Very small solid bodies often having an irregular shape. They may be a single substance or a mixture of chemical substances often as the product emanating from a process waste stream. They include dirt, dust, smoke, soot, fumes, aerosols, mists, and sprays. They are sufficiently small to be able to penetrate the lower reaches of the lungs. (p. 274) Dust is classified as particulates that have an aerodynamic diameter of less than 75 micrometres (75 μm), whereas smoke has an aerodynamic diameter of less 15 μm. Inhalable particulates are generally less than 15 μm while respirable dust is less than 5 μm. Cement dust ranges from 1 to 100 μm, while fly ash is approximately 10 μm. Fine particulates have aerodynamic diameters of less than 2.5 μm and abbreviated as PM_2.5. See air pollution.