Safe zone in isolation rooms

Madam,

We appreciate the comments of Profs Beggs and Kerr on our work, the main purpose of which was to understand how different airflow patterns affect virus diffusion in isolation rooms.¹ In practice, the risk of virus dispersal is controlled not only by airflow patterns but also by other factors, such as contagious transmissions, changing airflow directions caused by people moving or doors opening, and other hazardous situations including carrying the virus, etc. In our investigation, we simplified the airflow model for the analysis of virus dispersal in isolation rooms by assuming the following to be the normal situation: (1) closed doors; and (2) air flow maintained in a stable state without any movement of people. In our investigation, only coughs producing larger droplets (≈30 μm) were considered for the requirements of isolation rooms used for non-airborne diseases, e.g. severe acute respiratory syndrome. However, the results provided reasonable physical evidence that the appropriate airflow patterns with suitable operating parameters, e.g. 12 air changes/h, which dominates the average velocity inside the isolation rooms, can create a relative safe zone for staff. The value of 12 air changes per hour is determined by engineering experiments and specified in our national specifications (CDC of Taiwan) for the installation of hospital ventilation systems. In addition, our proposed computational fluid dynamics (CFD) technique is a simple and readily available method for analysing the air within isolation rooms that are subject to coughs producing larger droplets.

Furthermore, in our opinion, CFD still has some difficulty in simulating realistic cases of airborne diseases, such as a cough that produces smaller droplets. Realistic simulations would require complex conditions and physical models, and would be restricted by limitations of numerical models for various situations inside the isolation rooms, e.g. the effect of evaporation for droplets, a long-time computational model for tracing each nucleus (micro-particles), and the problems of mathematical instability for the CFD techniques. The physical mechanism for droplet nuclei drying out during the transition of virus dispersal by airflow patterns inside the isolation rooms is not clear. For coughs producing smaller droplet nuclei, we suggest that alternative approaches are more suitable, i.e. tracer containment testing.2, 3

As suggested by Profs Beggs and Kerr, in order to mark out the relative safe zone inside an isolation room, the fraction of relative hazardous zone (FHZ) is defined as:

$$\text{FHZ}\equiv \frac{\text{Area of Relative Hazardous Zone on Floor}}{\text{Total Area of Floor}}\times 100\%$$ (1)

The area of the relative hazardous zone is defined as the possible locations where the larger droplets (≈30 μm) may fallout on to the floor inside an isolation room. Based on the definition of FHZ, the fraction of relative safe zone (FSZ) can be calculated by FSZ = (100% – FHZ). The computed values of FHZ for models investigated in our work were: (1) FHZ = 58.7% for VC-01; (2) FHZ = 46.0% for VC-02; (3) FHZ = 76.5% for VC-03; (4) FHZ = 61.5% for VC-04; (5) FHZ = 61.7% for VC-05; (6) FHZ = 46.2% for VC-06; and (7) FHZ = 36.8% for VC-07.¹ The safest zones were in the area near the air supply vents for each airflow pattern.¹ Model VC-07 ( Figure 1) demonstrated the smallest relative hazardous zone of droplet fallout and virus diffusion of coughed gas. This particular ventilation arrangement provides the best results for controlling the dispersal of virus droplets, and provides a relative safe zone for staff performing their duties inside isolation rooms.

Figure 1.

The top view of the relative hazardous zone on the floor for Model VC-07.