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If you do have to exercise in the direct midday sun, the best thing you can do is ensure that you stay cool and in the shade for as long as possible before you start.Minerva Studio/Getty Images/iStockphoto

Two days before last month's Ottawa Marathon, runners received an unwelcome warning from the race director. The entire weekend event, including four shorter races, might be cancelled because of "serious concerns around heat-related illnesses," thanks to forecast race-day highs of over 30 C.

For the 47,000 runners gearing up to race, the threat of cancellation offered a stark reminder that summer heat is a serious and sometimes insurmountable barrier to prolonged exercise. With the risk of heat stroke rising progressively when temperatures exceed 21 C with 50 per cent humidity, the number of runners needing medical attention at big events can quickly swamp resources.

Similar challenges face Olympic athletes who will compete in Rio this summer. And their races won't be cancelled or rescheduled, despite forecast highs in the upper 20s and average humidity above 70 per cent in August.

That's why sports federations around the world have teams of physiologists studying the best ways to handle hot weather. It's not just Olympians who can put this knowlege to good use. For those lacing up running shoes, hitting cycle paths or hiking trails this summer, here's some of the latest research sports scientists are providing to their athletes. It ranges from scientifically-backed common sense to counterintuitive findings that throw cold water on age-old assumptions.

It's not just the heat …

And it's not just the humidity, either. When marathon race directors consider cancelling a race, they're looking beyond the humidex and considering instead the "wet-bulb globe temperature," a scale that incorporates air temperature, humidity, wind speed and solar radiation to assess how hot we actually feel.

Measuring the wet-bulb globe temperature isn't easy, requiring a specialized "wet-bulb" thermometer to measure the effects of humidity and wind and a "globe" thermometer to measure the effects of solar radiation – which is why you don't see it in weather reports. But the additional effects of solar radiation are more important than most people realize, as a study by researchers in Japan and Britain revealed.

The researchers had volunteers cycle to exhaustion several times in a chamber in 30 C heat and 50 per cent humidity. The only difference between the trials was the amount of simulated solar radiation, which ranged from zero to 800 watts per square metre, which is equivalent to a cloudless day at noon. (Intermediate values of 250 and 500 watts per square metre simulated thin and thick cloud cover.)

The results, which were published in the European Journal of Applied Physiology in April, showed that average time to exhaustion was cut nearly in half, from about 45 minutes with no sun to 22 minutes with full sun. While solar radiation didn't change the subjects' core temperature, it did raise skin temperature, making them feel hotter and possibly diverting blood from the muscles to the skin.

The study was prompted by the popular but contested belief that cloudy days produce the fastest marathons, says Dr. Ronald Maughan of Loughborough University, one of the study's authors. The size of the effect was surprising, he says, "but the folk wisdom of experienced runners often turns out to be correct."

That may have been a blessing for the Ottawa race organizers, who went ahead with their events after making minor adjustments to move start times later into the evening or earlier in the morning. Cloud cover on both Saturday and Sunday protected runners from the worst of the sun.

If you do have to exercise in the direct midday sun, the best thing you can do is ensure that you stay cool and in the shade for as long as possible before you start, Maughan says. If you're wearing a hat and other clothing, make sure they're light and well ventilated.

Douse yourself

Try this thought experiment: If you had a glass of ice-cold water and wanted to cool yourself down, would you be better off drinking it, or dumping it on your head?

That's the question that Nathan Morris and Ollie Jay of the University of Sydney's Thermal Ergonomics Laboratory tackled recently in the journal Temperature. "Drinking a cold fluid can certainly feel good on a hot day," Jay says, "but the notion that it actually physically cools you down once sweating has initiated seemed exaggerated."

By swallowing the cold water, Morris and Jay calculated, you lose 39 kilojoules of heat as the water warms up to body temperature inside you. If you instead swallow a cup of ice slush, you'll do even better, losing 81 kilojoules of heat.

But the best option, at least theoretically, is to take advantage of the incredible cooling power of evaporation. If you pour the water over your head and let it evaporate, you'll lose a massive 607 kilojoules.

Of course, there are some important caveats. One is that you'll probably spill some of the water on the ground when you pour it over your head – though, Jay points out, you could spill 85 per cent of it and still come out ahead compared to drinking ice slush.

Also, if you're already sweating so much that sweat is dripping to the ground, that means you're maxing out your ability to evaporate water, so pouring more on won't help as much. For that reason, pouring water on is likely most helpful on relatively dry, windy days – or if you're cycling, which creates its own wind.

Finally, in addition to keeping you cool, drinking helps you stay hydrated. The Ottawa race medical director Jon Hooper wisely advised runners to seek the best of both worlds: take two cups at each water station, one to drink and the other to pour over your head.

Precool with caution

At the American College of Sports Medicine (ACSM) annual meeting in Boston earlier this month, Kamiel Maase, the head of sports science and innovation for the Netherlands Olympic Committee, showed off the new cooling vests that his athletes will be wearing in Rio.

The form-fitting vests are filled with a special hydrogel that activates when it contacts water, and then provides evaporative cooling for up to 72 hours. They can also be prefrozen to provide extra chill for 10 to 20 minutes.

Such precooling is the latest iteration of a technique that has become standard practice over the past decade for athletes competing in hot conditions. In addition to cooling vests, some athletes use cooling gloves or scarves, or drink prerace ice slushies – an innovation introduced by scientists at the Australian Institute of Sport, who shipped cumbersome slushie machines to the 2008 Olympics in Beijing.

The pros and cons of precooling, however, are still being debated. At the ACSM meeting, researchers from the University of Alabama presented data suggesting that externally precooling your head, neck, torso and legs may make you feel cooler, but doesn't actually prevent your core temperature and heart rate from drifting upward during exercise. In contrast, they found, drinking a slushie before exercise does work to keep your heart rate lower.

In fact, drinking a slushie may actually be too effective, Jay and his colleagues at the University of Sydney have found. The ice seems to trigger previously unknown temperature sensors in your stomach, which in turn tricks your body into thinking you're cold and reduces your sweat rate. In some circumstances, that can actually lead you to heat up more.

(The same effect can happen in reverse when you drink a hot drink: your sweat rate increases, which cools you down. That may explain why drinking a cup of hot tea is seen as a good way to cope with midafternoon heat in some cultures.)

Once again, the devil is in the details of your environment. If you're sweating so much that sweat is dripping to the ground, then reducing your sweat rate won't hurt you, so a slushie can help cool you. And there are also psychological benefits to anything that makes you feel cool, beyond its subtle effects on your core temperature.

Ultimately, even with the best cooling technology and most up-to-date physiology, the most important thing you can do is adjust your pace and expectations to suit the conditions. Thanks to all the warnings, that seems to be what marathoners in Ottawa did. Times were slower than usual, and while twice as many runners were treated for heat exhaustion compared to normal years, there were no disasters.

"I would rate it as fantastic," Hooper told the CBC after the race. "Faced with what the weather was, it could have been terrible."