How heat waves and record-breaking temperatures can affect your health

How Heat Waves and Record-Breaking Temperatures Can Affect Your Health

What to know about the symptoms of heat stress as millions across the U.S. are under heat alerts

People young and old trying to beat the heat in a water-based sculpture at Rockefeller Center Plaza in New York City.PHOTO: YUKI IWAMURA/AGENCE FRANCE-PRESSE/GETTY IMAGES

By Dominique MosbergenFollow

Updated July 20, 2022 3:49 pm ETSAVEPRINTTEXT

Record-high temperatures have been stifling swaths of North America and Europe, as well as parts of Africa and Asia, this summer. Britain on Tuesday provisionally recorded its highest-ever temperature of over 40 degrees Celsius (about 104 degrees Fahrenheit).

The National Weather Service this week said that more than 100 million people across the U.S. are under heat alerts, with dangerous heat—in some locations up to 100 degrees Fahrenheit or higher—threatening large portions of the South-Central and Northeast U.S.

Public health officials have warned people in hot areas to take measures to prevent heat stress, which can lead to potentially serious and even deadly heat illnesses like heat stroke.

Here’s what to know about the effects of extreme heat on the human body:

What is extreme heat?

Extreme heat is defined by the Centers for Disease Control and Prevention as weather that is much hotter or more humid than average for a particular time and region. The U.S. Department of Homeland Security defines extreme heat as a period of high heat and humidity with temperatures above 90 degrees for at least two to three days.

Some public health experts, however, caution against pointing to a specific temperature as “extreme” as the same temperature could have different health impacts depending on the location and the person in question. Extreme heat is “what feels unusually hot to you, based on where you live,” said Gregory Wellenius, professor of environmental health at Boston University. “That’s because we all to some degree get used to a certain summertime heat. Some places are more accustomed to hot temperatures and others are not.”

Dr. Wellenius pointed to Britain as an example. “It’s hitting 40 degrees Celsius, which is unprecedented. It’s not accustomed to those high temperatures and won’t be as prepared as, say, somewhere in Southern Europe or the Southern U.S.”

In a community hit by a water outage and extreme heat, a man in Bixby, Okla​., this week used a mobile water-filling station.PHOTO: SEPTEMBER DAWN BOTTOMS/BLOOMBERG NEWS

What happens to the human body when exposed to extreme heat?

The human body is constantly trying to regulate its core body temperature, which is ideally maintained within a narrow range of about 97 degrees to around 99 or 100 degrees Fahrenheit.

When faced with extra heat, the body attempts to release some of it through sweating and by sending more blood from the warmer interior of the body to the skin’s surface, where heat can dissipate into the surrounding air.

As sweat evaporates, cooling can occur but excessive sweating can lead to dehydration and electrolyte imbalances, which can impact muscular and neurological systems, according to Chris Uejio, an associate professor at Florida State University who specializes in the health impacts of climate change.

Water Loss in the Body

Water makes up up to 60 percent of the human adult body, according to the U.S. Geological Survey. When an adult participates in physical activities or exercise outdoors in high temperatures, they can lose anywhere between a half liter (17 oz. ) to two liters of water every hour. If a person does not hydrate adequately, their health could be impacted.


Becomes thirsty



Water loss

from body*

16.9 oz.

water bottle



Dry mouth



Water loss

from body*



Increased heart rate,

Increased body temperature



Water loss

from body*



Swollen tongue,

Difficulty with speech,

Reduced mental and physical performance



Water loss

from body*



Rapid temperature increase,




Water loss

from body*


*For person weighing 150 pounds

Source: World Health Organization

To send more blood to the skin’s surface, the heart beats faster and blood vessels dilate to accommodate more blood flow. This can cause a strain on the cardiovascular system, increasing risks of strokes and coronary events like heart attacks, said Dr. Uejio.

He added that studies have linked extreme heat to other health impacts, including pulmonary and kidney-related problems, as well as mental-health issues.

If the body is unable to cool itself sufficiently, it can overheat, which when extreme can cause organ damage or failure and even death.

Higher temperatures have also been linked to diminished cognitive abilities and concentration. Some studies have shown an increase in traffic accidents and declines in academic performance when temperatures rise.

How hot is too hot for the human body?

Heat stress happens when the body isn’t able to adequately cool itself—but that ability depends on several factors, including the person’s age and health and the environmental conditions they are in, said Dr. Wellenius.

“There’s not a magical temperature. Different people have different abilities to cope,” he said.

High humidity can make it seem hotter than it is as the body isn’t able to cool itself as effectively through sweating in muggy conditions. Someone with pre-existing conditions or who may be taking certain medications may experience heat stress more quickly. That may also be true for people who aren’t able to seek shelter from prolonged heat exposure.

Some research has shown that multiple days in a row of high heat exposure can increase the risk of heat-related illness, Dr. Uejio said.

What role does humidity play in heat waves and extreme heat?

A measurement known as the wet-bulb globe temperature is sometimes used as an indicator of heat-related stress on the body, specifically when doing outdoor activities. The wet-bulb globe temperature, a type of perceived temperature, is a measurement that takes into account multiple environmental factors including temperature, humidity, wind speed, sun angle and cloud cover.

“Humidity decreases our ability to sweat effectively so higher humidity is more dangerous,” said Julia Gohlke, an associate professor of environmental health at Virginia Tech. 

When sweat evaporates from the skin, the body’s heat is transferred to the surrounding air, creating a cooling effect. The rate at which sweat evaporates, however, depends on how much water is already in the air, or how humid it is, and also how windy it is. 

Since wind increases the rate that sweat can evaporate from the body, the windier it is, the better the cooling effect, Dr. Gohlke said. 

The National Weather Service advises taking precautions when doing outdoor activities when the wet-bulb globe temperature is above 80 degrees Fahrenheit. Once the wet-bulb globe temperature exceeds 90 degrees, the body can become stressed after just 15 minutes of outdoor activity, the agency said.

The more hot, humid, stagnant and sunny it is, the higher the risk of heat stress, public health experts say. 

Who is most vulnerable to the effects of extreme heat?

While extreme heat can be a risk to anyone, some groups are at higher risk of health effects, experts say. Those groups include infants, older adults and people with pre-existing health conditions, such as cardiovascular or pulmonary diseases and mental-health conditions. People who have no access to adequate shelter or air-conditioning, such as the unhoused, and those who work or exercise outdoors could also be at higher risk, according to Dr. Wellenius.

“Everybody is at risk, even those who are young, physically fit and healthy,” said Dr. Wellenius.

What is the difference between heat exhaustion and heat stroke? What are the symptoms? 

Heat-related health effects include heat cramps, heat exhaustion and heat stroke. 

Heat cramps, per the CDC, are muscle spasms caused by a loss of water and salt from the body. Prolonged exposure to extreme heat combined with dehydration can cause heat cramps.

Heat exhaustion is similarly caused by extreme heat exposure and is exacerbated by dehydration. Symptoms include headache, nausea, dizziness, irritability, thirst and decreased urine output. The CDC recommends seeking medical help if symptoms include vomiting or if they worsen or last more than an hour.

Heat stroke is the most serious medical condition caused by extreme heat and, according to the CDC, requires emergency medical treatment. It occurs when the body is no longer able to regulate its core temperature and the body’s temperature rises rapidly to 103 degrees Fahrenheit or higher, the agency said. 

The symptoms of heat stroke can be similar to those of heat exhaustion but more severe, said Dr. Uejio. Heat stroke patients may experience either profuse sweating or hot, dry skin that is no longer able to produce sweat. They may also suffer from altered mental states or confusion, seizures and loss of consciousness.

Heat stroke can be preceded by symptoms of other heat-related illnesses like heat cramps and heat exhaustion, but public health experts say that it can also come on suddenly without prior symptoms. Monitoring one’s core body temperature can be a way to prevent heat stroke, said Dr. Uejio, though he recommended adding one or two degrees to the measurement as thermometers used at home may not be accurate at gauging core body temperatures. 

Without immediate medical attention, heat stroke can cause permanent damage to vital organs like the brain, heart and kidneys or death.

Every summer in the U.S., an average of more than 65,000 people visit an emergency room for acute heat illness, according to the CDC. Between 2018 and 2020, at least 3,000 people in the U.S. died from heat-related causes, the agency said.

What should you do in an extreme heat event? How can you prevent overheating? 

In an extreme heat event, give priority to staying cool and hydrated and taking time to rest, public health experts say. 

“The good thing about heat is that it really shouldn’t kill anybody, not at the levels we’re seeing. These are all avoidable deaths,” said Ben Zaitchik, professor of earth and planetary sciences at Johns Hopkins University. 

If possible, limit outdoor activities and stay in an air-conditioned place. Go to a shopping mall, public library or heat-relief shelter if there isn’t air-conditioning at home. Take cool showers and wear loosefitting, breathable clothing. If you have to be outside, wear sunscreen, take breaks often and find shade when able.

Drink plenty of fluids and avoid alcoholic, caffeinated or very sugary drinks, which can be more dehydrating than beneficial. 

If extreme heat lasts for several days, Dr. Gohlke recommended finding ways to cool down at night. “Even if you have to be outside in the day, research shows that it’s really important for the body to cool down and rest at night,” she said. 

Dr. Zaitchik also recommended checking in on neighbors and others in one’s community, particularly those who may be more vulnerable to the heat, such as the elderly.

“You can do so much by checking up on people,” he said. “Literally just getting someone a drink of water or getting them to a new location or opening a window can make all the difference.” 

In the longer-term, more efforts need to be made to reduce factors that exacerbate heat in places where people live, said Dr. Zaitchik. Steps should be taken, for instance, to mitigate urban heat islands, hotter areas in urbanized spaces, by planting more trees and using different building materials, he said. 

Sports teams and companies that involve outdoor work should also ensure that athletes and workers take precautions when heat is extreme. 

Where are heat waves happening?

Heat waves, which are defined by the Environmental Protection Agency as a series of days with significantly hotter than average temperatures in a particular place, are becoming more common and more intense in many places across the U.S. and the world, said Dr. Wellenius. more…

Extreme heat has been scorching parts of Europe, including the U.K., Spain, Portugal and France, in recent weeks, killing thousands. The Weather Channel said Wednesday that more than 200 million people in the U.S. will experience temperatures of 90 degrees Fahrenheit or higher this week.

Scientists say human-caused climate change is fueling the increase and intensity of heat waves as more heat is being trapped in the atmosphere by greenhouse gases, causing average temperatures to rise. “Climate change is not just a problem of the future,” said Dr. Wellenius. “The data is clear: It’s really hot right now. And people are dying from that and suffering from that.” 

Beyond the impacts of global warming, scientists are also exploring other possible factors, including urbanization and land-use changes, as well as meteorological factors like changes in jet streams.

Will people have to migrate in the future due to extreme heat?

Extreme heat could drive millions of people to migrate in the coming decades, said the International Organization for Migration in its 2017 report on extreme heat and migration.

If global temperatures rise by 1.5 degrees Celsius by the end of the century, which is the goal of the Paris climate accord, up to 60 million people globally are projected to live in areas where it could be too hot at times for the human body to properly function, the IOM report said. If temperatures rise by 2 degrees Celsius, that projection increases to more than 100 million people.

Dr. Uejio said the most vulnerable may be forced to move in the face of increasingly hot temperatures.

“If you have economic means, you could adapt—install air-conditioning and find other solutions. But for those who don’t have the financial, technological and educational resources to adapt, there will be more challenges,” he said.

Dr. Zaitchik said he didn’t expect many people to move in the near-term because of heat alone, but said other related problems like worsening drought and wildfires could prompt people to leave their homes.

Photos: Temperatures in the U.K. Break Records as Europe Swelters in a Heat Wave

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