The unwanted poisonous by-products of human activity, toxic wastes can arise from many sources. Dioxins are the unwanted by-products of the manufacture of herbicides. Heavy-metal residues from copper or silver mining can contaminate groundwater or streams and lakes and become concentrated in fish. Radioactive wastes are generated by nuclear power reactors, the manufacture of nuclear weapons, and, at low levels, by the industrial and medical application of radioisotopes.
Many toxic wastes can harm humans, animals, or plants.
Significant exposure to lead in paints and gasoline can lead to nervous disorders. Organic toxic wastes can lead to liver and kidney inflammation. Others, such as PCBs, are suspected of causing birth defects. Some organic materials, such as benzene, are known to cause cancer in laboratory animals and are listed as possible cancer-producing agents in humans. Moderately high levels of copper, silver, and other heavy metals are detrimental to the growth of algae.