There's something gentle and peaceful about a snowfall — that is, until it comes to shovelling the stuff, at which point it can become deadly.

The latest evidence of this is the recent storm in the northern U.S., which has led to eight deaths, several of which can be attributed to heart attacks sustained while shovelling snow, officials say.

Digging yourself out after a large snowfall can be "a shock to the system," says Matthew Mayer, a senior specialist of research at the Heart and Stroke Foundation of Canada.

"We know that most Canadians are inactive and in most cases unfit, and when we get these big snowfalls and go out shovelling, sometimes, the literal physical stress we put on our body is just too much."

Mayer says there's no clear research on what effect the cold has on the heart, but he says one of the theories is that the lower temperature "constricts the body and the blood flow, slowing it down, increasing the blood pressure and possibly giving rise to more [blood] clot formation."

This, coupled with the sudden physical exertion, can lead to heart attacks.

Restricted oxygen supply

The people most at risk are those who don't exercise, and for whom "snow-shovelling may represent the most strenuous activity they do in the year," says Dr. Neil Fam, a cardiologist in Toronto.

When a person starts vigorously shovelling snow, the blood flow to the heart becomes very fast. This may not cause problems for someone who is fit, but for someone who is unused to physical activity, the rush of blood can lead to chest pain, or angina, "because the heart muscle isn't getting enough oxygen supply," Dr. Fam says.

The individual may have plaque in their arteries that they don't know about. The rush of blood can cause some of this plaque to rupture, which can produce blood clots that squeeze the oxygen supply and consequently lead to a heart attack, Fam says.

Although snow-shovelling deaths are often associated with the elderly, age isn't as much as a factor as overall fitness, says Fam, who works at the Toronto Heart Centre.

"We have some patients who are very, very healthy into their 80s who still do everything for themselves," he says, adding "there are patients in their 30s and 40s who shovel snow and then have a heart attack."

While it might seem odd, shovelling snow requires a warm-up just like any other physical exertion, such as playing recreational hockey, says Mayer.

He suggests something as simple as taking a short walk beforehand to get the blood flowing.

"I often think a really good saying is, 'We should be getting fit to do these activities, not do these activities to get fit.'"