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Temperature-Related Mortality in London
Reference
Carson, C., Hajat, S., Armstrong, B. and Wilkinson, P. 2006. Declining vulnerability to temperature-related mortality in London over the 20th century. American Journal of Epidemiology 164: 77-84.

Background
The authors write that "temperature-related mortality is of current scientific and public health interest in the United Kingdom because of the persistently high number of excess winter deaths [our italics and bold] ... and also, more generally, because of debates about the vulnerability of European and other populations to the projected increased frequency of heat waves under global climate change."

What was done
Carson et al. analyzed London mortality and meteorological data for four periods of the 20th century that they "selected to avoid times of war and influenza pandemics: 1900-1910, 1927-1937, 1954-1964, and 1986-1996."

What was learned
The results indicated, according to the researchers, that there was "an increase in risk at low temperatures in each period, but the strength of association gradually declined over the century ... from a 2.5% increase in mortality for each degree-C fall in temperature below 15°C in 1900-1910 to approximately a 1.2% increase in mortality per degree-C fall in temperature in 1986-1996." At the other end of the temperature spectrum, they say their analyses "also provided some indication of heat-related mortality in the earlier periods of analysis, but not in 1954-1964 or 1986-1996 [our italics]."

What it means
These results suggest, first of all, that cold is a much more effective killer than heat. In addition, they suggest that the deadly effects of both extreme cold and extreme heat have been muted with the passage of time, "despite the aging of the population and a progressive increase in the prevalence of cardiorespiratory disease, as Carson et al. add, "which would otherwise tend to increase susceptibility" to temperature-induced death.

Another implication of their study results, in the words of Carson et al., is that "the decline in vulnerability to cold and heat is most readily explained by beneficial changes relating to increasing wealth." Some of the items they mention in this regard are improvements in health care, improved nutrition, improved housing, increased car ownership, climate-controlled transportation and shopping facilities, and improved clothing fabrics, although they say "we cannot quantify or even identify all of the modifying factors that have contributed to this reduced susceptibility." Last of all, they say "it is reasonable to conclude that a similar modification of risk will occur among populations in other settings, particularly in low- and middle-income countries, as they grow richer."

Reviewed 13 September 2006