The summer temperatures in Philadelphia in the last 10 years been on average higher than they were in the sizzling 1990s, yet the number of heat-related deaths, according to health department figures, has declined significantly: 72 over the last decade vs. 361 in the ’90s.

Heat deaths are down “not just in Philly, but everywhere,” said Laurence Kalkstein, a climatologist who is an expert on heat-related mortality. He and other experts believe that has a lot to do with this city.

What officials are calling a “dangerous” heat wave will reach an apex this weekend when temperatures will make a run at 100 in Philadelphia for the first time since 2012, and overnight lows will struggle to get below 80.

Some heat-related deaths are possible, particularly in rowhouse neighborhoods with many seniors living alone.

However, Philadelphia evidently has become quite good at mitigating the dangers of extreme heat, and its methods have been copied elsewhere.

In 1995, the city instituted an aggressive heat-wave response system, coordinated with the National Weather Service.

Perhaps more significantly, four years earlier Philadelphia also made a controversial decision to broaden the definition of heat deaths to include those in which heat is a contributing factor, as opposed to a strict criterion of hyperthermia.

That helped draw attention to the hazards of heat, said Gary Szatkowski, retired chief meteorologist at the Mount Holly weather service office.

“The first step to solving a problem is to recognize the problem," said Szatkowski, who arrived in Mount Holly in the 1990s.

The Centers for Disease Control and Prevention determined that the Philadelphia counting system not only was valid, but that it should be adopted nationwide. Other places have done so. “In general, that is becoming more common,” CDC scientist Paul Schramm said.

Why ‘heat-related’?

In the summer of 1993, when the temperature hit 100 for three consecutive days in July, the city reported 118 deaths. But Philadelphia’s numbers were viewed with considerable skepticism. Other cities that baked were reporting only a handful of deaths. Those cities were counting only deaths directly attributable to hyperthermia — meaning the deceased’s core body temperature had reached 105 degrees.

But Haresh Mirchandani, who came to Philadelphia from Detroit in the late 1980s as medical examiner, believed that policy greatly undercounted the victims.

On some heat-wave days, death rates spiked well above daily averages, suggesting heat was a factor.

Mirchandani, now deceased, reasoned that in many cases, medical examiners wouldn’t be able to get to victims in time to measure the body temperature at the moment of death. It might take investigators several days to locate a body. Most of the victims were likely to have been elderly and living alone.

He ordered his investigators to look for clues such as closed windows or the absence of fans and air conditioners.

The CDC validated his methodology. In the historic Chicago heat wave of 1995, heat was determined to have been a contributing cause to more than 500 deaths.

“People know a lot more about heat than they did in 1993,” Szatkowski said.

How the city responded

In 1995, Philadelphia established its low-tech, low-cost Hot Weather–Health Watch/Warning System.

When the heat alert is activated — and the Heat/Health Emergency has been extended until 11 p.m. Monday — the Philadelphia Corp. for Aging activates its Heatline (215-765-9040), which will be available from 8:30 a.m. until midnight through Sunday, and on Monday from 8:30 a.m. to 11 p.m.

In 2010, the Heatline received close to 2,000 calls over 11 hot days.

City officials also nudge residents to keep an eye out for elderly neighbors who live alone. It also opens a network of cooling centers citywide.

“I think Philly is a good city for this to be implemented in," said Szatkowski, who had enthusiastically involved the weather service in the heat-alert program. "It’s a collection of small neighborhoods, so you can reach down to local neighborhood level.”

Kalkstein, who helped develop models that triggered heat warnings, calculated that from 1995 to 1998, the city’s response saved over 115 lives.

Dangers in the night

This heat wave might be particularly dangerous because the nights are going to be quite steamy, forecasters fear.

“We focus on cooling options during the day,” said Anthony S. Mazzeo, chair of emergency medicine at Mercy Catholic Medical Center and Nazareth Hospital. “At night, that’s very difficult.

“Usually nighttime is when they get their break from the heat," he said, but without air-conditioning, when the hot air is saturated with sweat-fighting water vapor, that’s hardly a break. That has a carryover effect during the day.

“You’re really fatigued when you don’t sleep,” he said.

His advice for those trying to sleep through the heat without air-conditioning is to keep the windows open and a fan — preferably two — going, and to put some ice in front of the blades.

Instead of tossing and turning, get up and drink some cold water: Hydration is critical.

“We don’t recognize we’re dehydrated until we are,” said Stephen Merrill, a nursing supervisor at PCA.