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How hot is too hot to exercise outside?

Getting acclimated to the heat, exercising in shade and listening to your body can keep you safer if you’re exercising outdoors in a heat wave

Exercising at MacArthur Park in Los Angeles on Tuesday. (Marcio Jose Sanchez/AP)
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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 workout 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.

Tips to stay cool during summer workouts

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.

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