Dioxin-like Compounds (Dioxins, Furans, and some PCBs and PBBs)

Nature of Dioxin-like Compounds

Dioxin-like compounds are all halogenated aromatic hydrocarbons. "Halogenated" in this case means that they contain either chlorine or bromine. Chlorine and bromine each can create three different groups of these compounds. The group of polychlorinated dibenzodioxins (PCDDs) includes 75 individual compounds, all structurally similar but different in detail. These related compounds are called congeners. Seven of the 75 CDD congeners have dioxin-like toxicity. The group of polychlorinated dibenzofurans (PCDFs) includes 135 congeners, 10 of which have dioxin-like toxicity. (Non-chlorinated dibenzofurans are still toxic, but not as toxic as dioxin-like compounds.) Lastly, some polychlorinated biphenyls (PCBs) have a coplanar structure that gives them dioxin-like toxicity in addition to the usual toxicity of PCBs. Thirteen out of the 209 possible PCB congeners have this property. All of these dioxin-like compounds can be formed with bromine instead of chlorine; in that case they are called PBDDs, PBCFs, or PBBs. The property that appears to give dioxin-like properties is chlorine or bromine in the 2,3,7, and 8 positions within the compound.

The most widely studied of these compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), more commonly known as dioxin. Dioxin-like compounds usually are seen in a mixture of many different congeners. To estimate the toxicity of a mixture, the various congeners in it are assigned toxicities as fractions of the toxicity of dioxin. For example, a particular congener might have one tenth the toxicity of dioxin. Then the amount of each congener is converted into an equivalent amount of dioxin that would produce the same toxicity. These are known as toxicity equivalents or TEQ. So, for example, 20 grams of a dioxin congener that has one tenth the toxicity of dioxin would be listed as 2 grams TEQ. This conversion process provides a way of comparing sources of dioxin-like compounds in a common format, whether the mixture of congeners contains dioxins, polychlorinated furans, or dioxin-like PCBs or PBBs.

Uses

Dioxins and furans have never been intentionally produced as products. Instead they are byproducts of certain combustion and chemical processes. PCBs were produced in large quantities for use as dielectrics, hydraulic fluids, plastics, and paints. PBBs were produced as flame retardants and for other uses. Intentional production of PCBs and PBBs has been discontinued in the U.S., and most current releases are from the use and disposal of old products. Dioxin-like compounds will sometimes be included within products as byproducts of the production process, for instance, the herbicide 2,4,5-T (Agent Orange) that was used in Vietnam was contaminated with a low percentage of dioxin.

Health Effects

Dioxin-like compounds are observed to cause toxic effects at very low levels compared to other environmental toxins. Immune system function, learning behavior, and the reproductive system can all be affected by prenatal dioxin exposure. Postnatal dioxin exposure can also affect the immune system and may cause certain types of cancer. A well-known effect of dioxin is chloracne, a severe acne-like condition that develops within months of an exposure to high levels of dioxin. A range of varying but similar effects occurs in humans, other mammals, and fish and reptiles. Dioxin-like compounds are one of the most well-known endocrine disruptors: compounds that can have an effect at low levels of exposure by mimicking hormones in the endocrine system. These compounds may be implicated in lowered human and animal fertility in some areas.

Dietary intake is thought to be the main pathway of human exposure to dioxin-like compounds. Dioxin-like compounds have been found throughout the world in air, soil, water, sediment, fish, shellfish, and meat and dairy products. These compounds tend to bind to fat, and given the ubiquity of exposure, almost everyone carries a body burden of these compounds. Four to twelve percent of the lifetime intake may occur as a result of breast feeding in the first year of life. The average body burden is within an order of magnitude of the level at which toxic effects are seen. This is troublesome, since individuals vary in both their level of exposure and their sensitivity to these compounds.

In the Environment

The available evidence indicates that dioxins and furans are extremely stable compounds under most environmental conditions, with environmental persistence measured in decades. The only environmentally significant way in which these compounds are broken down is by sunlight, and this process can't occur when the toxins are bound to particles in the air or soil. Once broken down by sunlight to some degree, PCBs can be destroyed by slow bacterial biodegradation. It is thought that the primary way that dioxin-like compounds enter the food chain is through atmospheric deposition. Because of their ability to bind to fat, these compounds will bioaccumulate in the food chain.

As mentioned in the health effects section above, dioxin-like compounds are thought to cause problems for wildlife as well as humans. For instance, seals, mink, and other related species are thought to be extremely sensitive to adverse reproductive effects from dioxin and PCBs.

Sources

Current emissions of dioxins and furans to the environment result principally from man-made (anthropogenic) sources. EPA's current estimate is that forest fires are a minor source of emissions, and that these compounds can be formed under certain conditions by microbial action on chlorinated phenolic compounds, but direct anthropogenic emissions are much larger.

EPA's central estimate is that total known releases of dioxin-like compounds in the U.S. were 3,000 grams TEQ in 1995. This is a substantial drop from the estimate of 11,900 grams TEQ in 1987. However, this number does not include dioxin-like compounds in products, poorly understood sources, or waste sent to landfills. EPA assumes that permitted landfills achieve long term isolation from the environment, a questionable assumption at best. Studies of air deposition, while uncertain, estimate deposition of 20,000 to 50,000 grams TEQ per year in the U.S., so the estimate of known sources may be missing sources that have not yet been well studied. Insufficient data are available to properly characterize releases to water or land, for example, or many kinds of poorly controlled fires. It is also possible that much of the air deposition may be from reservoir sources of dioxin-like compounds that were released in previous years.

The major sources of environmental release in the U.S. are:

• Combustion sources such as waste incinerators, fuel burning, cement kilns, and fires.

• Metal production sources such as iron ore sintering and secondary copper production.

• Chemical manufacturing sources such as production processes for pentachlorophenol wood preservatives, chlorine bleached wood pulp, and phenoxy herbicides.

Greenpeace points out that many of these dioxin sources would not be sources unless chlorine-containing chemicals were introduced into the process. For instance, incinerators and recycling smelters may produce as much dioxin as they do because they burn PVC plastic, which contains chlorine. Steel mills and other metal production sources may introduce plastics and chlorinated solvents into their furnaces. For this reason, and because many other PBTs also arise from chlorine chemistry, Greenpeace advocates a focus on eliminating industrial processes that use elemental chlorine rather than cleaning up byproducts afterwards.

Pentachlorophenol deserves special note here, because it is the single largest source of dioxin-like compounds.

About 18,500 tons of pentachlorophenol were used for wood preservation in the U.S. in 1994. This pentachlorophenol contained about 25,000 grams TEQ of dioxin-like compounds as impurities from the manufacturing process.

Dioxins and chlorinated furans are not reported in EPA's Toxic Release Inventory database (TRI) because of TRI's high reporting thresholds. Sizeable transfers of PCBs and PBBs to treatment and disposal sites were reported in 1995, but no releases. Pentachlorophenol was reported released to air with 6,200 pounds and to water with 3,100 pounds in 1995 TRI; this would be 8.4 grams TEQ to air and 4.2 grams TEQ to water by EPA's conversion factor. EPA's database of hazardous waste (BRS) reports 62 million tons of hazardous waste generated in 1995 that contains dioxin-like compounds, although the concentration of dioxin-like compounds in the waste is not known.

References

* Environmental Protection Agency. Special Report on Environmental Endocrine Disruption: An Effects Assessment and Analysis. Office of Research and Development. EPA/630/R-96/012. February 1997.

* Environmental Protection Agency. The Inventory of Sources of Dioxin in the United States (external review draft). National Center for Environmental Assessment. EPA/600/P-98/002Aa. April 1998.

* Environmental Protection Agency. Health Assessment document for 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and Related Compounds: Volume III of III (external review draft). National Center for Environmental Assessment. EPA/600/BP-92/001c. August 1994.

* Environmental Protection Agency. Estimating Exposure to Dioxin-like Compounds (external review draft). Office of Health and Environmental Assessment. EPA/600/6-88/005Ca. June 1994.

* Greenpeace. Dow Brand Dioxin. Ed. by Jack Weinberg. Spetember 1995.

* EPA's Toxic Release Inventory (TRI) database, 1996 "frozen" version

* EPA's Biennial Reporting System (BRS) dataabase, 1995 final version

Detailed sources of emission of dioxin-like compounds

(Quantities are in grams TEQ, or the number of grams of dioxin that would be needed to produce the same toxicity as the actual number of grams of a mixture of dioxin-like compounds. This table does not include dioxin-like compounds sent to permitted landfills. EPA's known categories of waste sent to landfills include municipal waste incinerator ash (1,800 grams TEQ 1995), sewage sludge (194 grams TEQ 1995), and pulp and paper mill sludge (21 grams TEQ 1995). Sources for this table were Tables 2-2 and 2-5 in the EPA Inventory of Dioxin Sources. All numbers are either central estimates or order-of-magnitude estimates for 1995.)