Summer heat variations can be fatal for unhealthy seniors.

May 22, 2012

Zanobetti A, MS O'Neill, CJ Gronlund and JD Schwartz. 2012. Summer temperature variability and long-term survival among elderly people with chronic disease. Proceedings of the National Academy of Sciences http://dx.doi.org/10.1073/pnas.1113070109.

Synopsis by Kai Zhang

More elderly people with heart and respiratory problems die when there is a larger difference between summertime high and low temperatures, a new U.S. study found. Analyzing changes over two decades, the study is the first long-term one to link mortality and summer temperature fluctuations.

A small swing between the high and low temperatures in summer might increase the risk of death in older people with certain chronic diseases, report researchers who tracked around 10 million elderly residents in 135 U.S. cities. Those who had cardiovascular, respiratory diseases and diabetes were more vulnerable to the summer temperature variations.

This is the first study to look at the link between year-to-year temperature variations in the summer months and year-to-year variations in survival of elderly. Variation between seasonal  annual high and low temperatures are expected to increase with a warming climate. This work highlights the need to expand current public health intervention programs from a focus on extreme heat events to strategies that include warnings about the dangers of summer temperature fluctuations.

People naturally think more heat-related deaths occur in regions with year-round hot weather, such as in the southern United States. However, prior studies of those living in areas with consistently higher temperatures find the opposite. Fewer deaths are due to heat in southern compared to northern states.

Short-term studies focus on the exposures to temperatures on the day of death or hospital admission, or several days before. Long-term studies deal with temperature exposures over months, a year or even longer.

Few studies have looked at health effects of long exposures. This means summer temperature variations have not yet been linked to reduced life expectancy.

To establish if there is a connection, researchers made good use of the U.S. Medicare database, which includes all U.S. residents older than 65. They focused on data from 1985 to 2006 in 135 U.S. cities. They placed people into one of four groups based on pre-existing diseases. These four groups included 3.7 million older people with chronic obstructive pulmonary disease, 1.9 million with diabetes, 3.4 million with congestive heart failure and 1.5 million with myocardial infarction.

Researchers used statistical models to examine the associations between year-to-year summer temperature variations and year-to-year changes in mortality risk among these four populations. They also tested whether these associations varied by age and green space in a city area. They took into account winter temperatures, ozone levels, personal risk factors and others that might influence the associations.

They found the risk of death increased around 5 percent among these four populations for every 1 degree Celsius the yearly summer temperature variation increased. Considering an average of 270,000 deaths among these four populations, this increased risk is equivalent to around 14,000 more deaths per year than would be expected. Older people living in areas with less green space were twice as likely to die than those in areas with more green space. Those older than 75 years were more likely to die than those between 65 – 74 years old.

Given that temperature variations are expected to rise as the climate warms, this work highlights the importance of incorporating summer temperature variations into current adaptation and intervention strategies that currently focus on extreme hot weather.

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