Negative room pressure is a technique used in a hospital room to prevent the spread of infection. The room is ventilated in such a manner that air is exhausted from the room to the outside in quantities greater than it can be taken in through the only opening in the room - a small gap beneath the door. It is generally only used for patients with airborne infections such as tuberculosis, varicella and measles.
Rooms are tested by releasing smoke from the outside of the room in front of the door gap, which should quickly move into the room.