Heat wave hits the US

More than 15 million people from the Washington, D.C. area to parts of Florida were under a level 3 out of 4 major heat risk through last Thursday, according to the National Weather Service (NWS). This level of heat "affects anyone without effective cooling and/or adequate hydration," the agency said.

The most intense heat last week centered on Virginia and the Carolinas, but temperatures continue to climb as a larger heat dome is slated to hit the United States this week.

That heat dome is expected to expand across the central and eastern United States, covering parts of the southern Plains to the Midwest, Mid-Atlantic, and Northeast. Temperatures in these regions are expected to hit at least 15 degrees above normal, well into the 90s, and will mark the hottest temperatures of the year to date. In parts of the Mid-Atlantic, it could feel as hot as 110 degrees with humidity.

A level 4 out of 4 extreme heat risk, which signifies rare, long-lasting heat without much overnight relief, is expected this week for parts of those areas, including St. Louis, Chicago, New York City, and Washington, D.C.

The heat wave is being driven by an area of high pressure from the western Atlantic Ocean funneling hot, moist air up from the Caribbean into the United States.

8 tips to stay safe in the heat

According to a 2023 study, heat is the deadliest form of extreme weather, contributing to more than 800 deaths on average each year since 1999.

Multiple days of intense heat disproportionately lead to more severe public health impacts, even in places that are used to summer heat. ED visits for heat-related illnesses surge on "major" and "extreme" heat risk days, especially when high humidity and warm nights stop the body from cooling, according to NWS.

Extreme heat can cause heat exhaustion, heat stroke, or a life-threatening illness that can occur if heat exhaustion isn't treated, according to Renee Salas, assistant professor of emergency medicine at Harvard Medical School.

Experts say that if there's a heat advisory in your area, you should avoid outdoor activities entirely. But even if there isn't one, you should be cautious on a hot day, according to Ashley Ward, director of Duke University's Heat Policy Innovation Hub. "It's one of the big misconceptions about heat to think, 'I'm only in danger when there's a heat wave occurring.'"

If you go outside in the heat, here's how experts recommend staying safe:

1. Check the heat index.

Before going outside, make sure to check the heat index, as it includes the humidity and air temperature to give you a more accurate idea of how it will feel outside, Ward said.

Also, pay attention to cloud coverage, wind flow, and what time you plan on being outside. When it comes to heat-related illnesses, "if it's a very humid day and there's no wind speed, then you are more at risk, even if the air temperature is lower," Ward said. For example, a humid 88-degree-day without wind could be more dangerous than a low humidity 94-degree day.

If possible, plan any outdoor activities earlier in the day before the hottest time of the day in your area. "Three to 5 p.m. in the afternoon is one of the most dangerous times of day," Ward said.

2. Acclimate to the heat.

Neil Gandhi, an emergency medicine physician at Houston Methodist, said he recommends easing into any outdoor time on hot days by starting with short periods of time outside, then slowly increasing that time to allow your body to get used to the high temperatures.

"Don't jump directly into the frying pan; take some time to acclimatize during outdoor activities," Gandhi said.

4. Aim for shady, natural environments.

Where you are outside can make a big difference in how hot it feels. Dark surfaces like asphalt typically absorb more heat than natural environments, and shaded areas can feel much cooler.

If you're walking a dog on a hot day, aim for a tree-lined park or a wooded trail rather than the sidewalk. Or, if you're going to the pool, bring a sun umbrella or pop-up tent to provide some coverage, Provencio said.

5. Know the symptoms of heat exhaustion.

If you're out with friends on a hot day, stay safe by looking out for signs of illness among each other. "Somebody may notice something in you that you don't notice," Salas said.

Heat exhaustion is "your body's way of telling you to get out of the heat immediately," Salas said. Typically, symptoms include sweating more than normal, weakness, dizziness, fainting, pale skin, nausea, vomiting, and fatigue.

Especially pay attention to older people, children, people with certain pre-existing conditions, and people on commonly prescribed medications to help manage blood pressure and mental health conditions, as they're at a higher risk of heat-related illnesses, Salas said.

If you or someone you know is experiencing symptoms of heat exhaustion, Salas recommends going into an air-conditioned room. Provencio recommended public libraries as they're free and open to everyone.

If you're still experiencing heat exhaustion symptoms even after removing yourself from the heat, seek medical care.

6. Drink lots of water.

On hot days, make sure you bring water wherever you go. Salas said drinking water can help your body produce more sweat.

Gandhi noted that sparkling water and water-rich fruits like watermelon can also help, but ultimately, it's best to avoid sugary drinks that can make dehydration worse.

You should also avoid alcohol on hot days, Salas added. While you might want to drink a cold beer on the beach, alcohol acts as a diuretic that can dehydrate you and impact your ability to sweat and cool down.

If you're feeling hot and need to cool down quickly, you can also submerge yourself in a body of water. If that's not possible, dunk your feet and arms in cold water, Ward said. This can "lower your core body temperature."

7. Utilize cooling towels.

If you're feeling hot, put an ice pack, a wet towel, or a cooling towel, which stays extra cold when it gets wet, on the back of your neck, under your armpits, and on your groin, Ward said.

"Some of the body's major blood vessels closer to the skin live there," Salas said. And when you put something cold on those areas, "it can help cool the blood running through them."

8. Take breaks.

"If you're being active outside, whether that's working, exercising or playing sports, as you work, your muscles are producing heat," Salas said. Make sure you take time to stop producing heat and cool down either in the shade or in an air-conditioned room.

And while you're taking a break, drink more water, Provencio said.