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Dioxin: Summary of the Dioxin Reassessment
Scientists from the Environmental Protection Agency (EPA), other federal agencies and the general
scientific community have conducted a reassessment of dioxin exposure and human health effects since 1991. This information sheet summarizes the draft reassessment, which is entitled Exposure and Human HealthReassessment of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) and Related Compounds.
A more in-depthdiscussion can be found in the companion piece, Dioxin: Scientific Highlights from Draft Reassessment(2000)
The term “dioxin” refers to a group of chemical compounds that share certain similar chemical
structures and mode-of-action biological characteristics. A total of 30 of these dioxin-like compounds existand are members of three closely related families: the chlorinated dibenzo-p
-dioxins (CDDs), chlorinateddibenzofurans (CDFs) and certain polychlorinated biphenyls (PCBs). The term dioxin is also used for themost well-studied and one of the most toxic dioxins, 2,3,7,8-tetrachlorodibenzo-p
-dioxin (TCDD). CDDsand CDFs are not created intentionally, but can be produced inadvertently in nature and by a number ofhuman activities. Combustion, chlorine bleaching of pulp and paper, certain types of chemicalmanufacturing and processing, and other industrial processes all can create small quantities of dioxins. PCBsare no longer manufactured in the United States but formerly were widely used as coolants and lubricants inelectrical equipment.
Combining Risks from Dioxins - the Toxicity Equivalents Approach:
Dioxins are believed to cause toxic effects in similar ways; that is, they share a “common mechanism
of toxicity.” As a result, EPA and others use an approach that adds together the toxicity of individual dioxinsin order to evaluate complex environmental mixtures to which people are exposed. Because dioxins differ intheir toxic potential, the toxicity of each component in the mixture must be accounted for in estimating theoverall toxicity. To do so, international teams of scientists have developed Toxicity Equivalency Factors thatcompare the toxicity of different dioxins. Given these factors, the toxicity of a mixture can be expressed interms of its Toxicity Equivalents (TEQ), which is the amount of TCDD it would take to equal the combinedtoxic effect of all the dioxins found in that mixture. The use of the TEQ approach represents a keyassumption upon which many of the conclusions in the reassessment are based.
The reassessment finds that, based on all available information, dioxins are potent animal toxicants
with potential to produce a broad spectrum of adverse effects in humans. Dioxins can alter the fundamentalgrowth and development of cells in ways that have the potential to lead to many kinds of impacts. Theseinclude, for example, adverse effects upon reproduction and development; suppression of the immunesystem; chloracne (a severe acne-like condition that sometimes persists for many years); and cancer. EPAcharacterizes TCDD as a “human carcinogen” based on the weight of evidence of animal and human studiesand characterizes other dioxins as “likely human carcinogens.”
The reassessment proposes that most dioxin enters ecological food webs by being deposited from the
atmosphere, either directly following air emissions or indirectly by processes that return dioxins already in the
environment to the atmosphere. Once they reach the environment, dioxins are highly persistent and canaccumulate in the tissues of animals. EPA estimates that most dioxin exposure occurs through the diet, withover 95% of dioxin intake for a typical person coming through dietary intake of animal fats. Small amountsof exposure occur from breathing air containing trace amounts of dioxin on particles and in vapor form, frominadvertent ingestion of soil containing dioxin, and from absorption through the skin contacting air, soil, orwater containing minute levels. These processes result in widespread, low-level exposure of the generalpopulation to dioxins.
Dioxin levels in the environment have declined significantly since the 1970s following EPA
regulatory controls and industry actions. EPA’s best estimates of emissions from sources that can bereasonably quantified, indicate that dioxin emissions in the United States decreased by about 80% between1987 and 1995, primarily due to reductions in air emissions from municipal and medical waste incinerators,and substantial further declines continue to be documented. Dietary intake of dioxin also appears to bedeclining.
Dioxin Effects in Human Populations
EPA estimates that the amount of dioxin found in the tissues of the general human population (which
is known as the “body burden”) closely approaches (within a factor of 10) the levels at which adverse effectsmight be expected to occur, based on studies of animals and highly exposed human populations. Despite thepotential risks, currently there is no clear indication of increased disease in the general population attributableto dioxin-like compounds. This may be due to limitations of current data and scientific tools rather thanindicating that dioxin exposure is not causing adverse effects. For cancer, EPA estimates that the risks forthe general population based on dioxin exposure could be as high as the range of a 1 in 100 to 1 in 1,000increased chance of experiencing cancer related to dioxin exposure. Actual risks are unlikely to exceed thisvalue and may be substantially less. This range for cancer risk indicates an about 10-fold higher chance thanestimated in EPA’s earlier (1994) draft of this reassessment.
Children and Other Groups of Concern
Fetuses, infants, and children may be more sensitive to dioxin exposure because of their rapid growth
and development. Data on risks to children are limited, however, and it is not known if the children in thegeneral population are experiencing adverse effects from dioxin. Although breast milk appears to be asignificant source of dioxin exposure for nursing infants, the overwhelming body of evidence supports thehealth benefits of breastfeeding despite the potential presence of dioxin. Other populations have experiencedelevated exposures to dioxin as a result of food contamination incidents around the world, through theworkplace or from industrial accidents, or from consumption of unusually high amounts of fish, meat, ordairy products containing elevated levels of dioxins. In some cases, such as U.S. Air Force personnelexposed to the herbicide Agent Orange contaminated with dioxin during the Vietnam War, dioxin exposurehas been associated with adverse health effects.
William H. Farland, NCEA, ORD (8601D), Washington, DC 20460 E-Mail:firstname.lastname@example.org
Tel: 202-564-3322; FAX: 202-565-0090
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