BOSTON -- Excess deaths from the heat of global warming will not be counteracted by a corresponding drop in cold-related mortality, investigators here estimated.
BOSTON, June 29 -- Excess deaths in the U.S. from the heat of global warming will not be counteracted by a corresponding drop in cold-related mortality, investigators here estimated.
An analysis of the deaths of more than 6.5 million people in 50 U.S. cities during the 1990s showed a 1.59% increase in mortality rates during two-day cold snaps, yet a larger jump during extreme heat spells, found Mercedes Medina-Ramon, Ph.D., and Joel Schwartz, Ph.D., of the Harvard School of Public Health.
"Our findings suggest that decreases in cold weather as a result of global warming are unlikely to result in decreases in cold-related mortality in the U.S." they reported online in Occupational and Environmental Medicine, a BMJ journal. "Heat-related mortality, in contrast, may increase, particularly if global warming is associated with increased variance of summer temperature."
Even in the hottest sunbelt cities, where the effects of heat on mortality were lowest, an increase in temperatures caused a significant rise in deaths, whereas the population at large seems to be acclimated to cold weather. This may be because central heating is more universal than air conditioning, the authors felt.
The investigators conducted a case-crossover study comparing daily mortality figures (for a total of 6,513,330 deaths) with weather for the years 1989 through 2000 in 50 cities.
They assessed weather exposure by looking at city-specific variables in temperature distribution, and calculated piecewise linear variables to determine exposure to temperature on a continuous scale above or below a threshold.
They also looked at the effects of hot and cold temperatures in season-specific models, and performed a meta-analysis of city-specific results, using city characteristics as effect modifiers.
They found that deaths were higher in periods of both extreme cold and extreme heat. During two-day cold snaps there was a cumulative 1.59% increase in deaths (95% confidence interval, 0.56% to 2.63%), and during extreme heat there was a 5.74% mortality excess (95% CI, 3.38% to 8.15%).
During the cold periods, there was an uptick in deaths from myocardial infarctions and cardiac arrest; deaths from heat did not show a similar pattern, the authors found.
"Although myocardial infarction and cardiac arrest mortality increase considerably on cold days, increases in all-cause mortality appear to be more pronounced on extremely hot days," they wrote. "Our findings suggest that decreases in cold weather as a result of global warming are unlikely to result in decreases in cold-related mortality in the U.S. Heat-related mortality, in contrast, may increase, particularly if global warming is associated with increased variance of summer temperature."
They acknowledged that their study was limited by the inability to control for air pollution other than ozone, because there were insufficient data on the levels of particulate matter during the study period, and this could have confounded results.