Researchers agree that extreme temperatures, both hot and cold, stress the human cardiovascular system, as well as other parts of the body. It is commonly assumed that hot weather directly causes more deaths, but the reality is that more deaths generally occur in the winter. Winter also brings with it flu season and other wider patterns of disease — and furnishes physical challenges like snow-shoveling — and so the precise effect of freezing temperatures themselves has not always been clear.

Another difficulty in studying the physiological impact of winter weather is what researchers call the “lag effect,” where very low temperatures on a given day will affect someone days, even weeks, later. Heat, by contrast, frequently impacts the human body much more rapidly. The winter season typically sees the greatest shifts in temperature — and general weather variability — so it is crucial to account for such lags in effects.

A 2013 study published in the International Journal of Biometeorology, “High-mortality Days during the Winter Season: Comparing Meteorological Conditions across Five U.S. Cities,” analyzes data relating to rapid shifts in temperature and pressure and patterns of mortality from 1975 to 2004. The cities examined were Minneapolis-St. Paul, Pittsburgh, St. Louis, San Antonio and Miami. The authors, Michael J. Allen and Scott C. Sheridan of Kent State University, draw some original conclusions by using a methodology that allows them to look at how dramatic shifts in weather ultimately lead to higher death rates (“spikes”).

The study’s findings include:

• December saw the greatest number of mortality-spike days. Minneapolis-St. Paul saw the most, while Miami saw the fewest. Most cities saw higher rates of cardiovascular- and respiratory-related deaths on such winter days.
• The temperature on winter mortality-spike days was typically warmer than the preceding five days, while atmospheric pressure was lower and more precipitation was likely.
• “The relationship between temperature and mortality is not entirely consistent with previous research that suggests mortality increases the most during low temperatures…. Despite an increase in blood viscosity during low temperatures, this research showed that days with warmer temperatures were associated with higher mortality. However, the importance of the lag effect and a delayed physiological response to the environment should not be overlooked as a probable, contributing factor.”

The researchers note that the study looks at only shorter intervals of time, and thus longer-term lag effects are not accounted for. “While warmer temperatures were found to be associated with high-mortality events, the extent to which the weather on high mortality days could be attributed directly to these deaths remains unresolved,” Allen and Sheridan write. “Increasing temperatures and decreasing pressure may be associated directly with a strong cold frontal passage, which would typically precede the observed spike-day conditions by several days.”

Related research: A 2009 study published in Epidemiology reviews data on a wider set of U.S. communities and identifies subpopulations, such as the elderly and certain minorities, who may be more susceptible to cold weather and its effects. It finds that “cold-related mortality was most associated with a longer lag (average of current day up to 25 days previous), with a 4.2% (3.2%-5.3%) increase in risk comparing the first and 10th percentile temperatures for the community.” A 2007 study from the University of California, Santa Barbara, also finds that females in general are at greater risk — as well as males in lower-income areas — and that an estimated 0.8% of all deaths in the United States during the period studied were attributable to cold. That study also finds that as the U.S. population shifts toward Southern states and warmer climates, this trend will impact overall life expectancy nationwide.

Keywords: snow, elderly, aging, safety, blizzards, extreme temperatures, heat waves