Big shifts in summer heat tied to death risk

While extended heat waves are known to be dangerous for senior citizens, new research found that big changes in average temperatures from one day to the next can be harmful, too. For each increase of 1 degree Celsius in summer temperature variability, the death rate for infirm elderly residents rose between 2.8 percent and 4 percent, depending on the ailment, the study found.

The researchers estimated that greater summer temperature variability, a predicted consequence of climate change, is causing 10,000 additional deaths per year in the United States, a figure that is likely to rise along with the mercury.

Cities such as Boston are likely to experience among the highest public health threats as temperature swings increase, the researchers said, noting the lack of widespread air conditioning and the likelihood of rising temperatures.

They also found a 1 percent to 2 percent greater risk of dying for African-Americans, as well as the poor. The risk was 1 percent to 2 percent lower for people living in cities with more green space, which moderates the heat.

“The effect of temperature patterns on long-term mortality has not been clear to this point,’’ said Antonella Zanobetti, the lead author of the study and a senior research scientist in the department of environmental health at Harvard’s School of Public Health. “We found that, independent of heat waves, high day-to-day variability in summer temperatures shortens life expectancy.’’

Diabetics were 4 percent more likely to die with each 1 degree Celsius increase in daily temperature fluctuations, while people suffering from heart failure were 2.8 percent more likely to die, according to the study.

The researchers found that the mortality risk was higher in hotter regions, noting that the elderly and those with chronic conditions have a harder time than others adjusting to extreme heat and are likely to be less resilient to significant temperature swings. However, the impact was blunted in cities where air conditioning is common.

With temperatures estimated to be rising at 0.4 degrees Fahrenheit per decade because of global warming, the risks of greater temperature swings are rising, said Patrick Kinney, a professor of environmental health science sciences and director of the climate and health program at Columbia University.

“I think it’s an important study, because it shows for the first time that greater variability in temperature across days is associated with increased mortality risk,’’ said Kinney, who did not take part in the study. “The methods are strong, as are the data that went into the analysis.’’

The study, published Monday in the Proceedings of the National Academy of Sciences, used Medicare data from 1985 to 2006 to track the health of 3.7 million chronically ill people ages 65 and older in 135 cities.

The researchers accounted for other things that might influence mortality, such as individual risk factors, winter temperature variance, and ozone levels. Still, they found that in years when the summer temperature swings were larger, cities had higher death rates than years with smaller swings.

“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, professor of environmental epidemiology at the Harvard School of Public Health and an author of the study.

He called on city governments to consider ways to increase green space, such as planting more trees and adding parks.

“People do not adapt as well to increased fluctuations around the usual temperature,’’ he said. “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.’’

David Abel can be reached at dabel@globe.com. Follow him on Twitter @davabel.