Air cleanliness control in operating rooms is based on air filtration, and targets the lowest density of particules and bacteria. Legally, the equipment must be periodically verified during periods of inactivity. Most airborne bacteria are of human origin. There is no fixed ratio between the particle count and the level of bacterial contamination. A study of different types of operating room at Besançon Medical Center reached the following conclusions:--Controls are also required during periods of activity.--Bacteriological testing is crucial.--Unidirectional airflow, especially of the horizontal type, is superior to other forms of isolation. The authors propose a "Functionality index", calculated as follows: bacterial count during use of the room minus bacterial count during non use/the number of people in the room. Efficient equipment would have a low index, which would be unmodified by the number of persons present.