Clean rooms are traditionally classified as either laminar flow or mixed flow rooms. Although these two types of rooms operate quite differently, they have generally been used in the same manner, with the same rules applying to both types, on the assumption that there was a unidirectional flow in both types. Experiments have shown that this is not the case in the mixed flow rooms. In some cases vertical counterflows have been shown to carry contaminants all the way to the ceiling of such rooms.
In the dilution controlled clean room concept, the air is sufficiently stirred to achieve good mixing. With a homogeneous concentration of particles in the air, the steady state particle concentration can be calculated. Particle concentration may be controlled by adjusting the air flow to maintain the concentration at a constant value. Under these conditions, only enough air must be circulated to carry away the particles currently being generated. This means that energy consumption is directly related to the particles being generated in the room and is therefore a practical as well as theoretical minimum.