The principle
Air moves from higher to lower pressure. Setting a deliberate pressure gradient between adjacent rooms means leakage travels in a chosen direction: outward from a space that must stay clean, inward toward a space that must stay contained.
Positive, negative and the spaces in between
Operating theatres and sterile compounding areas hold positive pressure to keep contaminants out. Isolation rooms and containment laboratories hold negative pressure to keep hazards in. Anterooms and airlocks buffer between the two and are where most design errors show up.
Why cascades fail in service
Door undercuts get sealed by new flooring. Ceiling voids are shared between rooms that should be separated. Extract fans are switched off out of hours while supply continues. Control systems chase a differential the building fabric cannot physically sustain.
Verification that means something
Steady-state readings on a calm day prove little. Meaningful verification tests the cascade under door transients, with the building at operational occupancy and adjacent systems running as they will in service.
