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Summer Temperature Swings Linked to Shorter Lifespan

Fluctuations in daily summer temperatures may boost the risk of death in older people with chronic diseases, according to a new study. The finding could have important implications for the nation’s aging population.

People generally adapt to the usual temperatures where they live, but the sweltering heat waves of summer are known to raise death rates among susceptible people. Those who are very old, very young, overweight or infirm are especially vulnerable. Scientists have been less certain about the long-term effects of sudden temperature changes. Climate models predict that day-to-day summer temperature swings will become increasingly common.

A research team led by Dr. Antonella Zanobetti of the Harvard School of Public Health decided to take a closer look at the long-term impact of summer temperature variability in 135 U.S. cities. The scientists analyzed Medicare data on more than 3.7 million at-risk people, ages 65 and older. All were patients released after hospitalization for chronic obstructive pulmonary disease (COPD), congestive heart failure, diabetes or a heart attack. The study was funded by NIH’s National Institute of Environmental Health Sciences (NIEHS) and the U.S. Environmental Protection Agency.

The researchers compared survival data with summer temperature fluctuations in each city. The scientists adjusted for potentially confounding variables, such as individual risk factors, heat waves, ozone levels and winter temperature swings. They tracked the patients for up to 21 years. Results appeared in the April 9, 2012, advance online edition of the Proceedings of the National Academy of Sciences.

The researchers found that greater swings in summer temperatures were significantly associated with shorter survival times. The link between temperature variability and mortality was especially strong in those 75 and older. The associations also varied by location, with stronger links between temperature swings and mortality in cities in warmer regions.

Longer survival times were seen in cities with higher proportions of green space, including parks and tree-filled areas. Shorter survival times were seen in more densely populated cities and in areas with greater proportions of African Americans or poverty.

Further analysis showed that each 1 °C boost in temperature variability was linked to an increased death rate of 4% for the group with diabetes, 3.8% for the heart attack group, 3.7% for those with COPD and 2.8% for those with heart failure. Based on these numbers, the researchers calculated that a further 1 °C increase in temperature variability might lead to an additional 14,000 deaths per year nationwide.

“The effect of temperature patterns on long-term mortality has not been clear to this point. We found that, independent of heat waves, high day to day variability in summer temperatures shortens life expectancy,” says Zanobetti. “This variability can be harmful for susceptible people.”

—by Vicki Contie