Heat Stroke: What Causes It and How to Prevent It

What Is Heat Stroke?

Heat stroke is a medical emergency in which the body becomes dangerously overheated and brain function changes.

Core temperature is often around 40°C or 104°F or higher, but emergency action should not depend on obtaining that number.

The defining warning signs include confusion, unusual behavior, slurred speech, poor coordination, seizure, fainting, or loss of consciousness.

Heat stroke can develop rapidly during hard exercise or more gradually during prolonged environmental heat.

Without immediate cooling and hospital care, it can cause permanent disability or death.

How the Body Normally Cools Itself

The body controls heat through sweating and evaporation, increased blood flow to the skin, radiating heat into a cooler environment, moving air across the skin, and reducing physical activity.

These systems work less effectively when the air is very hot, humidity is high, air movement is low, the person wears heavy clothing, physical work creates extra heat, or dehydration reduces circulation.

Heat stroke develops when heat gain and heat production exceed heat loss.

Main Heat Stroke Causes

1. Extreme Environmental Heat

High air temperature makes it harder for the body to release heat. Hot indoor areas can be as dangerous as outdoor conditions.

2. High Humidity

Sweat cools the body only when it evaporates. High humidity slows evaporation, so someone may sweat heavily without cooling effectively.

3. Strenuous Physical Activity

Working muscles create heat. Running, football, military training, construction, farming, delivery work, and other intense activity can raise core temperature quickly.

4. Dehydration

Fluid loss can reduce blood volume and make sweating and circulation less effective. A fully hydrated person can still develop heat stroke when heat and workload are extreme.

5. Heavy or Impermeable Clothing

Protective suits, helmets, pads, uniforms, and non-breathable clothing can block sweat evaporation and trap body heat.

6. Lack of Acclimatization

The body needs time to adapt to hot work or training. New workers, returning workers, travelers, and athletes beginning summer training are at higher risk.

7. Inadequate Rest and Cooling

Continuous work or exercise without planned breaks allows heat to build faster than the body can remove it.

8. Hot Living Conditions

People living without air conditioning, ventilation, or access to cooler spaces may accumulate heat over several hot days and nights.

9. Alcohol and Drugs

Alcohol can worsen dehydration, judgment, and heat regulation. Stimulants can increase heart rate, activity, and heat production.

10. Health Conditions and Medicines

Heart, kidney, respiratory, neurologic, metabolic, and mental-health conditions may reduce the body’s ability to respond to heat.

Why Humidity Makes Heat More Dangerous

When humidity is high, sweat evaporates more slowly.

Risk therefore depends on more than air temperature. It also depends on direct sunlight, air movement, physical workload, clothing, exposure time, health, and acclimatization.

The heat index can help describe how hot conditions feel. Some workplaces use wet-bulb globe temperature for a fuller assessment.

Do not wait for an official heatwave declaration before using prevention measures.

Exertional Versus Classic Heat Stroke

Exertional Heat Stroke

Exertional heat stroke occurs during intense physical activity. It often affects athletes, military personnel, outdoor workers, firefighters, and people wearing heavy protective equipment.

It can develop within minutes or hours, and heavy sweating is common.

Classic Heat Stroke

Classic heat stroke usually results from prolonged environmental heat. It often affects older adults, infants, people with chronic illness, people taking heat-risk medicines, and people without adequate home cooling.

Both forms require emergency treatment.

Heat Stroke Symptoms and Warning Signs

• Confusion or disorientation
• Unusual behavior
• Slurred speech
• Poor balance or coordination
• Seizure
• Fainting or collapse
• Very high body temperature
• Hot skin
• Heavy sweating or dry skin
• Fast breathing
• Rapid heartbeat
• Headache
• Nausea or vomiting
• Severe weakness

Altered mental status after heat exposure is enough to treat the situation as possible heat stroke.

Heat can also worsen blood pressure, heart, kidney, and lung conditions. Read Blood Pressure Signs and Symptoms, Signs of a Heart Attack, and Kidney Disease Symptoms.

Heat Exhaustion Versus Heat Stroke

Heat exhaustion may cause headache, dizziness, nausea, heavy sweating, pale or clammy skin, muscle cramps, thirst, weakness, and a fast heartbeat.

If symptoms do not improve promptly with cooling, or if confusion, seizure, fainting, or reduced consciousness develops, call emergency services.

Who Is Most at Risk of Heat Stroke?

Older Adults

Older adults may have reduced thirst, less efficient temperature control, chronic disease, limited mobility, or medicines that affect heat tolerance.

Infants and Young Children

Children heat up faster and depend on adults for fluids, rest, shade, and safe scheduling.

Pregnant People

Pregnancy changes circulation, fluid needs, and heat tolerance.

Athletes and Workers

High-intensity training, competition pressure, heavy workload, and protective gear increase risk.

People With Chronic Illness

Risk can be higher with heart disease, kidney disease, diabetes, respiratory disease, neurologic disease, mental-health conditions, and obesity.

Visit the Heart and Cardiovascular Health Hub, Kidney Health and Disease Hub, and Diabetes and Blood Sugar Management Hub.

Medicines and Substances That May Increase Heat Risk

Some medicines can affect sweating, circulation, alertness, kidney function, thirst, or fluid balance.

• Diuretics
• Some blood-pressure medicines
• Anticholinergic medicines
• Some antidepressants
• Some antipsychotics
• Some antihistamines
• Stimulants
• Some medicines that affect kidney function

Alcohol and illicit stimulants may also increase risk.

Do not stop prescribed medicine because hot weather is expected. Ask a doctor or pharmacist how your medicines affect heat tolerance and hydration.

For more context, read What Causes High Blood Pressure?.

Possible Heat Stroke Complications

• Brain injury
• Seizures
• Kidney injury
• Liver injury
• Muscle breakdown
• Abnormal heart rhythms
• Low blood pressure or shock
• Bleeding and clotting problems
• Respiratory failure
• Coma
• Death

Damage may continue after body temperature starts to fall. Anyone with suspected heat stroke needs hospital evaluation even if they seem better after cooling.

How to Prevent Heat Stroke

Check Heat Conditions

Review temperature, humidity, direct sun, workload, clothing, and access to cooling.

Schedule Activity During Cooler Hours

Move work or exercise to early morning or evening when possible.

Use Planned Rest Breaks

Take breaks before symptoms begin. Break frequency should increase as heat, humidity, workload, or protective clothing increases.

Drink Regularly

Drink enough for the conditions and workload rather than waiting for intense thirst. People with heart, kidney, or fluid-balance problems need individual advice.

Wear Appropriate Clothing

Use loose, lightweight, breathable clothing when safety requirements allow.

Use Shade or Air Conditioning

Rest in a genuinely cooler space. A fan may be less effective when indoor temperatures are extremely high.

Use a Buddy System

Heat stroke can impair judgment. Coworkers, teammates, and family members may recognize confusion first.

Avoid Alcohol Before Heat Exposure

Alcohol can worsen dehydration and judgment.

Review Medicines

Ask about heat precautions before a heatwave or hot season.

Never Leave Anyone in a Parked Vehicle

Cars can become deadly within minutes, even when windows are partly open.

For broader daily prevention habits, read the Healthy Lifestyle Roadmap.

Heat Acclimatization

Acclimatization is the gradual process of adapting to repeated heat exposure.

New workers and athletes should increase heat exposure gradually over approximately 7 to 14 days.

Returning after illness, vacation, or a long cool period may require renewed acclimatization.

Acclimatization reduces risk but does not make someone immune to heat stroke.

Heat Stroke Prevention for Workers

• Cool drinking water
• Planned rest breaks
• Shade or cooled recovery spaces
• Heat-risk monitoring
• Acclimatization
• Supervisor and worker training
• A buddy system
• Emergency communication
• Cooling equipment
• Workload adjustment

Workers should be able to report symptoms without punishment or pressure to continue.

Heat Stroke Prevention for Athletes

• Schedule sessions during cooler hours.
• Build intensity gradually.
• Use heat-acclimatization plans.
• Provide water and cooling stations.
• Modify or cancel activity during dangerous conditions.
• Monitor teammates for behavior changes.
• Have emergency communication ready.
• Prepare rapid cooling equipment for high-risk events.

For lower-intensity activity, read Walking Exercise for Seniors. Outdoor exercise should still be moved indoors during dangerous heat.

Heatwave Safety at Home

• Keep blinds or curtains closed on sun-facing windows during the hottest hours.
• Ventilate when outdoor air is cooler.
• Use air conditioning or visit a cooled public space when available.
• Sleep in the coolest safe room.
• Reduce use of heat-producing appliances.
• Check indoor temperature.
• Drink regularly according to personal medical advice.
• Check on people who live alone.

Fans may provide comfort and increase evaporation, but they do not make an extremely hot room safe by themselves.

Protecting Children and Older Adults

Children

• Never leave a child in a parked car.
• Schedule play during cooler hours.
• Provide frequent water and shade breaks.
• Use light clothing and sun protection.
• Watch for unusual quietness, sleepiness, vomiting, or confusion.
• Do not cover a stroller with a blanket that blocks airflow.

Older Adults

• Check in daily during extreme heat.
• Confirm access to a cooled space.
• Review medicine and hydration advice.
• Watch for weakness, falls, confusion, reduced urine, or poor appetite.
• Arrange transport before indoor heat becomes dangerous.

Common Heat Stroke Myths

Myth: Dehydration Is the Only Cause

False. Heat, humidity, workload, clothing, health, medication, and acclimatization also matter.

Myth: Heat Stroke Happens Only Above 100°F

False. Humidity, direct sun, workload, and protective clothing can create dangerous heat stress at lower air temperatures.

Myth: Sweating Rules Out Heat Stroke

False. Exertional heat stroke often includes heavy sweating.

Myth: Fit People Cannot Get Heat Stroke

False. Highly trained athletes can develop exertional heat stroke.

Myth: Water Alone Prevents Every Case

False. Prevention also requires rest, shade, acclimatization, workload control, and early symptom recognition.

Myth: You Must Measure 104°F Before Calling

False. Altered mental status after heat exposure is enough to treat the situation as an emergency.

Myth: Fever Medicine Treats Heat Stroke

False. Heat stroke is not an infectious fever.

Myth: Sunscreen Prevents Heat Stroke

False. Sunscreen protects against ultraviolet injury but does not prevent dangerous core overheating.

Myth: Fans Always Prevent Heat Illness

False. Fans may not provide enough protection in extremely hot indoor conditions.

Myth: Someone Who Feels Better After Cooling Can Return to Activity

False. Suspected heat stroke requires medical evaluation and a supervised recovery plan.