Two types of materials cause pollution: natural and man-made. It might sound weird to associate “natural” and “pollution” but too much of any material in a given place in certain circumstances can make it a harmful substance.
Consider metals for example. Metals are natural materials that can be increased in the environment due to human activity such as shipyard activity or waste from industries. Natural processes such as volcanic and geothermal activity can also increase contamination levels, but usually to a lesser extent. This can occur in relation to the geological nature of the soil, sediment or bedrock, or through volcanic or geothermal activity.
Metal pollution can be extremely detrimental and harmful to organisms, including humans, especially if the metals enter the food chain. However, because metals can be natural and an integrated part of the biosphere and are often needed in small doses by living critters, organisms have adapted over time by developing various mechanisms to deal with increased levels. In addition metals can be present in the environment but not absorbed by organisms because the metals are trapped by organic matter or sediment particles.
Man-made pollution, on the other hand, is more harmful because it’s created in such a way that most organisms can’t evolve fast enough to cope with it. Man-made materials include Styrofoam and other kinds of plastics, and Polychlorinated biphenyls (PCBs), which were made from 1929 until they were banned in 1979 after their negative impact on health and the environment was established. When such material is released into the environment, the microorganisms that usually break down natural materials are not equipped to do so with man-made pollution. Therefore, these materials can persist in nature for an extremely long time, up to several hundred years! This is evidenced in the Great Pacific Garbage Patch where man-made materials, specifically plastic, have been accumulating for some time. The damaging effect of man-made pollution is increased by its long-term presence in the environment, giving it many more opportunities to be harmful and even enter the food chain. Removal of such material is usually very difficult because little is known about its chemical and physical fate in the environment over time.
-- Dimitri Deheyn, ecotoxicologist, Marine Biology Research Division