Factors involved in the aerosol transmission of infection and control of ventilation in healthcare premises

Abstract

The epidemics of severe acute respiratory syndrome (SARS) in 2003 highlighted both short- and long-range transmission routes, i.e. between infected patients and healthcare workers, and between distant locations. With other infections such as tuberculosis, measles and chickenpox, the concept of aerosol transmission is so well accepted that isolation of such patients is the norm. With current concerns about a possible approaching influenza pandemic, the control of transmission via infectious air has become more important. Therefore, the aim of this review is to describe the factors involved in: (1) the generation of an infectious aerosol, (2) the transmission of infectious droplets or droplet nuclei from this aerosol, and (3) the potential for inhalation of such droplets or droplet nuclei by a susceptible host. On this basis, recommendations are made to improve the control of aerosol-transmitted infections in hospitals as well as in the design and construction of future isolation facilities.

Figure 1

Droplet generation. A flash photo of a human sneeze, showing the expulsion of droplets that may be laden with infectious pathogens. Sneezing can produce as many as 40 000 droplets of 0.5–12 μm. These particles can be expelled at a velocity of 100 m/s, reaching distances of several metres. Smaller droplets with less mass are less influenced by gravity, and can be transported as a ‘cloud’ over greater distances by air flows. Larger droplets with more mass are more strongly influenced by gravity and less so by air flows, and move more ‘ballistically’, falling to the ground more quickly. Reproduced with the kind permission of Prof. Andrew Davidhazy, School of Photographic Arts and Sciences, Rochester Institute of Technology Rochester, NY, USA.

Figure 2

Smoke visualization of exhalation flow from nose of the right mannequin penetrating into the breathing zone of the left mannequin, which are 0.4 m apart. Reproduced from Figure 12 in Reference with the kind permission of Blackwell Publishing.

Figure 3

Droplet suspension. Illustration of the mechanics of suspension of droplet nuclei produced by an infected patient due to the effects of air friction and gravity.

Figure 4

Droplet transport. The dispersion of the droplet nuclei is affected by the air flows from an open window, the ventilation system and door opening. (a) The air flow from an open window is affected by temperature differences between the inside and outside. In this figure, inside the room is warm and outside is cold. (b) The ceiling-mounted ventilation vent injects clean air into the room, which is removed by the exhaust vent near the patient's head, diluting the total amount of contaminated air. This also generates a downward flow pattern. (c) The action of opening a door generates a large vortex that sweeps clean air into the room and ejects contaminated air. When there are temperature differences between inside and outside, this also leads to a buoyancy exchange flow indicated in (a).

Figure 5

A snapshot of the movement of one person in the corridor of a full-scale test room for a six-bed general hospital ward. The ward was ventilated by a downward supply system with exhaust at floor level. At the time of the experiment, the supply air stream was marked by smoke. A person walking forward at about 1 m/s would push a front layer of air creating a volume flux, F, of about 255 L/s, with an attached wake of 76–230 L behind (see main text for calculations). This introduces a significant mixing air flow into the room.

Figure 6

Illustration of the two commonly used air distribution methods in rooms. (a) Mixing ventilation: the cool air is supplied at ceiling level at high velocity and returned at either ceiling or floor level. The air in the room is generally fully mixed due to the strong mixing created by the overall air recirculation in the room, governed by the strong supply momentum. (b) Displacement ventilation: the cool air is supplied at floor level at low velocity and returned at ceiling level. The air in the room is divided into two parts: the upper part with ‘polluted’ air and the lower part with ‘clean’ air. Both parts of figure reproduced with the kind permission of CSIRO Australia. © CSIRO.