Major article
The ventilation of multiple-bed hospital wards: Review and analysis

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Background

Although the merits of ventilating operating theatres and isolation rooms are well known, the clinical benefits derived from ventilating hospital wards and patient rooms are unclear. This is because relatively little research work has been done in the ventilation of these areas compared with that done in operating theatres and isolation rooms. Consequently, there is a paucity of good quality data from which to make important decisions regarding hospital infrastructure. This review evaluates the role of general ward ventilation to assess whether or not it affects the transmission of infection.

Methods

A critical review was undertaken of guidelines in the United Kingdom and United States governing the design of ventilation systems for hospital wards and other multibed rooms. In addition, an analytical computational fluid dynamics (CFD) study was performed to evaluate the effectiveness of various ventilation strategies in removing airborne pathogens from ward spaces.

Results

The CFD simulation showed the bioaerosol concentration in the study room to be substantially lower (2467 cfu/m3) when air was supplied and extracted through the ceiling compared with other simulated ventilations strategies, which achieved bioaerosol concentrations of 12487 and 10601 cfu/m3, respectively.

Conclusions

There is a growing body of evidence that the aerial dispersion of some nosocomial pathogens can seed widespread environmental contamination, and that this may be contributing to the spread infection in hospital wards. Acinetobacter spp in particular appear to conform to this model, with numerous outbreaks attributed to aerial dissemination. This suggests that the clinical role of general ward ventilation may have been underestimated and that through improved ward ventilation, it may be possible to reduce environmental contamination and thus reduce nosocomial infection rates.

Section snippets

Ward ventilation

Although a plethora of guidelines on the ventilation of health care facilities have been published,7, 8, 9, 10 the vast majority of these are concerned with specialist facilities, such as operating theatres, isolation rooms, and bronchoscopy suites, where the risks associated with the airborne transmission of infection are well characterized. In comparison, guidelines regarding the ventilation of general ward spaces, patient rooms, and intensive care wards are much sparser and often vague in

Single- and multiple-bed rooms

It is impossible to address the issue of ward ventilation without first considering the nature of patient rooms. In many parts of the world, including the United Kingdom, it is common practice to have multiple-bed wards, often subdivided into bays containing 4 or 6 patients. However, in the United States, the practice is to place patients in single rooms where possible, with a maximum number of 2 patients per room.9 Indeed, the 2006 American Institute of Architects (AIA) guidelines now mandate

Evidence for aerial dissemination

Before focusing on ventilation systems, it is worth considering the evidence regarding the airborne transmission of infection in hospitals. A full discussion of this topic is beyond the scope of this review, however, and thus we give only a brief overview of the evidence for the aerial dissemination of pathogens within the ward environment. This overview is restricted to those infections that normally are not considered airborne in nature and thus does not cover TB, legionnaire's disease, or

Ventilation guidelines in the united kingdom and united states

Table 1 summarizes the ventilation and comfort standards for general ward spaces and ICUs as promulgated by the various regulatory bodies in the United States and United Kingdom. In the United States, the AIA guidelines regulate the design of health care facilities.9 The AIA guidelines are supplemented by the guidelines of the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE).10 In the United Kingdom, HTM 2025, published by the Department of Health, is used to

Drivers

Analysis of HTM 2025 reveals that with respect to the ventilation of general ward spaces, the guidance notes are driven by comfort and economic issues rather than by clinical considerations. As long as reasonable patient comfort conditions are maintained, the regulations are not concerned about whether ventilation is achieved by natural or mechanical means. Indeed, HTM 2025 actively encourages natural ventilation, although it is very vague as to how this should be achieved in practice. This

Patient density and activity

One potential weakness of simply quoting required air change rates is that this approach takes no account of patient density—the ventilation rate is determined solely by the room volume, rather than the number of occupants. In reality, as ward occupancy levels increase, bioaerosol production within the space also increases. Any increase in the number of beds in a ward space will be accompanied by a corresponding increase in the number of nursing staff and visitors, all of whom will liberate

Influences on pathogen transport and removal

Both the UK and US guidelines assume the use of dilution ventilation when ventilating general ward spaces. Such a strategy relies on good air mixing within the room space and generally is achieved by supplying clean filtered air in through diffusers in the ceiling and extracting contaminated air out through grills also located in the ceiling. With this type of ventilation system and full air mixing, the steady-state contaminant level, Ce, achieved in the ward space can be calculated asCe=qcQv,

Particle size distribution

The ability of any given ventilation system to remove particles from a room space does not depend solely on the air change rate. In reality, air in ventilated rooms usually is far from fully mixed,56 and the concentration of bioaerosols depends on the location of the bioaerosol source, the local airflow patterns, and the size distribution of the particles. Very small particles fall through the air very slowly and thus are much more likely to be removed by the ventilation system, whereas large

Ventilation strategy

Notably, the guidelines for ventilating general ward spaces in both the United Kingdom and the United States make no attempt to specify airflow patterns and assume that dilution ventilation will be used. This situation may be due in part to the complexity of airflow patterns in rooms, as alluded to earlier; however, it is perhaps worth considering the affect of air flow direction on bioaerosol concentration generated within a ward space. Consequently, we carried out a short CFD study to explore

Discussion

From the foregoing discussion, it is clear that ventilation systems for general wards and patient rooms are specified using criteria that differ little from those used for nonclinical spaces. The guidelines in both the United Kingdom and the United States avoid any discussion of the risks posed by airborne microorganisms, but focus on providing a comfortable environment. This is understandable, given that patient comfort is of great importance and that the clinical risk posed by many airborne

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