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A multi-institutional research effort led by the Center for Climate, Health and the Global Environment at Harvard TH Chan School of Public Health reports that short-term exposure to low and high ambient temperatures was associated with increasing cardiovascular mortality in Swedish heart failure patients, with heat-related risk strengthening in 2014-2021.
Climate change has increased the frequency and intensity of extreme weather events, with direct consequences on human health and mortality. Previous studies have identified low and high temperatures as key contributors to cardiovascular mortality.
In the study “Short-term exposure to low and high temperatures and mortality in patients with heart failure in Sweden,” published in JAMA CardiologyResearchers conducted a nationwide time-stratified case-crossover study to investigate the associations between short-term exposure to low and high ambient temperatures and all-cause and cardiovascular mortality in Swedish patients with heart failure.
A total of 250,640 heart failure patients who died from any cause between 2006 and 2021 were identified from the Swedish National Patient Register and Cause of Death Register.
The researchers mapped the daily outdoor temperature and air pollution in each participant’s residential area on a 1 × 1 km grid using a machine learning model. Local climate “low” and “high” temperatures were taken at the 2.5th and 97.5th percentiles for each municipality.
Temperature on each day of death was compared to controls surrounding death. The effects of the day of death and the previous six days were analyzed together. A statistical model estimated the risk over this seven-day window, using the temperature linked to the lowest mortality as a reference point. The researchers then estimated the proportion of deaths attributed to low and high temperatures.
Short-term temperature exposure showed a U-shaped relationship with mortality in heart failure patients in Sweden. Mortality risk increased in both cold and hot temperature extremes, with greater effects at lower temperatures.
Between 2006 and 2021, low temperatures increased all-cause mortality by 13% and cardiovascular mortality by 16%. High temperatures increased all-cause mortality by 5% and cardiovascular mortality by about 8% in the later years of the study.
Heat-related risk was higher between 2014 and 2021 than between 2006 and 2013. Men, diabetic patients and those using diuretics were more vulnerable to cold. Patients with atrial fibrillation or flutter and those exposed to higher ozone levels were at higher risk of mortality during heat.
The authors concluded that short-term exposure to low and high temperatures was associated with increased all-cause and cardiovascular mortality in Swedish heart failure patients, with heat-associated mortality risk appearing to increase over time.
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More information:
Wenli Ni et al, Short-term exposure to low and high temperatures and mortality in patients with heart failure in Sweden, JAMA Cardiology (2025). DOI: 10.1001/jamacardio.2025.3932
Sanjay Rajagopalan et al, Heart failure and suboptimal temperatures, JAMA Cardiology (2025). DOI: 10.1001/jamacardio.2025.3939
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