In reply to NobleWanderer:
i've camped on quite a few occasions in the snow, the longest being 3 1/2 days in a small 2-man tent below Suilven, when it snowed for nearly 3 days non-stop. There was never any risk of suffocation, even though the snow built up all round the eaves of the flysheet, and much higher on one side. Occasionally one would have to bang the inside of the tent to make the snow slide off the roof, but mostly it slid off of its own accord. Of course, as well as the air vent, one had the inner doors unzipped an inch or two. Further points:
1. If there's a blizzard you are unlikely to sleep well or for very long. There's too much to attend to. If you are cooking in the doorway under the flysheet, or even inside the tent near the door the heat of the stove will cause the snow around and under the front flysheet/door/s to melt, and a lot of time will be spent creating, and attending to drainage channels so that water keeps away from the entrance. You will also be spending quite a bit of time attending to the water supply i.e putting snow into big plastic bottles and bringing them into the tent to melt. Then storing it/them just outside the inner doors.
2. You will of course have to open the main front door/s occasionally (c. once a day) when you go out for a 'bowel motion', so a lot of fresh air will get into the tent then.. This is a very long procedure involving having to get fully dressed in all one's outdoor winter gear in the awkward confines of the small tent. And then leave the tent, typically for a nearby outcrop to support yourself while in a squatting position and crapping into a bag. (That's taken with you later). Then the return to the tent - also a very long procedure of taking off one's outers in the front awning and trying to let as little snow from your clothing get int the inner tent as possible. Whole procedure takes c. half an hour.
3. Having a pee is much easier. While in your sleeping bag (which you'll be the whole time in a small tent) you will be wearing very little, perhaps some long johns and a thermal vest at very most. You then crouch at the very front of the inner tent (or even the front of your sleeping bag), unzip the inner doors, then the outer door/s and then stand up so that upper body is the blizzard, and pee in a leeward direction from the tent - then get straight back down into your sleeping bag zipping the doors shut as you do so. Whole procedure takes less than a minute! Very exciting ... Again, plenty of air gets into the tent, and probably some snow ...