Exhausted athlete

How to Tell If You’ve Spent Too Much Time Out in the Heat

Heat illness can happen to anyone

Your body always wants to be as close to 98.6 degrees Fahrenheit, and usually, it does a pretty good job at regulating its own temperature. On a hot, humid day, though? Not so much.

When your body loses its ability to self-regulate, you run the risk of developing a heat illness, one of the biggest summer health hazards. Heat illness happens most often to people who are exercising or doing physical activity outdoors for long periods of time, to elderly people, and to people taking medications that increase their sensitivity to high temperatures. Given the right conditions, though, it can happen to anyone.

Here are the signs you should look out for.

Heat exhaustion vs. heatstroke

When your body has to work extremely hard to cool you down, you can develop heat exhaustion. During heat exhaustion, the body’s core temperature is usually less than 104 degrees Fahrenheit, but blood pressure is low and the heart is not pumping blood as efficiently as it should.

At this stage, the body is still doing what it’s supposed to. “You’ll be very fatigued and sweating a lot and thirsty—so those natural defenses against heat and dehydration are still working,” says Peter Shearer, MD, associate director of the Mount Sinai Hospital emergency department in New York City.

Heat exhaustion does not necessarily lead to heatstroke—but it could, says exercise physiologist Michael Bergeron, PhD, president and CEO of Youth Sports of the Americas. Bergeron describes exertional heatstroke as “a clear medical emergency affecting multiple body systems,” which usually occurs when the body’s core temperature rises above 104 degrees.

Heatstroke causes the central nervous system to malfunction. It can also damage the brain, heart, liver, kidneys, spleen, and muscular tissue. “Your body loses the ability to thermoregulate, so at that point it’s much more difficult to reverse itself,” says Dr. Shearer.

Warning signs: Cramping, nausea, or headache

The first signs of a heat illness are often stomach cramping or nausea, which can indicate dehydration or an electrolyte imbalance in the body. These are closely linked with heat illness, since the body needs water (and electrolytes like sodium) to properly regulate its temperature, but it also loses them through sweat. Headaches are also a common first sign of dehydration.

If can be hard to pinpoint the exact cause of GI distress while you’re exercising. “But, if conditions warrant you thinking this, best to assume that it is heat-related,” says Bergeron. His advice? “Take a break, hydrate, and when you’re feeling better, restart what you were doing.”

Warning signs: Thirst and heavy sweating

“Heavy sweating is a sign that your body is producing a lot of heat,” says Bergeron, which could lead to heat exhaustion if conditions don’t improve or get worse. In fact, heat is more dangerous on humid days, because sweat can’t evaporate off the skin—which normally produces a cooling sensation—as easily as it can with dry or breezy weather.

Feeling thirsty is also a sign that your body needs more water. Even if you don’t feel parched, it’s important to sip water frequently on hot days. If you’re out in the heat for more than an hour, a sports drink can also help replenish lost electrolytes. (Try one of these DIY sports drinks.)

Warning signs: Dark urine

Watch out for urine trouble. “If you’re out playing tennis for hours but you’re drinking between sets and you take regular bathroom breaks and your urine is fairly clear, that’s a sign you’re hydrating well,” says Dr. Shearer. “But if you go for a long run and you’re not urinating a lot, and when you do go it’s very dark and concentrated, those are sign you’re underhydrated. That means you won’t be able to release heat as efficiently through sweat and evaporation, which puts you at risk for overheating.”

Warning signs: Weakness, dizziness, or collapse

Heat exhaustion often makes people get dizzy, feel the need to sit down, or even pass out momentarily. “Which is sort of a good thing,” says Bergeron; “that is, it stops your body from continuing before you get into more serious trouble.”

The hotter it gets outside, the more common these symptoms are. But Bergeron says it’s important to remember that, for people doing strenuous workouts, exertional heat illness can occur in relatively cool conditions, as well.

Warning signs: Lack of sweating

When the body reaches a certain internal temperature, its natural defenses—like sweating to release heat—begin to shut down. “If a person’s been outside exercising in the heat and their skin is totally dry, that’s a red flag,” says Dr. Shearer.

However, says Bergeron, it’s not always the case that a person will stop sweating as they enter the danger zone—so don’t assume that just because someone is sweating heavily that they’re okay. (If they have other signs of heat illness, get them help immediately.)

Warning signs: Confusion, convulsions, or coma

If heatstroke progresses far enough, you can hurt your brain. “You start to experience confusion and delirium,” says Dr. Shearer. “And that’s really dangerous, because when you’re confused you won’t do the normal things to cool yourself down, like seek out shade or get water.”

In the most severe cases of heatstroke, fainting and convulsions can occur.

You don’t have to feel hot

The signs of heat illness aren’t always as obvious as they might seem. “In fact, with developing overheating—possibly leading to exertional heatstroke—one can feel chilled, even in the heat,” says Bergeron.

The reason? When the body experiences exercise- or heat-related stress (or both), it protects itself by producing inflammatory proteins. But these proteins can also interfere with the body’s thermoregulation, bringing on symptoms like chills, goosebumps, or cold, clammy skin.

You may not realize it’s happening to you

Even if you do feel overheated, nauseous, or a little “out of it,” it can be hard to stop what you’re doing and take a break—especially if you’re in the middle of a killer workout running a race. That’s why it’s so important to recognize the earliest symptoms of heat illness and address them while you still can.

“A challenge is that an overheated athlete is often the worst person to make an objective assessment,” says Bergeron. “As the body and brain heat up, the brain’s cognitive capacity and objective reasoning is often compromised.”