Much of the U.S. is in the midst of dangerous and potentially record-breaking daytime temperatures as a heat dome has settled over the Midwest and the Atlantic Coast. But nighttime temperatures are staying unusually warm, too, raising the risk of heat exhaustion and heat stroke even higher, especially for people without access to air conditioning.

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“We’re seeing temperatures well into the evening that are staying shockingly high and, in some cases, never dropping below 80 or 75 degrees. And this is really concerning,” according to Ashley Ward, director of the Heat Policy Innovation Hub at Duke University’s Nicholas Institute. Ward says that climate change is making the trend worse.

“We’re not getting the opportunity for recovery overnight, as we’ve historically seen,” she says. “Our bodies need it, of course, but plants and animals … also our energy infrastructure. We need to be able to cool off overnight.”

In the Baltimore-D.C. region and southeastern Pennsylvania, including Philadelphia, highs on Thursday were expected to be in the low to mid 100s with heat indices around 110F, according to the National Weather Service forecast. Overnight lows are expected to remain in the low 80s for many of those areas.

Friday’s forecast will be much the same.

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Climate change means that each summer we are seeing more heatwaves similar to the current one and also more hot nights. The Fifth National Climate Assessment, published three years ago, notes that nighttime temperatures are rising faster than daytime temperatures in the U.S. “[T]he number of nights where the temperature never falls below 70°F is increasing everywhere in the US except the Northern Great Plains,” it says.

According to Alex DaSilva, a meteorologist with Accuweather, “The trends have been for those overnight lows to be warmer and warmer and warmer, especially when we’re dealing with these big heat wave events.”

The reason? The atmosphere is retaining more heat at night, DaSilva says. With warmer nights, the daytime temperature rise starts from a hotter floor.

It’s made worse by generally more humid conditions, also attributable to climate change. For that reason, those afternoon summer showers may not help, he says. “It can even feel worse sometimes after a rain shower or a thunderstorm comes through.”

People “can’t evaporate the sweat off of our skin when there’s so much water content in the air,” Ward says, which can lead to heat stroke or heat exhaustion, she says.

“It’s really a combination of temperature and relative humidity,” W. Larry Kenney, a professor of physiology and kinesiology at Penn State University, tells NPR’s Morning Edition. “The sweating response is really only valuable if that sweat can evaporate.”

These temperatures are a concern for anyone exposed to them, but for people without access to air conditioning, the situation is especially dangerous. According to the U.S. Energy Information Administration, 88% of U.S. households had air conditioning installed as of 2020.

However, a separate study by KFF — a nonprofit organization focused on health policy, polling, and journalism formerly known as the Kaiser Family Foundation — found that Asian, Black, and Hispanic households were more likely than White households to lack air conditioning. The study also found that lower-income households were more likely to be without air conditioning than those with higher incomes.

The most vulnerable people are older men and women. And the problem is compounded by comorbidities such as heart disease and diabetes, Kenney says.

If you have to be out in the heat, he recommends taking it slowly and getting used to the heat gradually.

And, of course, staying hydrated. “Make sure they drink before their heat exposure and especially afterward to try to catch up.”