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. 

Toxic pollutants are materials contaminating the environment that cause death, disease and/or birth defects in the organisms that ingest or absorb them. The quantities and length of exposure necessary to cause these effects can vary widely.

Sources: wikipedia.org and oecd.org