When is it Just Too Hot Outside to Run?

Is it safe to run outside under soaring temperatures and high humidity? Or should we give ourselves a break? 

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Singapore’s running scene is heating up these days, quite literally. The period between late March and late September will definitely not be a breeze for outdoor runners. With the average temperature ranging from 33 – 34 °C daily, we can expect our sweat glands to work overtime. While we’re sure that die-hard runners won’t get discouraged by burning pavements, we can’t help but wonder when it is just too hot outside to run.

When the Wet Bulb Globe Temperature is All Black

The Wet Bulb Globe Temperature (WBGT), also known as the more sophisticated, well-rounded friend of the thermometer, measures heat stress by taking into account the relative humidity, ambient temperature, wind, and solar radiation. WBGT has been used by professional athletes and military personnel globally to determine appropriate work-to-rest ratios, the number of water breaks needed and length of practice when doing exercise under strong heat. You can calculate heat stress using WBGT calculator.

WBGT shows 4 different heat categories: green (WBGT index under 29.4 °C), yellow (WBGT index between 29.4 – 31.0 °C), red (WBGT index between 31.1 – 32.2 °C), black (WBGT index above 32.2 °C).

When the heat category shifts from green to yellow, it’s recommended to take at least three breaks with a minimum duration of four minutes for each hour of exercise.

Outdoor activity should be capped at a maximum of two hours when the WBGT index is in the red category.

When the WBGT index falls into the black zone, kick off your sneakers and chill at home or whichever cool, sheltered place you may prefer.

Bear in mind that body mass, sex and the intensity of exercise also play a role in determining one’s heat tolerance. Essentially, there’s no hard and fast rule as to when it is just too hot to run outdoors. But, when the temperature outside soars higher than our body temperature, there’s a good chance that we’re better off staying indoors.

When You’re Feeling the Burn

And not in a good way - but in a spasm-inducing, life-threatening way.  When we exercise, our muscles generate heat. As a result, we sweat. Our bodies cool down as our sweat evaporates.

On a particularly infernal day, on top of the heat produced by our bodies, we also gain some from solar radiation. The high humidity is especially tough on our bodies because it prevents sweat from evaporating since there is too much water in the air.  What can our bodies do?

This is when the problems begin. Our bodies’ failure to regulate temperature can cause different heat injuries. For one, we may get heat cramps around our thighs, calves and abdomens due to excessive sweating as our bodies try to lose heat quickly. 

Heat exhaustion is another common injury. Headaches, dizziness and profuse sweating are just some of the symptoms.

And the worst possibility: a heat stroke. It happens when the body temperature rises above 41 °C and the body stops sweating. Without proper and immediate medical care, one can lapse into a coma and die within minutes. Don’t say we didn’t warn you.

To prevent heat injuries, we must keep ourselves sufficiently hydrated. It’s recommended to drink about 500 ml half an hour before exercise and 250 - 500 ml every half an hour during exercise.

While some of us may dread going on a weighing scale, this is one of the few times that the weighing scale is our best friend. You should weigh yourself before and after exercise.

For every one kilogram lost during exercise, the body has lost approximately one litre of fluid, and we should replace it by drinking 1.5 times the fluid lost. It’s also important to wear loose and breathable clothing to prevent heat build-up around the body.

When You Haven’t Done Enough Prep

By the time you get to this part of the article, you probably don’t feel like running in the heat anymore. And you’re not alone. Even some running pros like Kara Goucher and Shalene Flanagan dread running in hot weather. But at the same time, these pros have shown us that running under the blazing sun can be done with enough preparation.

Our bodies have a self-adjusting mechanism. With rising temperatures in Singapore, it’s best to let our bodies acclimatise to the hot weather. Instead of going all out on the field, go slow and easy for about 7 - 10 days to ease our bodies into the new environment.

And of course, the old but gold saying “workout starts in the kitchen” applies here too. To prepare for running in hot weather, we should bid goodbye to that large plate of salted fish fried rice or Double McSpicy for a while. Heavy meals can add extra heat to the body. Alcohol and caffeinated drinks are a no-go either. They can cause dehydration and increase the risk of heat injuries.

"But, when the temperature outside soars higher than our body temperature, there’s a good chance that we’re better off staying indoors."

As uncomfortable as it may be, running in the hot and humid Singapore weather can be accomplished. There are some guidelines advising the best practices to run in the heat. But most importantly, to determine when it is too hot to run outside, listen to your body.


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LIV3LY Editor
LIV3LY Editor

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