Short-term exposure to both cold and hot temperatures may increase the risk of hospitalisation for heart failure, according to a study published in JACC, the flagship journal of the American College of Cardiology, and presented simultaneously at ESC Congress 2026. The findings from Sweden highlight how temperature extremes may contribute to sudden worsening of heart failure and underscore the need for health systems to prepare for environmental stressors.

Researchers said the study addresses an important gap in understanding how extreme temperatures affect people with heart failure in Nordic countries. Sweden and other high-latitude regions experience frequent cold weather. Still, they are also facing increasing exposure to heat as the climate warms, potentially creating new health risks in populations less accustomed to high temperatures.

“To the best of our knowledge, no previous study has examined the short-term associations between extreme temperature events or continuous non-optimal ambient temperatures and heart failure hospitalisations within a Nordic population,” said lead author Wenli Ni, PhD, a postdoctoral research fellow at the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health.

The researchers analysed approximately 482,000 heart failure hospitalisations recorded in the Swedish National Patient Register between 2006 and 2021. Cold spells were defined as at least two consecutive days with daily temperatures at or below the fifth percentile between October and March. Heat waves were defined as temperatures at or above the 95th percentile between April and September, based on municipality-specific temperature distributions.

Cold spells were associated with an increased risk of heart failure hospitalisation two to six days after exposure, with the association reaching statistical significance between three and five days. Exposure to lower ambient temperatures was similarly linked to increased hospitalisation risk within three to six days, with the strongest evidence appearing three to five days after exposure.

Higher ambient temperatures showed a different pattern. Exposure to hotter temperatures was associated with an increased risk of heart failure hospitalisation within zero to three days, suggesting that heat may have a more immediate effect. However, the researchers found no significant association between formally defined heat waves and heart failure hospitalisation.

The different timing of cold- and heat-related risks could have implications for how health systems respond to weather conditions. The authors said the delayed increase in hospitalisations following cold exposure may support closer monitoring of vulnerable patients in the days after cold-weather alerts. At the same time, the more immediate effects associated with higher temperatures could require faster responses during periods of hot weather.

In an accompanying editorial, Tiantian Li, PhD, deputy director of the National Institute of Environmental Health at the Chinese Center for Disease Control and Prevention in Beijing, argued that growing evidence on temperature-related cardiovascular risks should be translated into practical prevention strategies. “We can no longer practice cardiovascular medicine in a meteorological vacuum,” Li said, calling for stronger early-warning systems that can turn epidemiological evidence into timely action to protect people at cardiovascular risk.

More information: Wenli Ni et al, Cold Spells, Heat Waves, and Nonoptimal Air Temperature and Risk of Heart Failure Hospitalization in Sweden, Journal of the American College of Cardiology. DOI: 10.1016/j.jacc.2026.07.008

Journal information: Journal of the American College of Cardiology Provided by American College of Cardiology