Airborne staphylococci in the surgical ward

Abstract

The exposure of hospital patients to airborne staphylococci has commonly been estimated by collecting the bacteria from relatively large volumes of air over rather short periods with the use of one or other form of slit-sampler (Report, 1948). The design of the slit sampler was determined largely by the need to collect particles down to about 1-2 ,u in diameter, for which collection by sedimentation on to exposed culture plates is very inefficient (Bourdillon, Lidwell & Thomas, 1941), and it therefore employs a high velocity jet of air impinging on the culture plate. While the slit sampler is a very efficient instrument for collecting airborne particles, it has proved difficult to modify it to give really long-period sampling because of the drying effect of the air-stream on the agar plates. Recently it has been shown that only a very small proportion of the staphylococci in the air of hospital wards are associated with particles less than 4 ,t in diameter (Noble, Lidwell & Kingston, 1963) and that the median equivalent particle diameter for airborne staphylococci is about 14 ,t. For such particles the deficiency of sedimentation as a method of collection is small: the settling rate of 14 It particles is such that the number contained in 1 ft.3 of air is approximately equal to the number that settle on 1 ft.2 in 1 min. Since with settling plates it is simple to obtain an integrated sample over a considerable number of hours, it seemed worth exploring the use of this method to monitor the staphylococcal content of the air in hospital wards, as has also been reported by Alder & Gillespie (1964). This paper describes a study in a small surgical ward in which an attempt was made to recognize the sources of the airborne staphylococci and to use the method to study cross-contamination between the separate rooms of the ward. MATERIALS AND METHODSAlmost all the work reported here was done in a 14-bed surgical ward at St Mary's Hospital London between February 1964 and September 1965; the ward is divided into 4 rooms ( Fig. 1) and is used mainly for men and women undergoing thoracic and cardiac surgery. The ward has natural window ventilation; the corridor between the rooms leads to a stairway and the stack effect in this generates a flow of air along the corridor and out of the bedrooms in almost all weathers.A much shorter study was made in a 22-bed open 'Nightingale' ward, using 4 settling plates placed in the 4 quarters of the ward.