The literature on masks broadly looks at the efficacy of different types of masks and their efficacy at preventing particle penetration (controlled studies) and the likelihood of infectious spread (case studies of healthcare workers). Other studies question the detrimental effects of masks, particularly with prolonged use. Cloth masks, which have become the norm for public use, have been shown to have penetration rates as high as 97% according to a BMJ study (which used to stand for the British Medical Journal, but is now titled by its acronym). A study of the use of cloth masks during the far more serious 1918 influenza pandemic showed no beneficial results, and another study demonstrates that cloth masks are particularly ineffective compared with medical masks. Surgical and cotton medical masks fared better, but still with discouraging results overall (see here, here, here, here, and here).
As masks-for-all advocates are quick to point out, N95 respirators do show beneficial results in containing viral infections, but these are virtually unworn by the public (and they have only recently become available to those outside of the healthcare profession).
Hellbound and Down 