The deposition velocity of airborne microbe-carrying particles (MCPs) falling towards surfaces was
obtained experimentally in operating theatres and cleanrooms. The airborne concentrations of
MCPs, and their deposition rate onto surfaces, are related by the deposition velocity, and
measurements made by a microbial air sampler and settle plates allowed deposition velocities to be
calculated. The deposition velocity of MCPs was found to vary with the airborne concentration, with
higher deposition rates occurring at lower airborne concentrations. Knowledge of the deposition
velocity allows the deposition on surfaces, such as product or settle plates, by a known airborne
concentration of MCPs to be predicted, as well as the airborne concentration that should not be
exceeded for a specified product contamination rate. The relationship of airborne concentration and
settle plate counts of MCPs used in Annex 1 of the EU Guidelines to Good Manufacturing Practice to
specify grades of pharmaceutical cleanrooms was reassessed, and improvements suggested