The chlorinated dibenzo-p-dioxins are a class of compounds that are loosely referred to as dioxins. There are 75 possible dioxins. The one with four chlorine atoms at positions 2, 3, 7 and 8 of the dibenzo-p-dioxin chemical structure is called 2,3,7,8- tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). It is a colorless solid with no known odor. 2,3,7,8-TCDD does not occur occur naturally nor is it intentionally manufactured by any industry, except as a reference standard. It can be inadvertently produced in very small amounts as an impurity during the manufacture of certain herbicides and germicides and has been detected in products of incineration of municipal and industrial wastes. At the present time, 2,3,7,8-TCDD is not used for any purpose than scientific research.

Structural diagram: National Institutes of Health

Exposure Pathways

The main environmental sources of 2,3,7,8-TCDD are:

Use of herbicides containing 2,4,5-Trichlorophenoxy acids (2,4,5-T)

Production and use of 2,4,5-Trichlorophenol in wood preservatives

Production and use of hexachlorophene as a germicide

Pulp and paper manufacturing plants

Incineration of municipal and certain industrial wastes

Small amounts formed during the burning of wood in the presence of chlorine

Accidental transformer/capacitor fires involving chlorinated benzenes and biphenyls

Exhaust from automobiles powered with leaded gasoline

Improper disposal of certain chlorinated chemical wastes

Although 2,4,5-T, 2,4,5-Trichlorophenol and hexachlorophene are no longer produced commercially (except for certain medical purposes), disposal sites of past production wastes are still sources of present exposure. 2,3,7,8-TCDD has been found in at least 28 of the 1,177 hazardous waste sites on the National Priorities List (NPL). Very low levels of 2,3,7,8-TCDD have been detected in ambient air. Detection of 2,3,7,8-TCDD in drinking water has not been reported. 2,3,7,8-TCDD has not been detected in most rural soils examined, but it can be present at trace levels in urban soils. The highest concentration of 2,3,7,8-TCDD was detected in a waste-oil-contaminated soil in Missouri that contained a 2,3,7,8-TCDD level more than one million times higher than soils from normal urban areas. 2,3,7,8-TCDD was detected in fish obtained from the contaminated sections of Lake Ontario, Saginaw Bay, the Michigan rivers, and several watersheds including those from Maine, Wisconsin, and Minnesota. In human milk, minute amounts of 2,3,7,8-TCDD have been detected in the United States and in several European countries.

Consumer sources are:

Skin contact with surfaces such as soil and vegetation contaminated by the chemical

Skin contact and inhalation of wood dusts from use of pentachlorophenol-treated woods

Inhalation of air near improperly maintained dump sites or municipal incinerators

Consumption of fish and cow's milk from contaminated areas

Consumption of breast milk containing 2,3,7,8-TCDD by babies

Minute exposure from the use of paper towels, napkins, coffee filters, computer papers, and other contaminated paper products.

Workers at risk of contacting 2,3,7,8-TCDD are:

Workers who have been involved in the production or use of trichlorophenol and salts, hexachlorophene, and 2,4,5-T or other herbicides containing this chemical. The production of 2,4,5-T and 2,4,5-Trichlorophenol, however, has been discontinued in the United States.

Workers in the pulp and paper industry

Workers at certain municipal and industrial incinerators

Workers involved in the high-temperature/pressure treatment of woods with pentachlorophenol

Workers at certain hazardous waste sites

Workers involved in the cleanup of certain accidental capacitor/transformer fires and in the salvaging of transformers

Workers who have been involved in spraying of phenoxy herbicides such as Agent Orange

2,3,7,8-TCDD can enter your body through:

Absorption through skin from contaminated soils and other materials

Ingestion of 2,3,7,8-TCDD through the consumption of contaminated fish, cow's milk, foodstuffs, and, in the case of small children, soil

Breathing contaminated ambient air. This may contribute very small amounts to total body intake; however, particulates such as fly ash from municipal and industrial incineration may constitute a major source of exposure

Intake of 2,3,7,8-TCDD from the consumption of drinking water should be negligible

According to one estimate of ambient exposure, breathing air constitutes 2 percent, drinking water less than 0.01 percent, and consuming foods 98 percent of the total human exposure to 2,3,7,8-TCDD. No estimate of relative intake of 2,3,7,8-TCDD due to skin absorption is available.

Health Effects

In humans, 2,3,7,8-TCDD causes chloracne, a severe skin lesion that usually occurs on the head and upper body. Unlike common acne, chloracne is more disfiguring and often lasts for years after the initial exposure.

There is suggestive evidence that 2,3,7,8-TCDD causes liver damage in humans, as indicated by an increase in levels of certain enzymes in the blood, although these effects might also have resulted from the concomitant exposure to the chemicals contaminated with 2,3,7,8-TCDD or to the solvents in which these chemicals are usually dissolved. Animal studies have demonstrated severe liver damage in some species.

There is suggestive evidence that 2,3,7,8-TCDD causes loss of appetite, weight loss, and digestive disorders in humans, although these effects might also have resulted from the concomitant exposure to the chemicals contaminated with 2,3,7,8-TCDD or to the solvents in which these chemicals are usually dissolved. Animal exposure to 2,3,7,8-TCDD results in severe loss of body weight prior to death.

Although not demonstrated in humans, in animal studies 2,3,7,8-TCDD produced toxicity to the immune system. This toxicity can result in greater susceptibility to infection.

Although not demonstrated in humans, in some animal species exposure to 2,3,7,8-TCDD resulted in adverse reproductive effects including spontaneous abortions. The monkey is very sensitive to this toxic property of 2,3,7,8-TCDD.

Although not demonstrated in humans, in some animal species exposure to 2,3,7,8-TCDD during pregnancy resulted in malformations in the offspring. Low levels of 2,3,7,8-TCDD have been detected in human milk, but the effects on infants and children are unknown.

The human evidence for 2,3,7,8-TCDD alone is inadequate to demonstrate or reflect a carcinogenic hazard, although certain herbicide mixtures containing a 2,3,7,8-TCDD as an impurity provide limited evidence of causing cancer in exposed humans. Based on the positive evidence in animal studies, 2,3,7,8-TCDD is probably carcinogenic in humans.

Information excerpted from:

Toxicological Profile for Dioxin June 1989

Agency for Toxic Substances and Disease Registry
U.S. Dept. of Health and Human Services