In 2003, while French youth protested American imperialism, 14,000 mostly elderly people were allowed to die in a heat wave. Heat waves kill a lot more people, it is believed, except they don't. Instead, an analysis of  74 million deaths in 384 locations across Australia, Brazil, Canada, China, Italy, Japan, South Korea, Spain, Sweden, Taiwan, Thailand, UK, and the USA found that cold weather kills about 20 times as many people as hot weather.

The study analyzed 74,225,200 deaths between 1985 and 2012 in those countries and their wide range of climates, from cold to subtropical. Data on daily average temperature, death rates, and confounding variables (such as 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.

"It's often assumed that extreme weather causes the majority of deaths, with most previous research focusing on the effects of extreme heat waves," says lead author Dr Antonio Gasparrini from the London School of Hygiene&Tropical Medicine. "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.

"Current public-health policies focus almost exclusively on minimizing 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."

Writing in a linked Comment, Keith Dear and Zhan Wang from Duke Kunshan University, Kunshan, Jiangsu, China say, "Factors such as susceptibility or resilience have not been included in the analysis, including socioeconomic status, age, and confounding air pollutants...Since high or low temperatures affect susceptible groups such as unwell, young, and elderly people the most, 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."

This study was funded by UK Medical Research Council.