A heatwave is defined as a period of unusually hot weather, dry or humid, that begins and ends abruptly and lasts at least two or three days.
Historically, heatwaves occurred once or twice a year, affecting only certain countries and specific geographic areas within them. However, since 2000, an increase in the magnitude, duration, and intensity of heatwaves has been observed worldwide. Factors such as urbanization and climate change have contributed to more frequent and intense heatwaves, including in areas where they were previously uncommon.
Heatwaves can have adverse effects on human health and natural systems. Children under five years of age, older people, and people with chronic diseases are among the most vulnerable. People who engage in outdoor activities for work or recreation during periods of intense heat, as well as migrants and other people on the move who travel long distances on foot, also face increased risks. Socioeconomic factors and living conditions, such as residing in remote communities with limited access to health services, can further increase vulnerability to the health effects of heatwaves.
During periods of extreme heat, a person can lose up to two liters of fluid per hour through sweating, increasing the risk of dehydration.
Globally, heat-related mortality among adults aged 65 and older increased by approximately 85% when comparing 2000-2004 with 2017-2021.
Exposure to extreme heat can harm health from pregnancy through childhood, leading to a 26% increase in the likelihood of preterm birth.
Between 2000 and 2016, the number of people exposed to heatwaves globally increased by at least 125 million, while the average heatwave lasted 0.37 days longer than during the 1986-2008 period.
Over recent decades, several countries in the Americas have experienced heatwaves with major public health impacts, including associated deaths. Notable events include Brazil (2010), with 735-737 excess deaths; Argentina (2013-2014), with 1,877 excess deaths during three heatwaves; and the 2018-2019 period, when at least seven countries across the Region issued heatwave alerts, with temperatures exceeding 40°C in several countries and reaching as high as 45°C in Mexico.
Frequently Asked Questions About Heatwaves and Health
What is a heatwave?
A heatwave is a period of unusually hot weather, whether dry or humid, that begins and ends abruptly and lasts for at least two or three consecutive days. Heatwaves can occur during the day, at night, or both, and may have serious impacts on human health and natural systems.
What happens to your body during a heatwave?
The human body has natural mechanisms to regulate its temperature when exposed to heat. One of the first responses is sweating, which helps cool the body through evaporation. Another response is vasodilation, a process in which blood flow is redirected toward the skin to release heat into the environment.
While these mechanisms are essential for maintaining a stable body temperature, they can become overwhelmed during prolonged or intense heat exposure, increasing the risk of heat-related illness.
Why can extreme heat be dangerous?
Excessive sweating causes the body to lose water and electrolytes, which can lead to dehydration and muscle cramps. At the same time, the cardiovascular system has to work harder to circulate blood to the skin, placing additional strain on the heart and other organs.
If the body can no longer regulate its temperature effectively, serious heat-related illnesses may occur.
Who is most at risk during a heatwave?
Everyone can be affected by extreme heat, but certain groups are more vulnerable, including:
- Infants and young children, especially those under 5 years of age
- Older adults
- Pregnant women
- People with cardiovascular disease
- People with respiratory conditions
- People living with diabetes, obesity, or thyroid disorders
- Outdoor workers
- Athletes and individuals engaging in strenuous physical activity outdoors
What are the early signs that extreme heat is affecting your health?
Early symptoms may include:
- Excessive sweating
- Muscle cramps
- Fatigue or weakness
- Dizziness
- Headache
- Swelling of the feet, ankles, or legs
- Flushed or reddened skin
- Heat rash, including small blisters or skin irritation
These signs could indicate that the body is struggling to cope with heat stress.
What symptoms should never be ignored?
Seek immediate medical attention if a person develops:
- A body temperature of 40°C (104°F) or higher
- Altered mental status, confusion, or disorientation
- Hot, red, and dry skin
- Severe dehydration
- Loss of consciousness
- Seizures
These symptoms may indicate heat stroke, a life-threatening medical emergency.
What is heat stroke?
Heat stroke occurs when the body can no longer regulate its temperature and is the most serious health consequence of extreme heat exposure. It can lead to kidney damage, neurological complications, organ failure, coma, and death if not treated immediately.
What should I do if someone shows signs of being affected by extreme heat?
Move the person to a cool or shaded location, loosen or remove excess clothing, and apply cool water or wet cloths to the skin. If the person is conscious, encourage them to drink water. Seek medical attention if symptoms worsen or do not improve quickly.
How can I protect myself during a heatwave?
- Drink water regularly, even if you do not feel thirsty.
- Stay in cool, shaded, or air-conditioned spaces whenever possible.
- Avoid strenuous physical activity during the hottest hours of the day.
- Wear lightweight, loose-fitting, light-colored clothing.
- Avoid excessive alcohol and caffeinated beverages.
- Follow advice and warnings issued by health and meteorological authorities.
Why are cities often hotter during heatwaves?
Many urban areas experience the urban heat island effect, where buildings, asphalt, and concrete absorb and retain heat. The loss of trees, vegetation, and green spaces further reduces natural cooling. As a result, cities can be significantly hotter than surrounding rural areas, particularly at night.
Are heatwaves becoming more common?
Yes. Heatwaves are becoming more frequent, more intense, and longer-lasting in many parts of the world. They are also occurring in areas where they were previously uncommon, increasing health risks and placing greater pressure on health systems and communities.
What is a heatwave alert?
A heatwave alert is an official warning issued when weather conditions are expected to pose a significant risk to health due to extreme heat. These alerts help communities, health services, and individuals take preventive measures before temperatures reach dangerous levels.
Why is it important to check on vulnerable people during a heatwave?
Some individuals may have difficulty recognizing symptoms of heat-related illness or may be unable to take protective measures on their own. Checking on older adults, people living alone, infants, young children, and individuals with chronic medical conditions can help prevent serious illness and save lives.
