Dioxin: is an extremely toxic compound to most organisims. Due to its very low water solubility, most of the dioxin occurring in water will adhere to sediments and suspended silts. It is generally resistant to microbial breakdown, and tends to accumulate in aquatic life, from algae to fish. Dioxin is not produced or used commercially in the U.S., but is formed mainly during combustion of a variety of chlorinated organic compounds. Dioxin is released to the environment in emissions from the incineration of municipal refuse and certain chemical wastes, in exhaust from automobiles powered by leaded gasoline, in emissions from wood burning in the presence of chlorine, in accidental fires involving transformers containing PCBs and chlorinated benzenes, and from the improper disposal of certain chlorinated chemical wastes.
Mercury:Mercury is a naturally occurring element that can be toxic when consumed by animals and humans. Sources of mercury include weathering of the earth's crust, the burning of garbage and fuels, and industrial emissions.
PCBs:Polychlorinated biphenyls (PCBs) are a family of man-made chemicals that contain 209 individual compounds with varying levels of toxicity. Some are recognized carcinogens. Eating contaminated fish is a major source of PCB exposure for humans because PCBs bioaccumulate in some species of fish found in contaminated waters. PCBs were widely used as coolants and lubricants in transformers, capacitors, and other electrical equipment until they were banned in 1977. Although PCBs are no longer manufactured, exposure still occurs as a result of historical contamination and the decommissioning of older transformers and capacitors, which have lifetimes of 30 years or more.