Certain chemicals tend to accumulate in fish. According to U.S. EPA, the current major source of dioxins is from incineration. Dioxins are produced in small quantities during the combustion of fossil fuels, wood, and municipal and industrial waste. Bleaching processes used in pulp and paper production produce dioxins, and they occur as contaminants during the production of some chlorinated organic chemicals. Dioxins have been detected in soil, surface water, sediment, plants, and animal tissue in all regions of Earth.
Dioxins are also highly persistent in the environment.
Dioxins in surface waters and sediments are accumulated by aquatic organisms and bioaccumulated through the aquatic food chain. Past industrial activities in the lower Kanawha Valley, especially near the city of Nitro, West Virginia, have resulted in several dioxin-contaminated sites. Dioxin likely originated with the production of industrial solvents and the herbicide 2,4,5-T at facilities in and around Nitro. Areas downstream of Nitro likely became contaminated through the release and transport of dioxin into the Kanawha River and its tributaries. Extensive sampling for dioxin is continuing to take place at several known and suspected contaminated sites in the Nitro area.