BOSTON — Dangerously hot temperatures are expected to spread across the Central and Eastern United States on Wednesday through the weekend, with temperatures soaring above 100 degrees Fahrenheit in the hardest-hit places, the National Weather Service has warned.

And even when the sun dips below the horizon, temperatures in many places are expected to remain in the 80s.

The hottest part of the country? Smack dab in the middle.

Everyone living in the region stretching from northern Oklahoma and central Nebraska through Iowa, Missouri and western Illinois should brace for a “prolonged period of dangerously hot temperatures and high humidity,” the warnings say. People in central and south central Kansas should expect to endure highs of about 102 degrees; the temperature in Des Moines, Iowa, was expected to hover around 100.

Excessive heat warnings have also been posted farther east, for parts of New Jersey, Delaware and Pennsylvania.

All told, at least 15 million people across the United States are being warned of dangerously high temperatures that could affect human health between Wednesday and Friday.

By the weekend, what meteorologists are calling a “heat dome” in the middle part of the country is expected to spread into the Great Lakes and the East Coast.

Extreme heat can kill. Here’s what you can do to stay safe.

“The combination of heat and humidity can take its toll on someone who is outside and overdoing it,” said Richard Bann, a meteorologist for the National Weather Service Weather Prediction Center. “It can be life-threatening.”

Last year, 108 people died from extreme heat, compared to just 30 who died from cold, according to statistics on weather-related fatalities released by the National Oceanic and Atmospheric Administration.

Here are four safety recommendations from the National Weather Service:

— Drink plenty of fluids.

— Stay in an air-conditioned room.

— Stay out of the sun.

— Check on relatives and neighbors, especially the elderly.

Some of the country’s biggest cities can expect to swelter.

So far, Philadelphia is the only major city on the East Coast under an excessive heat warning. Meteorologists are predicting highs there of 100 degrees. But New York, Washington, D.C., and Boston are expected to be uncomfortably hot, with temperatures soaring into the high 90s and above; Chicago can expect the same.

Saturday’s expected highs are 97 in Boston, 100 in Washington and 98 in New York. Chicago is expected to see a high of 97 degrees Friday and 94 on Saturday.

Gentry Trotter, founder of Cool Down St. Louis, a nonprofit that provides utility assistance for low-income families and donates air conditioners to people who are elderly or have disabilities, said that the organization has assisted in more than 1,900 emergency situations over the past three weeks and received 63% more requests this year than it did last year.

“It has just been brutal,” he said. “Yesterday we went in a home of an 80-year-old lady, and the heat almost knocked us over.”

In Iowa, football practice is off, but the 2020 campaign rolls on.

Heat warnings have Iowa farmers worrying about their corn, after planting was delayed by a soggy spring that has not left the plants much time to take root.

Despite the scorching heat, Democratic candidates for president have continued to crisscross the state. Sen. Amy Klobuchar of Minnesota marched 3 miles over the weekend in saunalike heat, and earlier this week, Pete Buttigieg, mayor of South Bend, Indiana, sweated through a forum on gun violence. Sen. Elizabeth Warren of Massachusetts is expected to attend an AARP forum in Sioux City

But ordinary people are shuffling everything from church picnics to sporting activities.

Steven Rogers, director of operations for Eastern Iowa Elite, a middle-school football club, said that two football practices have already been canceled and the team tryouts this coming Saturday have been moved indoors.

“You can’t really expect the kids to perform at the highest level if they are outside in this heat, beyond the fact that its totally dangerous,” he said. “There have been a couple of kids having some pretty bad health issues related to heat and overexertion. It’s just not safe.”

Nighttime is unlikely to bring much relief.

More than 100 local heat records are expected to fall Saturday, according to the National Weather Service. Most won’t be record-daily highs but record-high nighttime lows, and that lack of cooling can be dangerous, meteorologists say.

Temperatures in parts of the East won’t drop below the mid- to upper-70s or even 80 degrees (26.7 Celsius) at night.

“Daytime hours when the sun is out is clearly our highest risk periods,” said Dr. Michael Kaufmann, EMS medical director with the Indiana Department of Homeland Security. “We’re not expecting the drops in temperature at night — or the humidity — that we often realize when the sun goes down.”

A high pressure system stretching from coast to coast is keeping the heat turned on. The heat and humidity are made to feel worse by the large amount of moisture in the air coming from the Gulf of Mexico, much of it left over from Hurricane Barry.

The heat will make air-quality problems worse.

Officials are also concerned about smog, which is exacerbated by the heat and makes it harder for certain people to breathe, including the very young, the elderly and people with asthma or lung diseases.

The Environmental Protection Agency’s live air quality tracker reported that the air was “unhealthy” Wednesday for sensitive groups in a stretch of the East Coast from Baltimore to Bridgeport, Connecticut, including Philadelphia and New York City.

The situation was particularly dire in parts of Connecticut, including Westport and Norwalk, where the air was listed as “very unhealthy.”

Is this heat wave caused by climate change?

Hot weather is nothing new, of course, especially in July. But climate change is making heat waves like this one more common.

“The meteorological ingredients that make heat waves today are the same ingredients that made them in the past, but climate change is bringing those ingredients together more often, generally speaking,” said Deke Arndt, chief of the climate monitoring branch of the National Centers for Environmental Information.

The 2018 National Climate Assessment, a major scientific report by 13 federal agencies, notes that while the peak of extreme heat in the United States occurred during the 1930s Dust Bowl, the number of hot days is increasing, and the frequency of heat waves in the United States jumped from an average of two per year in the 1960s to six per year by the 2010s. Also, the season for heat waves has stretched to be 45 days longer than it was in the 1960s, according to the report.

It is all part of an overall warming trend: The five warmest years in the history of accurate worldwide record-keeping have been the past five years, and 18 of the 19 warmest years have occurred since 2001; worldwide, June was the hottest ever recorded.

Jennifer Francis, a senior scientist at the Woods Hole Research Center, noted that the jet stream, which generally helps to move weather systems across the country, has been unusually wavy in June and early July. That “always spells trouble,” she said, and can lead to conditions like heat waves stalling in place.

This article originally appeared in The New York Times.