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.
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.