All moral rights asserted. Recolouring or alteration of the image is prohibited without permission. Light micrograph of FFP2 (filtering facepiece 2) mask material. FFP is a European standard. FFP2 masks filter at least 94 percent of airborne particles.. This is the middle layer, which consists of a fleece (thick lines) onto which nanofibers are spun. At only 0.3-0.1 micrometres, these are so fine that even a very thin layer provides a very good filtration rate while allowing easy breathing. The droplets help to fix the fibres to one another. Magnification: x500 when printed at 15 centimetres wide.