Heat Exhaustion and Heat Stroke: What the Evidence Actually Says

Key takeaways

• Heat exhaustion signs: heavy sweating, cold, pale and clammy skin, fast weak pulse, nausea or vomiting, muscle cramps, tiredness, dizziness, headache, fainting
• Heat stroke signs: body temperature of 103°F (39.4°C) or higher, hot red skin that may be dry or damp, fast strong pulse, confusion, losing consciousness
• Most useful single change: Stay in air conditioning during the hottest part of the day; use a cooling center if you need one.
• Commonest mistake: Leaving anyone, including children and pets, in a parked car.
• See a doctor if any of the signs under "When to speak to a doctor" below apply to you.

What it is

Two heat-related illnesses: heat exhaustion, which is a warning sign, and heat stroke, which is a medical emergency. It sits within Weather Safety. The figures come from the bodies named at the foot of this page.

The numbers, in one place

What helps

What follows is the practical part: the things guidance actually asks of you.

• Stay in air conditioning during the hottest part of the day; use a cooling center if you need one
• Drink more water than usual, without waiting until you are thirsty
• Wear loose, lightweight, light-colored clothing
• For heat exhaustion, move to a cool place, loosen clothing, use cool wet cloths and sip water

What to avoid

The common mistakes, including the ones that sound sensible.

• Leaving anyone, including children and pets, in a parked car
• Heavy outdoor activity in the hottest part of the day
• Giving a person with heat stroke anything to drink

In depth

Your body keeps its core temperature in a narrow range mainly by moving blood toward the skin and by sweating. As sweat evaporates, it carries heat away. In hot weather this system works hard, and it has limits. When the air is humid, sweat evaporates poorly, so you keep sweating without cooling much. When you lose a lot of water and salt through sweat without replacing them, blood volume drops, and the heart struggles to supply both the skin and the vital organs at once.

Heat exhaustion is the stage at which this strain shows. The heavy sweating, clammy skin, fast weak pulse, dizziness and nausea are signs that your cooling system is still working but is close to its limit. If you rest somewhere cool and replace fluids, most people recover within an hour or so. If you keep going, the system can fail. Heat stroke is that failure: body temperature climbs to dangerous levels, and the brain, heart, kidneys, liver and muscles can be damaged.

There are two broad patterns. Classic heat stroke tends to develop over several days of a heat wave, often in older adults or people with chronic illness who are indoors without air conditioning. Exertional heat stroke can come on within hours in young, healthy people who are working, training or playing sports in the heat. In both, the most important factor in survival is how quickly the body is cooled. That is why fast cooling and calling 911 matter more than anything else.

Signs and symptoms

• Muscle cramps or spasms during or after activity in the heat, often an early warning
• Feeling faint, light-headed or unusually weak when standing
• Headache, nausea or a sudden lack of energy that does not match the effort
• Dark yellow urine or passing much less urine than usual
• Clumsiness, irritability, odd behavior or slurred speech, which suggest the brain is affected
• Collapse during exercise in the heat

Causes and risk factors

• High heat index, the combination of heat and humidity that limits sweat evaporation
• Hard physical work or exercise in hot conditions, especially without rest breaks
• Not being used to the heat, such as during the first hot days of the season or a new outdoor job
• Dehydration from not drinking enough, vomiting, diarrhea or alcohol
• Heavy or dark clothing and protective gear that trap heat
• Health conditions such as heart disease, obesity and diabetes, and some medicines
• Homes without air conditioning, especially upper-floor apartments in cities

How it is checked

Heat illness is recognized mainly from what happened and how the person looks. In the field, the key question is whether the brain is affected: confusion, strange behavior, slurred speech, seizures or collapse point to heat stroke rather than heat exhaustion and call for 911 and immediate cooling. A temperature taken by mouth, forehead or ear can underestimate the true core temperature in someone who is overheated, so a normal-seeming reading should not reassure you if the person is confused.

In the emergency room, clinicians measure core temperature, usually rectally, and check blood tests for salt levels, kidney and liver function, blood sugar and muscle breakdown. They also look for other causes of fever or confusion, such as infection, drug effects or low blood sugar. Some people with heat stroke need hospital care for several days while organs recover, and follow-up checks may be advised before returning to hard work or sports.

Treatment and support options

First aid for heat exhaustion

Stop all activity and move to air conditioning or shade. Loosen or remove extra clothing, lie down with the legs slightly raised if you feel faint, and cool the skin with wet cloths, a cool shower or a fan with misting. Sip water or a drink with some electrolytes if you are not vomiting. Most people feel better within an hour. If symptoms worsen, you vomit or recovery takes longer than an hour, get medical help.

Cooling someone with heat stroke

Call 911 first, then start cooling right away rather than waiting for the ambulance. The fastest method is putting the person in a tub of cool or ice water up to the neck. If that is not possible, soak them with cold water, cover them with wet sheets or towels and fan them, and put ice packs in the armpits, groin and neck. Do not give anything to drink if they are confused or drowsy.

Hospital care

In the hospital, cooling continues until the core temperature is in a safer range, and fluids are given into a vein. Doctors monitor the heart, kidneys, liver and blood clotting closely, because heat stroke can affect many organs at once. Fever-reducing medicines such as acetaminophen (Tylenol) do not help with heat stroke, because the problem is not a reset thermostat, as in an infection, but a cooling system that has been overwhelmed.

Recovering safely

After heat exhaustion, take it easy for the rest of the day and keep drinking fluids. After heat stroke, the body may be more sensitive to heat for weeks, and a gradual, supervised return to work or training is generally recommended. Athletes and outdoor workers should ask their doctor or team medical staff for a return plan rather than going straight back to full effort.

Prevention

• Check the HeatRisk forecast and plan outdoor tasks for early morning or evening
• Build up heat tolerance gradually over one to two weeks when starting outdoor work or training
• Take regular rest breaks in shade or air conditioning and drink water on a schedule
• Know where the nearest cooling center is; calling 211 or your local health department can help
• Use a buddy system at work and on teams so someone notices early signs

Day to day

During a heat wave, think in terms of the whole day. Do errands and exercise early, spend the hottest hours in air conditioning, and keep your home cooler by closing blinds on the sunny side. Cool showers and damp cloths can bring your temperature down quickly. If your home has no air conditioning, even a few hours a day at a library, mall or cooling center gives your body a chance to recover.

Watch your urine color as a simple guide to hydration: pale yellow is a good sign, while dark urine suggests you need more fluid. If you are on a fluid restriction for heart or kidney disease, ask your doctor ahead of time how to adjust in hot weather. Avoid large amounts of alcohol and very sugary drinks, which do little to help hydration. Keep an eye on others, too, since people with heat illness may not notice their own confusion.

For specific groups

Outdoor workers

Construction, farm, landscaping and delivery workers face high risk, especially new workers who are not yet used to the heat. Employers are expected to plan for water, rest and shade. Speak up if you feel unwell, and look out for coworkers who seem confused or stop sweating normally.

Athletes

Exertional heat stroke is a serious, preventable cause of sudden illness and death in young athletes. Practices in hot weather should be shortened, include frequent water breaks and phase in equipment gradually. Teams should have a cold-water tub ready so a collapsed athlete can be cooled on site.

Babies and young children

Children heat up faster than adults and depend on others to keep them cool. Dress them lightly, offer fluids often and never leave them in a parked car, where the temperature can rise rapidly even with a window cracked. Call their doctor for fewer wet diapers, unusual sleepiness or fever.

Pregnancy

Pregnancy increases the body's heat load and fluid needs. Extreme heat is linked to pregnancy complications, so limit time in the heat, stay well hydrated and rest in cool places. Contact your obstetric provider if you feel faint, have contractions or notice reduced movement.

Questions to ask your doctor

• Do any of my health conditions or medicines raise my risk in hot weather?
• How much should I drink on hot days, given my heart or kidney condition?
• After heat stroke, when and how can I safely return to work or sports?
• Do I need any blood tests to check that my kidneys and liver have recovered?
• Are there signs that mean I should go to the emergency room rather than rest at home?
• Should I be using electrolyte drinks, or is water enough for me?

More questions, answered

Can you get heat stroke without being in the sun?

Yes. Heat stroke can happen indoors, especially in homes without air conditioning during a heat wave. Many heat deaths occur indoors in older adults living alone. Sun adds heat, but the main problems are high air temperature, humidity and the body's ability to shed heat.

Does sweating stop in heat stroke?

Not always. In classic heat stroke the skin is often hot and dry, but in exertional heat stroke people are frequently still sweating heavily. Do not rely on dry skin as a sign. Confusion, odd behavior or collapse in the heat are the most important warning signs.

Are sports drinks better than water?

For most people doing everyday activities, water is enough. During long, sweaty work or exercise, drinks with electrolytes can help replace the salt you lose. Drinking very large amounts of plain water without salt during prolonged exercise can, rarely, lower blood sodium to dangerous levels.

What is the heat index?

The heat index combines air temperature and humidity to show how hot it feels to the body. Because humid air slows sweat evaporation, a humid 90°F day can be more dangerous than a dry one. It is measured in shade, so direct sun can make conditions feel even hotter.

If I had heat stroke once, am I more likely to have it again?

Some people remain more sensitive to heat for a period after heat stroke. Your doctor may advise a gradual return to activity and, for athletes or military personnel, sometimes a formal heat tolerance assessment. Most people who are cooled quickly recover fully.

Is it safe to use an ice bath for someone with heat stroke?

Yes. Cold-water immersion is the fastest way to cool a person with heat stroke, and it is widely used by athletic trainers and emergency teams. Keep the person's head supported above the water, stay with them and call 911. Rapid cooling greatly improves the chance of survival.

Related topics

• Sun and Heat Abroad: What the Evidence Actually Says (Travel Health)
• The Evidence on Burns and Scalds, Without the Hype (Emergency Care)
• Stroke: What Published Guidance Supports (Brain Health)
• The Evidence on Fever, Without the Hype (Immunity)

Where to go next

• Making Sense of Heat Exhaustion and Heat Stroke
• Heat Exhaustion and Heat Stroke: A Practical Checklist
• Heat Exhaustion and Heat Stroke — the reference entry
• Weather Safety: The Complete Guide
• Heat and Medications: What Published Guidance Supports
• Hypothermia: What Published Guidance Supports

When to speak to a doctor

Call 911 for signs of heat stroke such as a temperature of 103°F or higher, confusion or passing out, then move the person somewhere cooler and cool them with wet cloths or a cool bath. Get medical help for heat exhaustion if the person is vomiting or symptoms get worse or last longer than 1 hour.

Where this comes from

• CDC — Heat-Related Illnesses
• CDC — Extreme Heat

How this page was put together

Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.

Medical disclaimer

This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.