This works well when it has been implemented correctly as a low level of any disease in the air is difficult to catch. The inlet should be in one ceiling corner, pushing ‘fresh air’ over the room and patient area with the outlet in the bottom opposing corner. This means the air change occurring is diagonal across the room, and the air change is actual, it really is changing the air 12 times an hour.
Historically though, very few rooms have air ducting installed in the optimal way. The inlet tends to be in one ceiling corner and the outlet is mounted also on the ceiling, meaning the air flow being measured is across the ceiling. In this situation the actual air change rate may far below what the hospital thinks it has achieved.