Table 7.2.1.

Definitions of respirable agents by physical form

Gas	A formless compressible fluid in which all molecules of the agent move freely at room temperature (25°C) and standard pressure (760 mmHg) to fill the space available
Vapour	Gaseous state of an agent which is normally liquid or solid at room temperature and standard pressure
Aerosol	Dispersion of solid or liquid particles of microscopic size in a gaseous medium. The following are all examples:
Dust	Dispersion of solid particles. Those of respirable size are not readily seen with the naked eye unless they are bathed in bright light
Fog	Dispersion of liquid particles generated by condensation from the vapour state
Fume	Dispersion of solid particles generated by condensation from the vapour state
Mist	Dispersion of liquid particles generated by condensation or mechanical means (e.g. nebulisation). The droplets are generally larger than those of a fog and may be visible individually to the naked eye
Smog	Mixture of smoke and fog, the former being the result of industrial pollution, the latter of natural climatic factors
Smoke	Dispersion of small particles (usually less than 0.1 µm diameter) resulting from incomplete combustion of organic substances