@grrttweaver @BrianLenzkes CDC has advised on graduated effectiveness of different types of masks for at least 9 months and probably longer (didn't check any earlier revisions of this page prior to April 2021).
https://www.cdc.gov/coronaviru...
@grrttweaver@BrianLenzkes疾病预防控制中心已就不同类型口罩的分级有效性提供建议至少9个月，可能更长（在2021年4月之前未检查本页的任何早期修订）。
https://www.cdc.gov/coronaviru...