As the polar vortex brings freezing temperatures to much of the United States, bundling up is not just a matter of comfort but of health. Sustained drops in temperature increase deaths from respiratory and cardiovascular illnesses, according to a new study. Published in the January issue of Environmental Health Perspectives, the study’s findings also have implications for how governments should protect those at risk.

The study looked specifically at cold spells — defined as severely low temperatures lasting two or more consecutive days. Prior studies have found that deaths rise on cold days, but authors were interested to learn how those patterns changed in a prolonged period of cold weather. Sustained heat waves have different effects on people’s health than single hot days, for instance.

“According to our literature search, the health effects of heat waves have received a lot of attention, but there was less information on the role of cold spells,” lead author Jouni Jaakkola at the University of Oulu in Finland said.

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Jaakkola and colleagues collected 26 studies spanning 13 countries that tallied up health impacts or deaths during cold snaps and compared them with expected mortality for days of average temperature.

When they re-crunched the data from nine of those studies, they were able to put a number to the risk: Cold spells increased overall deaths by 10 percent. Deaths by cardiovascular disease and respiratory disease were the most affected, rising by 11 percent and 21 percent, respectively. Cold temperatures cause blood vessels and airways to constrict, putting people who are already suffering these illnesses at risk.

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Some of the studies showed that the problem might not necessarily be the low temperatures themselves, but the sudden drop in those temperatures. The studies didn’t support the notion that longer cold spells had a compounding effect, with worse mortality than single cold days.

But many unknowns remain. The researchers’ next step is to fill those gaps by determining the length of cold exposure that is dangerous, and whether increased time spent indoors during cold weather might also be problematic by, for instance, increasing people’s exposure to viruses. Most important, said Jaakkola, is understanding who is most affected by cold exposure.

“We should develop protection for socially deprived and homeless [people] who are among the most susceptible,” he said.

But, he added, they are not the only ones.”Patients with chronic cardiovascular and respiratory diseases also constitute a susceptible population,” he said. “[They] should be informed about the health risks.”

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