http://www.authorstream.com/presentation/mat_nnl-817002-presentation-2

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Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. [1] Pollution can take the form of chemical substances or energy , such as noise, heat, or light. Pollutants, the elements of pollution, can be foreign substances or energies, or naturally occurring; when naturally occurring, they are considered contaminants when they exceed natural levels. POLLUTION