From the British Medical Journal on using cloth masks, which many people are making for themselves and for health workers who are running out of the disposables.
Finally, WHO concludes that “Cloth masks are not recommended under any circumstance.” This warning is perhaps based on the results of a trial comparing cloth masks with medical masks for healthcare staff in high risk hospital settings in Vietnam, which cautioned against recommending them for healthcare workers. It’s unclear how relevant this finding would be in community settings, especially if the objective is source control. There has been an unfortunate lack of research into cloth masks and no randomised controlled trials seem to have been done on them in community settings. However, homemade cloth masks (although not found to be as effective as surgical masks) have still been shown in laboratory tests to reduce source transmission and block external aerosols.