The first big heat wave of 2025 has arrived, coinciding with the official start of summer.

More than 40 million people in the U.S. were under heat alerts Friday, according to the National Weather Service. Temperatures in the Midwest are expected to crest well into the 90s Fahrenheit this weekend. Triple-digit highs could pop up in New York, Philadelphia, Washington and Denver. The peak will likely hit the East Coast early next week.

Meanwhile, British health officials are also issuing heat warnings for the U.K.

Climate change has been exacerbating heat waves, and the problem isn’t going away any time soon. An increasingly hot planet — due largely to burning fossil fuels such as coal, oil and natural gas — means already hot regions are getting worse.

Heat doesn’t just mean canceled events. It also poses a health risk, especially for children, older people and those with certain health conditions.

So here are some tips to stay safe.

When does heat

become dangerous?

The answer depends on more than the temperature. The most detailed measurement is called the wet bulb globe temperature (WBGT), which includes temperature, humidity, cloud cover and wind. The heat index, which measures temperature and humidity, is less descriptive but easier to find on weather apps. Both explain why a shaded soccer field on a 90 degree F day in arid Phoenix may be less risky than an exposed park on an 80 degree F day in soupy Little Rock.

Just based on heat index, NOAA has a chart that calculates how dangerous prolonged exposure can be. For example, a day where temperatures reach 96 degree F and 45% humidity would fall into the “danger” category for prolonged exposure or strenuous activity.

The WBGT threshold isn’t exact, but recent research suggests that even some young, healthy people can’t endure hours of exposure to high heat and humidity.

How to cool down

Overnight lows can be a particularly dangerous part of a heat wave, said Ashley Ward, director of the Heat Policy Innovation Hub at Duke University.

“Your body needs a reprieve,” she said. “You don’t get that overnight, we start the next day at a deficit.” Heat can worsen labor productivity and lead to more visits to the emergency room.

“When we have overnight temperatures that don’t drop below 75 degrees” F, she said, “You start to see some pretty extraordinary outcomes with respect to heat illness and heat stroke, and even mortality.”

Ward’s answer: Find air conditioning. That might be at home, but she said census data overcounts how many people have access.

If you can’t afford to cool the whole house, Ward said, create a “cool corner” and sleep there, so your body is prepared to tackle the next day.

Evaporative or “swamp” coolers can help in dry heat, but they increase humidity and can make it more difficult to cool down. In humid places, just use a fan.

If you don’t have air conditioning, find public places that do, including movie theaters, malls and libraries. Some communities set up cooling centers.

Depending on where you live, the Low-Income Home Energy Assistance Program can help you buy a window air conditioning unit, according to the National Council on Aging. Some local nonprofits and civic organizations can also help you access one if cost is a burden.

Know your rights

if you work outside

Knowing what workplace protections you have is important. But there are no federal heat rules to protect workers in the United States.

Some states have them, including Washington, Oregon, California, Nevada, Colorado, Minnesota and Maryland, according to the National Resources Defense Council. Other states don’t have any.

If your state has work rules, try to learn them — though there are still challenges to ensuring regulations are actually enforced, said Bharat Venkat, director of the University of California, Los Angeles Heat Lab.

He notes that sometimes shaded areas are too far for workers to take breaks without losing wages, or that management can make it impossible for workers to advocate for themselves. “Most workers don’t actually have control over their time or control over where they work,” he said.