DIF-OT is a ceiling filter system for operating theatres requiring an ISO5 Clean Room Classification, suitable for transplants, heart surgery, ortho-paedics and neurosurgery. The system provides differential airflow velocities from the centre towards the perimeter of the treated zone, ensuring a constant and efficient purging of the surgery area. The laminar airflow diffusion eliminates induction and prevents contaminated air from entering the operating area, without the use of potentially restrictive guide curtains.