the performance of the media used in cotton-mill filters has been examined. efficiencies were found to be only of the order of fifty percent for the test aerosols used, and this mainly reflects the high proportion of fibrous material, or lint, which is stopped at the surface of the filter. the finer dust component of the aerosol is captured with an efficiency of only about ten percent experimental evidence supports the view that such low efficiencies are partly due to particle bouncing, associated with the high reynold's number at which the medium operates. reynold's numbers are lower for the accumulated lint layer, which is composed of finer fibers than the medium, or when face velocity is reduced. both these effects can be obtained without reducing flow through the equipment by using pleated media, which give higher efficiencies. further increase in efficiency may be obtained by applying an external electric field near the medium, or by using a medium made of electret fibers. 15 refs