Heat Exhaustion vs Heat Stroke: How to Tell the Difference and What to Do First

Key points

• The difference between heat exhaustion vs heat stroke is not the temperature you feel. It is whether the person is still thinking clearly. Confusion, slurred speech, or stumbling means heat stroke until proven otherwise.

• Heat stroke is a 911 call. Make the call first, then start cooling. Do not wait for an ambulance to begin.

• Cold water immersion is the fastest field cooling there is. OSHA calls an ice bath the best method to cool someone rapidly in an emergency.

• Skip the fever reducers. MedlinePlus says do not give aspirin or acetaminophen for heat illness. They do not touch this kind of heat.

• Heat cramps and heat rash are the early chapters of the same story. Treat them as a warning, not a nuisance.

Two people can sit down in the same shade on the same August afternoon and look about the same. One of them will be fine in 30 minutes. The other one is dying, and the thing that tells you which is which is not sweat, and it is not a thermometer.

Sorting out heat exhaustion vs heat stroke is one of the few first aid calls where a regular person, with no equipment, can genuinely change the outcome. The window is short. The signs are learnable. Here is what actually separates them, what to do in the first 5 minutes, and the mistakes that cost people time they do not have.

What is the difference between heat exhaustion and heat stroke?

Heat exhaustion is your body losing the fight but still fighting. Heat stroke is your body having stopped fighting, with the brain now taking damage. That is the whole distinction, and everything below is just how it shows up on the outside.

Here is the sign-by-sign comparison, drawn from NIOSH heat illness guidance and the National Weather Service.

Heat exhaustion looks like this:

• Mental status: awake, oriented, miserable. Maybe irritable. Can answer you.

• Skin: heavy sweating. Cool, pale, and clammy to the touch.

• Body temperature: normal or mildly elevated.

• Pulse: fast and weak.

• Other: headache, dizziness, nausea or vomiting, muscle cramps, weakness, heavy thirst, dark urine or not much of it.

Heat stroke looks like this:

• Mental status: confused, slurring, acting strange, combative, staggering, seizing, or unconscious. This is the one that matters.

• Skin: hot. Either dry and red, or still soaked in sweat. Both happen.

• Body temperature: high, often above 103 degrees F, but you probably cannot measure it usefully.

• Pulse: rapid and strong.

• Other: throbbing headache, nausea, fainting, loss of consciousness.

Notice what is not on that list as a reliable divider. The old rule that heat exhaustion means sweating and heat stroke means hot dry skin is only half true. NIOSH lists heat stroke as presenting with either hot dry skin or profuse sweating. Somebody who collapses while running or working hard is often still drenched. If you are waiting to see dry skin before you take it seriously, you will wait too long.

What is the single most important sign of heat stroke?

Altered mental status. If the person is not thinking and speaking normally, treat it as heat stroke and call 911, no matter what their skin feels like.

The CDC Yellow Book puts it plainly in its heat and cold illness chapter: heat exhaustion can come with irritability or mild confusion, but major neurologic signs such as seizures or coma indicate heat stroke. NIOSH lists confusion, altered mental status, and slurred speech as the first heat stroke symptoms it names, ahead of anything about skin or temperature.

In the field, use a real test instead of a vibe. Ask three questions with answers you can check: what is your name, where are we right now, and what day is it. Then ask them to stand up and walk 10 steps. Somebody with heat exhaustion is grumpy about it and does it. Somebody sliding into heat stroke gives you a wrong answer, gives you a slow answer, gets belligerent about being asked, or cannot hold a straight line.

The part most guides skip: the person will not tell you they are in trouble. Impaired judgment is a symptom of the injury, so the guy who insists he is fine and wants to get back to work is giving you evidence, not reassurance. Trust the test, not the testimony.

Why is body temperature unreliable in the field?

Because the only temperature that means anything in suspected heat stroke is a core temperature, and you almost certainly cannot get one.

The clinical threshold for heat stroke is a core temperature at or above 104 degrees F combined with altered mental status. But the CDC Yellow Book notes that a rectal reading is the most reliable way to check, an underarm reading is only a reasonable estimate, and that even without a thermometer a person with heat stroke will feel hot to the touch. Oral and forehead and ear thermometers are all measuring in the wrong place on somebody whose skin blood flow has gone haywire. A normal looking oral reading on a confused, overheating person is a false negative that gets people killed.

So invert the usual order. Assess the brain first, the skin second, and the thermometer last or not at all. A 101 reading on a person who cannot tell you what day it is does not downgrade the emergency.

When should you call 911 for heat illness?

Call 911 immediately for any of these, and do not talk yourself out of it:

• Confusion, slurred speech, strange behavior, or agitation in someone who has been in the heat

• Any loss of consciousness, even brief, or any seizure

• Hot skin plus a person who cannot answer simple questions

• Vomiting that will not stop, so they cannot keep fluids down

• Symptoms that get worse, or that do not clearly improve within about an hour of cooling and rest

• Body temperature above 102 degrees F, which is the threshold MedlinePlus gives for calling emergency services

NIOSH is direct about heat stroke: call 911, stay with the person, and start cooling while you wait. OSHA's field guidance is even shorter. When in doubt, call 911.

For heat exhaustion that is not improving, NIOSH says get the person evaluated at a clinic or emergency room, and call 911 if that is not available. There is no prize for handling it yourself.

How do you cool someone down fast?

Cooling comes first, and cold water immersion beats everything else. OSHA's heat illness first aid guidance says to immerse the worker in cold water or an ice bath, and calls it the best method to cool a person rapidly in an emergency.

The principle underneath it is simple. Heat leaves a body into water far faster than it leaves into air, so anything that gets cold water against a large area of skin is worth more than anything that only moves air around. Cool aggressively, and keep cooling while help is on the way.

In order of how well they work:

1. Full immersion in cold water. A stock tank, a kiddie pool, a bathtub, a livestock trough, a cooler big enough to sit in. Dump in whatever ice you have. Keep the head above water and hold onto them.

2. Continuous cold water over the whole body. A garden hose or a shower running cold, kept on them, not a quick rinse.

3. Ice packs to the neck, armpits, and groin, plus the head and trunk. These are where big blood vessels run close to the surface. NIOSH and OSHA both name these spots specifically.

4. Wet cloths plus moving air. Soak sheets or towels, lay them over the person, keep re-wetting them, and put a fan on them. The evaporation is doing the work, so the cloth has to stay wet.

Before any of that: get them out of the sun into shade or air conditioning, and strip off outer layers, heavy work clothing, boots, and socks. A car with the air conditioning cranked is a legitimate cooling chamber if it is what you have.

One honest caveat on fans. Ready.gov points out that fans create airflow and a false sense of comfort but do not reduce body temperature on their own, and the National Weather Service advises against relying on a fan when the heat index is up in the high 90s. A fan on dry skin in very hot air is close to useless. A fan on wet skin is genuinely cooling. Wet the person, then run the fan.

What should you never do for heat illness?

Some of the reflexes people reach for make things worse. The MedlinePlus heat emergencies page lists these outright.

• Do not give fluids to anyone confused, vomiting, or unconscious. They cannot protect their own airway, and you can put liquid into their lungs. The National Weather Service says flatly not to give fluids in heat stroke. Sips of cool water are for the alert, oriented heat exhaustion patient only.

• Do not give anything with alcohol or caffeine. A beer on a hot porch is exactly the wrong move, and so is a big iced coffee.

• Do not give fever reducers. No aspirin, no acetaminophen, no ibuprofen. This is the one that surprises people, so it is worth understanding why.

• Do not give salt tablets. NIOSH says avoid them. Use water plus a normal snack or a sports drink instead.

• Do not use alcohol rubs on the skin.

• Do not leave the person alone, ever, once you suspect heat stroke. They can seize or lose consciousness in the time it takes you to go get help.

On the fever reducers, the reason is worth 30 seconds of your attention. A fever and heat illness are not the same physical event. In a fever, your body has deliberately moved its own thermostat setting up to fight an infection, and acetaminophen works by moving that setting back down. In heat illness, the thermostat setting never moved. The body is simply taking on heat faster than it can shed it, and it is already trying as hard as it can to cool off. There is no setting for the drug to adjust. You get zero cooling and you still get the drug's normal load on a liver and kidneys that are already under strain from the heat. Cool water works. Pills do not.

What are heat cramps and heat rash actually telling you?

They are the early stages of the same process, and they are your cheapest warning.

Heat cramps are painful spasms in the legs, arms, or abdomen during or after heavy work in the heat. NIOSH's advice is to drink water and take in something with carbohydrates and electrolytes, like a sports drink, every 15 to 20 minutes, and to avoid salt tablets. Get medical help if the person has heart problems, is on a low sodium diet, or the cramps go past an hour.

Heat rash is those red clusters of tiny pimples or blisters that show up on the neck, upper chest, groin, under the breasts, and in elbow creases. It means sweat ducts are plugged and that patch of skin is not cooling you anymore. Get to a cooler and less humid spot, keep the area dry, and here is the counterintuitive part: NIOSH says do not use ointments and creams on it. They trap more moisture in exactly the place you need to dry out. Powder is fine.

Heat syncope is fainting after standing a long time in the heat or standing up too quickly. Sit or lie down somewhere cool and sip water slowly.

There is a fourth one worth knowing because it hides: rhabdomyolysis, where overworked muscle tissue breaks down and floods the kidneys. The tell is muscle pain plus dark, tea colored or cola colored urine. NIOSH says stop activity, drink water, and get to a medical facility and specifically ask to be checked for it with a creatine kinase blood test. That is a strange sentence to say out loud in an ER, and saying it anyway can save a set of kidneys.

Who is most at risk for heat illness?

The CDC's risk factor list is short and specific:

• Adults 65 and older, who are more prone to heat related health problems and often do not feel thirsty when they should

• Infants and young children, who depend entirely on somebody else to keep them cool and hydrated

• People with chronic medical conditions, especially heart and lung disease and diabetes

• Pregnant people

• Outdoor workers and athletes, who generate a lot of internal heat on top of the outside heat

• Anyone without air conditioning at home

Medications belong on that list too, and this is the piece most heat articles leave out. The CDC's clinician guidance on heat and medications spells out the mechanisms. Antipsychotics, tricyclic antidepressants, anticholinergics, and many antihistamines interfere with central thermoregulation or cut down sweating. Diuretics drive fluid loss and blunt thirst. Beta blockers reduce the surface blood vessel dilation that sheds heat. NSAIDs and sulfonamides raise kidney injury risk when a person is dehydrated. Heat also degrades the medicine itself, damaging inhalers and reducing insulin effectiveness, which is one more reason a hot house is a medical problem and not just an uncomfortable one. If you or someone you look after takes anything on that list, that is worth a conversation with a pharmacist before the next heat wave, not during it. It is also a good reason to keep a plan for keeping medications stable when the refrigerator quits.

How long does it take to get used to working in the heat?

About 7 to 14 days of gradual exposure, and the first few days are the dangerous ones.

OSHA's numbers on this are stark. Almost half of heat related worker deaths happen on a person's very first day on the job or their first day back after time away, and over 70 percent happen in the first week. Not the hottest day. The first day.

NIOSH's acclimatization schedule is easy to remember:

• New to the heat: no more than 20 percent of a normal workload on day 1, then add no more than 20 percent each day after.

• Experienced but coming back after time off: 50 percent on day 1, 60 percent on day 2, 80 percent on day 3, 100 percent on day 4.

• Losing it: a weekend off does not undo your adaptation, but a week or more away can cost you a significant amount of it. The good news is that it usually comes back in 2 to 3 days.

This applies to your Saturday projects too. The person who spends the week in an office and then roofs a shed in August is, physiologically, a brand new worker on day 1. The same August yard work also puts you into the tall grass and the wasp nests, so it pairs well with knowing how to handle ticks, stings, and a reaction that turns serious.

On hydration, the guidance has gotten more careful than "drink as much as you can." The CDC Yellow Book recommends drinking enough to relieve thirst during exercise, and warns that forcing water on someone who is not thirsty raises the risk of hyponatremia, which is dangerously diluted blood sodium. Steady sipping through the day, plus food or an electrolyte drink if you are sweating hard for hours, is the target. Chugging a gallon at lunch is not.

What do you do during a heat wave with no power?

An outage during a heat wave is when heat illness stops being an outdoor problem. Indoor temperatures in a sealed house climb for hours after the sun goes down, and the people most at risk are the ones least able to leave.

The single highest value move is to go somewhere with air conditioning. Ready.gov's advice is to identify cooling centers before you need one, and libraries, shopping centers, community centers, and rec centers all commonly serve as one. Your local health department can tell you where. Even a few hours in a cool building resets your body and buys you the rest of the day.

If you are staying put, work the house:

• Close it down early. Shut windows, blinds, and curtains on the sunny side before the day heats up, not after. Blocking sun before it comes through the glass beats trying to cool the room later.

• Move down and to the north side. Basements and ground floors on the shaded side of the house run meaningfully cooler than upstairs bedrooms.

• Open up at night if it actually cools off outside. Cross ventilate after sundown, then close everything again at first light.

• Wet the person, not the room. Cool showers, wet towels on the neck and wrists, damp cotton clothing. This is the same physics as the field cooling above, and it works at home.

• Keep water cold without a fridge. A cooler holds ice far longer than a dead refrigerator does, and cold drinking water matters more than cold food does.

• Do not run a generator or a grill indoors, not in a garage, not on a covered porch. Carbon monoxide kills people during outages every year.

• Check on people. Older neighbors, anyone living alone, anyone with a chronic condition. Ready.gov and the CDC both put this near the top, and it is the step that most often finds trouble early.

Never leave a person or a pet in a closed car on a warm day, outage or not. A parked car heats far faster than most people expect, and "just a few minutes" is how nearly all of those deaths start. If you are building out your outage supplies more broadly, the volume problem is different from what a go bag solves, which is worth reading about in the shelter in place supply checklist.

Where does home first aid end?

It ends the moment mental status changes. Everything in this article is what a bystander can do in the first few minutes, and those minutes matter enormously. But cooling somebody in a stock tank is a bridge to an ambulance, not a substitute for one.

Be honest about the limits here. You cannot measure a core temperature reliably without a rectal thermometer. You cannot see the organ damage that heat stroke does, and it can keep unfolding after the person looks better. Someone who "came around" after cooling still needs to be evaluated, because improvement on the outside is not the same as recovery on the inside. Call 911, keep cooling, and let the people with IV fluids and monitoring take it from there.

None of this is medical advice, and I am not a clinician. It is a plain reading of what the CDC, NIOSH, OSHA, the National Weather Service, and MedlinePlus publish for the public. When there is a real person in front of you and you are unsure, the answer is the same as it always is: call. If a heat emergency also involves a swallowed substance or a suspected poisoning, Poison Control is free and staffed 24 hours at 1-800-222-1222. For the everyday injuries that happen alongside all this, the same calm approach applies to burns, cuts, and sprains around the house.

Frequently asked questions

Can you have heat stroke and still be sweating?

Yes. NIOSH lists heat stroke as presenting with either hot dry skin or profuse sweating. Sweating that has not stopped does not rule out heat stroke, especially in someone who collapsed during exertion. Judge by mental status, not by how wet their shirt is.

How long does heat exhaustion take to go away?

Most people improve substantially within about an hour of getting cool, resting, and taking in fluids. The National Weather Service advises getting medical attention if the person is vomiting, if symptoms worsen, or if they last longer than an hour. Failure to improve is itself a warning sign that this is something more serious.

What temperature is considered heat stroke?

Clinically, a core temperature at or above 104 degrees F together with altered mental status. In the field, do not rely on that number. Oral, ear, and forehead thermometers are unreliable in this situation, and a normal looking reading on a confused person does not rule out heat stroke.

Should you put someone with heat stroke in an ice bath?

Yes if you can do it safely. OSHA calls cold water or ice bath immersion the best method to cool a person rapidly in an emergency. Keep their head above water, hold onto them, call 911 first or have someone else call while you cool, and never leave them unattended.

Does Tylenol help heat stroke?

No. MedlinePlus specifically says not to give fever medicines like aspirin or acetaminophen in a heat emergency. Fever reducers work by lowering a thermostat setting that the body raised on purpose. In heat illness that setting never moved, so the drug has nothing to act on, and it adds a load on organs already stressed by the heat.

Is it safe to use a fan during a heat wave?

It depends on what else you are doing. Ready.gov notes that fans create airflow and a false sense of comfort but do not lower body temperature on their own, and the National Weather Service advises against relying on one when the heat index is in the high 90s. A fan blowing on wet skin cools well. A fan blowing very hot dry air on dry skin does not.

You never know, but you can always be ready.

Sources: CDC NIOSH, Heat-Related Illnesses, CDC Yellow Book, Heat and Cold Illness in Travelers, CDC NIOSH, Acclimatization, CDC, People at Increased Risk from Heat, CDC, Heat and Medications, OSHA, Heat Illness First Aid, OSHA, Protecting New Workers, National Weather Service, Heat Illness, MedlinePlus, Heat Emergencies, MedlinePlus, Heat Illness, Ready.gov, Extreme Heat.