Performance and use of electrically charged fibrous filters

Though toxic material is hazardous in any ingestable form, much of the health hazard of airborne dusts that are insoluble in body fluids, such as quartz or coal dust, is associated with particles that have aerodynamic diameters less than about 7 μm. Efficient mechanical filters can be produced from ultrafine fibres, and filters made from micrometre sized glass fibres capture fine particles effectively. However, the slight damage that such a filter, incorporated into a respirator, might incur during normal wear may cause fibres to be shed, unless the filter is encapsulated. The aerodynamic diameter of a fibre is close to its physical diameter, and it depends only weakly on its length, and so such shed fibres may constitute a respiratory hazard. The author describes how for this reason the enhancement of filtration efficiency by the use of fibres holding a permanent electric charge is an attractive proposition