The performance of a small, 52 square meter laboratory clean room at the University of Minnesota has been evaluated using a system of aerosol measuring and monitoring instruments. Aerosol in the ambient atmosphere and in the clean room has been measured simultaneously using a condensation nucleus counter and diffusion battery, a differential mobility particle sizer and two laser optical particle counters to provide concentration and size distribution data between 0.002 to 10 μm diameters. The diurnal variation of the condensation nuclei concentration in the clean room has been found to follow closely that in the ambient atmosphere. This suggests that particles in the CNC range measured in the clean room have their origin in the ambient atmosphere and that the clean room filter does not provide a perfect shield between indoor and outdoor air for these small particles. The ratio of clean room CMC to ambient CNC counts has been found to range from 7 × 10−7 when the ambient CNC count is low (1 × 105 per cc) to 2 × 10−7 when the ambient CNC count is high (1 × 106 per cc). This can be explained by the smaller size of the airborne particles in the ambient atmosphere under high concentration conditions and the greater efficiency of the HEPA filters for these particles.

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