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Air filtration: protection against respirable dust.

The principal health hazard associated with airborne dust is attributable to particles with aerodynamic diameters of a few micrometers or less, and filtration is, in principle, a good way of obtaining protection against such small particles. The elements of filtration theory can be deduced from simple arguments, which illustrate that it is difficult to remove particles that are 1 or 2 microns in diameter from air by filtration using a filter made from ordinary textile fibres. Either much finer or electrically charged fibres must be used and, in general, the latter solution is preferable. A number of different electrically charged filter materials exist, examples of which are materials that are charged triboelectrically, by corona, or by induction. The mechanism of action of these materials is described, along with conditions necessary for their successful use.