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(i) According to the form in which they persist after release into the environment.
♤ Primary pollutants: These persist in the form in which they are added to the environment e.g. DDT, plastic.
♤ Secondary Pollutants: These are formed by interaction among the primary pollutants.
♤ For example, peroxyacetyl nitrate (PAN) is formed by the interaction of nitrogen oxides and hydrocarbons.
(ii) According to their existence in nature.
♤ Quantitative Pollutants: These occur in nature and become pollutant when their concentration reaches beyond a threshold level. E.g. carbon dioxide, nitrogen oxide.
♤ Qualitative Pollutants: These do not occur in nature and are man-made. E.g. fungicides, herbicides, DDT etc.
(iii) According to their nature of disposal.
♤ Biodegradable Pollutants: Waste products, which are degraded by microbial action. E.g. sewage.
♤ Non-biodegradable Pollutants: Pollutants, which are not decomposed by microbial action. E.g. plastics, glass, DDT, salts of heavy metals, radioactive substances etc.,)
(iv) According to origin
♤ Natural
♤ Anthropogenic