In tests of around 11 masks, the F.D.A. had authorized to be sold to American hospitals, seven failed. A mask removed as little as 24 to 35% of the particles, according to a C.D.C. test on April 15. Others barely missed the cut, such as the masks from DaddyBaby, a company in Fuzhou, China, which generally makes diapers. These masks removed between 91% and 93.5% of the particles.