Pollution is the introduction of contaminants into an environment that causes instability disorder harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy such as noise heat or light energy. 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 is often classed as point source or non point source pollution. The Blacksmith Institute issues annually a list of the worlds worst polluted places.