Dioxins
March 2006
Term | Overview |
Dioxins |
Dioxins, or chlorinated dibenzo-p-dioxins (CDDs), are a class of structurally similar chlorinated hydrocarbons. The basic structure is comprised of two benzene rings joined via two oxygen bridges at adjacent carbons on each of the benzene rings. There are eight homologues of CDDs, monochlorinated through octachlorinated. Each homologous group contains one or more isomers or congeners. |
Dioxin |
A term used interchangeably with 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCCD or TCDD). This is the most toxic form of dioxins. |
Dioxin-like compounds |
These compounds are from a group of halogenated aromatic hydrocarbons that have molecules shaped like TCDD. They produce similar toxic effects as dioxin. They include certain chlorinated dibenzofurans (CDFs), polychlorinated biphenyls (PCBs), polybrominated biphenyls (PBBs), brominated dibenzo-p-dioxins (BDDs), and brominated dibenzofurans (BDFs). |
Not all dioxins have the same toxicity or ability to cause illness and adverse health effects. The most toxic chemical in the group is TCDD. It is the standard to which other dioxins are compared. The levels of other dioxins measured in the environment are converted to a TCDD-equivalent concentration on the basis of how toxic they are compared to TCDD. These converted dioxin levels are then added together to determine the total equivalent (TEQ) concentration of the dioxins in a sample.
The halogenated dioxins and furans are a diverse set of chemicals.
Dioxins are not intentionally produced and have no known use. They are the by-products of various industrial processes (i.e., bleaching paper pulp, and chemical and pesticide manufacture) and combustion activities (i.e., burning household trash, forest fires, and waste incineration). The defoliant Agent Orange, used during the Vietnam War, contained dioxins. Dioxins are found at low levels throughout the world in air, soil, water, sediment (the bottom of rivers, streams, and lakes), and in foods such as meats, dairy, fish, and shellfish. The highest levels of dioxins are usually found in soil, sediment, and in the fatty tissues of animals. Much lower levels are found in air and water.
CDDs are not manufactured commercially in the United States except on a small scale for use in chemical and toxicological research. They are unique among the large number of organochlorine compounds of environmental interest in that they were never produced intentionally as desired commercial products.
Dioxins are produced as by-products of incomplete combustion and some chemical processes, including natural ones.
Dioxins enter the physical environment by release during
The chart below lists specific sources and routes of exposure.
Specific sources and routes of exposure | |
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Exposure through diet |
The major sources of human exposure (96%) are
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Exposure through air |
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Exposure through occupation |
People who perform the following types of work can be exposed to dioxins:
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Accidental exposure |
Rare cases of high level exposures through industrial accidents such as occurred in Seveso, Italy. |
Exposure from waste sites |
Potential chronic low level exposure by living next to a hazardous waste site containing dioxins. Currently, in the United States, there are 126 Superfund sites (with a completed exposure pathway) contaminated with dioxins. |
Toxic effects of dioxin exposure | |
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Biochemical effects |
Dioxin is believed to exert its toxicity by acting on
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Cellular effects |
Dioxins are believed to affect the growth regulation of cells. They can induce or block programmed cell death (apoptosis). Thus, dioxins can induce either cell proliferation or differentiation. This may lead to tissue underdevelopment (hypoplasia), overgrowth (hyperplasia), transformation (metaplasia), or tumor formation (neoplasia). |
Carcinogenicity |
TCDD carcinogenicity in animals is well established. All cancer bioassays have been positive. They show that dioxin is a complete carcinogen that does not require another chemical exposure for full action.
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Confirmed human health effects (noncancers) |
Dioxins can cause several health effects:
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Suspected health effects |
Suspected health effects of dioxins include the following:
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Distribution Methods | |
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Accumulates in body fat |
After being absorbed, dioxin distributes to organs according to lipid (fat) content and readily accumulates in body fat. The 2000-2001 National Health and Nutrition Examination Survey (NHANES) survey found dioxin in serum at the range of 6.4 to 7.4 picogram/grams (serum lipid adjusted) in certain U.S. populations (5.8 pg/g lipid is the detection limit). |
Elimination |
Levels of dioxin and dioxins in the environment have been declining since the 1980s.
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Medical Tests | |
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Blood and fatty tissue tests for TCDD levels |
Fatty tissue and blood serum can be analyzed for the presence of TCDD by gas chromatography-mass spectrometry (GC-MS). This method can quantify TCDD to 100 parts per quadrillion. Serum levels have been correlated with fatty tissue levels in persons with long-term exposure. However, analyses of serum or fat TCDD levels by GC-MS are expensive and time-consuming. The testing is not done routinely, unless there is a valid concern regarding acute high dose or significantly increased exposure. |
Agency | Guidelines |
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WHO |
Total Daily Intake 1-4 picograms per kilogram per day (pg/kg/day) |
EPA |
No established reference concentrations (RfC) or reference doses (RfD) |
ATSDR Minimal Risk Levels (MRLs) | |||
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Name | Route | Duration | MRL |
Dioxins | Oral | Acute | 200 pg/kg/day |
Intermediate | 20 pg/kg/day | ||
Chronic | 1 pg/kg/day | ||
pg/kg/day = picograms per kilogram of body weight per day |
Factors that determine how severe the health effects are from dioxin exposure include
Physical and chemical properties |
Information on physical and chemical properties of certain congeners (particularly 1,2,3,7,8,-pentachlorodibenzo-p-dioxin and 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin) would help clarify the different fate and transport pathways of the homologous groups |
Production, import/export, use, and release and disposal |
CDD releases are not required to be reported to EPA, so there are no Toxics Release Inventory data for CDDs. |
Environmental fate |
A better understanding of how CDDs behave in the environment will help answer such questions as
Information regarding the degradation of other congeners, specifically octachlorodibenzo-p-dioxin (OCDD), and their degradation products in water, sediment, and soil would be useful in evaluating the various pathways of human exposure. |
Bioavailability from environmental media |
More information is needed about oral and skin exposures to determine the bioavailability of CDDs from food, water, and soil. Â More information is needed to examine the discrepancy in the mass balance from CDDs ingested from foods and eliminated in feces. For inhalation exposure, information on the bioavailability from fly ash and sediments would be useful. Information is also needed on the selective uptake of the 2,3,7,8-substituted CDD congeners. |
Food chain bioaccumulation |
Information is needed from a larger number of species on their retention of 2,3,7,8-substituted CDD congeners. General information on retention and distribution of other CDDs would be useful in better understanding aquatic and terrestrial food chains. |
Exposure levels in environmental media |
Reliable monitoring data for the levels so that the information obtained on levels known body burdens of CDDs to assess the vicinity of hazardous waste sites. |
Exposures levels in humans |
Additional biological monitoring data are needed for those U.S. populations surrounding hazardous waste sites or municipal, medical, or industrial incinerators. Comparisons are needed for urban versus rural exposures. Data are also needed for other potentially exposed populations such as subsistence fishers and hunters. Information on tissue levels in the general population worldwide is mostly deficient. As they are identified, exposed populations should be evaluated to characterize exposure levels and health effects. This information may show a need to conduct health studies on these populations. |
ATSDR online resources |
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This page was updated on 01/04/2008