Dust can enter into the air from numerous sources, e.g. as shown on illustration ST 13. When packaging is opened dust is frequently released. To get an idea of how much dust is present in a cubic metre of air, and to determine the nature of the dust particles, a known volume of dustcontaining air is sucked through a suitable filter. By weighing the filter before and after filtration of the air, the dust concentration can be calculated. If filters of differing porosity are used the dust can be separated into various fractions. The middle part of the illustration shows filter pores through which air is being sucked and where aggregates of dust particles is held back. At the bottom right of the illustration a magnified schematic drawing of this type of dust filter is shown. The left-hand figure illustrates the use of a protective dust mask that is, in itself, a simple example of the separation of a solid - gas mixture.

Illustration ST 14 gives some possible compositions of dust.