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Classification of Air borne Contaminants

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, xylene, trichloroethylene and other solvents. 

Gases and vapours are also classified as under-
1.Organic solvent vapours e.g. alcohol, acetone, CS2, CCl4, benzene, xylene. 
2.Pulmonary irritant gases e.g. CL2, NO2, phosgene. 
3.Upper respiratory irritant gases e.g. NH3, SO2, formaldehyde, acetic acid. 
4.Chemical asphyxiant gases e.g. CO, HCN. 
5.Simple asphyxiant gases e.g. N2, CO2, methane, it's homologous and acetylene. 
6.Other inorganic and organic gases e.g. H2S, arsine and pesticides vapours. 


(B) Particulate Matters:
These are solid tiny particles produced by blasting, crushing, drilling, grinding, mixing etc. and suspended in the air. Examples are as under:


1) Dusts: Solid particles generated by handling, crushing, grinding, rapid impact, detonation and de-crepitation of organic inorganic materials such as rocks, or metal, coal, wood, grain etc. Dusts do not tend to flocculate except under electrostatic forces. They do not diffuse in air but settle under the influence of gravity. The particle size varies from 0.1 to 1000 microns. Fly ash from chimney varies from. 3 to 80 microns. 

2) Fumes: Solid particles generated by condensation from the gaseous state, generally after volatilisation from molten metal's etc. And often accompanied by a chemical reaction such as oxidation. Fumes flocculate and sometimes coalesce. The particle size varies from 0.001 to 100 microns. Examples: lead, zinc, or nitrous fumes. 
3) Mists: Suspended liquid droplets generated by condensation from the gaseous to the liquid state or by breaking up a liquid into a dispersed state, such as by splashing, foam and atomising. The particle size varies from 50 to 100 microns. Examples: Sulphuric acid mist. 
4) Smokes: Small gas-borne particles resulting from incomplete combustion and consisting predominantly of carbon gaseous material are grouped in this category. The particle size varies from 0.5 to 1 micron. 
5) Smog and Fog: The air contaminants may be present in the forms of smog and fog which are not usually encountered in an industrial environment. The particle size varies from 1 to 50 micron. 
6)Aerosols: It is a colloidal system in which the dispersion medium is a gas and the dispersed phase is solid or liquid. The term aerosol is applicable till the solids or liquids remain suspended in the gaseous media. The particle size varies from 0.01 to 100micron. Dust, smoke or mist are examples. Aerosols affect weather, damage materials and impair health. Atmospheric Aerosols like hydrocarbons, lead, arsenic, sulphuric acid etc. may injure human health because of their toxic nature. 
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