Dioxins and dioxin-like compounds

Dioxins and dioxin-like compounds (DLC) are by-products of various industrial processes, and are commonly regarded as highly toxic compounds that are environmental pollutants and persistent organic pollutants (POPs). They include:

• Polychlorinated dibenzodioxins (PCDDs), or simply dioxins, technically a derivative of dibenzodioxin
• Polychlorinated dibenzofurans (PCDFs), or simply furans, technically a derivative of dibenzofurans. Whilst they strictly speaking are not dioxins, they have dioxin-like properties
• Polychlorinated biphenyls (PCBs), which also strictly speaking are not dioxins, but some have "dioxin-like" properties. They can under certain conditions form from the more toxic dibenzodioxins and dibenzofurans through partial oxidation
• Finally, it may refer to Dioxin (chemical), the basic chemical unit of the more complex dioxins

Because dioxins refer to such a broad class of compounds, that vary widely in toxicity, the concept of toxic equivalent (TEQ) has been developed to facilitate risk assessment and regulatory control. Toxic equivalent factors (TEFs) exist for selected congeners of dioxins, furans and PCBs. The reference congener is the most toxic dioxin 2,3,7,8-TCDD which per definition has a TEF of one.

In reference to their importance as environmental toxins the term dioxin or dioxins is used almost exclusively to refer to compounds from the above groups which demonstrate the same specific toxic mode of action associated with 2,3,7,8-TCDD. Incidents of contamination with PCBs are often reported as dioxin contamination incidents Dioxin Affair since it is this toxic characteristic which is of most public and regulatory concern.

Toxicity

The symptoms reported to be associated with dioxin toxicity are incredibly wide ranging, both in the scope of the biological systems affected and in the range of dosage needed to bring these about. Acute effects of single high dose dioxin exposure include chloracne and wasting syndrome, but chronic and sub-chronic exposure can be incredibly harmful at much lower levels, especially at particular developmental stages including foetal, neonatal and pubescent stages. An example of this variation in response is clearly seen in a study following the Seveso disaster indicating that sperm count and motility was effected in different ways in exposed males, depending on whether they were exposed before, during or after puberty.

Dioxin is also associated strongly with carcinogenesis although its precise mechanistic role is not clear. The US EPA along with the International Agency for Cancer Research have categorised dioxin, and the mixture of substances associated with sources of dioxin toxicity as a "likely human carcinogen"http://www.health.state.mn.us/divs/eh/risk/guidance/dioxinmemo1.html. It is thought that the presence of dioxin can accelerate the formation of tumours and adversely affect the normal mechanisms for inhibiting tumour growth, without actually instigating the carcinogenic event. As with all toxic endpoints of dioxin a clear dose-response relationship is very difficult to establish.

The LD50 of dioxin also varies wildly between species with the most notable disparity being between the ostensibly similar species of hamster and guinea pig. The oral LD50 for guinea pigs is as low as 0.5 μg/Kg body weight, whereas the oral LD50 for hamsters can be as high as 5 g/Kg body weight, a difference of as much as 10,000,000x.http://ces.iisc.ernet.in/energy/HC270799/HDL/ENV/enven/vol336.htm

The uncertainty and variability in the dose-response relationship of dioxins in terms of their toxicity, as well as the ability of dioxins to bioaccumulate mean that the tolerable daily intake of dioxin has been set very low, 1-4 pg/Kg body weight per day, to allow for this uncertainty and ensure public safety as far as possible.

Common uses

According to the Agency of Toxic Substances & Disease Registry:

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, 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.

Sources

Dioxins are produced as by-products of incomplete combustion and some chemical processes, including natural ones.

General sources of exposure

Dioxins enter the physical environment by release during:

• Combustion
• Metal smelting and refining
• Manufacturing of chlorinated chemicals
• Paper bleaching
• Natural biological and photochemical processes
• Mobilization from environmental reservoirs (e.g., stirred sediments)

Specific sources and routes of exposure

Exposure through diet

The major sources of human exposure (96%) are

• Animal fats found in meats
• Full fat dairy products
• Fatty fish (herring, mackerel, salmon, sardines, trout, tuna)

Exposure through air

• Breathing incineration gases released from medical, municipal, and hazardous waste incinerators.
• Breathing gases released by industrial processes from paper mills, cement kilns, and metal smelters.

Exposure through occupation

People who perform the following types of work can be exposed to dioxins:

• Production and handling of certain chlorinated phenols (such as 2,4,5-trichlorophenol or pentachlorophenol [PCP])
• Production or handling of chlorinated pesticides, such as 2-4-dichlorophenoxyacetic acid (2,4-D) and other herbicides
• Chlorinated pesticide application
• Pressure treatment of wood with PCP and handling of PCP-treated wood
• Production of chlorinated paper at pulp and paper mills
• Operation of municipal solid waste or hazardous waste incinerators
• Hazardous waste clean-up operations

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.

Intentional Toxic Equivalent Factors (ITEF) for the dioxin congeners of concern (NATO/CCMS)

2,3,7,8-TCDD	1
2,3,7,8-TCDF	0.1
1,2,3,7,8-PeCDD	0.5
2,3,4,7,8-PeCDF	0.5
1,2,3,7,8-PeCDF	0.05
1,2,3,4,7,8-HxCDD	0.1
1,2,3,6,7,8-HxCDD	0.1
1,2,3,4,7,8-HxCDF	0.1
1,2,3,7,8,9-HxCDD	0.1
1,2,3,7,8,9-HxCDF	0.1
1,2,3,6,7,8-HxCDF	0.1
1,2,3,4,6,7,8-HpCDD	0.01
2,3,4,6,7,8-HxCDF	0.1
OCDD	0.001
1,2,3,4,6,7,8-HpCDF	0.01
1,2,3,4,7,8,9-HpCDF	0.01
OCDF	0.001

(T = tetra, P = penta, Hx = hexa, Hp = hepta, O = octa)

For furan isomers

Isomer	Name	WHO/EPA	TEF
2378	TCDF Tetrachlorodibenzofuran	0.1	0.1
12378	PeCDF Pentachlorodibenzofuran	0.03	0.05
23478	PeCDF Pentachlorodibenzofuran	0.3	0.5
123678	HxCDF Hexachlorodibenzofuran	0.1	0.1
123789	HxCDF Hexachlorodibenzofuran	0.1	0.1
123478	HxCDF Hexachlorodibenzofuran	0.1	0.1
234678	HxCDF Hexachlorodibenzofuran	0.1	0.1
1234678	HpCDF Heptachlorodibenzofuran	0.01	0.01
1234789	HpCDF Heptachlorodibenzofuran	0.01	0.01
12346789	OCDF Octachlorodibenzofuran	0.0003	0.001

External links

• World Health Organization
• Dioxin Homepagezh:類戴奧辛物質