Nitrous oxide (N2O) and Carbon Dioxide (CO2) tracer gases were used to simulate airborne infection and to quantify and characterise the patterns of air movement produced within the isolation room. In the gas tracer tests (denoted as GTT) a known flow rate of gas was released from different points in the room (bed, sink, lobby, corridor) and the concentrations were measured around and outside the room when steady estate had been reached. Most of the tests were done with the door closed, but the room was also challenged by leaving the door open, repetitively opening and closing of doors and simulations of fan failure.
For both gas tracer and bacteriological experiments, the sampling points were located around the bed at the same distances from the source (0.5, 1.0 and 2.0 m). Two DIN men were used to simulate the heat loads from a nurse and patient. Six tubes were located at 1.45 m (breathing height) around the bed, and an extra tube was connected to the extract. Sample locations are marked with crosses in Figure 2. Aerosolised bacteria was released from the “mouth” of the DIN man-patient.
The investigations provided evidence to validate the PPVL isolation facility.
Results in the PPVL mock-up and real hospital ICU room showed that the rooms were fundamentally well mixed and showed rapid recovery from perturbations and simulated fan failure. Testing the same design under positive and negative pressure configurations showed that within the isolation room, the direction and magnitude of pressure gradients had no bearing on the conditions. GTTs also showed that there was no direct relation between the protection levels in the room and the pressure difference with its adjacent areas.
The quality and quantity of internal flow patterns determine the effectiveness of the dilution ventilation. In both designs, typical N2O levels in the extract represented a dilution of 103 relative to the source. Values measured in the rooms were normalised against the extract concentration and results show that concentration levels are within the same range in the same room under different pressure regimes. The concentration outside the room was under the minimum detectable concentration of the instruments used, which indicates that the protection levels were at least 105.
Bacteriological results and GTT results from the University of Leeds study were comparable, and as expected the concentration was higher near the release point with the bed in position 1 (See Figure 2). The effects of the ventilation design, particularly the fresh air supply location can be seen in position 2, which showed a lower concentration, in both bacteriological and CO2 experiments.