Heat Exhaustion

First aid for heat exhaustion

1. Stop activity and get out of the heat: into air conditioning or deep shade.
2. Lie the person down with legs slightly raised if they feel faint.
3. Remove extra clothing and equipment, and loosen tight clothing.
4. Cool the skin: cool, wet cloths or a cool shower, misting with water and fanning. Ice packs in the armpits, groin and neck help.
5. Give fluids if the person is fully alert: sips of cool water or a sports or oral rehydration drink. Avoid alcohol and large amounts of caffeine.
6. Watch closely. If they become confused, stop sweating, vomit repeatedly or do not improve within an hour, call 911 or your local emergency number.

Do not give acetaminophen (paracetamol), ibuprofen or aspirin to lower the temperature. They do not work for heat illness and can harm the liver or kidneys when the body is stressed.

Heat exhaustion symptoms

• Heavy sweating
• Cool, pale, clammy skin, sometimes with goosebumps
• Faintness and dizziness, especially when standing up
• Headache
• Nausea or vomiting
• Muscle cramps
• Weakness and fatigue
• Fast, weak pulse
• Thirst and dark urine
• Irritability

Symptoms can come on suddenly, or build over several days of heat exposure with poor fluid intake, which is common in older adults during heat waves.

Heat exhaustion vs heat stroke

The key difference is the brain. In heat exhaustion the person is still thinking clearly. In heat stroke the body's cooling system has failed and the core temperature rises high enough to disrupt brain function.

Sweating does not rule out heat stroke: athletes with exertional heat stroke often still sweat. Confusion is the deciding clue.

The heat illness spectrum

• Heat rash: small itchy bumps from blocked sweat ducts.
• Heat cramps: painful muscle spasms from salt loss during heavy sweating.
• Heat syncope: fainting from blood pooling in the legs.
• Heat exhaustion: the body is struggling, with water and salt depletion.
• Heat stroke: cooling has failed; a life-threatening emergency.

Why it happens

Your body cools mainly by sweating and by sending blood to the skin. In heat and humidity, sweat evaporates poorly, so you keep sweating without cooling. You lose water and salt, blood volume drops, and the heart struggles to supply both muscles and skin. The result is faintness, a weak racing pulse and exhaustion.

Risk factors

• Heat and humidity: a heat index above about 32 °C (90 °F) raises risk; above 39 °C (103 °F) risk is high.
• Not being acclimatized: the first few days of hot weather, a new outdoor job or arriving in a hot climate. Full acclimatization takes about 1 to 2 weeks.
• Age: infants, young children and adults over 65 regulate heat less well.
• Hard physical work or exercise, especially in heavy uniforms or protective gear.
• Medicines: diuretics, beta-blockers, antihistamines, anticholinergics, some antidepressants and antipsychotics, and stimulants.
• Health conditions: heart disease, kidney disease, diabetes, obesity, and a recent illness with fever or diarrhea.
• Alcohol and dehydration.

How heat exhaustion is diagnosed and treated medically

Diagnosis is based on symptoms and the setting. In an ER or athletic medical tent, staff check core temperature (a rectal temperature is the accurate method in suspected heat stroke), mental state, heart rate and blood pressure. Blood tests may check sodium, potassium, kidney function and muscle enzymes (CK). Treatment is cooling and fluids; intravenous fluids are given when vomiting prevents drinking or dehydration is significant.

Recovery: how long does heat exhaustion last?

With prompt cooling and fluids, most people feel much better within 30 to 60 minutes. Tiredness, a mild headache and reduced exercise tolerance can last 24 to 48 hours. Rest in a cool environment, keep drinking, and avoid hard work or exercise in the heat for at least a day or two. After an episode, you may be more sensitive to heat for about a week. Athletes should return gradually.

Complications

• Progression to heat stroke
• Rhabdomyolysis (muscle breakdown) after intense exertion, with dark urine
• Low sodium (hyponatremia) if large volumes of plain water replace heavy sweat losses
• Falls and injuries from fainting
• Worsening of heart or kidney disease

Preventing heat exhaustion

• Check the heat index and plan heavy activity for early morning or evening.
• Drink regularly before you are thirsty. During hours of heavy sweating, include electrolyte drinks or salty snacks, and do not overdo plain water.
• Wear light, loose, light-colored clothing and a wide-brimmed hat.
• Build up exposure gradually over 1 to 2 weeks.
• Take frequent breaks in shade or air conditioning. Workers should follow a "water, rest, shade" plan.
• Check on older neighbors and relatives during heat waves. Electric fans alone do not prevent heat illness when indoor temperatures are very high; cooling centers do.
• Never leave children or pets in a parked car, even with windows cracked.

Frequently asked questions

What are the first signs of heat exhaustion?

The earliest signs are heavy sweating, feeling unusually tired or weak, thirst, a headache, and dizziness when standing. Muscle cramps and nausea often follow. Stopping activity and cooling down at this stage usually prevents the illness from getting worse.

How do you tell heat exhaustion from heat stroke?

Look at mental state and temperature. A person with heat exhaustion is alert and has a temperature usually below 40 °C (104 °F). Confusion, odd behavior, slurred speech, seizures or collapse, or a temperature of 40 °C or higher, suggest heat stroke. Call 911 and start rapid cooling.

How long does heat exhaustion last?

Symptoms usually improve within 30 to 60 minutes of cooling and drinking. You may feel tired or headachy for a day or two. If you are not improving after an hour, or you get worse at any point, seek emergency care.

What should you drink for heat exhaustion?

Cool water is fine for most people. If you have been sweating heavily for hours, a sports drink or oral rehydration solution also replaces salt. Sip steadily rather than gulping large amounts. Avoid alcohol and energy drinks.

Can heat exhaustion make you sick the next day?

Yes. Fatigue, weakness, headache and poor appetite can linger for 24 to 48 hours, and you may be more sensitive to heat for several days. Rest, rehydrate and stay out of the heat. New confusion, dark urine or persistent vomiting need medical assessment.