A multi-institution research effort led by Harvard T. H. Chan School of Public Health’s Center for Climate, Health, and the Global Environment reports that short-term exposure to both low and high ambient temperatures was associated with increasing cardiovascular mortality among Swedish patients with heart failure, with heat-related risk strengthening in 2014–2021.
Climate change has increased the frequency and intensity of extreme weather events with direct implications for 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 Among Patients With Heart Failure in Sweden,” published in JAMA Cardiology, researchers conducted a nationwide, time-stratified case-crossover study to investigate associations between short-term exposure to low and high ambient temperatures and all-cause and cardiovascular mortality among Swedish patients with heart failure.
A total of 250,640 patients with heart failure who died from any cause between 2006 and 2021 were identified from the Swedish National Patient Register and the Cause of Death Register.
Researchers mapped daily outdoor temperature and air pollution to each participant’s home area on a 1 × 1 km grid using a machine-learning model. Local climate “low” and “high” temperatures were taken from the 2.5th and 97.5th percentiles for each municipality.
Each death day temperature was compared with controls surrounding the death. Effects from the day of death and the six preceding days were analyzed together. A statistical model estimated risk across this seven-day window, using the temperature linked to the lowest mortality as the reference point. Researchers then estimated the proportion of deaths attributed to low and high temperatures.
Short-term temperature exposure showed a U-shaped relationship with mortality among patients with heart failure in Sweden. Mortality risk rose at both cold and hot extremes, with stronger effects at low temperatures.
Across 2006–2021, 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 around 8% in the later years of the study.
Heat-related risk was greater during 2014–2021 than in 2006–2013. Men, patients with diabetes, and those using diuretics were more vulnerable to cold. Patients with atrial fibrillation or flutter and those exposed to higher ozone levels faced greater mortality risk during heat.
The authors conclude that short-term exposure to both low and high temperatures was associated with increased all-cause and cardiovascular mortality in Swedish patients with heart failure, 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 Among Patients With Heart Failure in Sweden, JAMA Cardiology (2025). DOI: 10.1001/jamacardio.2025.3932
Sanjay Rajagopalan et al, Heart Failure and Nonoptimal Temperatures, JAMA Cardiology (2025). DOI: 10.1001/jamacardio.2025.3939
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