Efforts were made to determine the optimum size for aerosol particles to be used in testing the effectiveness of particulate respirator filters. Possible influences of the test flow rate on the size determinations were considered. During these studies particles in the most penetrating size ranges were used to test filters which are commercially available against dust and mists (DM); paint, lacquer, and enamel mist (PLEM); and dust, fume and mist (DFM); as well as high efficiency (HE) types. Initial testing was performed in a continuous airflow of 64 liters/minute. Filters were tested without any preconditioning. Particle sizes in these tests ranged from 0.03 to 0.24 micrometers count mean diameter (CMD) for solid sodium-chloride and 0.03 to 0.30 micrometer CMD for dioctyl- phthalate (DOP). The findings indicate that respiratory filters follow many of the predictions of single fiber filtration theory. It was possible to identify a particle size at which minimum efficiency exists. This size was a function of the flow rate, filter type, and filter manufacturer. For HE filters these minimum efficiencies occurred at larger particle sizes than for other types of filters. Excellent efficiencies were demonstrated by the HE filters at or nearer to the worst case conditions than is possible for the other tested filters, resulting in efficiencies greater than 99.97 percent. The authors conclude that tests conducted of worst case possibilities are much more vigorous and are able to differentiate between good, medium, and low efficiency filters.
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