Trachea. Coloured scanning electron micrograph (SEM) of epithelial cells lining the human trachea (windpipe). Some of the cells are covered in cilia (blue), while others have tiny microvilli (brown) which increase the surface area of the cells. The epithelium is covered in mucus which traps large dust particles and removes gaseous pollutants such as ozone and sulphur dioxide from inhaled air. The cilia beat rhythmically, filtering particles suspended in the incoming air, and sweeping dust and other foreign material back towards the larynx. Magnification unknown.