Major types of masks: N95/KN95 respirators, surgical face masks, and cotton face masks.
N95/KN95 respirators can filter 95% of particles with an aerodynamic diameter greater than or equal to 0.3 μm, and block viruses. They can help prevent airborne diseases.
Disposable surgical face masks have 3 layers. The outer layer is hydrophobic non-woven layer that prevents droplets from entering the mask; the middle layer has a filter to block 90% of particles with a diameter greater than 5μm; and the inner layer in contact with the nose and mouth absorbs moisture. They are typically for sterile medical operations and be used to prevent airborne diseases.
Cotton face masks are heavy, stuffy, and do not fit closely to the face, and thus not effective against viruses
