idea of what this might look like: A WBGT of 82 degrees can occur in 75-degree weather with 90% humidity, 84-degree weather with 50 percent humidity, or 100-degree weather with 10 percent humidity.
As a general set of recommendations, the American College of Sports Medicine recommends for people who are not used to exercising in the heat:
- If WBGT is above 65 degrees Fahrenheit , take extra rest and pay attention to hydration.
- If WBGT is above 72 , take extra rest and limit the duration of the exercise.
- If WBGT is above 78 , take an extra rest, limit the duration, and also reduce the intensity (for example, don’t try to run that fast).
- If WBGT is above 82 , you should rest as much as you work, do all of the above, and watch closely for signs of heat illness because the risk is high.
- If WBGT is above 86 , pack it up and go home.
For people who are heat acclimatized, which means they have been safely exercising in the heat for at least a few weeks, safety rules can be relaxed a bit as your body learns to cool down a bit better. In these cases, cutoffs look like this:
- If your WBGT is above 72 , you can still train as usual, but pay special attention to hydration.
- If WBGT is above 82 , “plan for intense or prolonged exercise with caution” and watch for signs of heat illness in high-risk individuals.
- If WBGT is above 86 , limit intense exercise and limit exposure to hot weather. Watch for signs of heat sickness.
- If the WBGT is above 90 , the risk is too great even for acclimatized athletes.
All of these tips are combined with the usual precautions. Take extra fluids and electrolytes with you for your hydration breaks, and be sure to find places in the shade to cool off. (A kiddie pool with an ice bag is one of the suggestions in the US Soccer rules.)
It is also important to know the signs and symptoms of heat exhaustion and heat stroke and what to do if you experience them in yourself, a teammate, or a workout buddy. Heatstroke is a 911 level emergency, so seek help if needed.