The Centers for Disease Control and Prevention issued new guidelines Wednesday urging people to “double mask” and to properly fit masks to slow the spread of the coronavirus, noting that masking has been shown to be infective in cutting down transmission.

“Cloth masks and medical procedure masks fit more loosely than do respirators (e.g., N95 facepieces). The effectiveness of cloth and medical procedure masks can be improved by ensuring that they are well fitted to the contours of the face to prevent leakage of air around the masks’ edges,” the notice states.

The announcement was based on published papers and experiments done by the CDC on the fit, effectiveness and composition of masks commonly worn by members of the public.  The CDC notes that 17 states and the District of Columbia have mandatory mask requirements, as do federal grounds.

“During January 2021, CDC conducted experimental simulations using pliable elastomeric source and receiver headforms to assess the extent to which two modifications to medical procedure masks, 1) wearing a cloth mask over a medical procedure mask (double masking) and 2) knotting the ear loops of a medical procedure mask where they attach to the mask’s edges and then tucking in and flattening the extra material close to the face (knotted and tucked masks), could improve the fit of these masks and reduce the receiver’s exposure to an aerosol of simulated respiratory droplet particles of the size considered most important for transmitting SARS-CoV-2,” the CDC notes as to best ways to utilize masks.