https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440799/ “Any type of general mask use is likely to decrease viral exposure and infection risk on a population level, in spite of imperfect fit and imperfect adherence, personal respirators providing most protection. Masks worn by patients may not offer as great a degree of protection against aerosol transmission.”

https://www.ncbi.nlm.nih.gov/pubmed/19193267 “We found that adherence to mask use significantly reduced the risk for ILI-associated infection”

https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD... “Surgical masks or N95 respirators were the most consistent and comprehensive [physical intervention] supportive measures.”