Filters for Face Masks: What are they? They’re replacement filters for face masks with filter pouches, helping the mask stay as fresh & effective as possible. About the product PM2.5 filters are designed to stop dust particles as small as 2.5 microns. These help prevent the spread of Covid-19, though they’re not surgical grade. Why you should buy them Staying safe & protecting others is vital in the age of Covid-19. &, keeping your face mask as fresh as possible is the best way to do that. These filters make that task as easy as possible!