Originally Posted by mike

Actually, Velo, breathing can hurt and it can do damage in really cold temperatures. At around 20 F like you mentioned, it won't be too bad, but at temperatures in the teens and below, it can be really bad.

If you don't pre-heat the air with a scarf or face-mask in temperatures below 0 F, you can really be hurting if you are gulping in freezing air when running, biking, cross-country skiiing, etcetera.

It happens to me once in a foolish while. When it happens, your lungs just scream with pain - a freezing cold pain from the very center of your body which is difficult to describe with words.