A woman stretches her arm on a sunlit road.

The heat is on. As the National Oceanic and Atmospheric Administration reports, 2018 is well on its way to becoming the fourth-hottest year on record across the globe, exceeded only by 2017, 2016, and 2015. And that's only for January through June, without taking August's temps into consideration. 

So it's little surprise that you may feel uncomfortably hot if you’re exercising outdoors or exerting yourself in other ways—weeding the garden, for instance.

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But can being physically active in the heat also pose a risk?

In some cases, yes.

Most of the time, your body is quite good at regulating its internal temperature.

“The body’s main way that it cools itself is through sweat,” explains Michelle Cleary, Ph.D., associate dean of graduate programs at Chapman University, in Orange, Calif. If you aren’t drinking enough to keep up with the fluid lost in sweat, though, your body can get too hot and become dehydrated. 

When you're physically active under these conditions, you may begin to feel lethargic, dizzy, and even nauseous. In some instances, your temperature may spike, a possible sign of heat stroke—which can be dangerous, even deadly. Playing sports, exercising, and doing yard work were among the most common causes of heat-related emergency room visits, according to one 2011 study.

And new research published in Plos Medicine suggests that health-related deaths will increase. The study, which used a mathematical model to predict heatwave deaths in 412 communities around the world between 2031 and 2080, found that the number is likely to rise significantly in many areas. 

Here in the U.S. an estimated 1,756 people die each year as a result of heat waves, according to the study. In the future, the researchers predict anywhere from roughly 2,400 annual deaths to approximately 10,400. 

As the health risks from heat climb, it's important to know how to keep yourself safe, especially if you're active outdoors. 

Dress Right and Safeguard Your Skin

What you wear can really help to keep you feeling cooler during a summer workout.

“You want to avoid anything that traps moisture against your skin,” Cleary says. Opt for lightweight, loose-fitting items, which allow sweat to evaporate more easily. Also, choose light colors, which absorb less heat than dark ones. Moisture-wicking polyesters may help move sweat away from your skin, where it can evaporate and cool you down.

Protect yourself from the sun’s rays, too, by wearing sunscreen with an SPF of 40 or higher during your outdoor summer workouts. Apply it at least 15 minutes before you go outside, and reapply at least every 2 hours.

Not only does a sunburn raise your risk of skin cancer, but, according to Luke Pryor, Ph.D., professor of kinesiology at California State University, Fresno, it may damage your sweat glands, which can hamper your body’s ability to cool itself.

Stay Hydrated

Your body is about 60 percent water, which allows your kidneys to filter out waste and your blood to transport nutrients throughout your system. Your sweat mechanism also helps keep your body at the right temperature, between 97° F and 99° F.

How much water do you need? The Institute of Medicine recommends that men consume about 15 cups of water daily and women about 11 cups (from food and non-alcoholic beverages). 

Instead of trying to keep track of your liters, however, Sandra Fowkes Godek, Ph.D., director of the HEAT Institute at West Chester University of Pennsylvania, recommends that you rely on your sense of thirst to tell you how much water to drink.

“Our thirst mechanism is adequate and very well developed,” Fowkes Godek says. The exception to this guideline is older adults. Our sense of thirst diminishes as we age, so for seniors, relying on thirst may not be sufficient to keep them hydrated. 

As for what to drink, water is always best, says Fowkes Godek. We do lose important nutrients known as electrolytes—such as sodium, magnesium, and potassium—through sweating. But most of us have no need for sports drinks or other beverages fortified with electrolytes, because we usually can replace the nutrients lost in sweat through regular meals and snacks. Just as important, sports drinks often contain a lot of added sugars.

The exceptions: People who exercise for more than an hour at a time, and workers who labor for long hours outside in the heat.

When it comes to food, consuming water-rich foods like melon, citrus, and leafy greens can help keep you hydrated. And while the heat may blunt your appetite, try to have a small snack of about 150 to 200 calories an hour to 30 minutes ahead of your workout, if you haven’t had a meal within the prior 4 hours. Then refuel within an hour after a workout. (See our advice on what foods are best to eat before and after any workout.)

Time Your Activity Right

During the summer, do as much of your outdoor physical activity in the morning or evening, when it’s slightly cooler. When outside, stay in the shade as much as possible.

It’s also important to let your body acclimate to exercising or working in the heat, says Douglas Casa, Ph.D., CEO of the University of Connecticut's Korey Stringer Institute, which produces research and advice on the prevention of heat-related deaths for athletes and workers. 

That means, ideally, slowly working up to a full-intensity training session or workday. A 2016 analysis found that taking eight to 14 days to acclimatize to exercising or working in the heat may be most effective for minimizing heat stress to your body. But that's not always practical. Still, if you have an outdoor activity such as a big hiking trip, a long-distance run, or a major yard project planned, try to work up to it over a period of days.

Adults who supervise groups of children, at camps or in sports teams, for instance, should make sure they give youngsters a chance to adapt to the heat as well. (See tips from the American Academy of Pediatrics on how to keep youngsters safe in the heat.)  

Symptoms of Heat Stroke

If you notice any signs of dehydration or heat-related illness—including dizziness or lightheadedness, headache, fatigue, nausea or vomiting, and muscle cramps—take a break from your activity, find shade or a cool room, and drink water.

And note that while the ability to cope with heat and humidity varies from person to person, some weather conditions merit precautions for all. So, pay attention to heat watches, advisories, and warnings in your area (available through the National Weather Service). On days with these alerts, take extra care to stay hydrated, and consider modifying your activity level or moving your summer workout indoors for the day.

And be on the lookout for signs of heat stroke, which can be fatal if not addressed quickly. The two most important symptoms are body temperature above 104° F and central nervous system problems such as losing consciousness, irritable or irrational behavior, mood changes, and disorientation.

You may not have a thermometer on hand, but if you notice one or more of the behavioral symptoms mentioned above during a summer workout in yourself or someone else, take action: “Get body temperature down as fast as humanly possible,” says Casa at the University of Connecticut.

Have someone call 911 while you cool the person down. Move them out of the heat and direct sun, and into a cold bath or shower (or use water from the garden hose or any other water that’s available if you can’t get indoors). Flip on a fan or air conditioner to speed cooling.