Between Extremes: Health Effects of Heat and Cold.

Climate change is expected to have profound effects on weather patterns and temperatures worldwide in the coming decades, with serious implications for public health.1 Among the many ways in which global warming bears on human health,2,3,4,5,6 few are more readily apparent than the trend of increasing heat waves, which are often regarded as the deadliest of all natural disasters.7,8 And despite current and future adaptation efforts,9,10 the overall health burden of heat waves could grow as average temperatures continue their upward tick and extreme heat events become more frequent, severe, and long-lasting.11 Although heat waves and cold snaps pose major health risks and grab headlines when they occur, recent studies have uncovered a more complex and perhaps unexpected relationship between temperature and public health. But while isolated heat waves pose a major health risk and grab headlines when they occur, recent research has uncovered a more complex and perhaps unexpected relationship between temperature and public health—on the whole, far more deaths occur in cold weather than in hot. This reality is obscured by the fact that, unlike heat-related health effects, which spike during discrete events, cold-related illnesses and deaths are diffuse throughout the year, don’t require extreme temperatures, and can lag well behind cold snaps.12,13 An analysis by the Centers for Disease Control and Prevention (CDC) of U.S. temperature-related deaths between 2006 and 2010 showed that 63% were attributable to cold exposure, while only 31% were attributable to heat exposure.14 In Australia and the United Kingdom, cold-related mortality between 1993 and 2006 exceeded heat-related mortality by an even greater margin—and is likely to do so through at least the end of the century.15 Researchers who evaluated 74 million U.K. and U.S. deaths reported in May 2015 that low temperatures are associated with 7.3% of all deaths versus just 0.4% for high temperatures, a ratio of more than 18 to 1.16 “You try to focus on the high-exposure situations, and sometimes you don’t realize that the important impact can be much higher for mild exposures, not because they are more dangerous but because they are far more common,” explains Antonio Gasparrini of the London School of Hygiene and Tropical Medicine, lead author of the 2015 paper.16 “These results are quite surprising because we never address [the relationship of temperature and health] from this perspective.” In an effort to better understand the direct effects of cold temperature on human health in an era of climate change, researchers are fine-tuning their models and methods to account for nuances including seasonal public-health trends unrelated to the direct effects of temperature. One recent paper argued that cold-season deaths likely are largely associated with increases in respiratory infections such as influenza,17 thought to be facilitated both by cooler, drier air and by more hours spent indoors.18 In this view, future climate warming may bring less relief from the increases in mortality associated with cold weather than previously thought. Other recent research has examined links between public health and extreme temperatures (highs and lows alike), which are more likely to actually cause effects, not just be associated with them. One study concluded that physiological acclimatization to extreme heat is unlikely to keep pace with climate change.19 Another helped quantify the so-called mortality displacement effect—deaths “bunch up” after a heat or cold event as seriously ill individuals die days or weeks earlier than they otherwise would have, then dip below normal.20 But evolving technological and behavioral adaptation to extreme temperature by individuals and communities is a more difficult concept to integrate into models, says Weiwei (Vivian) Yu, an expert on statistical modeling and methods at the University of Queensland. “We’re not sure how to measure these factors, even though we know they’re very important to the effect on health,” she says. Ultimately, despite their huge scope of impacts, heat and cold exposures are individual phenomena and thus highly relative, researchers have found—from one person, home, neighborhood, city, and region to the next.21,22,23,24 That simple fact, it turns out, may dictate how researchers and public officials talk about temperature and health as climate change progresses.