About 63,000 different chemicals are thought to be in use worldwide with 3000 accounting for 90 % of the total production. Chemicals can enter oceans from runoff, drains, as rain collecting air pollution and from the breakdown of other substances to form toxic chemicals.
The major problems are from persistent organic pollutants (POPs). These breakdown very slowly and can accumulate in animals' tissues and can effect reproduction and cause cancers. POPs include the highly toxic dioxins and PCBs (Polychlorinated Biphenyls) together with various pesticides such as DDT and dieldrin.
Researchers have found that some types of plastic begin to break down in the ocean within one year, releasing potentially toxic bisphenol A (BPA) and other chemicals into the water.