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. 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 non-point source pollution. The Blacksmith Institute issues an annual list of the world's worst polluted places. Pollution became a popular issue after World War II, due to radio-active fallout from atomic warfare and testing.