London School of Hygiene & Tropical Medicine

The findings, published in The Lancet, also reveal that deaths due to moderately hot or cold weather substantially exceed those resulting from extreme heat waves or cold spells.

Lead author, Dr Antonio Gasparrini, Senior Lecturer in Biostatistics and Epidemiology at the London School of Hygiene & Tropical Medicine, said: "It's often assumed that extreme weather causes the majority of deaths, with most previous research focusing on the effects of extreme heat waves. Our findings, from an analysis of the largest dataset of temperature-related deaths ever collected, show that the majority of these deaths actually happen on moderately hot and cold days, with most deaths caused by moderately cold temperatures."

The study analysed over 74 million deaths between 1985 and 2012 in 13 countries with a wide range of climates, from cold to subtropical. The countries involved were Australia, Brazil, Canada, China, Italy, Japan, South Korea, Spain, Sweden, Taiwan, Thailand, UK, and USA. Data on daily average temperature, death rates, and confounding variables (eg, humidity and air pollution) were used to calculate the temperature of minimum mortality (the optimal temperature), and to quantify total deaths due to non-optimal ambient temperature in each location. The researchers then estimated the relative contributions of heat and cold, from moderate to extreme temperatures.

Around 7.71% of all deaths were caused by non-optimal temperatures, with substantial differences between countries, ranging from around 3% in Thailand, Brazil, and Sweden to about 11% in China, Italy, and Japan. Cold was responsible for the majority of these deaths (7.29% of all deaths), while just 0.42% of all deaths were attributable to heat.

The study also found that extreme temperatures were responsible for less than 1% of all deaths, while mildly sub-optimal temperatures accounted for around 7% of all deaths-with most (6.66% of all deaths) related to moderate cold.

Dr Gasparrini added: "Current public-health policies focus almost exclusively on minimising the health consequences of heat waves. Our findings suggest that these measures need to be refocused and extended to take account of a whole range of effects associated with temperature.

"In particular, adapting to this context the concept of 'population strategy' to public health, popularised by the late Prof Geoffrey Rose here at London School of Hygiene & Tropical Medicine, this study suggests that structural interventions for mitigating the risks across days of non-optimal temperature may be more effective than targeting high-risk days of extreme weather".

The study was funded by UK Medical Research Council.

Publication

• Antonio Gasparrini, Yuming Guo, Masahiro Hashizume, Eric Lavigne, Antonella Zanobetti, Joel Schwartz, Aurelio Tobias, Shilu Tong, Joacim Rocklöv, Bertil Forsberg, Michela Leone, Manuela De Sario, Michelle L Bell, Yue-Liang Leon Guo, Chang-fu Wu, Haidong Kan, Seung-Muk Yi, Micheline de Sousa Zanotti Stagliorio Coelho, Paulo Hilario Nascimento Saldiva, Yasushi Honda, Ho Kim, Ben Armstrong. Mortality risk attributable to high and low ambient temperature: a multicountry observational study. The Lancet. DOI: 10.1016/S0140-6736(14)62114-0