A long-term study in Sweden indicates that freezing temperatures can trigger heart attacks for people already with increased health risks.

Heart attack rates peak in winter, and decline in summer – and the team from Lund University undertook the analysis of 16 years of national data to determine whether meteorological factors determined the health outcomes at a population level.

The colder range of temperatures led to an increase in heart attacks, as the Swedish scientists report at the Congress of the European Society of Cardiology today.

“Our results consistently showed a higher occurrence of heart attacks in sub-zero temperatures,” said Moman A. Mohammad, a cardiologist at Lund University. “The findings were the same across a large range of patient subgroups, and at national as well as regional levels, suggesting that air temperature is a trigger for heart attack.”

Two sets of data were cross-referenced: the Swedish myocardial infarction registry known as SWEDEHEART, and weather data from hundreds of weather stations collected in the Swedish Meteorological and Hydrological Institute.

Weather data was available for 99 percent of the 280,873 heart attacks for the years 1998 to 2013, they report.

The days were broken down in to temperature ranges: either below 0 degrees Celsius, between 1 and 10 degrees Celsius, or above 10 degrees.

The average number of heart attacks on the coldest days was higher than the warmest range; that meant approximately four more heart attacks on those days throughout the Scandinavian nation.

Heart attacks also statistically correlated to stronger winds, less sunshine, and higher humidity, they added.

All the meteorological correlations were also found to be consistent across susceptible health subgroups in the population, including the elderly, those with hypertension or diabetes, and other categories, they report.

The Swedish team hypothesizes that colder temperatures forces smaller blood vessels near the surface of the skin to contract, thereby increasing pressure in the arteries – and boosting the risk of cardiac arrest if risk factors are already present, they add.

Other studies have found additional aspects to the correlation between heart attacks and colder weather – including respiratory-tract infections, pneumonia, and influenza as putting additional strain on hearts and blood vessels. Additionally, reduced physical activity could put people with increased risk factors at further risk. Another major danger documented has been the activity of snow shoveling – which has been shown to drastically increase heart attacks, especially for men.