As per the guidelines of the U.S. National Institute for Occupational Safety and Health, authentic N-95 masks should have at least three filters.
An N95 filtering facepiece respirator is a type of tight-fitting respirator that filters out all kinds of particles from the air, including bacteria and viruses breathed through it. These respirators filter out at least 95% of tiny (0.3 microns) particles. According to the FDA, the edges of the N95 respirator are designed to form a seal around the nose and mouth, and they are tested for fluid resistance, filtration efficiency (particulate filtration efficiency and bacterial filtration efficiency), flammability and biocompatibility. N95 mask brands have to be compliant with the N95 classification of the U.S. National Institute for Occupational Safety and Health (NIOSH).
The filters used in respirators are considered “fibrous” in nature —constructed from flat, nonwoven mats of fine fibers. In all fibrous filters, three ‘mechanical’ collection mechanisms operate to capture particles: inertial impaction, interception, and diffusion. Inertial impaction and interception are the mechanisms responsible for collecting larger particles, while diffusion is the mechanism responsible for collecting smaller particles. In some filters, an additional mechanism of electrostatic attraction also operates. This mechanism aids in the collection of both larger and smaller particle sizes. Hence authentic N95 masks have a minimum of three layers. Various N-95 variants also have five or more layers.
Non-N95 facemasks are loose-fitting and provide only barrier protection against droplets, including large respiratory particles. No fit testing or seal check is necessary with non-N95 facemasks. According to an advisory issued by the World Health Organization regarding the use of face masks in the context of COVID-19, a cloth facemask for non-medical usage should consist of three layers: an inner layer near the mouth that can get moist, a middle filtration layer and an outer layer exposed to the outside environment.
The Centers for Disease Control and Prevention (CDC) does not recommend that the general public wear N95 respirators to protect themselves from respiratory diseases, including coronavirus (COVID-19). The CDC says N95 masks are critical supplies that must continue to be reserved for healthcare workers and other medical first responders.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.