A protective air supply system for controlling air supply flows in a clean room, wherein the clean room comprises a clean area subject to contamination, comprising: - a first air supply diffuser and a second air supply diffuser, arranged to be installed within a ceiling of the clean room on opposite sides of the clean area and spaced from side walls of the clean room so that each of the supply air diffusers is configured to diffuse: -- a first air flow, having a first air flow volume A l/s, directed along a ceiling of the clean room and towards the clean area, and towards the first air flow from the other air diffuser so that the first air flows from the first and second air supply diffusers (2,3) collide inside the clean area and the clean area is flushed with the combined air flow, and -- a second air flow, having a second air flow volume B l/s, directed along the ceiling of the clean room and towards a perimeter of the clean room and in opposite direction to the first air flow from the same air supply diffuser (2, 3), and - a controller coupled to the air supply diffusers, characterized in that the controller is configured to adjust the first air flow volume A and the second air flow volume B and their ratio, and - switch between a normal patient mode and an isolation mode, wherein, in the normal patient mode, each air supply diffuser (2, 3) is arranged to diffuse the first air flow, and, in the isolation mode, each air supply diffuser (2, 3) is arranged to diffuse the first air flow and the second air flow. - a first air supply diffuser and a second air supply diffuser, arranged to be installed within a ceiling of the clean room on opposite sides of the clean area and spaced from side walls of the clean room so that each of the supply air diffusers is configured to diffuse: -- a first air flow, having a first air flow volume A l/s, directed along a ceiling of the clean room and towards the clean area, and towards the first air flow from the other air diffuser so that the first air flows from the first and second air supply diffusers (2,3) collide inside the clean area and the clean area is flushed with the combined air flow, and -- a second air flow, having a second air flow volume B l/s, directed along the ceiling of the clean room and towards a perimeter of the clean room and in opposite direction to the first air flow from the same air supply diffuser (2, 3), and -- a first air flow, having a first air flow volume A l/s, directed along a ceiling of the clean room and towards the clean area, and towards the first air flow from the other air diffuser so that the first air flows from the first and second air supply diffusers (2,3) collide inside the clean area and the clean area is flushed with the combined air flow, and -- a second air flow, having a second air flow volume B l/s, directed along the ceiling of the clean room and towards a perimeter of the clean room and in opposite direction to the first air flow from the same air supply diffuser (2, 3), and - a controller coupled to the air supply diffusers, characterized in that the controller is configured to adjust the first air flow volume A and the second air flow volume B and their ratio, and - switch between a normal patient mode and an isolation mode, wherein, in the normal patient mode, each air supply diffuser (2, 3) is arranged to diffuse the first air flow, and, in the isolation mode, each air supply diffuser (2, 3) is arranged to diffuse the first air flow and the second air flow.