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It was a hot September day almost 20 years ago, but Mark Tanaka remembers it like yesterday.

He was 36, just a few months into his first job as an ER doctor in the San Francisco Bay Area, running his first 50-mile race up and down the sweltering slopes of nearby Mount Diablo.

Around mile 38, approaching the summit, he started cramping. He felt nauseous and slightly disoriented. His ears were buzzing. A mile or two later, he collapsed at an aid station in spasms. After resting, he kept going, even as diarrhea and cramps wrecked his body. At the summit, race staff forced him off the course and called an ambulance.

By that point, he said, he was overcome by a symptom hard to ignore: an “impending sense of doom.” Blood tests in the ER showed dangerously low sodium levels and a serious condition called rhabdomyolysis, in which damaged muscle tissue leaches proteins into the blood that can cause heart damage, kidney failure, and death. Both are signs of heatstroke, the most serious and final stage of heat illness.

Despite medical training and knowing the dangers of heat, Tanaka, who has since finished at least 200 ultramarathons, nearly died that day after pushing through the early and escalating signs of heat illness. Countless others haven’t been so lucky.

According to the Environmental Protection Agency, heat is the “leading weather-related killer in the United States,” causing at least 700 deaths each year—though . Among those who die each year are hikers, cyclists, runners, and parks employees, mostly in scorching arid states across the West or the humid, hot Southeast. As climate change pushes average temperatures higher and causes longer, hotter heat waves, doctors and climatologists agree the danger is only increasing.

“It is very easy to underestimate heat’s effect and overestimate your ability to cope,” Tanaka says. “You’ve got to be really careful.”

But of all the hazards recreationalists face on the trail, death by heat is among the most preventable. It’s vital to understand the risk of heat illness, the early warning signs, what to do if you or your companions start to experience it, and when to modify your plans.

What Is Heatstroke?

Heatstroke is the final stage of heat illness, an umbrella term that includes heatstroke and exhaustion, as well as rashes and cramps, as your body runs out of ways to lower your core temperature, and athletes are among those most at risk. When you exercise on a hot day, your body’s cooling mechanisms are working double-time, burning through water and electrolytes to fight both the outside temperature and the heat generated by your own muscles.

“At some point, as you continue to exert yourself, your cooling system can’t keep up,” says Caleb Dresser, a Harvard faculty member, emergency physician, and expert on extreme heat. This can happen much more quickly than many people understand: internal temperatures can rise above 104 degrees—the widely-recognized threshold for heat stroke—in as few as 10 to 15 minutes.

The earliest symptoms usually include cramps, headaches, and dizziness. Athletes, often used to pushing through pain, don’t always recognize these warning signs. “You compensate pretty well until you crash,” Dresser says. Without immediate, aggressive medical intervention in an ER, Dresser says, that crash can end in seizures, organ failure, coma, and death. Minutes matter, he says, and without emergency care, cases can be fatal.

How Hot Is Too Hot to Exercise?

Experts hesitate to suggest any one temperature as the cut-off point for exercising when it’s hot out, because with proper preparation and precaution, you can train in even the hottest temperatures. Every year, highly experienced and heat-trained athletes compete in the Badwater 135, a 135-mile ultramarathon across Death Valley, the hottest place on earth, in the middle of July. Those athletes have undergone weeks of heat training to prepare (which often includes sitting or working out in a sauna for weeks before the race) and run with a support team icing them down at aid stations.

But while the most prepared athletes may safely be able to run in extreme heat, far lower temperatures are still plenty hot to kill you. Abby Wines, who has worked at Death Valley National Park for 17 years, says the days she’s most concerned for hikers are “when it’s hot—but not really hot.” In a park that regularly tops 120 degrees, she says most visitors know to avoid hiking in the hottest weather. But when temps are in the 90 to 100 range, she says, hikers get in over their heads and calls for rescue go up.

You need to pay attention to humidity, too. In places like the Midwest and Southeast—or on stormy days in the West—high humidity can make milder temperatures more dangerous, in part because evaporative cooling (sweating) doesn’t work as well when there’s lots of moisture in the air already.

The , a chart showing perceived temperatures based on the heat and humidity level, shows a red zone of temperatures when you should avoid activity. According to this chart, a 90-degree day at 40 percent humidity feels like 91 and falls in the yellow zone of “extreme caution,” when prolonged activity might lead to heat exhaustion or stroke. A 90 degree day at 95 percent humidity feels like 127—by which point heatstroke is “highly likely.”

“It surprises many people to learn that the heat index values…are for shady locations,” writes the National Weather Service. In direct sunlight, the perceived temperature can be 15 degrees higher. Exercising in anything over a perceived heat of 103, they say, is “likely” to result in heat illness.

How Do I Know I’m in Trouble? What Should I Do?

You should know the forecast, but you should also know your own body. “My first warning sign when I’m starting to overheat, is my arms start to get goosebumps,” Wines says. “When that happens, I always seek shelter, drink water, take a break, lower my heart rate, and just let my body calm down.”

Symptoms like confusion, dizziness, and headaches should not be ignored, Dresser explains. “One of the first organ systems that starts to produce symptoms is the brain,” he says. Same with cramps and other signs of dehydration, which signal that your body is struggling to compensate.

Fluids, electrolytes, finding shade or a cooler location, and stopping activity are key to recovering from the early phases of heat exhaustion, Dresser says. “This is a very correctable state.”

When a person shifts from heat exhaustion to heatstroke, it’s common for the body to transition from profuse sweating to an absence of sweat. If you or your companions have stopped sweating, have a racing heart rate, or experience a seizure, call for emergency help immediately and make every effort to cool down.

How Can I Prevent Heat Illness?

Take it slow, and give yourself time to acclimate. Wines recommends taking “baby steps” to test yourself in heat. Start with a shorter hike that’s near services before trying something challenging or remote. Like acclimating to altitude, it can take two to three weeks of regular exposure to heat before your body adapts. If you’re preparing for a longer trip or event in high heat, research heat training protocols to prepare for the temperatures you might experience. Also note that heat acclimation is lost very quickly. In Death Valley, park staff who spend more than three days outside the area are considered no longer heat-adapted, and must adjust their workloads.

The recommendation to carry water seems obvious, but it’s often overlooked. Ethan Veneklasen, an ultra runner and co-director of the near Lake Tahoe, says he rarely carries water on short runs. But on a hot day, he won’t leave that weight behind. “It’s really easy to make bad decisions on the front end, and then those bad decisions compound,” he says. Dehydration can lead to dizziness and confusion that impair further decision-making and push an athlete deeper into trouble. The Centers for Disease Control recommends during exertion in heat, you need about eight ounces of water every 15 to 20 minutes, spread out in sips.

Eating salty snacks is equally important. Sweat requires sodium too, so replacing water but not electrolytes can also lead to heat illness. The American College of Sports Medicine recommends taking in 500 to 700 milligrams of sodium per hour of exercise. Research has shown that most so check the labels of your favorite snacks to plan ahead.

Wearing lightweight, loose clothing and sunscreen can help, too: sunburned skin makes it much harder to cool off.

Make a Plan and Stick to It

After his friend Philip Kreycik died running on a hot day last summer, Chris Thoburn, a 33-year-old ultrarunner familiar with long, scorching runs, realized that on top of his usual preparation, the most important thing he can do to stay safe is to stick to his plans. He makes sure he knows how far he’s going, how long it will take him, and where he can access water along the way. He ensures his devices have battery power to last his run, that somebody knows where he’ll be, and that he won’t feel pressured to push hard to get back in time for another commitment.

Once he’s on the trail, the only change he’ll make is to turn around or end early, even if he’s feeling good. “As athletes, we often go out and go, ‘Hey, I’ve never been on that trail before,’ or ‘I’m feeling pretty good today, I could push a little bit further. But the trouble with all those decisions is they are taking away a piece of your safety net.  You go from being perfectly prepared, maybe even overprepared, to being not prepared at all,” he says.

“I can point to times before Phil’s passing when I made the wrong decisions. And understanding better now what the decision matrix should be, I think will keep me much safer into the future.”