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Environmental Factor, May 2012

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Temperature variability tied to shorter life expectancy

By Nancy Lamontagne

Graphic for Joel Schwartz's study

The study included people living in cities, show as dots, in a variety of climate zones. (Photo courtesy of Joel Schwartz)

Joel Schwartz, Ph.D.

Schwartz’s study confirms that increased weather variability is a serious public health concern. (Photo courtesy of Joel Schwartz)

Researchers led by NIEHS grantee Joel Schwartz, Ph.D., report that increasing variability in summer temperatures may reduce the life expectancy of elderly people with chronic medical conditions. Greater temperature variability is expected to occur because of climate change, and the researchers estimate that just a 1 degree Celsius increase in temperature variability could result in approximately 14,000 additional deaths per year in the groups included in the study.

Previous studies examined heat waves, or the effects of hot weather on day-to-day mortality, but these studies do not show whether the shortening of life expectancy from the heat-related event is significant. “If the lives lost on hot days were only shortened by, for example, a few weeks, then there would be no noticeable impact on the annual average mortality rate,” said Schwartz, a professor of environmental epidemiology at the Harvard School of Public Health. “Our study is the first cohort analysis of the effects of long-term differences in temperature and mortality rates.”

The study is the first publication to come from the NIEHS-funded Climate Change and Human Health research program (see story), which was launched in 2010. This program funds research examining the environmental risk factors linked to climate change that impact the health of people, including extreme heat and other changing weather patterns, and increased exposures, such as air pollution and harmful chemicals.

“A main goal of the NIEHS Climate Change and Human Health Program is to identify specific factors that may increase population vulnerability to health impacts of climate change,” said NIEHS Health Scientist Administrator Caroline Dilworth, Ph.D. “To this end, study findings like these help broaden our understanding of the populations at greatest risk.”

Looking long term

The Harvard researchers used Medicare data from 1985 to 2006 to follow the health of 3.7 million chronically ill people over age 65. They looked at whether year-to-year differences in the annual average mortality rates related to year-to-year differences in the variability of summer temperatures. The researchers analyzed each of the 125 cities included in the study separately. “This is important because people likely acclimate to usual patterns in their city,” said Schwartz. “It also avoids confounding factors that differ across cities.” The researchers also took into account other factors such as winter temperature variance, heat waves, ozone levels, and individual risk factors.

The results showed that each 1 degree Celsius increase in summer temperature variability increased the death rate for elderly people with chronic conditions between 2.8 and 4.0 percent, depending on the condition. The mortality risk was 1 to 2 percent greater for those living in poverty and for African-Americans.

Contrary to prior studies of heat waves and hot days, the summer temperature variability in this study seemed to have a greater effect in southern cities. Residents in warmer cities might be more acclimated to hot weather but less able to handle variable weather. “That is important for public health, because the heat wave studies suggested that these cities had less to worry about, and there may have, therefore, been less attention to the issue in these cities,” said Schwartz.


Incorporating more green space could help offset the effects of temperature variability. The study showed that mortality risk was 1 to 2 percent lower for people living in cities with more green space. Schwartz explained that the sensitivity to variable summer temperatures likely occurs because people cannot get into a routine in regard to activities, appropriate clothing, and air conditioning usage. Local weather broadcasts could help people adjust, by emphasizing when temperatures will be noticeably different and suggesting that people adapt their clothing and activity patterns accordingly. However, this type of intervention needs testing. 

The researchers plan a number of directions for future research. “We need to look at elderly people without pre-existing medical conditions and with different conditions, such as neurological diseases,” he said. “Why African-Americans are more vulnerable is a key question.” Further study is also needed to understand the physiology of responses to temperature variability and why certain effects occur.

Citation: Zanobetti A, O'Neill MS, Gronlund CJ, Schwartz JD. 2012. Summer temperature variability and long-term survival among elderly people with chronic disease. Proc Natl Acad Sci U S A; doi:10.1073/pnas.1113070109 [Online 9 April 2012]. Summary.

(Nancy Lamontagne is a science writer with MDB, Inc., a contractor for the NIEHS Division of Extramural Research and Training, Superfund Research Program, and Worker Education and Training Program.)

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