The heart attack danger is a perfect storm of two factors that do not normally occur together:

1) Extreme physical exertion - sudden, intense arm-heavy exercise often as a Valsalva maneuver (straining and holding the breath). This dramatically increases blood pressure, which puts acute stress on the heart and arteries.

2) Exposure to cold air exacerbating the strain on the heart with vasoconstriction (blood vessel constriction). Cold temperatures cause peripheral blood vessels (arteries and veins) to narrow. This forces the heart to work harder to pump blood through the constricted vessels to keep the body warm, leading to an increase in blood pressure. The combination of constricted vessels and high physical exertion means the heart needs significantly more oxygen to function, but the cold and high pressure can limit the blood and oxygen flow (myocardial oxygen demand is increased). Breathing cold air can also trigger constriction in the coronary arteries that supply the heart muscle itself, further increasing the risk of reduced blood flow and a heart attack.

Snow can be very heavy depending on the water content. So sometimes it's really light and basically effortless to shovel, but sometimes each shovel full of snow is 10-20 pounds of weight that you have to throw over and over. That gets taxing very quickly, plus people don't generally warm up before doing this intense exercise.

It's not like shoveling snow is super dangerous. Most people don't die from shoveling the snow. But it can happen and it's worth slowing down and taking breaks.

Physically it is very taxing. Snow is heavy, and the movements aren’t typical of daily activity. Even for a modestly sized property it can take awhile.

I think it’s a combination of incredible weight, lots of aerobic activity, and the cold which masks some of the fatigue that might tell you to take a break. I am over 40, and over an inch or two just pay someone to deal with the snow.