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Multicenter Study
. 2007 Feb;4(2):e68.
doi: 10.1371/journal.pmed.0040068.

Natural Ventilation for the Prevention of Airborne Contagion

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Free PMC article
Multicenter Study

Natural Ventilation for the Prevention of Airborne Contagion

A Roderick Escombe et al. PLoS Med. .
Free PMC article

Abstract

Background: Institutional transmission of airborne infections such as tuberculosis (TB) is an important public health problem, especially in resource-limited settings where protective measures such as negative-pressure isolation rooms are difficult to implement. Natural ventilation may offer a low-cost alternative. Our objective was to investigate the rates, determinants, and effects of natural ventilation in health care settings.

Methods and findings: The study was carried out in eight hospitals in Lima, Peru; five were hospitals of "old-fashioned" design built pre-1950, and three of "modern" design, built 1970-1990. In these hospitals 70 naturally ventilated clinical rooms where infectious patients are likely to be encountered were studied. These included respiratory isolation rooms, TB wards, respiratory wards, general medical wards, outpatient consulting rooms, waiting rooms, and emergency departments. These rooms were compared with 12 mechanically ventilated negative-pressure respiratory isolation rooms built post-2000. Ventilation was measured using a carbon dioxide tracer gas technique in 368 experiments. Architectural and environmental variables were measured. For each experiment, infection risk was estimated for TB exposure using the Wells-Riley model of airborne infection. We found that opening windows and doors provided median ventilation of 28 air changes/hour (ACH), more than double that of mechanically ventilated negative-pressure rooms ventilated at the 12 ACH recommended for high-risk areas, and 18 times that with windows and doors closed (p < 0.001). Facilities built more than 50 years ago, characterised by large windows and high ceilings, had greater ventilation than modern naturally ventilated rooms (40 versus 17 ACH; p < 0.001). Even within the lowest quartile of wind speeds, natural ventilation exceeded mechanical (p < 0.001). The Wells-Riley airborne infection model predicted that in mechanically ventilated rooms 39% of susceptible individuals would become infected following 24 h of exposure to untreated TB patients of infectiousness characterised in a well-documented outbreak. This infection rate compared with 33% in modern and 11% in pre-1950 naturally ventilated facilities with windows and doors open.

Conclusions: Opening windows and doors maximises natural ventilation so that the risk of airborne contagion is much lower than with costly, maintenance-requiring mechanical ventilation systems. Old-fashioned clinical areas with high ceilings and large windows provide greatest protection. Natural ventilation costs little and is maintenance free, and is particularly suited to limited-resource settings and tropical climates, where the burden of TB and institutional TB transmission is highest. In settings where respiratory isolation is difficult and climate permits, windows and doors should be opened to reduce the risk of airborne contagion.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Measurement of Ventilation
Illustrative carbon dioxide (CO2) concentration-decay experiment demonstrating a rapid rise in CO2 concentration during initial release to a peak of 6,000 parts/million (ppm) followed by slow decay calculated as 0.5 ACH until the windows and doors were opened. After windows and doors were opened, CO2 concentrations fell rapidly, indicating a calculated ventilation rate of 12 ACH. Repeated experiments of this type defined the effect of architectural and environmental variables on natural ventilation.
Figure 2
Figure 2. Effect of Window Opening and Wind Speed on Absolute Ventilation
The effect of partial and complete window opening and wind speed on natural ventilation is shown, compared with mechanically ventilated negative-pressure respiratory isolation rooms. The triplet of bars on the left of the graph represents absolute ventilation measured in naturally ventilated clinical rooms on days when wind speed was within the lowest quartile (i.e., ≤2 km/h), with windows and doors closed (n = 102), partially open (n = 167), or fully open (n = 86). The triplet of bars in the centre of the graph represents absolute ventilation at wind speeds in the upper three quartiles combined (i.e., >2 km/h) with windows and doors closed (n = 266), partially open (n = 74) or fully open (n = 240). “Partially open” was defined as at least one window and/or door open, but not all. The single bar on the right of the graph represents absolute ventilation in mechanically ventilated negative-pressure respiratory isolation wards at 12 ACH. The corresponding median ACH for the seven bars from left to right are: 1.0; 7.6; 20; 1.8; 17; 34; and 12.
Figure 3
Figure 3. Ventilation and Protection against Airborne TB Transmission in Old-Fashioned Compared with Modern Rooms
Ventilation and protection against airborne infection is shown for pre-1950 versus modern (1970–1990) naturally ventilated facilities versus mechanically ventilated negative-pressure respiratory isolation rooms. The triplet of bars on the left represents ACH in old-fashioned, high-ceilinged, pre-1950 naturally ventilated clinical areas (n = 22; 201 experiments), versus modern naturally ventilated facilities (n = 42; 125 experiments), versus mechanically ventilated negative-pressure facilities (n = 12). The left-centre triplet of bars represents the same comparison for absolute ventilation (m3/h/100); the right-centre triplet of bars represents that for absolute ventilation per person (m3/h/100); and the triplet of bars on the right that for the estimated risk of airborne TB transmission (percentage of susceptible persons infected), for 24-h exposure to infectious TB patients [17]. Data are shown for 64 naturally ventilated rooms with windows and doors fully open (the remaining six naturally ventilated rooms had windows that could not be fully opened).
Figure 4
Figure 4. Estimated TB Transmission Risk over Time for Three Sources of Increasing Infectiousness in Naturally versus Mechanically Ventilated Facilities
The estimated risk of TB infection over time for exposure to three TB source cases of different infectiousness is shown for pre-1950 naturally ventilated facilities (dotted lines) versus modern 1970–1990 naturally ventilated facilities (dashed lines) versus mechanically ventilated negative-pressure isolation facilities at 12 ACH (continuous lines). The three infectious sources are: q = 1.3 standard ward TB patients who infected guinea pigs studied by Riley [32] (lowest three lines); q = 13 an untreated TB case who infected 27 coworkers in an office over 4 wk [17] (middle three lines); and q = 249 for an outbreak associated with bronchoscopy of a TB patient [14] (uppermost three lines). Median values for all measures of absolute ventilation for each category of naturally ventilated room with all windows and doors open have been used in the model.

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