Researchers have revealed that heat waves are deadliest when they occur early in the summer and are longer and hotter than usual.
The news comes on the heels of other research predicting hotter and more frequent heat waves in coming years, reports Discovery News.
Michelle Bell and Brooke Anderson, both of the Yale School of Forestry and Environmental Studies and the co-authors of a study, studied heat mortality in 43 U.S. cities from 1987 to 2005.
In early summer heat waves, the risk of mortality increases 5.05 percent, compared to 2.65 percent for heat waves that occur later in the summer.
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.
This is not good news at a time when climate scientists are predicting more frequent and longer heat waves.
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 is 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 were 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.
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.
Air conditioning is very helpful, but is a double-edged sword when it comes to fighting heat mortality, said Kinney.
The study is published in the journal Environmental Health Perspectives