Philly will be hit by a high heat and humidity wave. Here’s what to expect and how to stay safe

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After a period of cool, dreary weather in the Philadelphia area, temperatures are expected to soar in the coming days.

The city is under a heat advisory Thursday, and the heat index is forecast to climb even higher starting Sunday into early next week.

“Monday, Tuesday, even into Wednesday … are going to be pretty oppressive days,” said Pennsylvania State Climatologist Kyle Imhoff.

Temperatures in the mid- to upper-nineties will come with higher-than-average humidity, forecasters say, making it particularly hazardous for people without access to air conditioning.

High heat and high humidity are a dangerous mix

High humidity makes heat especially dangerous, because it inhibits the body’s ability to cool off by sweating. When sweat evaporates, it releases heat from the skin and tissues below.

“It’s one of our great evolutionary benefits,” said Chris Uejio, a professor at Florida State University who studies how climate change affects human health. “That helps the body cool down much more than it does by just trying to pump blood towards the skin surface and having the air transfer that heat away.”

But when the air already holds a lot of moisture, sweat can’t evaporate from our skin as easily.

“Then, we see that our health risks increase greatly,” Uejio said.

Another way that high humidity makes heat more dangerous is by preventing temperatures from dropping at night, Imhoff said. High dew point, a measure of humidity, acts as a “floor” on nighttime temperatures, he said, making it harder for people to rest.

The levels of humidity expected over the next week will be higher than average, but not unheard of for the Philadelphia region, said Nick Guzzo, a meteorologist with the National Weather Service in Mouth Holly, New Jersey. But the sudden increase in temperatures after a mild period could make the weather feel more extreme.

“It might seem a little bit more of a shock,” he said.

Heat-related symptoms to watch out for

Extreme heat contributes to more weather-related health issues than any other type of hazard in the U.S., Uejio said.

Temperatures and humidity levels like the ones expected in the Philadelphia area in the coming days put people at risk of heat exhaustion and heat stroke.

Symptoms to watch out for include:

• Dizziness or lightheadedness
• Headache
• Intense sweating
• In severe cases, no sweating
• High internal body temperature
• Blurred vision
• Fast pulse
• Confusion
• Nausea
• Fainting
• Seizures

But more often than heat stroke, extreme heat causes kidney, heart or lung issues, particularly in people with pre-existing conditions such as cardiovascular disease, said Dr. Sameed Khatana, a professor of cardiovascular medicine at the Perelman School of Medicine and a physician at the Philadelphia Veterans Affairs Medical Center. Khatana’s research has found that extreme heat is associated with higher rates of death from cardiovascular disease nationwide, particularly for men, Black people and older adults.

Rachel Cottle, a postdoctoral scholar at the University of Texas Southwestern Medical Center who studies the effect of heat on the body, said in extreme heat, the heart works harder to try to cool off the body.

“The heat and the humidity together places a lot of strain on our heart,” she said.

Who’s most at risk?

People over the age of 65 and those with certain pre-existing medical conditions are most vulnerable to extreme heat.

Asthma, Chronic Obstructive Pulmonary Disease, heart disease and diabetes are a few conditions that put people at higher risk of heat-related health issues, Khatana said.

Certain medications can also increase a person’s vulnerability to heat. These include diuretics and beta blockers, which are commonly prescribed for high blood pressure, heart disease and heart failure, Khatana said.

People living unsheltered outside, people who work outdoors in industries such as construction, landscaping or agriculture, people who work in commercial kitchens, people exercising outside and households without consistent access to air conditioning are also at higher risk.

How to stay safe

Experts say the best way to stay safe during a heat wave is to avoid extreme heat altogether, by staying in air conditioned spaces.

But many Philadelphians lack access to air conditioning, either because they don’t have a functioning air conditioner or they can’t afford the high electricity bills that come with running one. This summer, there are even fewer resources available, as the state doesn’t have enough federal funding to run its free air conditioning program.

If you’re unhoused or don’t have air conditioning at home, experts recommend sheltering in an air conditioned public space, such as a library or mall. As of Wednesday, the city of Philadelphia had not declared a heat health emergency, during which some senior centers, libraries and rec centers extend their hours to act as cooling centers.

Remember to stay hydrated. This helps lessen the cardiovascular strain that can come from overheating and allow your body to produce plenty of sweat, Cottle said.

Submerging your hands and forearms in cool water can also help lower your body temperature and heart rate, she said. Try filling your sink with cool water, and placing your hands and forearms inside for periods of 10 minutes at a time, she said. Placing cool, wet towels on parts of your body with lots of blood flow, such as your neck, armpits or groin, can also help.

Using a fan can help cool you off. But above certain temperatures, fans can actually increase the danger by acting like the blower in a convection oven, Cottle said. When the temperature inside a room is above 90 degrees, the National Weather Service advises against directing a fan toward yourself and recommends only using fans to exhaust hot air from a room or to bring cooler air in.

If you’re working outside, drink lots of water, take breaks when you can and stay in the shade if possible.

If your neighbors are elderly and live alone, Uejio recommends checking on them during the hottest part of the day.

“One reason extreme heat is so tricky and potentially deadly is that we sometimes don’t understand how we’re being affected by it until it’s too late,” he said.