Door-opening motion can potentially lead to a transient breakdown in negative-pressure isolation conditions: the importance of vorticity and buoyancy airflows

Abstract

A patient with severe chickenpox was admitted to a negative-pressure isolation room. He remained sedated, intubated and mechanically ventilated throughout his admission. He was managed only by nurses immune to chickenpox. A non-immune male nurse occasionally handed equipment through the doorway, without entering the room. Ten days later, he also developed chickenpox. Sequencing of viruses from the patient and nurse showed the same rare genotype, indicating nosocomial transmission. An experimental model demonstrated that, despite negative pressure, opening the door could have resulted in transport of infectious air out of the isolation room, leading to a breakdown in isolation conditions.

Figure 1

A series of photos taken from vertically above the experimental perspex water tank constructed to simulate the flow of air across a full-size doorway. The left-hand side of the tank simulates an isolation room, and the dark food dye simulates infectious air. The right-hand side of the tank simulates the corridor or anteroom (a) (t=0 s). The model door was built to open inwards, like that of the intensive care unit, and was opened by hand via an attached stiff rod. (b) shows how the dye has moved (long arrow) when the door (short arrow) has stopped opening (t=0.7 s). Transport of the dye into the adjacent room is shown in (c) (t=2.5 s).

Figure 2

(a) Top-down view of the possible airflow currents that result from the opening (inwards, in this example) of a standard hinged door. W, angular velocity at which the door is opened; θ, angle through which the door moves; red (dark) face, person moving towards the doorway; red (dark) arrows, air moving out of the room; blue (light) arrows, air moving into the room. (b) View from inside the room of the situation illustrated in (a), showing a three-dimensional view of possible airflow trajectories induced by the movement of the door opening into a hypothetical isolation room. The red (dark) face represents a person entering the room, through the doorway, who may be exposed to infectious air. (c) Same view from inside the room as illustrated in (b), but this time showing the possible airflow trajectories taking into account the temperature and density differences between cooler air outside (in the corridor) and warmer air inside a hypothetical isolation room. At the doorway itself, differences in air temperature and density with height are marked by the faces: red (dark) face, head height; blue (light) face, just above the floor.