Small changes in summer temperature — as little as 2 degrees more than usual — may put elderly people with chronic medical conditions at significantly greater risk, a new Harvard study suggests.
The research, published in the Proceedings of the National Academy of Sciences, found that for each increase in summer temperature of 1 degree Celsius – just under 2 degrees Fahrenheit -- the death rate for seniors with chronic conditions rises between 2.8 percent and 4.0 percent.
The upshot: Greater summer temperature variability in the U.S. could result in more than 10,000 additional deaths per year.
"We found that, independent of heat waves, high day to day variability in summer temperatures shortens life expectancy," said lead researcher Antonella Zanobetti, a research scientist with the Harvard School of Public Health. "This variability can be harmful for susceptible people."
While previous research has focused on the short-term effects of heat waves, but Zanobetti said the new study is the first to examine the effects of climate change on life expectancy.
Scientists have predicted that climate change will increase world temperatures as well as summer temperature variability.
For the new study, Harvard researchers analyzed Medicare data from 1985 to 2006 to track 3.7 million chronically ill people over age 65 living in 135 U.S. cities. They sought to evaluate whether mortality rates were related to variability in summer temperature.
They found that in years with wide summer temperature swings death rates were higher than in years with smaller swings. For each 2 degree Fahrenheit increase in temperature variability mortality risk increased 4 percent for diabetics; 3.8 percent for heart attack survivors; 3.7 percent for people with chronic lung disease; and 2.8 percent for those with heart failure.
Mortality risk was higher in hotter regions, the researchers found, noting the elderly and those with chronic conditions have a harder time than others adjusting to extreme heat and may also be less resilient.
"People adapt to the usual temperature in their city. That is why we don't expect higher mortality rates in Miami than in Minneapolis, despite the higher temperatures," said Joel Schwartz, one of the researchers. "But people do not adapt as well to increased fluctuations around the usual temperature. That finding, combined with the increasing age of the population, the increasing prevalence of chronic conditions such as diabetes, and possible increases in temperature fluctuations due to climate change, means that this public health problem is likely to grow in importance in the future."
The study was funded, in part, by the U.S. Environmental Protection Agency and the National Institute of Environmental Health Sciences.