School of Biological Sciences
Natural Resources: depletion, pollution and toxicology
[ BIOL202 Home | Aquabiology at Liverpool ]
|
|
School of Biological Sciences |
|
|
Natural Resources: depletion, pollution and toxicology[ BIOL202 Home | Aquabiology at Liverpool ] |
Dioxins (polychlorinated dibenzo-para-dioxins, or PCDDs) and furans (polychlorinated dibenzofurans, or PCDFs) are a group of toxic chemical compounds which are inadvertently generated and released into the environment as by-products of various combustion and chemical processes. Due to their toxicity, tendency to bioaccumulate, and persistence in the environment, dioxins and furans have been the subject of ongoing public health and environmental concern. Despite existing controls, they are distributed widely in the environment, sometimes at levels which may pose risk. For example, dioxins/furans have been the cause of numerous fish consumption advisories in the Great Lakes region, and the U.S. Environmental Protection Agency (EPA) has recently estimated that the risks for the general population based on dioxin exposure could be as high as the range of a 1 in 100 to 1 in 1,000 increased chance of experiencing cancer related to dioxin exposure (USEPA, 2000b). In response, various local, state, regional, and national efforts are focusing on achieving further reductions in dioxin contamination. One of these efforts is the Great Lakes Binational Toxics Strategy (Binational Toxics Strategy or GLBTS), which encompasses various activities and strategies being considered under the guidance of a multi-stakeholder GLBTS dioxin/furan workgroup.
Dioxins and furans are halogenated aromatic hydrocarbons which can have from one to eight chlorine substituents. There are 75 individual chlorinated dioxins and 135 individual chlorinated furans. Each individual dioxin and furan is referred to as a congener. Both the number of chlorine atoms and their positions determine the physical and chemical properties, and therefore, the fate and toxicity of a given congener. In addition to dioxin and furan congeners, coplanar polychlorinated biphenyls (PCBs), a subset of PCBs, also exhibit dioxin-like toxicity due to their structural and conformational similarities to dioxin compounds. Dioxins, furans, and dioxin-like PCBs are commonly found as complex mixtures when detected in environmental media, biological tissues, or as releases from specific sources. Generally, dioxins and related compounds are colorless crystals or solids that have a low water solubility, high fat solubility (i.e., are lipophilic), and low volatility. They bind strongly to soils and sediments and are extremely stable under most environmental conditions, making them persistent once released in the environment. Because they are lipophilic, they also tend to bioaccumulate.
Only dioxin/furan congeners with chlorines attached at a minimum in the 2,3,7, and 8 positions, as those shown in Figure exhibit the high toxicity associated with dioxin. One compound, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), is the best studied of this class of compounds and serves as the reference compound for assigning toxicity equivalence factors for related congeners. For risk assessment purposes, estimates of the toxicity of sources which contain a mixture of PCDD and PCDF congeners are often expressed as toxicity equivalents (TEQ). TEQ is calculated by multiplying concentrations of each dioxin and furan congener present in a source with a toxicity equivalency factor (TEF). The TEF is an estimate of each congener's toxicity relative to the toxicity of 2,3,7,8-TCDD. The TEQ values for each congener are added together for the total TEQ concentration. Thus, concentrations of dioxins and furans represented as a TEQ concentration provide a quantitative estimate of toxicity as if all congeners present in the mixture are a toxic equivalent mass of 2,3,7,8-TCDD. Thirteen of the total 209 PCB congeners are also thought to have dioxin-like toxicity, and are often included in the calculation of dioxin/furan TEQs in toxicity assessments.
For
further study and Reading - related links 
Historically, various TEF schemes have been defined and used to present results. The different TEF schemes, and a new uniform TEQ nomenclature that clearly distinguishes between the different TEF schemes, are discussed in detail in the updated draft Dioxin Reassessment (see USEPA, 2000b: Section 1.2 of the Integrated Summary, or Part II of Chapter 9 "Toxicity Equivalence Factors (TEF) for Dioxin and Related Compounds"). In the updated draft Dioxin Reassessment, the nomenclature I-TEQDF is used to denote the International TEF scheme adopted by EPA in 1989, and TEQDF-WHO98 is used to refer to the 1998 WHO update to the TEFs previously established by WHO for dioxins, furans, and dioxin-like PCBs (USEPA, 2000b). The I-TEQDF abbreviation is equivalent to the TEQs reported in the 1998 Draft Dioxin Inventory (USEPA, 1998). For this reason in this Step 3 report, the calculations of the percent contribution of a given source to the total inventory were performed based on I-TEQDF.
On April 7, 1997, Canada and the United States signed the Great Lakes Binational Toxics Strategy: Canada-United States Strategy for the Virtual Elimination of Persistent Toxic Substances in the Great Lakes. The Binational Toxics Strategy identified twelve bioaccumulative substances having sufficient toxicity and presence in water, sediments, and/or aquatic biota of the Great Lakes system to warrant concerted action to eliminate their input to the Great Lakes. They are called "Level 1 substances". Dioxins/furans are one of the classes of Level 1 substances, and are the subject of this report, which was prepared in response to the U.S. challenge goal for dioxins and furans written in the GLBTS:
U.S. Challenge: Seek by 2006, a 75 percent reduction in total releases of dioxins and furans (2,3,7,8-TCDD toxicity equivalents) from sources resulting from human activity. This challenge will apply to the aggregate of releases to the air nationwide and of releases to the water within the Great Lakes Basin, using the September 1994 draft Dioxin Reassessment as an interim baseline. Once U.S. EPA has completed and released its final Dioxin Reassessment, the Reassessment's 1987 emissions inventory will be used as the challenge baseline.
To guide Environment Canada (EC) and the U.S. EPA, along with their partners, as they work toward virtual elimination of the strategy substances,the GLBTS outlined a four-step analytical framework:
1.
Gather information
2.
Analyze current regulations, initiatives, and programs which manage or
control substances
3.
Identify cost-effective options to achieve further reductions
4.
Implement actions to work toward the goal of virtual elimination
In accordance with Step 3 of the four-step process, this report (the Step 3 report) documents the analysis of available information on dioxin sources and regulations with the goal of identifying the best options for further reductions. Specific goals of this report include the identification of opportunities for new or modified approaches, pollution prevention programs, or other alternative measures, which may accelerate the pace or increase the level of dioxin/furan reduction, while taking into account cost-effectiveness.