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How To Stay Safe in Extreme Heat: 11 Things To Know > News > Yale Medicine

Jan 31, 2024

BY KATHY KATELLA July 28, 2023

A sweltering summer day can do more than make you hot and irritable. Heat is the number one weather-related killer in the United States, accounting for more than 700 deaths each year. And now there’s the threat of “extreme heat,” when temperatures are much hotter and last longer—plus, there is often more humidity in the air.

All of this impacts your health, from making you feel sluggish to putting you at risk for such serious conditions as heat stroke.

“When temperatures soar, steps like drinking enough fluids and heading to an air-conditioned location can be critical to avoiding serious health emergencies,” says David Della-Giustina, MD, a Yale Medicine emergency medicine specialist with expertise in wilderness medicine. “However, if you take appropriate action, you should be fine.”

Below, Dr. Della-Giustina discusses extreme heat and how we can stay safe.

Extreme heat is defined differently by two U.S. government agencies. It’s considered a period of two to three days above 90 degrees Fahrenheit (according to the U.S. Department of Homeland Security) or summertime temperatures that are much hotter and/or humid than average (according to the Centers for Disease Control and Prevention [CDC]). Under the latter definition, an extreme heat temperature varies depending on geography, since a particular location’s average temperature at a given time of year may be different than another’s.

In addition to the temperature, experts consider the “heat index,” a measure of how hot it feels when air temperature is combined with relative humidity. “It's the combination of heat and humidity that stresses the human body,” Dr. Della-Giustina says, explaining that when humidity rises, sweat doesn’t evaporate as quickly. As a result, the body is unable to release heat efficiently.

When the temperature outside reaches about 90 degrees, the body’s ability to offset body heat dissipates, Dr. Della-Giustina adds. “That’s where the heat index becomes important because if it's very humid and 85 degrees—but it feels like 100 degrees—you may not be able to offload your heat, which can lead to heat illness,” he says.

Hot weather can limit the body’s ability to cool itself, leading to dehydration in as little as half an hour. It can also lead to heat illness, an umbrella term for a range of conditions, including heat rash, heat cramps, heat exhaustion, and heat stroke, as well as damage to the brain and other organs in serious cases. Here’s more on each:

Yes. A heatwave, a prolonged period of hot weather, can last more than two days—in recent years, average heatwaves in U.S. urban areas have lasted about four days. This uninterrupted heat can raise the risk of dehydration and deplete the body’s electrolytes (essential minerals vital to key bodily functions), making it hard to catch up.

“It’s easier for the body to deal with heat for one day; it’s much harder when you experience multiple days of heat,” Dr. Della-Giustina says. “You don’t feel as well and are less inclined to drink more water. You may have a headache and feel lightheaded or nauseated—you may be throwing up. That is when heat illness may progress from heat exhaustion to heat stroke.”

Anyone can develop heat illness, but two groups are especially likely to end up in the emergency room, explains Dr. Della-Giustina. One is people who work outdoors, such as farmers, utility workers, or construction workers, who may be unable to leave their jobs to move into the shade or inside with air conditioning.

The second group is older adults, especially those living alone in an apartment or house, sometimes without air conditioning. Older adults don’t adjust to sudden temperature changes as fast as younger people—in some cases, chronic illnesses and certain medications they take affect their ability to regulate body temperature, and research has also shown that sweat gland function deteriorates with aging. “They also may not be able to get to an air-conditioned place like a mall or a designated cooling center in their community,” says Dr. Della-Giustina.

Other high-risk groups include children younger than age 2, who sweat less and generate more heat than adults when they are physically active. That’s why no child should be left in a parked car even with the window open, since young children left in hot cars are at especially high risk of having heat stroke or dying.

According to the CDC, people who are chronically ill are at risk, too, including those who are obese, use drugs or alcohol, or have conditions such as mental illness, heart disease, poor circulation, or diabetes.

Yet another group is people who take medications such as diuretics, which are common high blood pressure treatments that cause them to lose electrolytes such as sodium and potassium, Dr. Della-Giustina adds.

Drinking water and other fluids is number one, Dr. Della-Giustina explains, adding that in the hot weather, you need to replenish fluids lost through sweat as frequently as every hour to prevent dehydration.

Don’t rely on old standards, such as eight glasses of water a day, to guide your consumption—an athlete, for instance, will need more fluid than a sedentary person. “We generally tell people to use the color of their urine to gauge how hydrated they are,” Dr. Della-Giustina says. “If your urine is yellow, you’re already one or two liters behind. If you are drinking enough water, your urine should be clear to pale yellow.”

One of the mistakes people make is they don’t replenish the sodium they’re losing when they sweat, Dr. Della-Giustina explains. “Salt is the primary electrolyte in your blood. Allowing your sodium level to get too low can cause significant problems, including an altered mental status and even cardiovascular collapse, among other things,” he says.

Sports drinks offer the replenishment of electrolytes, including sodium, which you can’t get from water alone, he adds. “The best way to rehydrate yourself is not to drink the sports drink alone but to dilute it—half water and half sports drink. Then, you get the hydration and electrolytes you need,” says Dr. Della-Giustina.

You should avoid the sun as much as possible, especially between 10:30 a.m. and 2 p.m., when people are most likely to develop heat exhaustion, Dr. Della-Giustina explains.

Anyone who must be outside in the middle of the day should take frequent breaks and stay hydrated. People who aren’t required to be outside should stay indoors as much as possible, he adds. “Athletes can be at really high risk if their mental attitude is ‘I’m just going to keep pushing myself,’” says Dr. Della-Giustina. He recommends that they train in the early morning or evening when it’s cooler.

Also, if you’re outside, use sunscreen; a sunburn can cause sweat pores to clog, limiting the body’s ability to cool down, Dr. Della-Giustina explains. “Getting a sunburn on top of the heat will make things much worse for you,” he says.

If you are anticipating extremely hot weather, check with your state, city, or county for a list of cooling centers in your area, and go to a cooling center when the weather changes, Dr. Della-Giustina explains.

You can also go to a mall, library, or restaurant—anywhere you can cool off. Spending as little as a few hours in an air-conditioned environment can help your body stay cooler when you return to the heat.

It’s important to know that electric fans may provide comfort, but once the temperature reaches the high 90s, they will not prevent heat-related illness. Other advice includes taking cool baths or showers and avoiding the use of stoves or ovens to keep the temperature down inside.

Loose-fitting clothing leaves room for air to circulate underneath it and allows you to sweat, and a vented hat will help release heat from your head, as well as keep the sun from beating down on your face.

Clothing that is light in color reflects the sun and deflects the heat, explains Dr. Della-Giustina. “In desert settings, many people wear large, white robes. That’s more cooling than going out in shorts and a T-shirt,” he says. Wear UV-protective clothing, if possible, since that blocks heat and prevents UV exposure, he adds.

There are a number of early warnings that you need to cool down, including headache, sweating, fatigue, dizziness, nausea, and lightheadedness.

For symptoms of specific heat-related illnesses, some of which can occur alone or simultaneously, check the CDC’s Warning Signs and Symptoms page.

“One of the common misconceptions is that if you're sweating, you don't have heat illness,” Dr. Della-Giustina says. “But most people with heat illness will continue sweating.”

Everyone in the house should know the signs and symptoms of heat stroke, which is characterized by an extremely high body temperature of about 104 degrees or higher; hot, red, dry, or damp skin; a fast, strong pulse; and confusion, among others. If you notice that someone is experiencing heat illness symptoms, Dr. Della-Giustina recommends the following:

The CDC’s Warning Signs and Symptoms for specific heat-related illnesses page also includes instructions on how to respond if someone is exhibiting symptoms of different types of heat illnesses.

Families can create a heat action plan, Dr. Della-Giustina explains. It can include making a list of cooling centers in the area. Covering windows with drapes or shades and weather-stripping doors and windows can help keep a house cool inside. Insulation can also keep heat out.

The Low Income Home Energy Assistance Program (LIHEAP) is a government program that helps people who need assistance preparing their homes for hot weather.

Also, the National Weather Service provides heat warnings and other information on local weather (you can enter your zip code into the search box). It also provides a Heat Index chart.

Heat rash: Heat cramps: Heat exhaustion: Heat stroke: