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.
