A "Clean Room" is what it sounds like, a room which is very clean. There are varying types of them, from the variety used to make watches to those used to produce satellites. The general idea is to prevent contamination of the air, which is typically dust. In order to enter a clean room, one typically has to wear a full body suit that is meant to contain everything within the suit, so no dust enters the room. No makeup is allowed inside, as those types of particles easily come off and float around in the air. The systems that filter what little dust gets into the air are full room air circulation, meaning that the clean room is inside another larger room, one that houses the filtration and air circulation. This allows for different types of air flow, such as air that only flows directly downward, through a grated floor and back into the system. This is effective because larger particles will fall into the filtration rather than staying in the room, possibly contaminating something. The reason these need to be so clean is dependent on what the room is being used for. For example, in one that mechanical watches are assembled in, it's necessary to keep out any kind of dust from the minuscule gears to prevent any kind of possible mechanical failure. Clean rooms are used all over the world, and in a variety of ways. One of the largest is NASA's cleanroom in the Goddard Space Center, which is about the size of a small warehouse. Each employee in the room also wears a wristband that discharges static electricity to a ground wire, preventing any electrical damage and even possible attraction to dust as a result.