Too darn hot ... but not so deadly

This June was the hottest on record in Philadelphia, July was No. 2, and August is projected to end in the top 10. Yet only 15 city residents have died of heat-related causes.

While any preventable death might be considered a tragedy, that number pales in comparison to the toll in other recent heat waves: 40 in 2002, 67 in 1999, 77 in 1995.

What's different this year?

No one fully understands the interplay of climate, medical conditions, and other factors that can be lethal for any given individual. Experts can, however, point to several weather facts that may have made this summer less dangerous even as it was more hot.

Heat early in the season, for example, can make people less vulnerable later. Shorter heat waves are less dangerous than longer ones. And high humidity, while uncomfortable, appears to be less deadly than dry heat.

"I personally haven't seen any emergencies that I attribute to heat," said Suzanne Shepherd, an emergency room doctor at the Hospital of the University of Pennsylvania.

Having a detailed response plan in place long before the heat hits helps - and the city has frequently been praised for the system that it developed following a 1993 heat wave that killed more than 100 people.

"Philadelphia is one of the leaders with heat-wave preparedness," said George Luber, an epidemiologist who works on weather-related health issues at the U.S. Centers for Disease Control and Prevention. "Perhaps some of the decrease should be attributed to their outreach."

The National Weather Service considers a heat wave to be three days in a row at 90 degrees and above. At some point during that period, if risks to health appear to be escalating, the service issues an excessive heat warning.

"There are plenty of days when it's 90 and it's not going to be a problem," said Gary Szatkowski, chief meteorologist at the weather service's Mount Holly office. "It depends on the time, the humidity, heat index. You have to look at more than just temperature." No warning was contemplated this week, the weather service said Sunday.

A heat warning from the federal meteorologists triggers the city's emergency heat plan: thousands of block captains who check in with elderly neighbors, extended hours at senior centers, and mandatory continuation of water and power for people whose late payments might otherwise mean a shutoff.

The chronically ill, particularly the elderly, are in the most danger. Heat also affects the circulatory systems of diabetics. It puts stress on people with cardiovascular and respiratory problems. And various medications reduce the body's ability to detect and respond to temperature extremes.

An early-summer heat wave can have a mixed impact.

"In the same weather, more people die earlier in the summer than later in the summer," said Laurence Kalkstein, a climatologist at the University of Miami and an expert on heat mortality, "because they haven't acclimated, and it kills off the people more susceptible."

In the first week of June 2010, high temperatures reached the low 90s, well above the norm. Two heat-related deaths were reported that week. But no heat-related deaths have been reported anywhere in the region since July 26, even though the mercury rose as high as 96 degrees the second week of August.

"Bodies adapt physiologically by producing more sweat glands over the span of a few weeks," said the CDC's Luber. Sweating brings water to the skin's surface to evaporate, helping the body cool.

The first heat wave of the season also prompts people to "start to take it easy," said Luber. Construction workers may begin at daybreak and then quit before the afternoon peak.

The longest heat wave so far this year began July 4 and lasted six days, shorter than some stifling periods in other years. At least nine deaths in the region - six in the city, two in Montgomery County, and one in Delaware County - were attributed to that hot period, which included two days over 100 degrees.

None of this summer's shorter heat spurts was anywhere near as deadly.

"In years when we have had high numbers, it's because we've had a succession of days," said Jeff Moran, spokesman for the Philadelphia Department of Public Health.

Research shows that the duration of the heat wave plays a role, Luber said, especially in places where many residents lack air conditioning. Cities retain much of the day's heat into the night, and rowhouses can amplify that effect, offering no break not just between heat waves but also between day and night.

Humidity also is a factor, although not necessarily a simple one. High humidity can interfere with the relief that sweating brings, adding to the risk on hot days.

Yet looking at 30 years' worth of Philadelphia data on "excess mortality" - more deaths than normal - it's not consecutive humid days that stand out but ongoing hot and dry conditions, said Kalkstein.

There is little excess mortality during the first three days of a heat wave, regardless of humidity. If the high temperatures continue into a fourth day, however, dryness is riskier. By the fifth day, excess deaths in dry heat are quadruple the number in humid heat.

This summer's heat waves have featured both high and low humidity, but just one - beginning July 4 - had four dry days in a row.

Kalkstein, who did much of his research while at the University of Delaware, helped develop the system used by the National Weather Service to determine when an excessive heat warning is warranted. He also worked on the emergency plan that Philadelphia activates in response to a heat warning that came out of the disastrous heat wave of 1993.

The 118 heat-related deaths reported that year by the Philadelphia Medical Examiner's Office - much higher than in many other cities - brought a review by the CDC of the criteria that this and other cities used to classify a death as heat-related.

Many coroners and medical examiners classified a death as heat-related only if the core body temperature was 105 or above, which would be the case if someone had died a short time before. When someone dies and is not quickly found, however, the body has cooled; core temperature alone will not prove that heat was a factor.

To get around that issue, Philadelphia considered not just body temperature but also environmental factors like closed windows and lack of air conditioning before determining that a death was or was not related to the heat.

The CDC concluded that Philadelphia's way of doing things was correct. Many medical examiners have since changed their guidelines, raising the number of heat-related deaths, although there is no uniformity on the issue either nationally or in this region.

Nevertheless, heat may be responsible for many times more deaths than officially recorded anywhere.

A wide range of health conditions can be worsened by heat. While someone might have died from cardiac arrest, for example, it might be the heat that caused the heart to fail.

"But medical examiners are reluctant to make that conclusion where the circumstances are unknown," said Luber.

Plus, most of those deaths are never investigated by a medical examiner, who by law gets involved only in certain circumstances, such as when someone died alone at home or when foul play is suspected.

Kalkstein estimates that this summer's true death toll from the heat could be as high as 225 in Philadelphia - 15 times the official number.

Yet that, in a way, is still good news.

"You got a bad summer, with a lot of hot days," said Kalkstein. "But the city's system has been effective in lessening the amount of the deaths."