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."
Contact Brooke Minters at brookeminters@gmail.com.
