Signs of Heat Exhaustion (and the Heat Stroke Red Flags)

Knowing the signs of heat exhaustion can save a workout, a race, or a life. Heat exhaustion is your body’s warning that its cooling system is falling behind — heavy sweating, dizziness, nausea, weakness, a pounding headache. Catch it early and you recover fully in under an hour. Miss it, keep pushing, and it can tip into heat stroke, which is a true medical emergency. This guide walks through exactly what to watch for, how to tell heat exhaustion from heat stroke, what to do in each case, and when to call 911.

Quick answer

• Heat exhaustion signs: heavy sweating, cool and clammy or pale skin, dizziness, nausea, headache, weakness, muscle cramps, fast weak pulse. You stay mentally clear.
• Heat stroke red flags (emergency): core temperature above 40°C/104°F plus confusion, slurred speech, agitation, collapse, or seizure. Sweating may stop or continue.
• Heat exhaustion response: stop, move to shade or AC, lie down, cool the skin, sip cool fluids. Improve within 30 minutes or escalate.
• Heat stroke response: call 911 immediately and start aggressive cooling — don’t wait for the ambulance.
• When in doubt, treat it as serious. Any change in mental state during heat illness is a 911 situation.

What heat exhaustion feels like

Heat exhaustion sets in when your body has been losing fluid and salt through heavy sweating and can’t keep its temperature in check during exertion or hot conditions. The hallmark is that you feel genuinely unwell but you’re still thinking clearly. Watch for:

• Heavy sweating that soaks your clothes
• Cool, pale, clammy skin — sometimes with goosebumps despite the heat
• Dizziness or lightheadedness, especially standing up
• Nausea or vomiting
• Headache
• Weakness and fatigue out of proportion to the effort
• Muscle cramps
• Fast, weak pulse
• Heavy, struggling breathing
• Irritability or just feeling “off”

The key feature: you’re miserable, but you know where you are and you can answer questions sensibly. That mental clarity is the line between a warning and an emergency.

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What to do for heat exhaustion

Act on the first signs — don’t try to “push through.” According to sports medicine guidance, the response is to stop and cool:¹

1. Stop exercising immediately. No “finishing the set.”
2. Get out of the heat — shade, indoors, air conditioning, a cool car.
3. Lie down and elevate your legs if you feel faint.
4. Remove excess clothing and gear.
5. Cool the skin — cool wet cloths, a fan, a cool shower, ice packs to the neck and armpits.
6. Sip cool fluids with some sodium if you have them. See ways to rehydrate and electrolytes for sweating.

You should feel meaningfully better within 30 minutes. If you don’t — or if symptoms get worse, you start vomiting repeatedly, or your thinking starts to cloud — treat it as heat stroke and call for help.

Suggested read: Intra-Workout Nutrition: When Mid-Session Fuel Helps

Heat exhaustion vs heat stroke

This table is the most important part of the page. The difference is mostly about core temperature and brain function.

The defining feature of heat stroke is the combination of a very high core temperature (above 40°C/104°F) with central nervous system dysfunction — confusion, disorientation, strange behavior, loss of coordination, fainting, or a seizure.² An old rule that heat stroke means “dry skin” is misleading: in exertional heat stroke, the person is often still sweating. Don’t use sweating to rule it out. Judge by temperature and mental state.

Suggested read: How Much Water Should You Drink Per Day? | Hydration Guide

Heat stroke: a medical emergency

Heat stroke kills, and survival hinges on how fast the body is cooled. If you see the red flags — high temperature plus confusion, collapse, or seizure:²³

1. Call 911 (or local emergency number) immediately.
2. Start cooling now — don’t wait for the ambulance. The fastest method is cold-water immersion (a tub, a pool, even a cooler) if available. If not, douse the person with cold water and fan them, and pack ice on the neck, armpits, and groin.
3. Move them to shade and remove clothing/equipment.
4. Don’t force fluids if they’re confused or vomiting — they could choke.
5. Keep cooling until help arrives or until their mental state clearly improves.

“Cool first, transport second” is the principle emergency and athletic-training guidance emphasizes for exertional heat stroke, because minutes of high core temperature cause organ damage.³

Who’s at higher risk

Some people slide into heat illness faster: those who aren’t heat acclimatized, people exercising at high intensity in hot, humid conditions, older adults, young children, anyone who is dehydrated or under-slept, people with heart or lung conditions, and those on medications like diuretics or certain blood pressure and psychiatric drugs. Heavy gear (football pads, hazmat suits, military kit) traps heat and raises risk sharply.¹ Big sweat-and-salt losses also set the stage — see electrolytes for sweating for why sodium matters here.

How to prevent it in the first place

• Acclimatize gradually over 10–14 days instead of going hard on the first hot day. See heat acclimatization.
• Hydrate on a plan — match your sweat losses, don’t over- or under-drink. See hydration during exercise.
• Replace sodium on long, sweaty sessions with electrolyte drinks.
• Train in cooler hours, wear light breathable clothing, and slow your pace in the heat.
• Build a buddy system for hard sessions in extreme heat so someone can spot trouble.

The full hot-weather playbook lives in exercising in heat.

Suggested read: Wim Hof Breathing: The Method, Science, and Safety

Heat cramps and the milder warning signs

Before full heat exhaustion sets in, your body often gives quieter hints. Heat cramps — sudden, painful muscle spasms, usually in the legs, abdomen, or arms — frequently show up during or after heavy sweating and are linked to fluid and sodium losses. They’re not dangerous on their own, but they’re a flag that you’re losing more salt and water than you’re replacing. Stop, stretch and gently massage the muscle, get into the shade, and take in fluids with some sodium; the answer is often in electrolytes for sweating.

Other early hints worth respecting: feeling unusually irritable or flat, a headache creeping in, your pace suddenly feeling far harder than the effort should be, or skin that goes clammy. Heat illness exists on a spectrum, and the smartest move is to act on the mild end — back off, cool down, drink — rather than waiting to see how bad it gets. The further along the spectrum you let it run, the closer you get to the emergency end.

How long recovery takes

After a bout of heat exhaustion, you may feel washed out for the rest of the day and even into the next. Rest, keep rehydrating, and avoid going back into the heat or hard training for at least 24–48 hours. If you had a genuine heat stroke, recovery is a different matter entirely — it’s a serious medical event that can carry lingering effects, and any return to training should happen only under medical guidance.¹

Bottom line

The signs of heat exhaustion — heavy sweating, dizziness, nausea, headache, weakness, cramps, and a fast weak pulse while still thinking clearly — are your cue to stop, cool down, and rehydrate. Do that and you’ll bounce back within half an hour. The line you must not cross is heat stroke: a core temperature above 40°C/104°F combined with confusion, slurred speech, collapse, or seizure. That’s a 911 emergency, and you should start aggressive cooling — ideally cold-water immersion — before the ambulance even arrives. When you’re unsure which one you’re looking at, treat it as the emergency. Any change in mental state during heat illness means call for help and cool fast.