How hot is too hot to exercise outside?

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As climate change leads to record-high temperatures around the world, dangerously hot days are prompting questions about when it’s too hot to exercise outdoors.

The stakes are high. The consequences of a too-hot workout “range from feeling thirsty to death,” said Clare Minahan, a sports scientist at Griffith University in Australia. But figuring out when to trade that woodsy running path for the old treadmill is not as easy as glancing at a thermometer.

Is there a simple rule I can follow?

Every body and every environment is different, experts caution. “There’s no magic formula,” Minahan said. “When I talk about a hard-and-fast number, the amount of gray area around that is quite enormous.”

But there is some basic guidance about when you should move your work indoors.

When it comes to reconsidering an outdoor workout, Minahan said she would be “starting to think about” moving indoors when the temperature climbs above 86 degrees Fahrenheit. She would “really” think about it as it moves closer to 90 degrees, she said.

In Australia, if the temperature hits 95 degrees, sporting events start getting canceled, said Minahan.

And unless you are specifically conditioned to exercise in the heat, “at 95, I would absolutely think about doing it another time or finding an air-conditioned room to do it in,” she said.

Early signs you’re getting too hot during exercise include developing a headache, feeling thirsty, general muscle weakness and even irritability, she said. If you start to feel chills, that can be an early sign of heat stroke.

Be aware of humidity

You also should consider your location. Are you up against the sticky, humid climate of the East Coast? Or the dry heat of the American West?

A hot day is likely to feel worse in humid climates. “It’s hard to breathe, almost. It feels like a pea soup,” Minahan said.

The reason: During high humidity, there’s so much moisture in the air that your sweat cannot evaporate to cool your skin. That is why it is often more comfortable to exercise in dry heat than in humid conditions.

Many people “would absolutely think twice about exercising” in 86 degrees and 85 percent humidity, Minahan said, but 90 degrees at 30 percent humidity is “going to feel fine for some people.”

Use a weather app

Weather apps often give more information about the safety of outdoor activity. And some apps may include something called “wet-bulb” temperature, which accounts for heat and humidity together.

Ollie Jay, the director of the Heat and Health Research Incubator at the University of Sydney, said the risk of working out in the heat depends on a complex set of variables, including posted temperature, actual temperature in the sun, humidity, wind and the kind of clothing or equipment a person is wearing.

Jay has devised a heat tool that allows users to select their city or Zip code and an activity such as cycling, walking or distance running to determine the level of risk over the course of the day. Type in Dallas, Tex., for instance, and you’ll be told that temperatures even late into a July evening are in the “red” zone for long-distance running, which indicates extreme risk for heat stress.

Why you should exercise in the shade

Jay notes that outdoor temperatures are typically measured in shaded areas. On its own, a straight temperature reading “means not a great deal,” he said.

The temperature in sunny areas may be significantly higher than you expected on the basis of the weather report, he said. Jay advises exercising in shade “because that will at least expose you to the temperature that you think you’re being exposed to.”

If you can, find an area that has a breeze. “That’s going to make a big, big difference,” he says. And wear loosefitting clothing, which will enable sweat to evaporate from the skin more efficiently.

You also should consider the surface on which you are exercising, Minahan says. Asphalt and concrete retain heat. “The climate temperature might be [95] degrees,” she said, “but if you’re running on a black road, at ground level, it’s a lot hotter. ”

Sheri Belafsky, a physician and director of the medical surveillance program at the University of California at Davis, noted that time of day matters, too. She encourages exercisers to be “very choosy” and to select the coolest part of the day to exercise, even if it means shaking up your schedule.

Get acclimated to the heat

Before going for the hot workout, it is important also to think about how much you’ve previously exercised in the heat and your level of aerobic fitness, Minahan says.

“If you’re reasonably fit, not carrying a lot of extra weight, you’re going to be much better off,” she said, noting that those who are in good physical shape typically start sweating earlier and are better at cooling themselves.

Be patient and give your body time to acclimate to the heat. You can acclimate to steamier conditions by building up the intensity and duration of your workouts over a few weeks, said Belafsky, the UC-Davis doctor.

“Our bodies are very good at adapting to heat; it just takes time,” she said.

Stay hydrated by drinking water a few hours before exercise, and listen to your body during the workout. Individuals’ hydration needs vary depending on the activity and weather in which it is being done, and you do not want to overdo it. The best advice is to have water available during your workout and to drink when you’re thirsty.

The elderly are more vulnerable to heat. And many people do not know recognize that some medications can affect heat regulation and sweating. Some oral contraceptives, for example, can increase the resting body temperature by nearly 1 degree Farenheit, Minahan says.

Jay advises us to be mindful of our heart rate and perceived exertion, and how they compare to how we normally feel doing the same activity in cooler conditions. If your heart rate is noticeably higher in the heat, consider reducing intensity to maintain a heart rate to which you are accustomed.

The bottom line: During outdoor exercise, listen to your body, and scale back when needed.

“‘Do less’ is my shortest advice,” Belafsky said.

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Do you have a fitness question? Email YourMove@washpost.com and we may answer your question in a future column.