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What is Dioxin?

Dioxin is the name given to a group of 200+ chemicals that are formed as unwanted by-products of industrial manufacturing and burning activities. The major sources of dioxins include chemical and pesticide manufacture, burning household trash, forest fires, and burning of industrial and medical waste products.  In the case of the Tittabawassee River flood plain, the source is known: Dow Chemical

Click here to listen to more of the Dr. Linda Birnbaum (EPA)  presentation: "Dioxin, are we at risk?".   You are hearing a clip from the video when this page is opened.

Chemical Structure of a few of the most toxic dioxins:  TCDD & TCDF

2,3,7,8-Tetrachlorodibenzo-p-dioxin 2,3,7,8-Tetrachlorodibenzofuran
2,3,7,8-Tetrachlorodibenzo-p-dioxin 2,3,7,8-Tetrachlorodibenzofuran
410d biohazard label.gif (869 bytes)poison.gif (455 bytes)        larrow.gif (281 bytes)   2,3,7,8-Tetrachlorodibenzo-p-dioxin  
                                                     Explaination of NFPA Chemical Hazard Label
 

Dioxin belongs to a family of chemicals with related properties and toxicity. There are 75 different dioxins, or polychlorinated dibenzodioxins (PCDDs), 135 different furans, or polychlorinated dibenzofurans (PCDFs), and 209 different polychlorinated biphenyls (PCBs). Each different form is called a "congener."

Not all of the "dioxin-like" chemicals have dioxin-like toxicity, and the toxic ones are not equally toxic. Only 7 of the 75 dioxins, 10 of the 135 furans, and 12 of the 209 PCBs have dioxin-like toxicity. These 29 different dioxins, furans, and PCBs all exhibit similar toxic effects caused by a common mechanism: binding to a particular molecule known as the aryl hydrocarbon or "Ah" receptor (see Chapter 5 of the CHEJ TSD).

It is believed that the tighter the binding to the Ah receptor, the more toxic the chemical. The most potent member of this family is 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD, which also has the greatest affinity for the Ah receptor.

The word "dioxin" is often used imprecisely. Some people restrict its use only to 2,3,7,8-TCDD, the most toxic and most studied dioxin. Others extend its use to the whole class of chemicals with similar toxicity and whose effects are controlled or triggered by the Ah receptor. In this report, the terms "dioxin" and "dioxins" are used to refer to any of the dioxin family members that bind to the Ah receptor and elicit dioxin like effects.

Toxic Equivalents

Although all dioxin-like compounds are thought to act in the same way, they are not all equally toxic. Their different toxicities may be due to their unique properties of absorption, distribution, metabolism, and elimination in a body and/or strengths of binding to the Ah receptor. Therefore, the health risk of each congener is assessed by rating their toxicities relative to TCDD, the most potent of the dioxins. TCDD is assigned a value of "1" and each of the 17 toxic dioxins/furans and 12 PCBs is assigned a "toxicity factor" that estimates its toxicity relative to TCDD. The resulting estimates are called toxic equivalency factors (TEFs), which have been recently updated by the World Health Organization.1 The toxic equivalency (TEQ) is determined by multiplying the concentration of a dioxin congener by its toxicity factor. The total TEQ in a sample is then derived by adding all of the TEQ values for each congener. While TCDD is the most toxic form of dioxin, 90% of the total TEQ value results from dioxin-like compounds other than TCDD.

The TEQ system is not perfect, but it is a reasonable way of estimating the toxicity of a mixture of dioxin-like compounds. There is good experimental support for the assumptions that underlie the TEQ system.1,2 TEQs make it possible to take toxicity data on TCDD, a compound about which our knowledge is vast, and estimate toxicity for other compounds about which much less is known.

The links below offer quite a bit of information on what it is, how it can harm you, and what to do to avoid it.

bulletAmerican People Dioxin Report Technical Support Document 
bulletMichigan Dept Community Health Dioxin Fact Sheet  (pdf)
bulletMichigan DEQ Information about Dioxin and the Tittabawassee River Flood Plain (pdf)
bulletATSDR Dioxin Fact Sheet
bulletThe Science behind Michigans 90 ppt RDCC for dioxin
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"Cleanup criteria for environmental contamination are determined under Part 201, Environmental Remediation, of the Natural Resources and Environmental Protection Act, 1994 PA 451, as amended (Act 451). The soil generic residential direct contact criterion (DCC) for dioxin is 90 parts per trillion (ppt). That criterion was developed in 1995 using the best information available at that time. The scientific information that has developed since 1995 indicates that dioxin poses even more of a risk than considered in 1995. Recent work conducted by the World Health Organization (WHO),  the United Nations Food and Agriculture Organization, the European Commission Scientific Committee on Food, and in the United States Environmental Protection Agency’s (EPA’s) draft dioxin reassessment supports standards even lower than those in effect in Michigan. The United States Department of Health and Human Services, the National Institute for Occupational Safety and Health and the WHO’s International Agency for Research on Cancer, as well as the EPA, have concluded, based on literally hundreds of animal and human studies, that 2,3,7,8-TCDD is a potent human carcinogen."  Source: MDEQ response to Midland 5/26 meeting

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The Part 201 DCC of 90 ppt for dioxin is based on exposure assumptions and toxicity information available in 1995. The toxicity of dioxin is currently being re-evaluated in a major reassessment done by the EPA, including review by the National Academy of Sciences. When promulgating the Part 201 cleanup criteria rules in 2002, the DEQ determined that it was more scientifically defensible to continue to apply the 1995 DCC of 90 ppt than to update the criterion before the results of the federal dioxin reassessment are available. It is anticipated that revision of the dioxin DCC to reflect current science and risk assessment would result in a generic residential soil DCC in the range of 10 to 70 ppt. An update of the soil DCC for dioxin would require: (Source: MDEQ response to Midland 5/26 meeting)

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A re-evaluation of the cancer potency value.

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An evaluation of noncancer toxicity.

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An appropriate animal-dose to human-dose conversion to account for differences between species.

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Selection of the most sensitive toxicity endpoint.

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Identification of an appropriate relative source contribution factor (which accounts for the fact that a significant source of dioxin exposure is from the diet).

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Incorporation of the updated generic exposure assumptions (i.e., the exposure assumptions used in the Part 201 Administrative Rules).

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The 90 ppt calculation simplified

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Part 201 Generic Soil Direct Contact Criteria

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Carcinogenicity Slope Factor

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EPA Great Lakes Water Quality Criteria for protection of Human Health

 

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