Paula Cannon, PhD, professor of molecular microbiology and immunology the Keck School of Medicine of USC, says her favorite type of mask is a KN95, which she started wearing last March when there was an N95 shortage.

KN95 masks are typically manufactured in China and are made from a rigid material. "When you put them on your face, it looks like a beak," Cannon says. "Those are good because they're a good quality filtering material and they fit snugly around people's faces. They're rated (at least in China) to prevent 95 percent of particles from coming through."

The KF94 is a little different. This type of mask is produced in South Korea and filters 94 percent of particles. It's long and wide, with wings covering the nose and chin, so there's a good seal around your face.

However, Cannon notes that no mask is 100 percent effective—even the gold standard N95 masks are named "N95" because in tests, they exclude 95 percent of particles.

If there's a pocket for a filter in your cloth mask, that means it has multiple layers, and that's a good thing."Common sense tells us that having three layers of fabric is better than two," Cannon says. "The more [layers] the merrier."

However, it's important to ask yourself: Does your mask move around on your face after you add the filter? Does the fit change at all? If there are any gaps around your mask, it doesn't matter how many layers you have on the inside.