Clean rooms use HEPA or ULPA filters to control particles in the air using laminar (unidirectional) or turbulent (turbulence, not unidirectional) flow principles. A laminar or unidirectional air flow system directs filtered air in a constant downward or horizontal direction to a filter located in a wall near the cleanroom floor, or recirculates through a raised perforated floor. Laminar air flow systems are typically used over 80% of the ceiling of a clean room to maintain a constant air flow. Stainless steel or other non-shedding materials are used to construct laminar air flow filters and hoods to prevent excess particles from entering the air.
Turbulent, or non-unidirectional air flow uses laminar air flow hoods and non-specific velocity filters to keep the air in a clean room in constant motion, although not all in the same direction. Rough air tries to capture particles that may be present in the air and drive them to the floor, where they enter the filter and leave the cleanroom environment.
In some places, vector clean rooms will also be added: air supply in the upper corner of the side of the room, using fan-type hePA filter, or ordinary HEPA filter with fan-type air supply outlet can be used, and the return air outlet is set at the lower part of the other side, and the height to length ratio of the room is generally between 0.5-1. The cleanroom can also achieve grade 5 (grade 100) cleanliness.