Cleanrooms are essentially controlled environments in which manufacturing can take place. They are designed to control the levels present of contamination, be it physical (dust), biological (micro-organisms) or chemical (cleaning agents/solutions) present within the defined area. After gowning you enter through an airlock. Each zone prior to the cleanroom itself is designed to reduce the level/risk of contamination within the room. The images below show the type of simple layout of a cleanroom and the filters (known as HEPA or High Efficiency Particulate Air filter) used in cleanrooms to reduce contamination from the air both inside (through cycling of the air within the room) and outside (air being brought into the room is cleaned).