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Positive vs Negative Operating room and protective isolation = Positive Airborne isolation = Negative When the total CFM from supply air (1&2) > than the return air (3), the room is under positive pressure and the air will flow out of the room. When the CFM from the return air (3) > than the supply air (1&2), the room is under negative pressure and the air will flow into the room

Two principles for air-conditioning operating rooms are that air should be supplied at the ceiling, in a unidirectional or laminar air pattern, and that higher air change rates result in lower bacte­rial counts within the room. However, these principles are applied along a wide spectrum, and ongoing research is being conducted to optimize air distribution airflow patterns and quantities.