Colder weather kills more people than hot spells, a probe into a key issue of public-health policy said on Thursday.

Researchers looked into 74 million deaths between 1985 and 2012 in 13 countries where there was a wide variety of climate, from chilly to subtropical.

They matched these against data on temperatures, the average death rates and factors such as humidity and air pollution, which can affect mortality.

A total of 7.7 percent of the deaths were related to the impact of "sub-optimal" temperatures, but almost all of that number -- 7.3 percent -- were attributable to the cold. The remaining 0.4 percent were tied to heat.

Extreme temperatures, as opposed to those that were moderately above or below average, accounted for less than one percent of all deaths.

"It’s often assumed that extreme weather causes the majority of deaths, with most previous research focusing on the effects of extreme heatwaves," said lead author Antonio Gasparrini from the London School of Hygiene and 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."

In its landmark Fifth Assessment Report, the UN's Intergovernmental Panel on Climate Change (IPCC) last year said it was "likely" that heatwaves had already become more frequent in Europe, Asia and Australia.

The IPCC said it was "very likely" heatwaves would become more frequent and last longer in the future, with big implications for health, businesses and urban design.

The study is published by The Lancet.