Coloured scanning electron micrograph (SEM) of a cross-section through a surgical face mask. The outer layers (blue) of this mask are made from a spunbond fabric. The internal layer (white) is made from very fine meltblown fabric. Spunbond and meltblown materials are both nonwoven fabrics formed from fibres of thermoplastic resin, typically polypropylene, bonded by heat and or pressure. This type of mask filters out bacteria and viruses in liquid droplets and respiratory aerosols, but not airborne bacteria or viruses. Magnification: x40 when printed at 15cm wide.