Coloured scanning electron micrograph (SEM) of the outer layer of fabric from a surgical face mask. The outer layers are made from a spunbond fabric, a nonwoven fabric 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: x50 when printed at 15cm wide.