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.
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.
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 LD 50 . LD 50 stands for "lethal dose—50 percent." That is, the LD 50 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 LD 50 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 LD 50 for hamsters is 0.045 milligrams per kilogram, making them thousands of times more resistant to TCDD than guinea pigs.
The LD 50 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 that—except for massive exposures to the chemical—TCDD 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.
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 ]