Heavy snowstorms are linked to a higher risk of being admitted to hospital for a heart attack, especially in men, a large Quebec study suggests.

Shovelling snow has long been suspected of straining the heart, partly because it combines sudden aerobic activity and a workout for the muscles during cold weather that can boost blood pressure and increase the risk of clots.

Now researchers have analyzed data on hospital admissions and deaths from heart attack (myocardial infarction) in Quebec from 1981 to 2014 to see whether the amounts of snowfall and duration were linked with risk of heart problems.

Over the study period, there were more than 65,000 deaths from heart attack and more than 128,000 hospital admissions during November to April when there were snowfalls in the province.

Dr. Natalie Auger of the University of Montreal Hospital Research Centre and her co-authors combined Environment Canada and hospital data to match each day when a patient had a heart attack with the days around when it occurred. That way, patients acted as their own controls.

snowfall St. John's shovelling out winter weather

Men with significant risk factors for heart disease may want to opt out of digging out after a major snowfall. (Peter Cowan/CBC)

Auger said they think shovelling is behind the association between snowfalls and heart attacks, although there's no way to be sure.

"Shovelling snow, particularly when wet and heavy, is really quite a physically demanding," study co-author Dr. Brian Potter, an interventional cardiologist at the University of Montreal Hospital Research Centre, said in a journal podcast.

A heavy snowfall of 20 cm was associated with a 34 per cent relative increased risk of death in men compared with no snowfall, the researchers said in Monday's issue of the Canadian Medical Association Journal.

"To have the conclusion that women are not at risk for snow shovelling and that we would actually say that all the men should stay home and not shovel and have the women go out would actually be a dangerous conclusion," Potter added.

The association was stronger when the flakes flew for two to three days.

In the study, snowfall was more important than temperature, Auger said.

When to avoid shovelling

People should be aware of their risk factors and physical capacity, Potter suggested.

At a policy level, Auger said municipal governments may want to consider the findings when reviewing how and when to clear snow, such as not blocking driveways.

Dr. David Alter of the Toronto Rehabilitation Institute is a cardiologist who wrote a journal commentary that accompanies the study.

"Perhaps it would be wise for men over 50 years of age who have cardiovascular risk factors or established heart disease and who are physically unfit or habitually sedentary to avoid the activity," Alter wrote.

The researchers didn't have data on the size of areas shovelled or whether snow was removed manually or with a snowblower.

Alter said the study's authors also had no information on patients' symptoms or other behaviours before, during or after a snowfall. Heart attacks that were a complication or trauma, such as car collisions, may have been misclassified.

Heavy snowfalls may have also made some people less likely to seek urgent medical care, which has been raised in previous studies

Paramedics advise shovelling sooner rather than later when possible since fresh snow is lighter. If you feel any pain, stop and take a break. Call an ambulance if you feel pressure or squeezing in the chest, shortness of breath, sweating, nausea and feeling faint.

The study was funded by the Canadian Institutes of Health Research and Quebec's health research foundation.