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Dioxin

Dioxin


Dioxin, formed by burning chlorine-based chemical compounds with hydrocarbons, is one of the most toxic chemicals known. Dioxin is a general term that describes a group of hundreds of chemicals that are highly persistent in the environment. It was the highly toxic impurity of Agent Orange, a defoliant used during the Vietnam War, and was the basis for evacuations in Times Beach, Missouri, and Seveso, Italy.

Dioxin has become the synonym for polychlorinated dibenzo[1,4]dioxins (PCDD) in general or the most toxic congener 2,3,7,8-tetrachloro dibenzo[1,4]dioxin (TCDD) in particular. PCDD comprise a group of seventy-five structurally similar compounds, so-called congeners. The individual congeners differ in the number and positions of the chlorine atoms in the molecule. Structurally related are the polychlorinated dibenzofurans (PCDF) (135 congeners). PCDD and PCDF are stable and fat-soluble molecules classified as persistent organic pollutants (POPs). The central element of the PCDD is the dioxin 6-ring, containing two oxygen atoms, while in PCDF it is the furan 5-ring, containing one oxygen atom. Apart from the structural similarity, they are often formed in the same processes, such as combustion.

PCDD and PCDF are formed incidentally in combustion and chemical production processes. Combustion processes include municipal and backyard waste incineration, residential and industrial burning of wood and coal, internal combustion in vehicle engines, and forest fires. Prerequisites for PCDD and PCDF formation are the presence of carbon compounds and inorganic or organic chlorine sources (e.g., sodium chloride or polyvinyl chloride). Their creation generally requires temperatures around 300°C and an excess or lack of oxygen. PCDD and PCDF are present as impurities in a variety of chemical products.

Chlorophenol-based pesticides like 2,4,5-T, a phenoxy herbicide, were responsible for up to half the environmental emissions of PCDD and PCDF in the early 1970s. A 1:1 mixture of 2,4-D and 2,4,5-T was used in Agent Orange; it contained elevated levels of PCDD. An estimated 110 to 170 kilograms (kg) of 2,3,7,8-TCDD alone were sprayed with herbicides over Vietnam. Consequently, acute and chronic health effects were observed in the exposed population and military personnel. Further examples of PCDD-contaminated chemical products are the wood, leather, and textile preservative pentachlorophenol, the widely used antimicrobial triclosan, and PCDF in polychlorinated biphenyls (PCBs). Certain thermal metallurgical processes as well as chlorine-based pulp and paper bleaching emit PCDD and PCDF into the environment. In its 1995 inventory, the U.S. Environmental Protection Agency (EPA) identified municipal and medical waste incineration, backyard refuse barrel burning, secondary copper smelting, and cement kilns burning hazardous waste as the largest current sources of dioxin-like compounds.

It has been claimed that 2,3,7,8-TCDD is the most toxic human-made compound, an ultra-poison. Although some evidence supports this view, the results of many studies are still controversial. 2,3,7,8-TCDD is extremely toxic to guinea pigs (the lethal dose is approximately 1 microgram or μg). For other animals and apparently for humans the acute toxicity is considerably lower. Long-term effects appear to be more serious. 2,3,7,8-TCDD was found to be the most potent multisite carcinogen in test animals, and PCDD and PCDF are also a human carcinogen. Noncancer effects may pose an even greater threat to human health. These include effects on reproduction and sexual development, teratogenic effects, endocrine disruption, suppression of the immune system, and neurological effects. PCDD and PCDF bioaccumulate and are stored in the body fat of higher organisms. About 95 percent of human exposure originates from food, especially fish, meat, eggs, and dairy products.

A 1990 global emission inventory estimated that municipal waste incineration was the major source of PCDD and PCDF in the environment. Abatement strategies focus on this issue. State-of-the-art incinerators may act as sinks for dioxins, but they are expensive to construct and operate. At a higher cost-efficiency, PCDD and PCDF reduction could be achieved by a reduction in the amount of garbage to be destroyed (e.g., by manufacturing long-lived products and using less packaging), a cessation of open waste burning, and technical improvements in as of yet less strictly regulated facilities such as cement kilns and metallurgical industries.

The remediation of dioxin-contaminated sites requires sophisticated abatement methods. Cleaning the Times Beach Superfund site was a ten-year effort that included the relocation of its inhabitants, construction of a series of spur levees surrounding the site to prevent floodwater from carrying contaminated soil off-site, installing a temporary incinerator, and the excavation and burning of 265,000 tons of contaminated soil.

see also Burn Barrels; Cancer; Endocrine Disruption; Health, Human; Incineration; Medical Waste; Persistent Organic Pollutants (POPs); Superfund; Times Beach, Missouri; War.

Bibliography

IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. (1997). Polychlorinated Dibenzo-para-dioxins and Polychlorinated Dibenzofurans. Lyon, France: World Health Organization, International Agency for Research on Cancer.

Safe, Stephen; Hutzinger, Otto; and Hill, T.A., eds. (1990). Polychlorinated Dibenzo-pdioxins and -furans (PCDDs/PCDFs): Sources and Environmental Impact, Epidemiology, Mechanisms of Action, Health Risks. New York: Springer-Verlag.

Young, Alvin L., and Reggiani, G.M., eds. (1988). Agent Orange and Its Associated Dioxin: Assessment of a Controversy. New York: Elsevier.


Internet Resource

Activists' Center for Training in Organizing and Networking (ACTION). Available from http://www.ejnet.org/dioxin.

Stefan Weigel

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Dioxin

Dioxin

The term dioxin refers to a large group of organic compounds that are structurally related to benzene (a colorless, flammable, and toxic [poisonous] liquid hydrocarbon, meaning it contains both carbon and hydrogen atoms) and may contain one or more chlorine atoms in their structures. Those compounds that do contain chlorine are known as chlorinated dioxins and are of the greatest environmental interest today.

Production and use

Dioxins have no particular uses. They are not manufactured intentionally but are often formed as by-products of other chemical procedures. Two such processes involve the manufacture of 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) and hexachlorophene. 2,4,5-T was once a popular herbicide (weed-killing agent), while hexachlorophene was an antibacterial agent used in soaps and other cleaning products. The use of both compounds has now been banned in the United States.

Dioxins are also formed as by-products of other industrial operations, such as the incineration of municipal wastes and the bleaching of wood pulp.

Toxicity

All 75 chlorinated dioxins known to science are believed to be toxic to some organisms at one level or another. The most toxic of these compounds is believed to be TCDD, or 2,3,7,8-tetrachlorodibenzo-p-dioxin. The differences in toxicities of the chlorinated dioxins is illustrated by the effects of TCDD on guinea pigs, hamsters, and humans.

The toxicity of a substance is commonly measured by a property known as LD50. LD50 stands for "lethal dose50 percent." That is, the LD50 for a substance is the amount of that substance needed to kill onehalf of a test population of animals in some given period of time, usually a few days.

The LD50 for TCDD for guinea pigs is 0.0006 mg/kg (milligrams per kilogram). That is, adding no more than 0.0006 milligram of TCDD per kilogram of body weight will kill half of any given population of guinea pigs. In contrast, the LD50 for hamsters is 0.045 milligrams per kilogram, making them thousands of times more resistant to TCDD than guinea pigs.

The LD50 for TCDD for humans cannot be determined the way it is for experimental animals. (Scientists can't just add TCDD to the diet of humans to see how much is needed to kill half the individuals in a sample.) However, researchers do have data about the health effects of TCDD on humans from other sources. The most important of these sources are studies of: (1) industrial exposures to toxins of chemical workers, (2) people living near a toxic waste dump at Times Beach, Missouri, and (3) an accidental release of TCDD at Seveso, Italy, in 1976.

The accident in Italy involved an explosion at a chemical plant that released between 2 to 10 pounds (1 and 5 kilograms) of TCDD to the surroundings. Residues as large as 51 ppm (parts per million) were later detected in environmental samples. This accident caused the deaths of some livestock and 187 cases of chloracne among humans. Chloracne is a skin condition caused by exposure to chlorine or certain of its compounds. But scientific studies failed to find increased rates of disease among those exposed to TCDD or a higher rate of birth defects among the offspring of pregnant women in the population.

Overall, studies suggest that humans are among those animals least affected by TCDD. Chloracne is probably the most common symptom of exposure to TCDD. The data on other health effects, such as disease (primarily cancer), deaths, and birth defects are much less clear. Some scientists argue thatexcept for massive exposures to the chemicalTCDD should be of little or no concern to health scientists. Other scientists are especially troubled, however, by possible effects resulting from long-term exposures to even small doses of TCDD.

TCDD in Vietnam

Some of the most troubling questions about dioxin concern the use of Agent Orange during the Vietnam War (a civil war between communist North Vietnam and noncommunist South Vietnam, fought mainly in the 1960s and 1970s; the United States began bombarding the North in 1964, but U.S. troops were withdrawn in 1973, shortly before the North's victory).

Agent Orange is a 50:50 mixture of 2,4,5-T and a related compound, 2,4-D (2,4-dichlorophenoxyacetic acid). The U.S. military sprayed large quantities of Agent Orange over the Vietnam countryside during the war in order to deprive the Vietnamese of food and cover. According to some estimates, more than 56,000 square miles (1.5 million hectares) of Vietnamese land were sprayed at least once.

Authorities believe that the Agent Orange used in Vietnam was contaminated by TCDD at concentrations averaging about 2 parts per million. If true, a total of 240 to 375 pounds (110 to 170 kilograms) of TCDD was sprayed with herbicides onto Vietnam.

Many veterans of the Vietnam War have claimed that exposure to TCDD caused them serious medical problems. A number of studies have been carried out by both governmental and private organizations, but so far those studies have not provided clear and convincing proof of the veterans' claims. Veterans' groups and other interested citizens, however, continue to push their cases about possible health effects from exposure to Agent Orange and TCDD.

[See also Agrochemicals ]

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dioxin

dioxin A member of a range of about 300 compounds produced as by-products of certain industrial chemical processes and also by the incomplete incineration of chlorinated hydrocarbon compounds, especially polychlorinated biphenyls, when the incineration temperature is lower than about 1200°C. The name ‘dioxin’ usually refers to one member of the group, 2,3,7,8-tetrachlorodibenzo-p-dioxin (C12H4Cl4O2), severe exposure to which can cause chloracne in humans.

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"dioxin." A Dictionary of Ecology. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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"dioxin." A Dictionary of Ecology. . Retrieved August 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/dioxin

dioxin

dioxin Any of various poisonous chemicals. The compound most commonly known as dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a by-product and impurity in the manufacture of various disinfectants and herbicides. It is also produced in the burning of chlorinated chemicals and plastics. Dioxin causes skin disfigurement and is associated with birth defects, cancer and miscarriages. Accidental releases of dioxin from chemical plants have caused several major disasters.

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