Dioxin molecules, or particles of dioxin, contain three linked rings of atoms. The term dioxin can describe all dioxins, any single dioxin, or any group of dioxins. In many cases, the term dioxin refers to the seven most toxic dioxins or only the most toxic dioxin, called TCDD. Some scientists believe that dioxins are the most toxic compounds made by humans, but others do not believe that research proves this to be true. However, dioxins are undesirable byproducts formed during chemical processes that involve chlorine and compounds composed of hydrogen and carbon. Dioxins form when chlorine gas is used to bleach paper, during the production of chlorine-containing compounds such as pesticides and antiseptics, and when substances that contain chlorine are burned. Forest fires also produce dioxins, but they are a relatively minor source. Dioxins have no commercial use and there is no evidence that they are produced by organisms.

Dioxins are chemically stable, that is, they do not easily break down into other compounds. Therefore, once produced, dioxins last in the environment for a long time. Scientists have found dioxins almost everywhere, in waterways, air, plants, fish, mammals, birds (especially fish-eating birds), bird-eggs, and humans. Dioxins that escape into the atmosphere can travel to and contaminate remote areas of the world. Chemicals that form dioxins usually contain a phenolic group that is a ring of six carbon atoms connected to an oxygen atom and a hydrogen atom. Two phenols can link together by forming a bond between the oxygen atom on one phenol and a carbon atom on the other phenol, and the oxygen atom on the second phenol also bonds with a carbon atom on the first. If chlorine atoms are also connected to the molecule, it forms a chlorinated dibenzo-p-dioxin, usually simply called dioxin.

 

Seventy-five different types, or congeners, of chlorinated dioxin molecules have been found to exist. Each dioxin molecule contains 12 carbon atoms, 2 oxygen atoms, and a total of 8 atoms of chlorine and hydrogen. The chlorine atoms cannot attach to the four carbon atoms that are attached to oxygen atoms. They can, however, attach to any of the eight remaining carbon atoms in the benzene rings. Dioxin molecules can therefore contain from one to eight chlorine atoms. Scientists describe different dioxins by naming the carbon atoms that have attached chlorine atoms. They number the carbon and oxygen atoms in a molecule, starting with a carbon atom that is next to a carbon atom connected to an oxygen atom. The numbers continue up to ten (for a total of ten carbon and oxygen atoms), counting counterclockwise around the outside of the molecule. Each dioxin, or congener, is designated by the numbers of the carbon atoms that have attached chlorine atoms and the total number of chlorine atoms in the molecule. The congener that contains chlorine atoms at positions 1, 3, and 6 (for a total of three chlorine atoms) is called 1,3,6-trichlorodibenzo-p-dioxin. The congener that contains chlorine atoms at positions 2,3,7, and 8 is called 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD.

Dioxins do not dissolve easily in water, but they do dissolve easily in many fats and oils. For this reason, they tend to accumulate in the set of food chains that circulates energy and material in the environment. Dioxins in the air fall onto plants. Animals eat these plants, and the small amount of dioxin from each plant adds up to higher concentrations in the tissue of the animals. These animals are in turn eaten by other animals, in which the dioxins concentrate further. Dioxin concentrations reach higher levels in organisms higher in the food web, a process called biomagnifications.

Some scientists believe that dioxins are extremely toxic compounds. Laboratory research has shown that guinea pigs can die from eating tiny quantities. Hamsters, however, can live even after eating much higher amounts. Scientists have conducted extensive research to determine how toxic dioxins are to humans. Humans appear to be in the middle range of sensitivity to the effects of dioxins. Because dioxins last for a long time and do not break down, even small daily doses can accumulate into levels that may be dangerous. Also, since they spread throughout the environment, many scientists and environmentalists have shown concerns about their effects on other species.

Scientists believe that dioxins are toxic because their size and shape lets them mimic the natural chemicals that act as messengers in the body. Hormones control growth, development, reproduction, and the function of tissues. When toxins act like hormones, they disrupt the body's normal growth and functions, possibly leading to birth defects, cancer, and many other problems. TCDD and the other toxic dioxins have four chlorine atoms in the far corners of the molecule (designated as positions 2, 3, 7, and 8), and they are nearly flat, or planar. Dioxins without chlorine in these positions are less planar, and they are either less toxic or nontoxic. Also, dioxins with more chlorine atoms than TCDD has are less planar and less toxic than TCDD. Researchers believe that the planar dioxins must act more like hormones than the other dioxins do.

Dioxins were first discovered in 1970 as an impurity in a herbicide, or plant-killing chemical. The United StatesSouth Vietnam during the Vietnam War to destroy the thick jungle and make it easier to find hidden enemy troops. Agent Orange contains the chemicals 2, 4, 5-trichlorophenoxyacetic acid (commonly referred to as 2, 4, 5-T) and 2, 4-dichlorophenoxyacetic acid (commonly referred to as 2, 4-D), along with dioxin impurities. armed forces sprayed this herbicide over

South Vietnamese newspapers reported that many babies born in areas sprayed with Agent Orange had birth defects. In addition, the offspring of laboratory animals that were fed Agent Orange had serious deformities. Scientists guessed that the deformities might be caused by the dioxin impurity, not the chemicals 2, 4, 5-T or 2, 4-D. Now it is believed that dioxin causes cancer in animals and probably leads to cancer in humans. Dioxins may cause a severe form of acne, severe headaches, weight loss, liver damage, and nerve damage. They may also cause problems with reproduction, the immune system, enzymes, and genes. In 1979 the EPA started to develop rules to lower dioxins in the environment. For example, factories that produce dioxins must control the amount that is released into the air, and paper bleaching must be carefully controlled so that dioxins are not released.