Dioxin

Chlorinated dioxins are a diverse group of organic chemicals. TCDD, or 2,3,7,8-tetrachlorodibenzo-p-dioxin, is a particular dioxin that is toxic to some species of animals in extremely small concentrations. As such, TCDD is the most environmentally controversial of the chlorinated dioxins, and the focus of this entry.

TCDD and other dioxins

Dioxins are a class of organic compounds, with a basic structure that includes two oxygen atoms joining a pair of benzene rings. Chlorinated dioxins have some amount of substitution with chlorine for hydrogen atoms in the benzene rings. A particular chemical, 2,3,7,8-tetrachlorodibenzo-p-dioxin (abbreviated as TCDD, or as 2,3,7,8-TCDD), is one of 75 chlorinated derivatives of dibenzo-p-dioxin. There is a very wide range of toxicity within the larger group of dioxins and chlorinated dioxins, but TCDD is acknowledged as being the most poisonous dioxin compound, at least to certain species of animals.

Dioxins have no particular uses. They are not manufactured intentionally, but are synthesized incidentally during some industrial processes. For example, under certain conditions relatively large concentrations of dioxins are inadvertently synthesized during industrial reactions involving 2,4,5-trichlorophenol. A well known case of this phenomenon is in the manufacturing of the phenoxy herbicide, 2,4,5-T (2,4,5-trichlorophenoxy acetic acid ). This chemical (which is no longer used) was once manufactured in large amounts, and much of the material was badly contaminated by TCDD. Concentrations in the range of 10-50 parts per million (ppm, or mg per liter) occurred in 2,4,5-T manufactured for use during the Vietnam War. However, there was a much smaller contamination (less than 0.1 ppm) in 2,4,5-T manufactured after 1972, in accordance with regulations enacted by the U.S. Environmental Protection Agency.

TCDD is also a trace contaminant of other products manufactured from trichlorophenol, including hexachlorophene, once commonly used as a topical antibacterial treatment. TCDD is incidentally synthesized when wood pulp is bleached using chlorine-based oxidants. The low-temperature combustion of chlorine-containing organic materials (for example, in cigarettes, burning garbage dumps, and barbecues) also produce dioxins, including TCDD. The incineration of municipal waste synthesizes small quantities of TCDD, although the relatively high temperatures reduce the yield of dioxins compared with the smoldering kinds of combustion just mentioned. Dioxins are also synthesized naturally in trace quantities, mostly during forest fires.

TCDD is a persistent chemical in the environment, and because it is virtually insoluble in water , but highly soluble in fats and oils, it strongly biomagnifies and occurs in especially large concentrations in predators at the top of the ecological food web. Moreover, TCDD is globally distributed, meaning that any chemical analysis of a biological tissue , especially of the fat of an animal , will detect residues of this dioxin (assuming that the analytical chemistry is sensitive enough).

Toxicity

TCDD is the most toxic of the chlorinated dioxins, while octachlorodioxin may be the least so. However, there are large differences in the susceptibility of species to suffering toxicity from TCDD. The guinea pig, for example, is extremely sensitive to TCDD, thousands of times more so than the hamster.

Short-term or acute toxicity is often indicated by a laboratory assay known as LD50, or the dose of a chemical required to kill one-half of a test population of organisms over a period of several days. Guinea pigs have a LD for TCDD in food of only 0.0006 mg/kg (that is, 0.0006 mg of TCDD per kg body weight). In comparison, rats have a LD50 for TCDD in food of 0.022-0.045 mg/kg, while hamsters have a LD50 of 1 mg/kg. Clearly, TCDD is a highly toxic chemical, although species vary greatly in sensitivity.

Depending on the dose and biological sensitivity, the symptoms of TCDD toxicity in mammals can include severe weight loss, liver damage, lesions in the vascular system, stomach ulcers , a persistent acne known as chloracne, birth defects , and ultimately, death.

Much of what is known about the toxicity of TCDD to humans has come from studies of: (1) industrial exposures of chemical workers, (2) people living near a toxic waste dump at Times Beach, Missouri, and (3) an accidental event at Seveso, Italy, in 1976. The latter case involved an explosion at a chlorophenol plant that released an estimated 2.2-11 lb (1-5 kg) of TCDD to the surroundings, and caused residues as large as 51 ppm to occur in environmental samples. This accident caused the deaths of some livestock within 2-3 days, but remarkably it was not until 2.5 weeks had passed that about 700 people were evacuated from the severely contaminated residential area near the factory. The exposure of humans to TCDD at Seveso caused 187 diagnosed cases of chloracne, but there were apparently no statistically detectable increases in the rates of other human diseases, or of deformities of children born to exposed women.

Overall, studies of humans suggest that they are among the least-sensitive mammals to suffering toxicity from TCDD. While chloracne is a common symptom of an acute human exposure to TCDD, the evidence showing increased rates of TCDD-related disease , mortality, cancer , or birth defects are equivocal, and controversial. Some scientists believe there is no evidence that a human has ever died from an acute exposure to TCDD. However, there is unresolved scientific controversy about the possible effects of longer-term, chronic exposures of humans to TCDD, which might result in increased rates of developmental abnormalities or cancers. Unless large, these effects would be difficult to detect, because of the great environmental and genetic variations that must be overcome in epidemiological studies of humans.

TCDD in Vietnam

To deprive their enemy of food and cover during the Vietnam War, the U.S. military sprayed large quantities of herbicides . More than 547,000 sq mi (1.4 million ha) of terrain were sprayed at least once. The most commonly used herbicide was a 50:50 mixture of 2,4,5-T and 2,4-D, known as Agent Orange . More than 46 million lb (21 million kg) of 2,4,5-T and 55 million lb (25 million kg) of 2,4-D were sprayed during this extensive military program.

An important aspect of the military use of herbicides in Vietnam was contamination of the 2,4,5-T by TCDD. A concentration as large as 45 ppm was measured in Agent Orange, but the average concentration was about 2 ppm. In total, 243-375 lb (110-170 kg) of TCDD was sprayed with herbicides onto Vietnam.

Because TCDD is known to be extremely toxic to some laboratory animals, there has been tremendous controversy over the possible short and long-term effects of exposure of soldiers and civilians to TCDD in Vietnam. Although claims have been made of effects in exposed populations, the studies have not been convincing to many scientists, and there is still controversy. The apparent, mainstream opinion from the most rigorous epidemiological studies suggests that large toxic effects have not occurred, which is encouraging. It is also likely that the specific effects of TCDD added little to the very substantial ecological effects caused by the use of military use of herbicides, and other weapons of mass destruction, during the Vietnam War.

See also Biomagnification.

Resources

books

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

periodicals

Harris, W.E. "Dioxins—An Overview." Tappi Journal (April 1990): 267-69.

Bill Freedman

KEY TERMS

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Acute toxicity

—A poisonous effect produced by a single, short-term exposure to a toxic chemical, resulting in obvious tissue damage, and even death of the organism.

Chronic toxicity

—This is a poisonous effect that is produced by a long period of exposure to a moderate, sub-acute dose of some toxic chemical. Chronic toxicity may result in anatomical damages or disease, but it is not generally the direct cause of death of the organism.

Epidemiology

—The study of the incidence, control, and transmission of diseases in large populations.