Air borne Contaminants. When chemicals are disseminated in air and contaminate it, they are called air-borne contaminants. They are classified according to their physical state as under: A) Gases and Vapours: 1. Gases: Normally formless fluid which occupy the space of enclosure and which can be changed your the liquid or solid state only by the combined effect of increased pressure and temperature. Gases diffuse. The particle size varies from 0.0005 to 0.01 micron. Examples are chlorine, ammonia, sulphur dioxide, hydrogen sulphide, hydrogen cyanide, carbon monoxide etc. Main pollutants are oxides of carbon, sulphur and nitrogen. 2.Vapours : The gaseous form of substance which are normally in the solid or liquid state and which can be changed to these states by either increasing the pressure or decreasing the temperature alone. Vapours diffuse. The particle size varies from 0.005 to 0.01 micron. Examples are vapours of lead oxide, benzene, ...