Extreme heat is a killer. In fact, extreme heat is the leading cause of weather-related death in the United States.

As the city coped with record-setting temperatures over the holiday weekend, and with summer just beginning, it’s more important than ever to take precautions, said Dr. Samuel Eldrich, medical director of the Temple Health-Chestnut Hill Hospital Emergency Department.

Heat in the 90s is bad enough, he said, but the kind of extremes we’re talking about are especially life-threatening.

“When you have heat in the 90s, we give the advice that you can go outside, you can do your activities, but pay attention to your body, make sure you drink plenty of fluids, and take breaks inside to cool down,” he said. “Don’t change your lifestyle, but definitely change some patterns during the day.

“But when it’s this hot, when we’re talking about 110 heat index, and over 100 degrees for a couple of days, try to avoid going outside at all costs, especially if you’re elderly or have underlying medical conditions (for example, cardiovascular or respiratory conditions or diabetes). Stay in the cool air and continue to hydrate, including electrolyte drinks such as Pedialyte or Gatorade. And importantly, if you don’t have air conditioning in the house, please go to a cooling center — they’re open throughout the city — to make sure you can stay cool during this time.”

Any place cool will do, he adds: a supermarket, the mall, or a movie theater. But if you don’t want to go there, then by all means find a cooling center. In the Northwest, centers include Emlen Arms Community Center at 6733 Emlen St., Germantown House, 5457 Wayne Ave, and the Joseph E. Coleman Northwest Regional Library, 68 West Chelten Avenue.

The elderly in particular are much more sensitive to heat changes, Eldrich said, and they tend not to drink enough fluids. Infants, too, can be especially vulnerable, but anyone can be sickened by the heat. He expects to see an uptick in emergency department visits as people spend more time outside during the Fourth of July holidays.

Extreme heat can cause dangerous changes in the cardiovascular system. Heart rate and blood pressure increase. The kidneys are also placed under strain. These issues can lead to heatstroke, which is a life-threatening emergency.

Here’s what symptoms to look for, according to Eldrich: fatigue, dizziness, vomiting and muscle spasms. In the most severe cases, heatstroke can lead to breathing issues and confusion. If any of these issues arise, call 911 immediately.

Of course, some people will be exposed to extreme heat by virtue of their outdoor jobs.

“What’s hard is, life doesn’t stop for business,” he said. “Maybe roofers would take a day off, but construction workers, people like that, they have to be outside. And so the best advice for them is finding shade, drinking plenty of fluids, trying to get out of the heat if you can, even for short periods of time. The body may not reset to 100%, but it replenishes you enough so you can tolerate heat for a longer period.”

The biggest takeaway, Eldrich says, is this: “Make sure you look out for your neighbors and family, especially those most at risk, like the elderly.”