Heat Waves: How Record-High Temperatures Can Affect the Human Body

Punishingly high temperatures have been stifling much of the Northern Hemisphere this summer, with record heat reported in swaths of North America and Europe, as well as parts of Africa and Asia. Last month was the hottest June ever recorded, according to the World Meteorological Organization. Early July saw the Earth’s hottest week on record.

The National Weather Service said this week that dangerous heat waves would continue to stifle parts of the U.S.’s South and Southwest.

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 conditions such as 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.”

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 affect muscular and neurological systems, according to Chris Uejio, an associate professor at Florida State University who specializes in the health impacts of climate change.

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 such as heart attacks, said 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.

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

Health effects from high temperatures 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 Uejio. Heat-stroke patients might experience either profuse sweating or hot, dry skin that is no longer able to produce sweat. They might also suffer from altered mental states or confusion, seizures and loss of consciousness.

Heat stroke can be preceded by symptoms of other illnesses related to high temperatures such as 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 Uejio, though he recommended adding one or two degrees to the measurement as thermometers used at home might not be accurate at gauging core body temperatures. 

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

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.

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 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 takes certain medications might experience heat stress more quickly. That might also be true for people who aren’t able to seek shelter from prolonged heat exposure.

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

Can extreme heat cause long-term health effects?

Heat stress not only causes immediate health effects but also can have long-term consequences, research shows. 

People who experience heat-related illnesses are at higher risk of chronic heart and kidney disease, studies suggest. 

Experiments in mice have shown that heat stress can trigger changes at a cellular level, which linger long after the animals have recovered from the heat exposure and are associated with immune suppression and long-term metabolic disorders.

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 several 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 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, 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 Wellenius.

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

Zaitchik also recommended checking in on neighbors and others in one’s community, particularly those who might 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 long-term, more efforts need to be made to reduce factors that exacerbate heat in places where people live, said 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 intense in many places across the U.S. and the world, said Wellenius. 

More than 61,000 people died from heat-related causes in Europe last summer, according to a recent analysis. The continent is being scorched by extreme heat again, with triple-digit temperatures reported in countries including Spain, Greece and Italy in recent weeks.

Heat domes are exacerbating stifling conditions in Europe, the U.S. and elsewhere. A heat dome occurs when high-pressure atmospheric conditions trap heat over an area, creating a “dome” of stagnant, hot air.   

The National Weather Service said in June that El Niño conditions, which tend to be associated with hotter-than-average years, were developing and expected to strengthen in the coming months. El Niño is the periodic warming of sea-surface temperatures across parts of the Pacific Ocean. Meteorologists said El Niño increases the odds of record temperatures and other extreme weather events.

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 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.

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.

Uejio said the most vulnerable might be forced to move because 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.

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

This article might be updated.

Write to Dominique Mosbergen at [email protected]