Ah yes, breathing hazards. Maybe when it gets down to -40 there are breathing hazards but breathing hard feels just fine to me at -15 (though I know that's uncomfortable for some people, everyone is different.) We've recently had a long discussion
over on Sam's log about running in the cold. I'm way more concerned about skin exposure (and sweat freezing when I stop) than breathing cold air... my throat and lungs may not mind the cold but I do lose a bit of my enviable shine.