Extreme Heat and Your Health: 7 Warning Signs You Should Know

Extreme heat is not just a problem in one part of the world. In 2026, major heat events have affected countries across Europe, Asia and the United States. In the U.S., a late-June and early-July heat event affected large parts of the country and was associated with a marked increase in heat-related emergency-department visits.

Extreme heat can feel uncomfortable, tiring, and draining. But when temperatures stay very high, heat can do much more than make you sweat.

Your body has to work harder to keep its temperature within a safe range. If it cannot get rid of enough heat, you can develop heat-related illness.

According to the World Health Organization (WHO), extreme heat can worsen existing health conditions and increase the risk of heat exhaustion, heatstroke, dehydration, kidney problems, and other serious health effects.

The good news is that many heat-related illnesses can be prevented if you know the warning signs and act early.

How does your body normally keep cool?

Your body works constantly to keep its internal temperature within a narrow range.

When you become hot, your body increases blood flow to the skin and produces sweat. As sweat evaporates from your skin, it helps remove heat from your body.

But this cooling system has limits.

Very high temperatures, high humidity, direct sunlight, strenuous activity, and poor air movement can make it harder for your body to lose heat. High humidity is particularly important because sweat does not evaporate as easily when the air already contains a lot of moisture.

If your body continues to gain heat faster than it can lose it, your temperature can rise to dangerous levels.

What happens to your body in extreme heat?

Heat affects more than your skin.

Your heart works harder as your body sends more blood toward the skin to release heat. You also lose water and salts through sweating.

With prolonged or severe heat exposure, this can put stress on the heart, kidneys, brain, and other organs. Heat can also worsen some existing medical conditions, including cardiovascular, respiratory, kidney, and diabetes-related conditions.

This is why extreme heat should be taken seriously, particularly by people who are more vulnerable.

7 warning signs of heat-related illness

Knowing the early warning signs can help you respond before the situation becomes more serious.

1. Heavy sweating

Sweating is one of the body's normal ways of cooling itself.

But unusually heavy sweating, especially together with weakness, dizziness, headache, or nausea, can be a warning sign that your body is struggling with the heat.

Move to a cooler place, stop strenuous activity, and begin cooling down.

2. Headache

A headache can occur when you become overheated or dehydrated.

If a headache develops after spending a long time in the heat, especially when accompanied by dizziness, weakness, nausea, or heavy sweating, do not simply ignore it.

Get somewhere cool and drink fluids.

3. Dizziness or weakness

Feeling dizzy, light-headed, unusually weak, or unsteady can be a sign of heat-related illness.

You may feel as though you need to sit or lie down.

Move to a cool or shaded place and rest. If symptoms are severe, persistent, or getting worse, seek medical care.

4. Muscle cramps

Heat and heavy sweating can cause loss of water and salts.

This can contribute to painful muscle cramps, particularly after physical activity in hot conditions.

Stop exercising or working in the heat and move somewhere cooler.

5. Nausea or vomiting

Feeling sick or vomiting can occur with heat-related illness.

If someone is becoming increasingly unwell in hot weather, especially if several symptoms occur together, they should not continue exercising or working in the heat.

Cooling and medical assessment may be needed.

6. Confusion or unusual behaviour

This is a much more serious warning sign.

Confusion, unusual behaviour, difficulty speaking clearly, loss of coordination, or altered mental status can indicate heatstroke.

Heatstroke is a medical emergency.

Do not wait for the person to "sleep it off."

Seek emergency medical help immediately and begin cooling the person while waiting for help.

7. Loss of consciousness or seizures

Fainting, loss of consciousness, or seizures in someone who has been exposed to extreme heat is an emergency.

Heatstroke can cause serious brain and organ damage and can be fatal if treatment is delayed.

Get emergency medical help immediately.

Heat exhaustion or heatstroke?

These terms are sometimes used interchangeably, but they are not the same.

Heat exhaustion can develop when the body loses too much water and salt, often through heavy sweating. Symptoms can include heavy sweating, weakness, headache, dizziness, nausea, thirst, and reduced urine output.

Heatstroke is much more serious. The body's ability to control its temperature breaks down, and the person may develop confusion, altered consciousness, seizures, or very high body temperature.

If you suspect heatstroke, treat it as a medical emergency.

Who is most vulnerable to extreme heat?

Heat can affect anyone, but some people are at greater risk.

These include:

• Older adults

• Infants and young children

• Pregnant women

• People with heart, lung, kidney, or other chronic diseases

• People who work or exercise outdoors

• People who live alone

• People taking certain medicines that can affect hydration or the body's response to heat

WHO also highlights the importance of checking on vulnerable people during periods of extreme heat.

If you take regular medication, do not stop or change it because of hot weather without speaking with your doctor. Some medicines can affect how your body responds to heat, while others need special storage conditions.

How can you protect yourself?

You do not have to wait until you feel unwell before taking action.

Stay cool

Try to avoid strenuous outdoor activity during the hottest part of the day.

Stay in the shade when you are outside and spend time in a cool place when possible. WHO recommends staying out of the heat during the hottest part of the day and keeping your home as cool as possible.

Drink fluids regularly

Drink water regularly, particularly when you are sweating or spending time outdoors.

If you have a medical condition that requires you to restrict fluids, follow your doctor's advice rather than increasing your fluid intake on your own.

Wear suitable clothing

Light, loose-fitting clothing can help your body lose heat.

A hat and shade can also reduce direct exposure to the sun.

Take breaks

If you work or exercise outdoors, take regular breaks in a cooler or shaded place.

Do not wait until you feel extremely thirsty, dizzy, or weak before stopping.

Check on others

During very hot weather, check on older relatives, neighbours, friends, and anyone who lives alone or has a chronic medical condition.

Children and older adults may need particular attention during periods of extreme heat.

What should you do if someone becomes unwell in the heat?

Move the person to a cooler or shaded place and stop their activity.

Help cool them by removing unnecessary clothing and using cool water, wet cloths, or other appropriate cooling methods.

If the person has confusion, altered consciousness, seizures, or loss of consciousness, seek emergency medical help immediately. Heatstroke requires rapid treatment.

Do not leave a seriously unwell person alone.

Extreme heat is more than just an uncomfortable day

A very hot day may seem like something we simply have to tolerate.

But extreme heat can place significant stress on the body. WHO describes heat as an important environmental and occupational health hazard and notes that its effects can be particularly serious for vulnerable people.

The important thing is to recognize the warning signs early.

Heavy sweating, headache, dizziness, weakness, muscle cramps, nausea, and confusion should not be ignored when you have been exposed to extreme heat.

And if someone develops confusion, seizures, or loss of consciousness, think heatstroke and emergency medical care.

Professional tip

You do not need to wait until you feel extremely hot to take action.

On very hot days, stay cool, stay hydrated, take breaks, and pay attention to how you feel. A few simple precautions can help prevent a hot day from becoming a medical emergency.

Related Articles:
1. Excessive Heat and Your Health: Why City Living Feels Hotter — and More Dangerous

Sources

1. World Health Organization (WHO) – Heat and health
   WHO – Heat and health

2. World Health Organization (WHO) – Heatwaves
   WHO – Heatwaves

3. World Health Organization (WHO) – Prevention and management of heat-related illness at mass gatherings
   WHO – Heat-related illness at mass gatherings

4. Centers for Disease Control and Prevention (CDC) – About Heat and Your Health
   CDC – About Heat and Your Health

5. Centers for Disease Control and Prevention (CDC/NIOSH) – Heat-related Illnesses

6. Harvard Health Publishing – Heat Stroke (Hyperthermia)
   Harvard Health – Heat Stroke

7. Harvard T.H. Chan School of Public Health – Extreme Heat: Who’s Most at Risk and Ways to Stay Safe
   Harvard T.H. Chan School of Public Health – Extreme Heat

8. Johns Hopkins Medicine – Heat-Related Illnesses
   Johns Hopkins Medicine – Heat-Related Illnesses

9. CDC – Heat and Your Health
   CDC – About Heat and Your Health

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