One week after a record-cool weekend, the first heat wave of the season is flaring across the region. And while it won’t challenge records, this one might be especially hazardous to elderly people who live alone and those with serious medical issues, health officials warn.

“This is very dangerous weather,” said William D. Surkis, who runs Main Line Health system’s internal-medicine residency program and is interim vice president of Lankenau Medical Center.

The coronavirus is a compounding factor, he added, in that it has made isolation the norm for many who might be most vulnerable to heat, some of whom live in rowhouses without air-conditioning.

The National Weather Service has posted a “heat advisory” for Sunday.

The hot spell might fall short of official warning criteria — typically, a heat index forecast of 105 — but Surkis cautions that those discomfort measures tell you only how you would feel in the shade.

» READ MORE: Climate change makes some Philadelphia neighborhoods extra steamy. The city has a plan.

And he said it is particularly worrisome that the heat wave is coming so early in the season and that it follows a raw Memorial Day weekend when Philadelphia set daily records for cool temperatures. Temperatures Sunday are forecast to be 40 degrees warmer than last Sunday’s.

“It is definitely going to be a contrast,” said Jonathan O’Brien, meteorologist at the weather service office in Mount Holly.

Temperatures in the city Saturday — the first day of the Philadelphia Flower Show, and the first one ever held outdoors, or in June — reached the 90s and are expected to rise into the mid-90s Sunday through at least Wednesday.

Hot nights

But perhaps of more concern is the lack of overnight cooling, allowing those rowhouses to heat up all the more rapidly after the sun appears.

The so-called urban heat island effect makes the city all the more punitive. Buildings and paved surfaces soak up heat during the day and are slow to give it up after sunset.

» READ MORE: Nights in Philly getting more sultry, data show

Readings aren’t expected to make it below 70 from Sunday morning through Thursday morning,

And that would be a symptom of a buildup of humidity, said O’Brien. The region will be on the western flank of a so-called Bermuda high over the Atlantic. Winds blow clockwise around centers of high pressure, so Philadelphia will experience steamy winds from the southwest.

About heat deaths

Pre-solstice heat waves aren’t all that unusual, and this one won’t be in a league with what occurred in 1991. Starting May 25 that year, for seven days the highs went over 90.

A 2011 study of 18 summers across the country, published in Environmental Health Perspectives, found that nearly twice as many deaths occurred in early heat waves, compared with those later in the summer.

One positive trend, other researchers have noted, is that although world temperatures and nighttime warming continue to increase, heat-related deaths worldwide have declined. That might have a lot to do with Philadelphia, where the numbers have declined dramatically.

In 1995, the city established its low-tech, low-cost Hot Weather–Health Watch/Warning System, which included activating a “heat line” operated by the Philadelphia Corporation for Aging, urging neighbors to look in on the elderly, and setting up a network of cooling centers.

That came two years after Philadelphia became embroiled in a controversy over how it calculated its heat death toll.

Traditionally, fatalities had been attributed to heat only when hyperthermia was verified, meaning the deceased core body temperature had reached 105 degrees.

The late Haresh Mirchandani, who became Philadelphia’s medical examiner in the late 1980s, believed that requirement led to gross undercounts of heat deaths.

» READ MORE: Summers are hotter, but heat-related deaths have dropped. Philadelphia has a lot do with that.

He ordered his investigators to look for proxy evidence such as closed windows or the absence of fans, reasoning they couldn’t get to all the bodies before they had cooled.

In 1993, the temperature hit 100 for three consecutive days in July and the city reported 118 deaths. Other cities that baked were using the traditional standard, thus reporting only a handful of deaths.

Mirchandani’s methods were vindicated by the Centers for Disease Control and Prevention, which validated his methodology. Chicago used that standard in 1995, when heat was determined to have been a contributing cause to more than 500 deaths.

The CDC also held up the Philadelphia heat-response system as a model for the nation, and several other cities adopted it. Over the last 10 years, the city has recorded 87 heat-related fatalities; in the 10-year period that began in 1993, 399 deaths were blamed on heat.

The trends notwithstanding, Surkis and others believe that heat-related deaths are still being grossly undercounted.

For example, it would be all but impossible to know what role heat played in the death of someone with emphysema.

“Sometimes the contribution of heat is never considered,” he said. “The impact it has on our community has been under-recognized.”

Weathering it

In the coming days, Surkis urges people to “think about older relatives,” especially those living by themselves. It wouldn’t hurt to stop by and check to see if the house is ventilated and has full ice trays in the freezer, or have them over for some air-conditioning.

People who are taking medications that inhibit sweating need to be careful, he said. Sweating, as Benjamin Franklin discovered in 1750, is the body’s air-conditioning system. As sweat evaporates, it gives off a critical cooling effect.

Surkis also advises the healthy to proceed with caution. “The human body is an amazing thing,” he said, but heat takes some getting used to.

And for those looking to fine-tune their bodies, he says, don’t be afraid to say, “Maybe this is not the best day to work out.”