Depends on how you're wearing it. If you wrap any thick fabric around your face or wear a balaclava and depending on how they sit on your face it can either not fog at all, or if you have a small nose like me fog heavily, but there are a few ways to avoid fogging if it does. 1. Buy a set of ventilated M Frame lenses or 2. become a mouth breather for the duration of use, but ONLY if you have heavy fabric covering your nose and mouth. If you're wearing a mesh mask, or plastic one, or one of those balaclavas that have a nose vent, you'll be fine.