In a 2015 randomized controlled trial involving 1,607 healthcare workers, researchers compared the rate of influenza-like illness in healthcare workers wearing cloth masks or medical masks to those in a control group (which sometimes used masks). Wearing cloth masks resulted in significantly higher rates of infection, the authors found. They also noted that in their test, the cloth masks were only 3 percent effective at blocking particles.

jdc wrote:
A similar study, also in 2019, confirmed that respiratory viruses are often found on the outside surface of medical masks and can be a source of self-contamination.

If you do wear a mask, be sure to remove it from behind your head. Don’t touch the surface of the mask. If you do touch the surface, immediately wash your hands or use hand sanitizer.

cidrap.umn.edu wrote: Sweeping mask recommendations—as many have proposed—will not reduce SARS-CoV-2 transmission, as evidenced by the widespread practice of wearing such masks in Hubei province, China, before and during its mass COVID-19 transmission experience earlier this year. Our review of relevant studies indicates that cloth masks will be ineffective at preventing SARS-CoV-2 transmission, whether worn as source control or as PPE.

PeterB wrote:
The problem with this article is that it's not really an unbiased view of the scientific literature; and the ArsTechnica author is not an expert (she has a Ph.D. in microbiology, but this really requires specialized knowledge of a virologist or infectious disease expert). ArsTechnica itself is also not unbiased.