Masks with a one-way valve (typically a raised plastic cylinder about the size of a quarter on the front or side) designed to facilitate easy exhaling allows droplets to be released from the mask, putting others nearby at risk per @SF_DPH #COVID19
https://www.washingtonpost.com/health/why-simple-cloth-masks-without-valves-are-better-at-fighting-the-spread-of-covid-19/2020/06/26/c445fca6-b574-11ea-aca5-ebb63d27e1ff_story.html …
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