The relationship between surface contamination (cfus/m²/h) with particles carrying aerobic bacteria and corresponding air contamination rates (cfus/m³) was evaluated in operating rooms (OR) equipped with ultra clean vertical or horizontal laminar airflow (LAF). For the evaluation we collected data during strictly standardized sham operations using non-woven disposable or cotton clothing. Air contamination in the wound and instrument areas (Casella slit sampler) was related to the surface contamination rate (settle plates) in the same areas and in addition, on the patient chest.

Typically, the mean surface counts were 20–70 cfus/m²/h and the air counts 1–2 cfus/m³in disposable clothing experiments, whilst the use of cotton clothing resulted in higher counts of 100–200 cfus/m²/h (wound P>0·05, patient P>0·05, instruments P<0·01) and 4 cfus/m³(P<0·02–0·001). In the vertical LAF, taking both disposable and cotton clothing operations together, the surface and air contamination rates (surface/air ratio SAR) were highly correlated (P =0·02–0·004) and the ratio varied between 18:1 and 50:1 with a mean for wound air of 36:1. Using only disposable clothing in the vertical LAF, the number of significant correlations was reduced. With cotton clothing experiments in vertical LAF and in the horizontal LAF using disposable clothing, no significant correlation between surface and air contamination was found.

The wide variation of SAR values and the inconsistent relationship between surface and air counts indicates that measurement of OR air contamination represents an unhelpful method for assessment of surgical site contamination in LAF units. We propose instead that colony counts on sedimentation plates is a clinically more relevant indicator of bacterial OR contamination in LAF units. In addition to the current bacteriological standard for ultra clean OR air of (<10 cfus/m³) we suggest a corresponding standard for the surface contamination rate of <350 cfus/m²/h.