|On each number may by a chlorine atom present.
Giving 209 different combinations
Between the time of initial (1929) and final production (1977) the USA produced an estimated 0,54 billion kg of PCB's. They were widely used in transformers and capacitors, lubricants, fire retardants, plastics and other materials. PCBs can enter the marine environment by adsorption to particles and atmospheric transport. 
They have been contaminants in the marine environment for more than 60 years. In this time they have become universally distributed in marine and estuarine environments. They occur in nearly all marine plant, algae and animal species. Like other organochlorine compounds they are a hazard to marine ecosystems because of their extreme stability, low biodegradability and lipid solubility, which causes them to bioaccumulate. Concentrations of PCBs in surface waters vary between 0,035 n/l (in open ocean), to 10 ng/l in highly polluted coastal waters. However, decreasing trends are being observed since concentrations peaked in the 1970s. 
PCBs tend to act as endocrine disrupting compounds, which disrupt the hormone balance of animals. This might cause immunodeficiency and or reproductive problems (which have been demonstrated in marine mammals). These effects are suspected to occur at much lower concentrations than those which cause acute toxicity. PCBs have been shown to cause chronic diseases in humans (such as skin lesions, reproductive disorders and liver damage)ans is suspected to be carcinogenic. Concentrations which (in laboratory conditions) cause lethal effects in fish range from 10 ppm to 300ppm. The concentrations measured in the 1970s in wild fishes and seal varied between 0,03 and 212 ppm.
There are 209 different forms of PCBs. Therefore, to asses the risk of PCB exposure, the sum of all these forms needs to be taken into account. 
See alsoPCB and heavy metals in beached sperm whales
- Lawrence E (ed.), 2000. Henderson’s Dictionary of Biological Terms. 12th edition. Prentice Hall, Pearson Education Limited. Harlow, Great Britain.
- Clark, R,B., 1999. Marine pollution. Oxford University press, Fourth edition, pp 161
- Kennish, M. J. (1996): Practical Handbook of Estuarine and Marine Pollution, CRC Press 524 pp
- OSPAR Commission 2000. Quality Status Report 2000, OSPAR Commission, London