As the mercury in some U.S. cities hits blistering highs, a new study involving Emory University finds that more than five million deaths each year can be attributed to hot and cold temperatures around the world.

The multi-institution study, published in Lancet Planetary Health, is the first to look at how temperature affects mortality across the world over 20 years: from 2000 to 2019, a period when global temperatures rose significantly.

Researchers found that while heat-related deaths increased in all regions, it was exposure to cold that caused most deaths during the study period. In the U.S., temperature changes were responsible for the deaths of more than 173,000 people each year, according to the study.

The findings come on the heels of another largescale study which concluded that human-induced climate change was responsible for a third of all heat-related deaths.

Emory’s Rollins School of Public Health assistant professor Noah Scovronick, PhD, who contributed to both studies, says the Lancet paper is unique for a few reasons:

• It looks at the health impact of temperature exposure for the entire year rather than just summers
• It considers the death toll from cold temperatures
• It provides estimates for the whole world, rather than just the countries for which data is available 

Scovronick says the finding that close to 10 percent of all global deaths are because of cold and hot temperatures is significant because it “more than doubles current estimates and cements temperature as a top 10 risk factor for mortality.” 

Of the global deaths attributed to outdoor temperatures, the study found:

• More than half occurred in Asia, particularly in East and South Asia
• Europe had the highest death rates due to heat exposure
• Sub-Saharan Africa had the highest death rates due to exposure to cold 

Researchers also found that cold-related death rates declined during the study period while heat-related deaths increased, a trend they expect will continue as the world gets warmer each year. The 10 warmest years on record have all occurred since 2005, and the five hottest years occurred in the second half of the last decade. 

The study, led by Australia-based Monash University, used data from 43 countries across five continents to produce their estimates.