In combination with hand washing and social distancing measures, it is important to wear masks to prevent the spread of COVID-19 because the virus spreads primarily through respiratory droplets released when people speak, cough, sneeze or sing. Some infected people may not appear ill or may develop symptoms at a later time, but they can still spread the virus without symptoms. Masks prevent people from spreading the virus to others, especially when we can't be 6 ft / 2m apart. A recent lab study conducted by the U.S. Centers for Disease Control and Prevention found that by wearing two masks, people's protection against virus in the air (also called aerosolized particles) was dramatically increased. The study demonstrated that wearing any kind of mask provides significantly more protection against infectious aerosols than not wearing a mask. Additionally, when dummies who wore two masks - like cloth face masks over surgical masks - were exposed to infectious aerosols, their level of protection was roughly 92%. (The group now recommends fitting a cloth mask over a medical procedure mask, and knotting the ear loops of a medical procedure mask and then tucking in and flattening the extra material close to the face. However, the U.S. CDC does not recommend wearing two disposable masks at one time or another mask on top of a KN95 or N95 mask.)

Cloth masks primarily protect people in close proximity to the person wearing the mask by trapping droplets that are released from the wearer. Cloth masks can also protect the wearer by filtering out some virus particles trying to enter. Medical masks (N95 or FFP2) filter virus particles from entering more effectively than cloth masks and are designed to protect the wearer. Supplies of those are limited and generally reserved for doctors.

Masks are a type of personal protective equipment (PPE), along with face shields, protective eye goggles, gloves, gowns, and other equipment. Proper use of PPE can help reduce the spread of COVID-19. Wearing PPE is a risk reduction measure, not a risk avoidance measure, so masks can help lower (but not completely eliminate) the chances of getting infected or infecting someone else. Maintaining hygiene, physical distancing, and other measures are still recommended.