The Behaviour of Fibrous Filter Media in Dust Respirators

The mechanical behaviour of filters is briefly reviewed, without complicated mathematics, and the general principles illustrate that large area filters of fine fibres perform well. The various types of electrostatic filter material available for use in respirators at the present are illustrated, one of which is a filter made of fine charged fibres, combining the advantages of the best type of mechanical filter with the beneficial effects of electric charge on the fibres. The way that deposited aerosol affects filter performance is described both from the point of view of clogging of mechanical filters and loss of efficiency of electrostatic filters. The latter is a drawback to electrostatic filters, but in the case of fine charged material the mechanical efficiency reduces the effect. Existing filter test methods are outlined and the need for a standard test of electrostatic filters to assess the likely performance in service is discussed.