The Royal Society’s Face mask report says if correctly used face masks, including homemade cloth masks, can contribute to reducing viral transmission. Masks can offer an important tool for contributing to the management of community transmission of Covid19 within the general population. The report says evidence supporting their potential effectiveness comes from analysis of (1) the incidence of asymptomatic and pre-symptomatic transmission; (2) the role of respiratory droplets in transmission, which can travel as far as 1-2 meters; and (3) studies of the use of homemade and surgical masks to reduce droplet spread.
Link to report: https://rs-delve.github.io/reports/2020/05/04/face-masks-for-the-general-public.html