Correlation between surface and air counts of particles carrying aerobic bacteria in operating rooms with turbulent ventilation: an experimental study.

Airborne contamination with bacteria-carrying particles (cfu/m3) and their sedimentation rate (cfu/m2/h) was compared in an operating room (OR) equipped with two turbulent ventilation systems. One was a thermally based system with inlet of cool clean air at the floor level and evacuation of the air at the ceiling by convection (17 air changes/h). The other was a conventional plenum pressure system with air supply at the ceiling and evacuation at the floor level (16 air changes/h). The study was made during rigidly standardised sham operations (N = 20) performed in the same OR by the same six member team wearing non-woven disposable or cotton clothing. Airborne contamination in the wound and instrument areas was related to the surface contamination rate in the same areas and in addition, on the patient chest and in the periphery of the OR. With the exception of the periphery of the OR, the surface and air contamination rates were highly correlated in both ventilation systems (P = 0.02-0.0006, r2 = 0.52-0.79). This was also true particularly when disposable clothing was used while the correlation was weaker in cotton clothing experiments. An equation describing the relation between surface and air counts is given. Typically, the surface counts were numerically 16-fold the air counts, i.e., the number of colonies sedimenting on four 14 cm-diameter agar plates during 1 h will almost equal the number of airborne cfu per m3. We propose, that sedimentation plates represent not only a technically easier method than air sampling but when correctly used, are also the most realistic indicator of airborne bacterial OR contamination in areas critical for surgery.