Cold temperatures the biggest culprit in climate-related deaths

The number of extreme weather events has risen globally in recent decades - an issue that is primarily attributed to global warming. But while popular notion holds that heatwaves are the greatest cause of climate-related deaths, a new study claims people are at much higher risk of death from cold spells.

The analysis found that most climate-related deaths are caused by cold temperatures.

What is more, the findings - published in The Lancet - reveal that moderately hot and cold weather conditions are much more likely to cause death that extremely hot or cold conditions.

The researchers at the London School of Hygiene & Tropical Medicine in the UK, reached their findings by analyzing more than 74 million deaths that occurred over 384 locations in 13 countries between 1985 and 2012.

The countries included in the analysis were the US, UK, Australia, Canada, Thailand, Sweden, Spain, Brazil, Italy, China, South Korea, Taiwan and Japan, covering a wide range of climates from cold to subtropical.

Data on the average daily temperature of each country was gathered, as well as data on death rates and factors that may have affected results, such as humidity and air pollution.

This information was used to calculate the optimal temperature in each location - the temperature linked to the fewest deaths - as well to calculate the total number of deaths caused by non-optimal temperatures. The researchers also estimated the number of deaths attributable to cold and hot conditions, ranging from moderate to extreme temperatures.

Cold weather kills 20 times as many people as hot weather

The results revealed that approximately 7.71% of all deaths across the 13 countries were caused by non-optimal temperatures.

Rates varied significantly between countries. While around 3% of deaths in Thailand, Brazil and Sweden were attributable to non-optimal temperatures, such temperatures were the cause of around 11% of deaths in China, Italy and Japan.

Perhaps the most interesting finding, however, was that cold temperatures were the cause of most of these deaths, accounting for 7.29% of them. Hot temperatures were the cause of only 0.42% of all deaths, meaning cold weather kills around 20 times as many people as hot weather.

What is more, the team found that less than 1% of all deaths were caused by extreme temperatures. Around 7% of all deaths were caused by moderate temperatures, according to the results, with approximately 6.6% accounted for by moderate cold.

In fact, the higher proportion of deaths attributable to moderately hot and cold temperatures may be explained by the fact that days with such temperatures are more frequent. "Although the risk of mortality due to extremely cold or hot days is actually higher, they are less frequent," he added.

"It's often assumed that extreme weather causes the majority of deaths, with most previous research focusing on the effects of extreme heatwaves.

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."

While this study challenges conventional beliefs about the health implications of temperature, the findings are not altogether surprising.

This is the first epidemiological investigation that directly compares the health burden due to heat and cold in multi-city multi-country data set, and the first one separating the associated risk between days with extreme and non-extreme temperatures. 

Although the results of this study are somewhat unexpected, they are not conflicting with evidence already available in the literature.

Furthermore, while current public health policies primarily focus on reducing the health implications of extreme heat, their results indicate a change of priority is needed. This study suggests that these interventions need to be refocused and extended to consider the whole range of effects associated with non-optimal temperature, in particular, accounting also for the health burden occurring in days with non-extreme temperatures.

In an editorial linked to the study, another team of researchers at Duke Kunshan University in China, point out that the researchers at the London School of Hygiene & Tropical Medicine in the UK, did not account for individuals' resilience or susceptibility to certain temperatures, which may have influenced the results.

"Since high or low temperatures affect susceptible groups such as unwell, young, and elderly people the most," they add, "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."

The UK Medical Research Council have provisionally awarded the researchers a grant to take this research further. Specifically, they aim at providing a better characterization of the geographical and temporal differences in temperature-health associations, and to use this information to derive more informed predictions of the health burden under scenarios of climate change.

A report from November 2014 warned that the elderly population is particularly vulnerable to heatwaves, and by the year 2100, climate change could raise the number of heatwave exposure events this population elderly individuals experience by threefold.

References:

1.  Mortality risk attributable to high and low ambient temperature: a multicountry observational study, Antonio Gasparrini, et al., The Lancet, doi: http://dx.doi.org/10.1016/S0140-6736(14)62114-0, published online 20 May 2015, abstract.
2.  The Lancet news release, accessed 20 May 2015 via AlphaGalileo.