Nanofiber filters from the Czech company NANOTEX Group s.r.o. The filters are three-layer, where the middle layer is a nanolayer based on PVDF polymer. In short, the filter captures up to 90% of all bacteria and viruses. WHEN TO REPLACE THE FILTER? It must be replaced after each sterilization of the mask, at the latest after eight hours of use. HOW TO INSERT FITLR IN A MASK? The filter is easy to insert into the cotton mask. The back of the mask has an oblong hole into which you insert the supplied filter and carefully align it in the hole so as to cover as large a surface as possible. The filters are sold in 8 pieces. We also suppy face masks with pocket for filter.