The Deadly Toll of High Temperatures

Why are some heat waves more harmful than others?

Newswise — Moscow grabbed headlines last summer as thousands perished during an unprecedented heat wave. However, a retrospective study of heat waves in 43 U.S. cities indicates mortality climbs even during the less dramatic heat events that hit many cities almost every year, according to research published online November 18 ahead of print in the peer-reviewed journal Environmental Health Perspectives (EHP). “Our findings have implications for decision makers addressing the health burden of heat waves and for researchers estimating health effects from climate change,” said study co-author Michelle L. Bell of the Yale School of Forestry & Environmental Studies.

“We found higher mortality risk from heat waves that were hotter, longer, or earlier in the summer,” Bell said. The average daily risk of nonaccidental death increased by an average of 3.74% during the heat waves studied, which occurred in 43 U.S. cities from 1987 through 2005. Risk increased by 2.49% for each 1°F rise in mean temperature and 0.38% every day a heat wave dragged on.

The definition of “heat wave” varies depending on the city in question. Bell and coauthor G. Brooke Anderson of the Yale Environmental Engineering Program defined heat waves as two or more days for which the average mean temperature exceeded the 95th percentile of temperatures for warm-season days (May through September) for a given city during the 19-year study period. They defined catastrophic heat waves as those with a mortality rate at least three times higher than any other heat wave in that city during the study period. Chicago and Milwaukee experienced catastrophic heat waves during the study period, both in July 1995.

Overall, daily risk of mortality was highest during the first heat wave of a season, increasing 5.04% compared with 2.65% for heat waves later in the summer. Bell explained that people are less accustomed to heat earlier in the season and may not take appropriate precautions. “Also, those most vulnerable to heat waves may succumb during the first event,” she said.

Risk of mortality was greater in the Northeast and the Midwest than in the South. Mortality did not increase at all during heat waves in several of the southern cities studied, including Charlotte, North Carolina; Dallas/Fort Worth, Texas; and Tulsa, Oklahoma.

In addition to variation across regions, the authors also found that mortality rates during heat waves of equivalent intensity and duration could vary from one year to the next within the same region or even the same city.

But heat duration, intensity, and timing were not enough to explain the high death rates during catastrophic heat waves. During Chicago’s catastrophic heat wave of 1995, when temperatures averaged 87.2°F over five days, daily mortality increased by 134%, 36 times the average increase in heat-wave mortality during the study period. In the Milwaukee event, average temperatures of 87.7°F over three days increased mortality by 93%.

The authors continue to examine additional factors that can intensify the danger of these events to help public officials learn which interventions can be most effective in preventing deaths during heat waves. “These can include differences in building structures, the use of air conditioning, and adaptation as people become accustomed to higher temperatures,” Bell said. Because the health burden of heat waves can vary so greatly, “the best intervention program for one city may not be the most appropriate in another,” Anderson said.

The research was funded by a U.S. Environmental Protection Agency (EPA) Science to Achieve Results graduate fellowship, a National Science Foundation graduate fellowship, the National Institute of Environmental Health Sciences, and the U.S. EPA through the Johns Hopkins Particulate Matter Research Center.

The article, “Heat Waves in the United States: Mortality Risk During Heat Waves and Effect Modification by Heat Wave Characteristics in 43 U.S. Communities,” will be available November 18 free of charge at http://ehponline.org/article/info:doi/10.1289/ehp.1002313.

EHP is published by the National Institute of Environmental Health Sciences, National Institutes of Health, U.S. Department of Health and Human Services. EHP is an open-access journal, and all EHP content is available free online at http://www.ehponline.org/. Brogan & Partners Convergence Marketing handles marketing and public relations for the publication and is responsible for creation and distribution of this press release.

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