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Dr. Martin Fischer, from Duke University, developed a simple, low-cost technique to visualize the effectiveness of different face coverings on droplet emissions during normal wear. Testing several face coverings, the researchers found that the particles can be blocked by some, but not all recommended face coverings.
N95s without valves, and surgical, and polypropylene masks worked best. Cotton face coverings provided some coverage. Bandanas and neck fleece didn’t block the droplets very much, and indeed the "filter" smaller droplets that remained longer airborne. The experiments clearly shows the importance of wearing masks to prevent spreading of droplets that cary the COVID-19 virus, and also suggest that some face covers don 't do the job at all.
See also original study in Science Advances (August 7, 2020):
