This field study was organised to track differential pressure relationships in critical care units of a metropolitan hospital. Maintaining design pressure balances is essential to control risk of airborne disease transmission within these units. the pressure differentials that exist between two patient isolation facilities (positive and negative) and their surrounding spaces were monitored continuously for one week. In addition, individual room challenges were recorded that tracked pressure changes associated with opening and closing doors in the patient room, the hallway, the anteroom of the patient room, and the adjacent patient room. Containment was excellent in the positively pressurised, occupied room. It was unacceptable in the negatively pressurised, unoccupied room. Room design features contributing to these observations were discussed.