"People die not because it's cold, but because it's winter," said Robert E. Davis, a University of Virginia environmental scientist who was the report's lead author.
His finding, and the puzzle it presents, caused a hubbub at the International Congress of Biometeorology in the fall in Kansas City, Mo., where the study was released. For a week, 200 researchers from around the world convened there to consider potential cause-and-effect connections between weather and health.
Between, for instance, this El Nio winter and a possible run of stomach bugs; frigid cold and heart disease; wind chill and frostbite; not to mention temperature and the elimination habits of one-hump camels.
Going back four millennia, ancient medicine was linking weather to well-being - laying the foundation for the latter-day science of biometeorology. It has been well-accepted in Europe for 50 years. And although still dismissed in some U.S. quarters as wackiness, it has been surfacing more and more in the mainstreams of American medicine and science - spurred in part by worries over global warming.
Biometeorology's disciples have long considered winter to be a stocking full of sticks, of aches and pains and blahs. But Davis' study of mortality rates has cast an even darker shadow over the season.
Given the searing heat waves during the 1990s, logic would point to higher death rates in the summer.
So much for logic.
There probably is no single solution to the mystery of winter's deadly toll, Davis said. One reason could be that people spend more time indoors, sharing their diseases - a phenomenon recognized in the 18th century by Benjamin Franklin.
Holiday tension could be another factor.
And then there's influenza. Flu shows up virtually every winter - a curiosity in itself. "Influenza is not obviously related to specific weather or climate conditions, nor is it understood why it is a winter disease," Davis' team wrote.
Joel Schwartz, a Harvard University epidemiologist, speculated that winter mortality might be tied to vitamin D deficiencies, from lack of sunlight. On this very day, solar energy in the Northern Hemisphere is at its lowest point of the year.
Lack of vitamin D, which helps the body absorb calcium, is linked to serious bone diseases, according to the National Institutes of Health.
"It may be a contributing factor" to the winter death rate, said Peter Hoeppe, an environmental scientist with Ludwigs-Maximilian University in Munich, Germany, and president of the International Society of Biometeorology.
Yet how it might kill someone at a specific time of year, he said he could not imagine.
As Paul Knappenberger, a member of Davis' research team, said of the findings: "We ourselves don't completely understand all the implications."
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Although death's seasonality is one of the more profound issues to surface recently in biometeorology, not all of the science's adherents have time to sit and ponder the mystery. When it comes to winter and other potential health effects, they have plenty of hot leads to keep them busy.
Coldhearted season
A study of more than 58,000 Scots has shown a dramatic increase during winter of so-called chaotic heartbeat, or atrial fibrillation, a sometimes fatal condition afflicting about 10 percent of the over-75 population.
Among those 75 to 84, hospital admissions for the ailment were 25 percent higher in winter than in any other season, and nearly 40 percent higher for those 85 and older.
"It is certainly possible that cold is an important trigger," said Simon Stewart, a heart specialist at the University of South Australia who conducted the recently released study.
Other suspects include heater gases, he said, and too many sips of wine for auld lang syne. He advises the elderly to restrict their holiday tippling.
Water bugs
Forecasters believe that this winter, while not gentler, will be warmer than normal throughout much of the United States. The reason: El Nio, the warming of surface waters in the tropical Pacific.
Parts of the country also are likely to be dramatically wetter. El Nio already has hammered the West and East Coasts with storms, and there probably are plenty more to come.
With them, scientists warn, could come health problems created when storm runoff contaminates drinking supplies.
Researchers at Johns Hopkins University in Baltimore looked at 548 outbreaks of serious diseases - primarily "acute gastrointestinal illness" - during the last 50 years nationwide. They found that more than two-thirds had followed heavy rainfalls, including one in Wisconsin in 1993 in which more than 400,000 people got sick and 54 died.
But even after El Nio fades, the threat of waterborne disease might not. Computer models suggest that global warming will bring more downpours - leading, the Johns Hopkins team warns, to more outbreaks.
Nipping at your nose
Biometeorology researchers pioneered the concept of comfort indexes, which gauge what cold and heat feel like to the human body. Without them, TV weather forecasters would have just regular old temperatures to recite.
During the 1990s, U.S. and Canadian scientists decided to take one index - the wind chill - a step further by attempting to compute the frostbite threshold for the first time.
They found that frostbite is not that easy to get, even at temperatures as low as 15 degrees Fahrenheit.
In the name of scientific discovery, 12 men had risked their noses. With sensors inside their cheeks, their bare faces were subjected to six temperatures, from 32 above zero to 58 below, and three wind speeds. They were checked for "frostnip," a white mark that precedes frostbite by about one minute.
Even with the wind chill at 22 below zero for 45 minutes, none of the subjects suffered frostnip. At 40 below, however, frostnip occurred within 15 minutes.
The index went into effect last winter, and is available online at www.nws.noaa.gov/om/windchill.
Randall Osczevski, one of the Canadian researchers who developed it, said the frostbite thresholds suggest we're tougher than we think.
Or, as he put it, "someone once said that a sweater is something you put on when your mother is cold."
Contact staff writer Anthony R. Wood at 610-313-8210 or twood@phillynews.com.
This series will return in the spring, with a box of tissues.
