These are small solid particles and liquid droplets suspended in the air. They are present in the air in the form of soot (produced due to incomplete combustion of fossil fuels), fly ash (coming with the furnaces), inorganic particles (metallic, metal oxides, lead halides, asbestos dust, sulphuric acid and nitric acid) and organic particles (paraffins, olefins, polycyclic aromatic hydrocarbons, i.e., PAH, etc.). The particulates may also be classified as viable and nonviable. Viable particulates are small size living organisms such as bacteria, fungi, moulds, algae, etc. Non-viable particulates are formed by disintegration of large size materials or condensation of small size particles or droplets. These include mist, smoke, fumes and dust. These particulates pass through the nose (bigger than 5 microns) easily and enter the lungs (having the size of about 1.0 microns), where they act as sites for adsorption of carcinogenic compounds causing lung cancer and bronchital asthma.
This disease is common in industrial workers and is called pneumoconiosis, (asbestosis due to asbestos, silicosis due to silica, etc.). They scatter light and hence affect visibility on foggy nights. They counteract the greenhouse effect as they reflect back heat of sunlight.
Smog and its kinds, The word 'smog' is a combination of 'smoke' and 'fog' because earlier, it was found to be formed by condensation of fog on carbon particle.