DogBoy

In through the nose, out through the mouth. I've felt like someone took a chisel to my lungs once when I went xc skiing in -25F as a teen because I winded myself and took off my facemask and was inhaling cold air something fierce. Since then, I try to concentrate on taking air in through the nose. Its a longer passage to the lungs so the air is warmed just a tad more than if it comes in the mouth. I also wear a balaclava, so the air around my mouth tends to be warmed a bit more than outside air. But like Machka says, breathing is not a problem in and of its self.