See the answers below from this New York Times piece.
The benefits of solo masking
One study from the C.D.C. found that a standard surgical mask protected the wearer from only about 7.5 percent of the particles generated by a simulated cough. But knotting the loops and tucking in the sides of the medical mask reduced exposure by nearly 65 percent. (Watch this video to see the “knot and tuck” method.) Covering the surgical mask with a cloth mask, a technique known as double masking, reduced exposure to the simulated cough particles by 83 percent.
It is true that masks work best when everyone in the room is wearing one. That’s because when an infected person wears a mask, a large percentage of their exhaled infectious particles are trapped, stopping viral spread at the source. And when fewer viral particles are floating around the room, the masks that others are wearing are more likely to block those particles that have escaped.
But there is also plenty of evidence showing that masks protect wearers even when others around them are mask-free. The amount of protection depends on the quality of the mask and how well it fits.
Given that the Delta variant is far more contagious than other variants, experts recommend wearing the highest-quality mask possible when you can’t keep your distance or aren’t outdoors — especially when nobody around you is masking up.