Dust

Dust, fine particles of organic and inorganic substances suspended in the atmosphere. The substances include animal and vegetable fibers, pollen, silica, bacteria, and molds. In some cities atmospheric dust also contains a large number of smoke particles and tarry soot particles. In an industrial city the air may contain more than 3 million particles per cu cm (more than 50 million per cu in), but above the middle of the ocean or in high mountains the count may be just a few thousand per cu cm. The size of dust particles varies from about half a micrometer (0.00002 in) to several times this size. The particles remain suspended in the air for long periods of time and may be carried great distances.

Atmospheric dust has two important physical properties: its ability to scatter light of short wavelengths (see Color) and its ability to serve as nuclei for the condensation of water vapor. Mist, fog, and clouds would never occur but for the presence of dust particles in the air.

The heavy concentration of dust in the air over large cities is a serious pollution problem. In places such as flour and sugar mills and coal mines a concentration of flammable particles constitutes an explosion hazard. Silica particles in dust are destructive to machinery because of their hardness; they can also be injurious when inhaled (see Silicosis). To obtain dust-free air, filters have been devised using either cloth or water. Dust and smoke may be removed from exhaust stacks of industrial plants by such devices as the Cottrell precipitator. See Air Pollution; Smog.