Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of chemical substances or energy, such as noise, heat or light. Pollutants, the components of pollution, can be either foreign substances/energies or naturally occurring contaminants. Pollution is often classed as point source or nonpoint source pollution. In 2015, pollution killed 9 million people in the world.
Major forms of pollution include: Air pollution, light pollution, littering, noise pollution, plastic pollution, soil contamination, radioactive contamination, thermal pollution, visual pollution, water pollution.
Until the 16th century, the verb 'to pollute' and the noun 'pollution' were used mainly in relation to morals and religion, thus pollution was defined as ceremonial impurity or defilement, or as profanation of some thing or place held to be sacred.
The term pollution is generally used to describe the presence of chemical substances produced by man's activities but not to describe the natural existence of even higher levels of the same substance. Thus, if a factory discharges lead into a river, even if it is diluted down to harmless levels, this is considered to pollute the river; but the lead naturally leached from rocks, even yielding high levels which can be dangerous to fish, is not considered to be a pollutant. Such loose usage of the term 'pollution' is biased and the word should be employed only when damage occurs.