Measurements of particle size distributions smaller than 0.1 μm in Class 100 clean rooms are summarized. The size distributions were measured in operational rooms during periods of time with little activity—the so-called "at rest" conditions. A simple particle number balance model is proposed, illustrating the importance of filter penetration and atmospheric aerosol on the concentration of submicrometer particles. Preliminary calculations are used to explain the absence of < 0.1 μm diameter particles in the clean rooms tested. A ratio of condensation nucleus counter concentration to optical particle counter concentration is suggested as a parameter to provide an indication of changes in clean room particle size distribution.

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