Deposition velocities of airborne microbe-carrying particles

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.