As more Americans don face masks to venture outside during the COVID-19 pandemic, many of those who wear glasses are finding that their lenses fog up. It's a problem that bespectacled surgeons, as well as goggle-wearing skiers, have long experienced.
Why does it happen? In a 1996 article in Ophthalmic & Physiological Optics, Tom Margrain, a professor at Cardiff University's School of Optometry and Vision Sciences, explained that in general “when a spectacle wearer enters a warm environment after having been in a cooler one, his/her spectacles may ‘mist up’ due to the formation of condensation on the lens surface.” He went on to say that polycarbonate lenses demisted more rapidly than those made of glass.
With that in mind, if your eyeglasses are fogging when you put on a face mask, it's because warm, moist air you exhale is being directed up to your glasses. To stop the fogging, you need to block your breath from reaching the surfaces of your lenses.