Climate and health: mortality attributable to heat and cold
Authors: Keith Dear, Zhan Wang
Journal: The Lancet (2015)

The European heatwave of 2003 showed the potentially disastrous effects of heatwaves. About 70 000 premature deaths resulted, notably in France, and a series of lethal heatwaves have subsequently occurred in several countries.1Christidis N Jones GS Stott PA Dramatically increasing chance of extremely hot summers since the 2003 European heatwave.Nat Clim Chang. 2015; 5: 46-50Crossref Scopus (326) Google Scholar However, less well appreciated is the similar burden of mortality from moderately hot weather, which, depending on definitions, could be greater than that of mortality attributable to extreme heat in some places.2Bull G Morton J Environment, temperature and death rates.Age Ageing. 1978; 7: 210-224Crossref PubMed Scopus (140) Google Scholar Evidence is accumulating about the health risks of abnormal ambient temperatures. Conventionally, studies either focus on the effects of extreme temperature events, or aim to define exposure–response associations between temperature and mortality.1Christidis N Jones GS Stott PA Dramatically increasing chance of extremely hot summers since the 2003 European heatwave.Nat Clim Chang. 2015; 5: 46-50Crossref Scopus (326) Google Scholar, 2Bull G Morton J Environment, temperature and death rates.Age Ageing. 1978; 7: 210-224Crossref PubMed Scopus (140) Google Scholar However, whether data that are collected at country, or even city, level are relevant to other climatic settings is doubtful. Moreover, a focus on extreme weather (such as heatwaves) might ignore the incremental risk of moderately unusual temperatures. Both high and low temperatures have been reported to be associated with mortality and morbidity from causes such as cardiovascular disease and respiratory disease.3Basu R Samet JM Relation between elevated ambient temperature and mortality: a review of the epidemiologic evidence.Epidemiol Rev. 2002; 24: 190-202Crossref PubMed Scopus (958) Google Scholar Although deaths attributable to cold are substantially more common in most places than are those attributable to heat, they attract far less public attention.3Basu R Samet JM Relation between elevated ambient temperature and mortality: a review of the epidemiologic evidence.Epidemiol Rev. 2002; 24: 190-202Crossref PubMed Scopus (958) Google Scholar In The Lancet, Antonio Gasparrini and colleagues4Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; (published online May 21.)http://dx.doi.org/10.1016/S0140-6736(14)62114-0PubMed Google Scholar use a very large multicountry database and novel methods to present a quantitative analysis of the attributable risk of ambient temperature for mortality. They collected data for daily mortality, temperature, and other confounding variables from 13 countries, which included more than 74 million deaths recorded in 384 locations across temperate and tropical climates (roughly a third of locations were in the USA). The investigators analysed the data with a time series quasi-Poisson model to obtain the cumulative exposure–response association of temperature and mortality for each location. The total effect of a given day's temperature on the death rate was defined as the sum of the effect on that day and 21 subsequent days. These associations were adjusted for average temperature, range of temperature, and country indicator; the minimum mortality temperature—or optimum temperature—and the fraction of deaths attributable to non-optimum temperature were calculated for each country. The effect on mortality was divided into cold versus heat, and moderate versus extreme. Gasparrini and colleagues4Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; (published online May 21.)http://dx.doi.org/10.1016/S0140-6736(14)62114-0PubMed Google Scholar report that, first, cold-related deaths outnumbered heat-related deaths by a factor of nearly 20, overall. Second, deaths attributable to moderately non-optimum temperatures substantially outnumbered those attributable to extreme temperatures; and third, the minimum mortality temperature was usually well above the median temperature. In the study, most attributable deaths were associated with cold (7·29%, 95% empirical CI 7·02–7·49), while only 0·42% (0·39–0·44) of deaths were attributable to heat, a large disparity between the effects of high and low temperatures compared with previous studies.5Anderson BG Bell ML Weather-related mortality: how heat, cold, and heat waves affect mortality in the United States.Epidemiology. 2009; 20: 205-213Crossref PubMed Scopus (878) Google Scholar, 6Guo Y Barnett AG Pan X Yu W Tong S The impact of temperature on mortality in Tianjin, China: a case-crossover design with a distributed lag non-linear model.Environ Health Perspect. 2011; 119: 1719-1725Crossref PubMed Scopus (355) Google Scholar The investigators mention that their results could be extended to projections of temperature-related mortality in climate-change scenarios. In 2014, the Intergovernmental Panel on Climate Change7IPCCClimate Change 2014: synthesis report: contribution of working groups I, II and III to the fifth assessment report of the Intergovernmental Panel on Climate Change.in: Core Writing Team Pachauri RK Meyer LA Intergovernmental Panel on Climate Change, Geneva2014: 151Google Scholar reported that, although climate change will lead to an increase in heat-related mortality and a decrease in cold-related mortality in some parts of the world, the current health burden of climate change was insufficiently known and the magnitude of such effects had not been addressed globally. The high proportion of deaths attributable to cold rather than heat seems to offer an alternative perspective on global warming and human health. However, if—as the data seem to show—extreme cold is relatively unimportant, then a few degrees of warming will not yield a large reduction in cold-related mortality. Moreover, if extreme heat is important, then the same few additional degrees might cause a substantial increase in heat-related mortality.8Staddon PL Montgomery HE Depledge MH Climate warming will not decrease winter mortality.Nat Clim Chang. 2014; 4: 190-194Crossref Scopus (43) Google Scholar, 9Bennett CM Dear KBG McMichael AJ Shifts in the seasonal distribution of deaths in Australia, 1968–2007.Int J Biometeorol. 2014; 58: 835-842Crossref PubMed Scopus (22) Google Scholar Gasparrini and colleagues4Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; (published online May 21.)http://dx.doi.org/10.1016/S0140-6736(14)62114-0PubMed Google Scholar also report that moderately non-optimum temperatures accounted for nearly 7% of the overall mortality, while extreme temperatures accounted for less than 1%, which suggests that efforts related to adaptation to temperature risk should focus on moderate non-optimum conditions instead of record-breaking weather. However, this finding applies more to cold than to heat, and, crucially, depends on the definition of extreme. The definition used by Gasparrini and colleagues for extreme cold was daily average temperature less than the 2·5th percentile for each location, while they used the 97·5th percentile to define extreme heat. Because the minimum mortality temperature tends to be near the 80th or even 90th percentile, extreme heat occurs on a large proportion—possibly as many as a quarter—of days that have temperatures higher than the minimum mortality temperature, while extreme cold occurs on a much smaller fraction of cool days—only about one in 30. Moreover, the mortality–temperature response functions tend to become increasingly steep at high temperatures, but become less steep at low temperatures in most places, with London, UK, and Madrid, Spain, being notable exceptions. Overall, deaths attributable to extreme heat are roughly as frequent as those attributable to moderate heat, while those attributable to extreme cold are negligible compared with those caused by moderate cold. To address the different risks for heat and cold and to develop adaptation strategies according to local conditions, it would be more helpful to treat the two extremes separately. The reported results4Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; (published online May 21.)http://dx.doi.org/10.1016/S0140-6736(14)62114-0PubMed Google Scholar have remarkably narrow confidence intervals: eg, the overall mortality attributed to non-optimum temperature was reported to be 7·71% (95% empirical CI 7·43–7·91). Although such narrow confidence intervals reflect the very large dataset used, it is worth considering what random process gave rise to this minimum uncertainty, and whether other random inputs that would increase it have been ignored. In fact, empirical CIs depend on the structure of the available data, and so show only the random (Poisson-distributed) occurrence of deaths in these locations over the measured time spans. As the investigators explain, their results are not globally representative, so should be interpreted with caution when applied to other regions that were not included in the database, especially by policy makers. How representative the results are of temperature-attributable mortality worldwide would be interesting to know. The CIs would certainly be much wider if the analysis took into account the process of sampling countries, but the main results might still be robust. Other limitations exist, and further work is needed. As Gasparrini and colleagues4Gasparrini A Guo Y Hashizume M et al.Mortality risk attributable to high and low ambient temperature: a multicountry observational study.Lancet. 2015; (published online May 21.)http://dx.doi.org/10.1016/S0140-6736(14)62114-0PubMed Google Scholar admit, factors such as susceptibility or resilience have not been included in the analysis, including socioeconomic status, age, and confounding air pollutants.10Dear K Ranmuthugala G Kjellström T Skinner C Hanigan I Effects of temperature and ozone on daily mortality during the August 2003 heat wave in France.Arch Environ Occup Health. 2005; 60: 205-212Crossref PubMed Scopus (62) Google Scholar For example, although the authors used each country as a proxy to control for regional characteristics in the meta-analysis, the reported total attributable mortality fraction of each country is still positively associated with the proportion of the population aged 60 years or older in 2013.11United NationsDepartment of Economic and Social AffairsPopulation DivisionWorld Population Ageing 2013. United Nations, New York2013Crossref Google Scholar China and Sweden are exceptions to the trend, possibly because of their extreme population and development situations. Since high or low temperatures affect susceptible groups such as unwell, young, and elderly people the most,12McGeehin MA Mirabelli M The potential impacts of climate variability and change on temperature-related morbidity and mortality in the United States.Environ Health Perspect. 2001; 109: 185-189Crossref PubMed Scopus (536) Google Scholar, 13Basu R High ambient temperature and mortality: a review of epidemiologic studies from 2001 to 2008.Environ Health. 2009; 8: 40Crossref PubMed Scopus (766) Google Scholar, 14Yu W Mengersen K Wang X et al.Daily average temperature and mortality among the elderly: a meta-analysis and systematic review of epidemiological evidence.Int J Biometeorol. 2012; 56: 569-581Crossref PubMed Scopus (141) Google Scholar attempts to mitigate the risk associated with temperature would benefit from in-depth studies of the interaction between attributable mortality and socioeconomic factors, to avoid adverse policy outcomes and achieve effective adaptation. Nonetheless, despite these limitations, the methods and results of Gasparrini and colleagues' study set a benchmark and starting point for future research. We declare no competing interests. Mortality risk attributable to high and low ambient temperature: a multicountry observational studyMost of the temperature-related mortality burden was attributable to the contribution of cold. The effect of days of extreme temperature was substantially less than that attributable to milder but non-optimum weather. This evidence has important implications for the planning of public-health interventions to minimise the health consequences of adverse temperatures, and for predictions of future effect in climate-change scenarios. Full-Text PDF Open Access