Masks don't stop viruses, but they stop some droplets and slow spread. Click on my name and look in particular at the salon.
They stop droplets but they don’t stop aerosols (droplets smaller than 10um). Aerosols make up about half the liquid expelled in a typical cough and are known to carry the virus. Aerosols don’t follow a ballistic trajectory to the ground like bigger droplets but instead waft all over the place on air currents. The fact of aerosols plus the massive leakage around most masks makes me think masks are a nice gesture and may provide a sense of “doing something” but aren’t anything like decisive in the course of the epidemic.