Running in the heat is dangerous, but the right training can prepare you.

How Hot Is Too Hot to Run Outside? Depends on How Comfortable You Are in the Heat.

How Hot Is Too Hot to Run Outside? Depends on How Comfortable You Are in the Heat.

Health and medicine explained.
June 21 2017 4:50 PM

How Hot Is Too Hot to Run Outside?

With enough preparation and training, it’s possible to run long distances in the heat.

Li Zhongfei/Thinkstock
Not too hot.

Li Zhongfei/Thinkstock

This story is republished with permission from the Conversation.

The Conversation

Most of Britain is experiencing a heat wave, with temperatures reaching up to 89.6°F. (The Southwest of the United States is also experiencing a heat wave, with temperatures up to 120°F.) The public health watchdog for England has issued an amber health warning, advising people to take care in the hotter weather. But what does it mean for runners? Is it ever too hot to go for a run?

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With the recent high temperatures in parts of the U.K. soaring above 86°F, people may find themselves questioning the safety of running in the heat. Running in hotter temperatures, though, is not uncommon, with many runners competing in warmer climates such as Kenya, Ethiopia, and Japan.

But while running in the heat may be considered a risk to some people—such as children, the elderly, and pregnant women—as long as precautions are taken, running in temperatures as high as 86–95°F is fine.

A number of running events take place in extreme heat (more than 95°F), such as Badwater, the 135-mile ultramarathon that takes place in Death Valley, California, where temperatures can soar to more than 122°F. There is also the annual Marathon des Sables, a six-day run across the Sahara Desert in Morocco, where temperatures can reach 122°F. This 156-mile run is considered the toughest foot race on Earth. Our experience at Kingston University with people running and training in our heat chamber for events such as the Marathon des Sables and Badwater demonstrates that with enough preparation, hydration, and sensibility about how hard you run, it is possible to run safely in high temperatures. But it is important to note that these races do take a lot of preparation and acclimatization, and running in such temperatures is certainly not recommended without thorough training.

Running in the 86°F heat does not come without its risks, it can very easily cause dehydration and  overheating, which can lead to muscle cramps, excessive sweating, headaches, nausea, tiredness, and dizziness. Your performance may be impaired, and you may find you are not able to run at the same pace or cover the same distance as you might have run in milder temperatures. Also, there can be serious health consequences to exercising in the heat, such as heat exhaustion and heat stroke. But these can be avoided if you listen to your body and take sensible precautions to avoid getting too hot, such as drinking enough fluids to stay hydrated; avoiding running at the hottest times in the day (between 11 a.m. and 3 p.m.); wearing light, breathable clothing; slowing down your normal pace; and consider acclimatizing to the temperature (which can take up to 14 days).

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Running in the heat causes the body’s core temperature to rise. The body works best when the core temperature is maintained at 98.6°F, so to help keep the body cool, the body starts to sweat,

allowing the heat to evaporate. This sweating causes water loss from the blood and can lead to dehydration.

To help with the sweating, blood vessels dilate to allow more blood to be diverted to the surface of the skin by enabling more heat loss as a way to reduce this rising temperature. This is why people go red and their blood vessels may be more visible in hotter conditions. The issue is that less blood is available to be delivered to the working muscles, which in turn puts a strain on the body, especially the heart. Sweating can lead to dehydration and so exercising in the heat may make you feel tired and unable to exercise as well as you usually can at cooler temperatures.

The hotter the environment, the higher the dependency on sweating and heat loss to maintain the core body temperature. Typically, people will lose up to one liter of sweat per hour when exercising in hot environments, but it can be more than four liters of sweat in other people.

For humans, though, one of the greatest things is that we are well designed at regulating our temperature compared with other animals. This enables us to run long distances in the heat. With regular exposure to high temperatures, the body learns to adapt, and the stresses and strains of running in the heat can be reduced. Adaptations to the body include increased sweat rates and blood volume, decreased losses of electrolytes (important salts and minerals) in the sweat, reduction in resting and exercising core temperatures as well as a reduction in heart rate and perceived effort levels when running in the heat. With preparation and common sense, you should be able to run safely in hot temperatures.

*Correction, June 22, 2017: This article originally omitted Chris Howe as one of the authors.

The Conversation
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Chris Howe is senior research technician at Kingston University.

Hannah Moir is a senior lecturer in health and exercise prescription at Kingston University.