Virus diffusion in isolation rooms

Abstract

We note that Kao and Yang's numerical analysis confirms the generally held opinion that isolation rooms should be negatively pressurized, with a piston ventilation system that promotes horizontal air flow over the patient and away from any healthcare workers (HCWs) present in the room.1Kao P.H. Yang R.J. Virus diffusion in isolation rooms.J Hosp Infect. 2006; 62: 338-345Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar We also note, with interest, that by off-setting the extract grilles behind the patient, it appears to be possible to concentrate any infectious particles produced to one side of the bed, thus leaving the other side relatively free of contamination. This finding may be of some importance because it would enable a ‘safe zone’ (or, perhaps more correctly, a safer zone) to be created close to the patient. The creation of such a zone would be a significant advance because it would enable nurses and doctors to attend to the patient's needs in relative safety, provided that they stayed within this area. Perhaps the area of the safe zone could be marked out on the floor? In order to create a truly safe zone to the side of the patient, it would be necessary to ensure that large respiratory droplets (>50 μm) are removed as well as smaller droplet nuclei (<10 μm). These larger droplets, which may contain infectious virus, tend to fall to the ground within 1–2 m of the patient, whereas the smaller droplets evaporate rapidly to form nuclei that can remain airborne for many hours.2Beggs C.B. The airborne transmission of infection in hospital buildings: fact or fiction?.Indoor Built Environ. 2003; 12: 1-10Crossref Scopus (154) Google Scholar It is unclear from Kao and Yang's work whether or not the movement of these larger droplets was simulated. Being relatively heavy, the effect of room air currents on these larger droplets is much less than that on droplet nuclei, and they tend to be removed from the air by gravitational deposition. They are nonetheless of considerable importance as a number of infections, including severe acute respiratory syndrome, are known to be transmitted by the droplet route. In order to protect HCWs in regions close to the patient, it is necessary to ensure that they are not exposed to these larger respiratory droplets. We would therefore encourage Kao and Yang to consider this issue in their future work. Notwithstanding our comments above, we believe that computational fluid dynamics is an important tool in analysing the spread of airborne infection. Indeed, our own analysis suggests that the positioning of supply diffusers and extract grilles can have a profound effect on the movement of infectious particles within isolation rooms, and that careful positioning of these can lead to significant improvements.3Noakes C.J. Sleigh P.A. Escombe A.R. Beggs C.B. Use of CFD analysis in modifying a TB ward in Lima, Peru.Indoor Built Environ. 2006; 15: 41-47Crossref Scopus (31) Google Scholar