At a time when climate scientists are predicting more frequent and longer heat waves, this isn't good news.
Deaths from early summer heat waves are higher than later in the summer.
People in northern cities are more vulnerable to heat than southern cities.
Air conditioning helps, but poses some big problems as well.
Early summer heat waves reap more lives, say researchers who have studied heat mortality in 43 U.S. Cities from 1987 to 2005. The news comes on the heels of other research predicting hotter and more frequent heat waves in coming years.
In early summer heat waves, the risk of death increases 5.05 percent, say Michelle Bell and Brooke Anderson, both of the Yale School of Forestry and Environmental Studies and the coauthors of a study which appears in Environmental Health Perspectives. Later in the summer, however, that risk drops by almost almost half -- to 2.65 percent.
In fact, over the years they studied, the risk of death during heat waves increased by 2.59 percent for each 1 degree Fahrenheit above a more normal temperature. For every day a heat wave drags on, the mortality risk increased 0.38 percent.
That isn't good news at a time when climate scientists are predicting more frequent and longer heat waves.
Global climate change is already priming summer heat waves that are predicted to get both more frequent and hotter over the next 30 years, based on high-resolution climate simulations of temperatures across the United States reported recently in the journal Geophysical Research Letters.
The climate model research looked at the potential effects of a global two-degree Celsius (3.6-degree Fahrenheit) warming and found it causes more extreme summer heat waves all over the United States, and especially in the West. Those heat waves will be more a lot more than that two degrees hotter than previous heat waves, incidentally.
"What makes this (mortality) information particularly useful is that it is based on analysis of several dozen cities across the U.S.," said epidemiologist Patrick Kinney of Columbia University.
The new study found, for instance, that the risk of death rose more in northern cities than in southern cities. Part of that difference is that there are more air conditioners in southern cities, and so people have a way to escape from the heat. It's also possible that people in the South are more acclimatized to warm weather.
"As an example, our results indicate differences in risk by region, which implies that programs that work well for one community may not work well for another," said Bell.
The new study builds on others in recent years that looked specifically at who was most vulnerable to heat waves.
"Our previous work found higher risk to heat-wave-related mortality for older ages groups, and for communities with higher unemployment or higher percentage of African-Americans," said Bell. This earlier work, which looked at Sao Paulo, Brazil; Mexico City, Mexico; and Santiago, Chile, also found that risk of weather-related death varied with a person's sex and gender, and from community to community.
As for exactly why more heat deaths happen earlier in the summer, it could be that there are simply more vulnerable people at the beginning of the summer. And if more of them die early, there are fewer of them around to succumb to late summer heat, said Bell. That would theoretically leave people who are somewhat more heat-tolerant or able to find ways to escape the heat.
Then there is the matter of air conditioning. It is very helpful, but is a double-edged sword when it comes to fighting heat mortality, said Kinney.
"While air conditioning is clearly protective, air conditioning uses a lot of energy and thus contributes to global warming and regional air pollution," said Kinney. "Also, the protection afforded by air conditioning can be lost during power failures, which occur most frequently during hot spells."