They are called negative pressure rooms because the air pressure inside the room is lower than the air pressure outside the room. This means that when the door is opened, potentially contaminated air or other dangerous particles from inside the room will not flow outside into non-contaminated areas.

Instead, non-contaminated filtered air will flow into the negative pressure room. Contaminated air is sucked out of the room with exhaust systems, which are built with filters that clean the air before it is pumped outside of and away from the healthcare facility.