In order to assess the efficiency of pollutant elimination in a pilot scale clean room, a numerical study is carried out, which permits to quantify contaminant dispersion in several configurations. The results are presented for several Air Change rate (ACH) usually recommended in such clean rooms and we call for the ventilation effectiveness concept to evaluate the ambiance quality. The contaminant removal effectiveness appears to be strongly related to the contaminant source position and does not vary significantly as the ACH decreases from 80.2h-1 to 20.1h-1. Moreover, even if the mean concentration of pollutant in the room increases with the reduction of the supply air velocity, the cleanliness class can remain unchanged in working areas, primarily because the contaminants are not evenly distributed in the whole room. These numerical results corroborate previous experimental observations and can lead to new recommendations.

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