While snow is beginning to fall in parts of the northern hemisphere, it wasn’t too long ago that frosty mornings seemed impossible. This summer was the hottest on record for Europe and China, the second hottest for North America and Asia, and the and the fifth-hottest June-to-August period for the planet since record-keeping began in 1880, according to NOAA.

Scientists are working to better understand what extreme temperatures do to the human body. A study published today in the American Heart Association’s journal Circulation finds that exposure to extremely hot or cold temperatures increases a heart disease patient’s risk of death.

[Related: Extreme heat days have tripled in cities over the past 40 years.]

“It underscores the urgent need to develop measures that will help our society mitigate the impact of climate change on cardiovascular disease,” study co-author Haitham Khraishah, a cardiovascular disease fellow at the University of Maryland School of Medicine (UMSOM) and University of Maryland Medical Center (UMMC), said in a statement.

The team representing more than 35 international research institutions analyzed over 32 million global cardiovascular deaths over 40 years and found that more deaths occurred on days when temperatures were at their highest or lowest compared to more moderate climate days.

These findings used health data from 567 cities, in 27 countries, and on 5 continents between 1979 and 2019. The definition of “extreme weather” differed from city to city, but was defined in the study as the top 1 percent or bottom 1 percent of the “minimum mortality temperature,” or the temperature at which the lowest death rate occured.

They found that for every 1,000 cardiovascular deaths on extreme hot days (above 86° F in Baltimore) accounted for 2.2 additional deaths and extreme cold days (below 20° F in Baltimore) accounted for 9.1 additional deaths.

People with heart failure were most likely to be negatively impacted by very cold and very hot days, compared to other conditions such as stroke and arrhythmia. Those with heart failure saw a 12 percent greater risk of dying on extreme heat days compared with the optimal temperature days in a city and the risk of heart failure death increased by 37 percent during extreme cold. Additionally, there was a rise of 2.6 additional deaths on extreme hot days and 12.8 on extreme cold days for those with heart failure, compared to the other cardiovascular diseases studied.

“While we do not know the reason why temperature effects were more pronounced with heart failure patients it could be due to the progressive nature of heart failure as a disease,” said Khraishah. “One out of four people with heart failure are readmitted to the hospital within 30 days of discharge, and only 20 percent of patients with heart failure survive 10 years after diagnosis.”

Some of this particular study’s limitations include an underrepresentation of data from South Asia, the Middle East, and Africa. According to the team, it’s possible that extreme heat had more of an impact than initially measured due to this lack of data. 

[Related: The polar vortex is about to split in two. But what does that actually mean?]

The study factored humidity and air pollutants into their calculations, which also could have accounted for excess deaths in places experiencing temperature extremes. They also controlled for the lag effect, or the delayed effect of temperature on human health and climate zone.

While climate change is more associated with extreme heat, studies have found that climate change can cause extreme events on both ends of the temperature spectrum. One example is the disruption of the polar vortex, that brings frigid cold air from the arctic down to the northern hemisphere.

The team on this study expanded the heart disease mortality database within the Multi-Country Multi-City (MCC) Collaborative Research Network, a group of epidemiologists, biostatisticians, and climate scientists studying the impact of climate change and environmental stressors on death rates.

“This study provides an indisputable link between extreme temperatures and heart disease mortality from one of the largest multinational datasets ever assembled,” said Mark T. Gladwin, Dean, UMSOM, Vice President for Medical Affairs, University of Maryland, Baltimore, in a statement. “The data can be more deeply mined to learn more about the role of health disparities and genetic predispositions that make some populations more vulnerable to climate change.”

The answers to these kinds of questions will be addressed in future research, according to Khraishah.