When Should I Worry About...

5 Questions to Ask Before Running in Humidity

May 27, 2021 - Katie McCallum

Whether you're training for a marathon or just headed out for a long weekend run, there's nothing like running outdoors.

That being said, Houston can get hot. Oppressively hot. Houston can also be humid. Suffocatingly humid.

"Any time you're running outdoors in the summer, it's important to take steps to prevent heat illness," says Dr. Vijay Jotwani, sports medicine doctor at Houston Methodist. "Humidity is actually a bigger issue than heat when it comes to the factors that can cause a person to overheat, but of course we deal with both here in Houston."

Running raises your core body temperature and running in hot weather can raise it even more. But getting hot is only half the issue — not being able to properly cool down is the other.

"The problem with adding humidity into the mix is that it limits your body's natural cooling mechanism — sweating. Since the air already contains a significant amount of moisture, humidity prevents sweat from evaporating off your skin to some degree," explains Dr. Jotwani.

This means less heat release from your body, making it more likely for you to overheat.

"It's not impossible to run in Houston's heat and humidity. Plenty of people do it. But doing so safely requires preparation," Dr. Jotwani adds.

Here are five questions to ask yourself before running in humidity:

1. Do you have the right gear?

You can't control the weather, but you can control the clothes you wear on your run — which affect how your body regulates its temperature.

"Opt for running clothes that are lightweight and breathable. Moisture-wicking fabrics are best since they actually help move sweat from your skin to the surface of the fabric where it can easily evaporate," explains Dr. Jotwani.

Whatever you do, avoid wearing layers or clothing items made of thick fabrics. These actually trap sweat, making it much harder to release heat.

2. Are you starting your run well-hydrated?

You probably have good instincts for when you should carry water on a run. (Hint: If it's a long training run and/or when it's hot outside).

But running safely in the humidity is about more than grabbing a bottle of water on your way out the door.

"Being hydrated is critical for proper regulation of your body temperature. If you're starting even mildly dehydrated, your body is ill-equipped to cool itself down as you run, and your risk of heat illness increases," warns Dr. Jotwani.


Before you start your run, Dr. Jotwani recommends checking the color of your urine to determine your hydration status. Pale yellow urine means you're hydrated. Anything darker means you're dehydrated and that you should approach a hot, humid outdoor run with caution.

And, even if you are well-hydrated, it's important to carry water with you so you can help replace the water you sweat out during your run.

"If you are someone who sweats a lot, you may even consider drinking a sports drink after your run to help replenish the electrolytes lost in your sweat," Dr. Jotwani adds.

3. Do you know when heat and humidity become too hot and too humid?

The primary factors that contribute to unsafe running conditions are:

  • High temperature
  • High humidity
  • Significant amounts of direct heat from the sun (also called radiant heat)
  • Lack of wind

"The more of these factors put together at a time, the less safe an outdoor run becomes," says Dr. Jotwani. "Keep these factors in mind and use them as your checks and balances. Are the odds stacking in your favor for a safe run? Or are they trending towards risky? It's best to err on the side of caution, especially if you're planning a long run."

For instance, if it's an incredibly hot, humid, cloudless, breezeless day, running at noon when the sun is directly overhead is much riskier than waiting until the evening when the sun is down and the temperature drops (even if just a little).

4. Are you acclimated to this level of heat and humidity?

The problem with saying a certain temperature is "too hot" and a certain humidity level is "too humid" is that what's "too hot, too humid" varies from person to person.

"It's amazing what a person's body can acclimate to over time, including fairly harsh weather. Here in Houston, this means people are able to run in levels of heat and humidity that non-Houstonians wouldn't even consider attempting," says Dr. Jotwanti.

But getting used to running in humidity takes time, as well as a commitment to listening to your body along the way.

"Any time the heat and humidity are well beyond what you've successfully run in before, you may want to rethink your approach to an outdoor run," warns Dr. Jotwani.

For instance, consider running the first few miles outdoors and then finishing the rest of your mileage on a treadmill indoors. On your next run, add a few minutes or a mile to your outdoor portion, listening to your body and turning around if you feel that you are overheating.

5. Do you know the signs and severity of heat illness?

As an endurance runner, you likely have a determined, can-do attitude. And while your mind is a powerful thing, your body can only do so much in high heat and humidity.

Heat illness is when the body overheats as a result of overexertion and/or hot weather. It starts with muscle cramps but can progress into heat exhaustion and then heat stroke — which is a medical emergency — if you don't stop and find a way to cool your body down.

"Some people think running with a cold towel or ice cubes can help prevent overheating. But in reality, this isn't an effective way to control your core body temperature," says Dr. Jotwani. "The only way to truly cool your body down if you are overheating is to stop running and seek cooler temperatures, such as by taking a cold shower or applying cold towels or ice."

The symptoms of heat illness and heat exhaustion include:

  • Weakness
  • Muscle cramps
  • Fatigue
  • Headache
  • Nausea and vomiting

If heat exhaustion progresses to heat stroke, it's now a medical emergency.

"Heat stroke is when the brain and other vital organs become affected by the high temperatures, and it's the most severe form of heat illness. If you or someone you love is showing signs of heat stroke, call 911 immediately and begin the cooling process by applying cold towels or ice to the main areas of blood flow (neck, armpits and groin)," says Dr. Jotwani.

Preventing heat illness and heat stroke while running begins with asking yourself the questions above. If you answer 'no' to any of these questions, you're risk of developing heat illness is increased and you should consider opting to do your run on a treadmill indoors or shortening the mileage of your outdoor run.

"If you're training for a marathon and pushing it on a long, outdoor run, just know that you can make yourself seriously ill," Dr. Jotwani warns. "As a safer alternative, I recommend going into your run well-prepared — listening to your body along the way and stopping if you feel like you're overheating."

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