This summer's murderous heat wave in Europe, which may have caused more than 11,000 deaths in France alone, is enough to make you look forward to the season of mittens, hot cocoa, and frostbite. Ah, for kind, crisp winter, when people aren't baked like blackbirds!
But not so fast: Demographically speaking, cold is actually a far bigger killer than heat. In the Northern Hemisphere, the Grim Reaper makes more house calls in December, January, and February, while—this year's statistically anomalous summertime mortality excepted—he tends to take time off during July, August, and September (naturally, this is reversed in the Southern Hemisphere). One study calculated that in the United Kingdom in the 1990s, more than 80,000 people died of cold-related causes per year—or more than 100 times the figure for heat-related deaths. All bets are off in the developing world, where the hottest months (whichever part of the calendar they fall on) are more deadly.
Some of the demographers' kernels of insight on the seasonality of mortality hold up across climates, cultures, and time zones: For example, the cold can kill you—and in some climates and conditions, national death rates are as much as 30 percent higher in the winter. Although cancer (No. 2 on the list of leading causes of death in the United States) knows no season, the body's reaction to cold—particularly among elderly men, who are most susceptible to climatic change—can increase the chances of blood clotting, which in turn leads to a higher incidence of death from heart attacks and strokes (Nos. 1 and 3 on the hit list). Similarly, the cold facilitates the transmission of respiratory diseases (No. 4) and ensures that they're more severe than strains contracted at other times of the year. And winter sometimes gives a final push to people who were already ill and vulnerable and likely to die soon anyway (this is impolitely called "harvesting" in the academic literature).
On the surface, then, logic suggests that the colder the climate, the greater the number of wintertime "excess deaths"—that is, the difference between mortality in the winter and the average expected number of deaths in the summer. Or not: In what Roland Rau at the Max Planck Institute for Demographic Research terms the "seasonality paradox," levels of cold-weather excess deaths in populations in relatively mild climates, such as Ireland or Portugal, are actually higher than in populations in truly eyeball-freezing climes like Canada, Russia, and Scandinavia. Generally speaking, people who are used to the cold know how to take care of themselves when the mercury drops. That said, the fruits of economic prosperity—such as more people using toasty warm private cars instead of drafty public transportation and being able to afford heat for their houses—are gradually reducing the levels of wintertime excess deaths in relatively mild climates.
One study of Yakutsk—one of the most bitterly chilly cities in eastern Siberia, and thus the world, where the average temperatures between October and March sink to a positively unbalmy minus 16 degrees Fahrenheit—concluded that lower temperatures did not cause any significant increase in mortality. The frosty denizens of Yakutsk exercised the seemingly obvious safety measures of wearing layers (more than four, on average), staying where it's warm, and keeping the heat cranked up. A small increase in mortality stemming from respiratory disease due to breathing cold air was offset by a decrease in death from accidents—presumably because during chilly spells cold enough to freeze bone marrow, few people go anywhere or do much at all, significantly reducing the opportunities for accidents. (Falling icicles, which each winter skewer roughly 100 Russians who happen to be under the wrong building eave at the wrong time, haven't—yet—been the subject of extensive demographic research.)
The dynamics of the seasonality of death change for folks at the more youthful end of the population spectrum, as not many young people die from cold-inspired heart attacks. Mortality among the young is relatively higher in the summertime, in part because their key killers—motor vehicle accidents, poisoning, drowning, and other unintentional injuries, as well as suicide, homicide, and other external causes—generally tend to strike more when it's warm. In Russia, summertime mortality soars for the demographic groups prone to combine imbibing copious amounts of alcohol with a dip in the local swimming hole; seasonal diving teams make a sport of dredging for the bodies of the drowned after every weekend. Mortality among the sober, but unsupervised, children of drunk-drowners also escalates. This notwithstanding, overall, excess deaths of young people in the summertime are far outnumbered by those of the elderly in the wintertime.
The cold-equals-death equation is turned on its head for much of the developing world, where gastrointestinal problems (such as diarrhea, gastroenteritis, and dysentery) that flourish during the summer account for a significant portion of all deaths. Also, in much of the developing world, winter-month temperatures remain semi-tropical, making it difficult to die of cold-related causes. Furthermore, many of the rich persons' diseases that fell the elderly in the developed world, with a bit of help from the cold—like strokes and heart disease—figure less prominently on the list of the leading causes of mortality in the Third World.
Not everything bad happens during the chillier times of year, though: One study found that among people older than 50 years of age in Austria and Denmark, those born in the autumn months (and, best of all, November) have a slightly higher life expectancy than those born in the spring (or, worst of all, May). The mothers of fall babies enjoyed better nutrition during the summer months, immediately prior to giving birth, while spring-baby mothers had a comparatively poor diet—which in turn affected the long-term health of their offspring.
One of the few silver linings of the seasonality of mortality is the impact of global warming on wintertime deaths. One study suggests that an increase in temperature of roughly 3.6 degrees Fahrenheitby the middle of this century would boost total heat-related deaths in the United Kingdom more than threefold, to just under 3,000, but the number of cold-related deaths would drop by 25 percent, or 20,000, to 60,000. In the meantime, keep that coat zipped.
Thanks to Roland Rau at the Max Planck Institute for Demographic Research, Craig Feinstein at the U.S. Social Security Administration, William R. Keatinge of Queen Mary and Westfield College of the University of London, and Martin McKee of the European Centre on Health of Societies in Transition at the London School of Hygiene and Tropical Medicine, University of London.