Serious environmental health problems and disease may arise from toxic elements in our water, air, soil, or even the rocks on which we build our homes.  This section should enhance your understanding of the material in the text regarding the process of biomagnification, why it is important in toxicology, and why there is controversy and concern about synthetic organic compounds such as dioxin.

A CLOSER LOOK  - Dioxin: The Big Unknown. 

   A colorless crystal made up of the elements oxygen, hydrogen, carbon, and chlorine, dioxin is classified as an organic compound because it contains carbon.  About 75 types of dioxin are known; they are distinguished from one another by the arrangement and number of chlorine atoms in the molecule.  Dioxin is not normally manufactured intentionally but is a by-product resulting from chemical reactions (burning of compounds containing chlorine) in the production of herbicides.  

   Although dioxin is known to be extremely toxic to mammals, its actions on the human body are not well known.  What is known is that sufficient human exposure to dioxin (usually from consumption of meat or milk containing the chemical) produces a skin condition that may be accompanied by loss of weight, liver disorders, and nerve damage.  

   Studies of animals exposed to dioxin suggest that some fish, birds, and other animals are sensitive to even very small amounts of the chemical; as a result, it is capable of causing widespread environmental damage to wildlife, including birth defects and death to young fish and birds.  However, the concentration necessary to cause human health hazards is still controversial.  A lack of data, some argue, still precludes the establishment of a specific threshold concentration of dioxin at which health hazards begin.  Because of these uncertainties the toxicity of dioxin will remain an unknown until further studies better delineate the potential hazard.

   Dioxin is a stable, long-lived chemical that is accumulating in the environment.  As yet we have not been able to determine a safe, reliable, and economically feasible way to clean up areas contaminated by dioxin.  Many old waste disposal sites are contaminated by dioxin; it may also be found in soil and streams several kilometers around the sites.  In 1992 the Environmental Protection Agency convened a panel to reevaluate the risk to the environment and people from exposure to dioxin.  The report from the panel concludes that dioxin is a probable carcinogen but not a widespread cancer threat at ordinary exposure levels, that risks to workers exposed to high concentrations may be higher than previously thought, and that very small levels of dioxin can cause serious damage to wildlife, potentially causing significant damage to ecosystems.