Summer is already heating up, and so are concerns about health and safety—especially for more at-risk individuals like elderly or vulnerable populations.

While much of the country is already being blanketed by soaring temperatures, by Sunday the central and eastern U.S. will be coping with crushing heat that could break records and impact millions. The National Weather Service predicts that from Chicago to Boston, as far south as Florida, and across the central states, temperatures will be well into the 90s. The hottest days may fall between June 17 and 20, with overnight lows still in the 70s.

According to the Climate Prediction Center, excessive heat is most likely to occur in the Ohio valley and mid-Atlantic. “Highs in the low- to mid-90s are forecast, possibly reaching daily record highs in many locations,” the organization posted on X (formerly Twitter).

The high-pressure system, called a heat dome, will move from the southern Great Lakes region to the northern mid-Atlantic, and even expand into New England. More than 260 million people are expected to see temperatures above 90, or even 100 degrees Fahrenheit.

The heat dome won’t just be hot, it will hold steady for days on end. Even if the temperatures aren’t record-breaking, the duration of intense heat could pose substantial public health challenges.

Experts take heat waves seriously. That’s because they come with the risk of illness. According to a 2022 study, extreme heat is more than just uncomfortable—it can be dangerous, claiming about 154 lives a day. Heat is actually the top weather-related killer, but the first wave of the season comes with heightened risks.

The National Weather Service has warned of “a high risk of heat stress or illnesses for anyone without effective cooling and/or adequate hydration.”

Extreme heat impacts people over 65 and people with preexisting health conditions like heart disease and diabetes more frequently. But it’s not just the sick or the elderly who need to be careful. Extreme heat can also be dangerous for children, pregnant persons, and those who have longer exposure times to the conditions, like construction workers.

Heat waves disproportionately impact Black Americans, too. According to a 2021 report from the Environmental Protection Agency, Black people are 40% more likely than non-Black people to live in areas with the most mortality due to extreme heat.

Summers are getting hotter, along with just about every other part of the year. Studies have clearly indicated that the kinds of crushing heat waves we’ve seen in recent years wouldn’t have been possible without human-caused climate change. Climate scientists have long been asserting that fossil fuels, the main contributor to global warming caused by humans, should be phased out to prevent the earth from continuing to warm.