Aerosol is a general term used to denote solid particles or liquid droplets suspended in the atmosphere. Particles can originate from numerous sources, both natural and Industrial. Also, they can be emitted as primary pollutants or formed in the atmosphere as secondary pollutants from precursor gases. Natural sources include wind-blown dust, sea spray, volcanoes and forest fires. Combustion of fossil fuels, weather in power station or motor vehicle engines, is a major anthropogenic source of dust particles. Many other activities also generate particulate pollution: smelting and processing of metals, cement manufactures, agriculture activities, construction etc. They can extend in diameter from fine particles less than 0.1 μm to large particles greater than 100μm. Particles greater than 10 μm in diameter do not penetrate the respiratory system as they are removed in the nasal region. Particles less than 10 μm can be deposit in the respiratory system and are called inhalable. Particles less than 2.5 μm can penetrate into the pulmonary region, consisting of bronchioles and alveoli, and they are called respirable.

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