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.
Best Face Mask Materials: Cotton With Chiffon
If you are making a homemade mask, a new study published in the scientific journal ACS Nano found that homemade face masks that use a combination of tightly woven cotton and polyester-spandex chiffon or silk will provide a very effective filter for the aerosol particles that spread the COVID-19 virus. Masks made with one layer of cotton and two layers of chiffon (a netlike fabric often found in evening gowns) or silk will filter out some 80 to 99 percent of particles — similar to the effectiveness of the N95 mask material — due to the electrostatic barrier of the fabric. But here’s the kicker: The mask must have a snug fit. Even a 1 percent gap reduces the filtering of all face masks by 50 percent or more.
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. (See instructions on how to make your own cloth face mask at home.)
The Annals of the Royal College of Surgeons of England published an article in 2011 that offered a simple method to prevent fogging, suggesting that, just before wearing a face mask, people wash their spectacles with soapy water, shake off the excess and then allow the lenses to air-dry.
"Washing the spectacles with soapy water leaves behind a thin surfactant film that reduces this surface tension and causes the water molecules to spread out evenly into a transparent layer,” the article reveals. “This ‘surfactant effect’ is widely utilised to prevent misting of surfaces in many everyday situations.” Antifogging solutions used for scuba masks or ski goggles also accomplish this.
Another tactic is to consider the fit of your face mask, to prevent your exhaled breath from reaching your glasses. An easy hack is to place a folded tissue between your mouth and the mask. The tissue will absorb the warm, moist air, preventing it from reaching your glasses. Also, make sure the top of your mask is tight and the bottom looser, to help direct your exhaled breath away from your eyes.
If you are using a surgical mask with ties, a 2014 article in the Annals of the Royal College of Surgeons of England advises going against your instincts. Tie the mask crisscross so that the top ties come below your ears and the bottom ties go above. It will make for a tighter fit.