Some Heat Waves Can Be More Harmful Than Others, Study Says

By Alyson Kenward

Heading into this last week of November, when the air is cool and the daylight hours are growing shorter, the idea of a heat wave might sound like a pretty good thing. But it was only a few months ago, back in July and August, that many parts of the country sought relief from long spells of abnormally hot weather. And the heat waves that hit the entire Atlantic coast were relatively mild compared to record-breaking temperatures felt elsewhere around the world this past summer.

Record high temperatures for 17 nations that have broken their national records so far in 2010. Previously, the largest number of countries to break such records in a single year was 14, according to Weather Underground and The Guardian newspaper. If verified, the record set in Pakistan would also stand as the warmest temperature ever recorded in the continent of Asia.

Thousands of deaths were attributed to the most extreme of these summer heat waves, like those in Pakistan, Russia and throughout the Middle East.

Now a new study from Yale University has put some some real numbers on the health risks from U.S. heat waves. In a review of heat waves in 43 American cities between 1987 and 2005, environmental scientists Michelle Bell and Brooke Anderson found that there is a slight but significant increased risk of dying during a heat wave, and that the risk can increase considerably if high temperatures persist for many days or affect areas where people haven't adapted to warm weather by, for example, commonly adopting air conditioning.

Also interesting was the finding that heatwaves occurring early in the summer were found to carry relatively larger risks for human mortality than heat waves with a later onset. In a press release for the new study, published in the journal Environmental Health Perspectives, Bell said:

We found a higher mortality risk from heat waves that were either hotter, longer or earlier in the summer.

In the United States, the researchers found that the average daily risk of dying was 3.74 percent higher during an average heat wave compared to other summer days, but this risk is tiny compared to what can happen during the most severe of heat waves. By comparison, death rates in Moscow doubled during this summer’s Russian heat wave — in other words, the mortality risk had increased by 100 percent. In France, during the soaring summer temperatures in 2003, the rate has been cited as being at least 130 percent higher. 

For the purpose of their study, the Yale researchers counted U.S. heat waves that involved two or more days in a row where the average daily temperatures were among one of the top five percent hottest days ever recorded in a particular city. They also discovered that every additional day that the heat wave lasted increased risk by nearly half a percent and that each additional 1°F rise in temperature increased the risk of death by another 2.49 percent.

Furthermore, the risk of death was greater in areas where hot weather is less common throughout the year, including the Northeast and Midwest. In their study, the researchers suggested this discrepancy could be because people living in those areas may be less acclimated to very hot weather, or are less prepared for it since fewer people own air conditioners in those areas. And though Bell and Anderson’s study did not break down the risk of death according to age, previous studies have found that both infants and the elderly are at greater health risk during heat waves.

By mid-century in New York, the sweltering heat of July 2010 may be thought of as cooler-than-average conditions, as more days above 90°F routinely occur. The 2010 dates on this “postcard from the future” for New York are actual; 2050 dates are representative of average projections.

Regardless of region or age, the overall risk of death was found to be only a few percent higher than during times with average weather.

The findings from this new study are based on an analysis of past heat waves, which lays a foundation to address how the risk of death or other health problems will change if the average summer temperatures increase in the coming decades because of climate change, as they are predicted to do, likely making episodes of extreme heat more frequent, intense and longer lasting. “Our findings have implications for researchers estimating health effects from climate change,” says Anderson. Though it is possible that heat wave related deaths could increase if such events become more frequent and severe, it is also possible that people will become more acclimated to these higher temperatures around the country.

Though there are certainly other heat-related health concerns that pick up during periods of abnormally hot weather, like heat exhaustion and dehydration and the enhanced effects of air pollution, it’s interesting — if not somewhat surprising — to learn that the risk of death during heat waves in the U.S. has remained very small during the past 20 years. 

Homepage image credit: Flickr/Vasilios Sfinarolakis.