Why do my glasses fog up under a dust mask, and what stops it?

Fogging is not a sign you fitted the mask badly. On a sealed half mask, exhaled air has to go somewhere, and the path of least resistance runs up the bridge of your nose and straight onto your lenses. Anti-fog wipes and tape treat the symptom for about ten minutes.

The mechanism that actually prevents it is airflow. In a powered hood a blower runs continuously, and air moves across the inside of the visor the whole time it is running, so warm moist air never sits still long enough to condense.

Ron McGee, who wears both glasses and a beard, put it plainly in writing: the positive airflow keeps his glasses clear. That is his experience, in his words, and we publish it with his permission.

Worth being straight about two things. First, this is a property of powered hoods generally, not something unique to one brand — if you already own a powered respirator and it fogs, the problem is more likely a blower or filter issue than the concept. Second, if you only need eye protection and not respiratory protection, a face shield and a fan is a far cheaper answer than anything on this site.