Several weeks after some health experts recommended that Americans wear two layers of masks to help reduce the spread of coronavirus, the Centers for Disease Control and Prevention has officially addressed the strategy.

Their verdict? Double up and tighten up.

Unlike medical-grade N95 masks, which are used by health care professionals and are typically tested for a proper seal to the face, homemade cloth masks allow air to both enter and exit through the sides or around the nose. So do surgical-style masks. The solution, according to the CDC, is to cover a surgical mask with a cloth mask to improve the fit. If the cloth mask is open on the sides, crossing the ear loops and then tucking in the corner—which the CDC has dubbed “knotting and tucking”—can often improve how snugly it conforms to your face.

The guidance comes in part from research conducted by Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco, and colleague Dr. Lindsey Marr of Virginia Tech. The CDC’s own study of simulated aerosol transmission found that a single surgical mask blocked 42 percent of particles and a cloth mask blocked 44.3 percent. The two worn together, with the cloth over the surgical mask, blocked 92.5 percent. Wearing a single surgical mask and tucking the corners was also observed to be effective, but only when both parties were wearing them. When two people observed either of these protocols, the risk of transmission was reduced by 96.4 percent (for two layers of mask) or 95.9 percent (for the single knotted surgical mask).

While the CDC presents double-masking or a single “tucked” surgical mask as largely interchangeable, it’s important to note that a two-layered mask appears to be significantly more effective overall. In the CDC study, a double-masking cover reduced the cumulative exposure risk for an unmasked person nearby by 82.2 percent, compared to just 62.9 percent for a surgical mask alone. A person wearing a two-layer mask in proximity to an unmasked person had their cumulative exposure reduced by 83 percent. Wearing a single surgical mask offered just a 64.5 percent reduction.

The bottom line? A two-layer mask is superior. A surgical mask may be better if you find two layers uncomfortable or if they obstruct your peripheral vision. People with larger faces, however, may find a tucked surgical mask too small. It’s also important to remember that the surgical mask should be labeled “medical grade” or “surgical grade” to ensure it’s made of polypropylene, a superior material for protection against infectious particles.

Dr. Gandhi previously explained to Mental Floss that the two kinds of masks work differently. “The surgical mask made of polypropylene electrostatically repulses the virus, whereas the cotton mask fibers physically block it,” Gandhi said. “The fit is improved by tamping down the masks with two layers.”

In all scenarios, the guidance is intended to improve the fit of a mask, which leads to decreased opportunities for transmission. Outside of a laboratory setting, different fabrics and situations could impact results. But the principle remains—a snug two-layer mask (one polypropylene, one cotton) that conforms to the face and doesn’t impede breathing is the most practical way of slowing the spread of coronavirus when an N95 mask is unavailable.

[h/t The New York Times]