most virus transmission occurs via larger particles in secretions, whether aerosol (<5 µm) or droplets (>5 µm), which are generated directly by speaking, eating, coughing, and sneezing; aerosols are also created when water evaporates from smaller droplets, which become aerosol-sized droplet nuclei. The point is not that some particles can penetrate but that some particles are stopped, particularly in the outward direction. Every virus-laden particle retained in a mask is not available to hang in the air as an aerosol or fall to a surface to be later picked up by touch. https://www.acpjournals.org/doi/10.7326/M20-2567

Nevertheless, the ability of cloth masks to intercept about 70% of infectious particles is a very meaningful benefit for any public health intervention https://www.theunion.org/news-centre/news/body/IJTLD-June-0244-Leung-FINAL.pdf

Both orgs are unsurprisingly much more reputable than the anti-vaxx, HIV denialist quacks at the AAPS.