A few weeks ago, I showed up to a workout class I take regularly, only to find that—surprise!—the gym’s air conditioning was out of commission. It was, of course, an unseasonably hot and humid early June day in New York City. Sweat was running down my chest as I simply sat outside the studio waiting for class to start. How was I going to take a 75-minute circuit-training class in this? I wondered. I was already so hot, the thought of jumping rope and doing jump squats was unimaginable.
But I had just come back from vacation and really was craving some strength work, so I decided to stay and stick it out.
The moment class started, I was worried I wouldn’t make it through. It was just so hot. There were a few other people in class who seemed to be similarly bothered by the stifling temperature, taking lots of water breaks and endlessly wiping the sweat from their brows, arms, chest, pretty much any inch of visible skin. But then there were people who seemed almost indifferent, going through the motions as usual. Sure, if you looked closely, every single person was sweating, but it seemed like I was faring much worse than 50 percent of the class. I’m not one to compare myself to other people in a group fitness class, but I couldn’t help but wonder why I personally seemed to be having a harder time handling the heat.
How our bodies deal with heat
To be able to understand why I suck at exercising in the heat, I first wanted a primer on what our bodies actually do to keep cool when we’re exercising in hot environments.
The human body releases heat through a few key energy-exchange processes, explains Stephen S. Cheung, Ph.D., kinesiology professor at Brock University in Ontario, Canada, and author of Advanced Environmental Exercise Physiology. Those include radiation, convection, conduction, and evaporation (here’s a brief science lesson on how all those work). “The body is working to do all of these at once,” Cheung says. He adds, though, that these processes all rely on a temperature gradient between your body and the environment around you—basically, when the air is cooler than your body, you can release heat via the first three methods (we’ll get to evaporation in a minute) more effectively. “The hotter the environment, the smaller the temperature gradient, and therefore those pathways become less viable,” Cheung says.
Which brings us to evaporation. When the air around you is really hot, the main way your body loses heat is through sweating and evaporation, says Cheung. “What happens with sweat is that your body produces it onto the skin, and then the body is heating up each water droplet and turning it into water vapor.” When water vapor, or steam, evaporates off your body, it produces a cooling effect. The process doesn’t rely on a temperature gradient but rather a difference in humidity between your skin and the air, says Cheung. “That’s why high humidity is a challenge—you can be sweating a lot but that sweat isn’t evaporating, so you’re just getting dehydrated and it’s dripping off your body and making you feel uncomfortable.” (That also means you shouldn’t wipe your sweat off if you want it to work—who knew?!)
In contrast, Christopher T. Minson, Ph.D., professor of human physiology at the University of Oregon and codirector of the Exercise and Environmental Physiology Labs, explains that your body can typically cool itself off way more effectively via sweat in hot, dry climates, where there’s plenty of space in the air for the water vapor to go.
What determines your personal heat tolerance
So it seems safe to say that most mortals would have been struggling at least a little bit in that workout class, considering how hot and humid it was. But why was I on the verge of giving up? I asked both Cheung and Minson if there is any explanation for why exercising in the heat might be more challenging for some people than others. And more important, if I could do anything to make it easier on me.
Of course, they say, genetics plays a role here—everybody is different, so of course all of our bodily functions vary right off the bat. But the biggest factor in determining how much physical stress the heat puts on you is how acclimated you are to it. Heat acclimation basically just describes the changes that happen in your body as you adapt to heat stress, and it’s something you have to work towards. “Humans have an incredible ability to adapt to high temperatures and perform well in them, provided we stay hydrated and it’s not too humid,” Minson says. “If someone has had prior exposure (especially recently) to heat stress, they’re going to have a better tolerability to heat stress.” Simply put, the only way to acclimate is by exposing yourself to heat consistently and basically building up a tolerance.
“There is a progressive timeline for becoming adapted to heat and different parts of your system will respond at different rates,” says Cheung. For example, he says, after about four or so days of exercising for an hour or two in a hot environment, your resting heart rate will likely start to lower. Sweat rate takes a little longer to ramp up, so it could take about two or so weeks to notice a difference.
If you’re really committed to exercising in the heat, you have to do it pretty much every day to get better at it. But, Minson notes, it’s likely you’ll notice a natural difference in your tolerance from, say, the beginning of the summer to the end. Even if you’re not diligently trying to acclimate, if you’re out spending time in the heat and exercising regularly, you’ll end up naturally getting more comfortable with it. (Also, Cheung notes, if you live somewhere that’s often hot and humid, you’ll be more acclimated. But that’s sort of pointless when talking about my situation in a room full of people who all live in the same city.)
Some of this also is thanks to another important factor: psychological conditioning. Or, what Minson calls, your “perception of how hot you are.” “Clearly there’s a physiological aspect to [heat acclimation]. But a large part is the mental side of it.”
He explains: “As you become more fit and heat acclimated, your ability to perceive how hot you are—and maintain exercise—will change.” For example, Minson says, if you exercise in a very hot room and rate your hotness a 9 on a scale of 1 to 10, the next day, the same environment and workload may feel like an 8. “Progressively, over about five to 10 days, you’re going to feel cooler in the exact same circumstances,” he adds. “Part of that is because of [physiological changes] but also your perception of how hot you are is going to change. At any given point, you just won’t feel as hot.”
My husband is a good person to ask about how I handle being uncomfortable (I don’t like it very much, and I very much like to complain). So this is all sort of making sense.
How to feel more comfortable exercising in heat and humidity
While some people will be better at tolerating heat from the start thanks to genetics, Minson is reassuring to me and anyone else who feels my pain: “I’ve never seen anybody who couldn’t get better at exercising in heat.” It’s really just all about acclimating, mentally and physically.
In the short term, though, there are a few things you can do to make hot workouts more bearable. (Other than vowing to only exercise indoors with the AC blaring until it’s cool outside again…which, I admit, is very tempting.)
“Have a hydration plan, have as much airflow as possible, and consider your clothing choice,” says Cheung. Minson also suggests drinking cold water before a hot workout or even putting ice packs on the back of your neck to cool your body down pre-workout. Take breaks often and drink more water when you’re feeling especially hot and thirsty. Ask your instructor if there’s a fan they can put on to circulate the air a bit. Wear light, breathable clothing.
And of course, stay safe. It’s OK to be hot and uncomfortable—dealing with that is part of the process of acclimating. What’s not OK are symptoms of heat stress, like change in mood, mental fogginess or confusion, decrease in coordination, higher than normal heart rate, or breathing a lot quicker than you normally would for that given level of exercise (you’re panting or hyperventilating). Cheung says all of these are early signs that the heat is putting too much stress on your body and you may be moving toward heat exhaustion (which can be dangerous). If you notice them, you should stop, drink water, and try to cool down either by finding a cold room, standing in front of a fan, or pouring cold water on yourself.
“A little discomfort isn’t going to hurt as long as you take the proper precautions,” says Cheung. Just be aware of the signs of potential danger and always listen to your body. If it’s telling you to stop, don’t try to be a hero—take it from someone who is completely unashamed to stop and sit for a water break while everyone else does sled-pushes past me.