Fig. 1. The inner and outer surfaces of the KF94 mask and surgical mask after inoculating 3 drops of 5 µL of methylene blue dye on the inner surfaces of both masks. The dye diffused from the inner surfaces (A) of the KF94 mask to the extent that it was visible on the outer surface (B), while the dye diffused only in inner layer (C) of the surgical mask, leaving the outer surface (D) dry.