The CDC has released data that found double-layering face masks or wearing tightly-fitting masks were more effective at preventing COVID-19 than single, loose-fitting masks alone.

Figure 1. Masks tested, including A, unknotted medical procedure mask; B, double mask (cloth mask covering medical procedure mask); and C, knotted/tucked medical procedure mask. Source: CDC

In laboratory testing, researchers simulated coughs and breathing and tested how well different masks worked to block aerosol particles — comparing no mask, a cloth mask or a surgical mask. They also tested two methods to optimize the fit of cloth and medical masks: wearing a cloth mask over a surgical mask, and tying knots on the ear loops of surgical masks and then tucking in and flattening the extra material against the face. Both methods produced substantially improved protection against transmission of and exposure to infectious aerosols.