What is Dioxin?
Initially dioxin was an abbreviation for a single substance, with the formidable chemical name, 2,3,7,8-tetrachlorodibenzo-p-dioxin. It is one member of a "family" of chemical compounds with an identical carbon-oxygen framework.
Chlorine atoms (four in this case) are attached at specific carbon atom sites as indicated by the numbers 2,3,7,8 in the name. The geometry of the molecule is shown in the figure below.
![[IMAGE OF TCDD MOLECULE]](images-TCDD/dioxin_image006.gif)
This pictorial shorthand describes the spatial relationship of the atoms that make up the molecule. How does the name relate to the structure? The dibenzo part comes from the two hexagons with inscribed circles. This is shorthand for benzene sub-structures. There is a carbon atom at the vertex of each hexagon.
The two benzene rings are joined by two oxygen atoms (5 and 10). By joining the benzene rings in this way the center of the molecule, containing the oxygens, is also a six-membered ring. A six-membered ring like this with two oxygens in it is called dioxin.
When the oxygens are at opposite sides of the ring, as in the figure, they are called para to each other, hence the -p- in the name. Voila! Two benzene rings joined with two oxygens in this fashion is dibenzo-p-dioxin, the so-called "parent" compound or structure or back-bone.
The four chlorine atoms are joined to the carbon atoms labeled 2,3,7, and 8. If the four chlorines were attached to different carbons (1, 2, 3, and 4 for instance), then the substance would be an isomer of 2,3,7,8-tetrachlorodibenzo-p-dioxin.
If nothing is indicated as attached to a carbon that has room for something to be attached, it has hydrogen atom(s) attached there. Therefore, there are hydrogens attached to carbons 1, 4, 6, and 9. (I won't go into how to tell if there is room, just trust me.) Counting up atoms leads to the empirical formula: C12H4O2Cl4.
You have just had a short-course in chemical nomenclature. The concept is that a chemical name refers to a specific molecular geometry and chemical formula.
The name is a complete description of the molecule, and the rules for naming them are by international convention (IUPAC - International Union of Pure and Applied Chemists). As you can see, however, the names tend to be long and complicated.
Abbreviations are quite common, especially when a substance with a long name impacts the public domain in some fashion. 2,3,7,8-tetrachlorodibenzo-p-dioxin has gone under the pseudonyms dioxin, tetrachloro dioxin, 2,3,7,8-tetrachloro dioxin, 2,3,7,8-TCDD, and TCDD (often with the prefix "deadly").
The term chlorinated dioxins refers generically to chlorinated dibenzo-p-dioxins.
TCDD was originally discovered as a by-product in the manufacture of trichlrophenol, an intermediate chemical in the manufacturing process for some pesticides. It does not occur in a pure form in nature.
Public interest was initially aroused when employees were exposed to 2,3,7,8-TCDD as a low concentration (part per million) constituent in industrial chemical exposure incidents involving trichlorophenol.
Since TCDD displays high toxicity to some experimental animals, there was concern over the possible health effects on these people.
Toxicology studies of the effects of chlorinated dioxins on test animals indicate that the toxicity is not the same for each possible isomer (placement of chlorine around the molecule).
Those isomers which contain chlorines in the 2,3,7, and/or 8 positions are more toxic than when hydrogen is attached to those positions.
This led to using the term dioxin to describe all isomers of chlorinated dioxins containing from 1 to 8 (the maximum number) chlorines, and 2,3,7,8-dioxins to refer to the "toxic" isomers.
Pure 2,3,7,8-TCDD was synthesized in 1968. It is a white, micro-crystalline solid (looks like table salt) which is insoluble in water and sparingly soluble in some organic solvents.
In summary, the term dioxin does not have a single, simple definition. It has referred to a group of related substances and to individual members of the group.
The arrangement of the atoms in these substances is known, and they all have the same carbon-oxygen basic structure and geometry with chlorine atoms attached to various carbon atoms. 
Where is Dioxin? Part 1
TRICHLOROPHENOL
COMBUSTION
FISH
HUMANS
This issue starts a discussion of where dioxins have been found. Much of it is based on work done by the Expert Advisory Committee on Dioxins commissioned by the Canadian Government.
The discussion is complex since the presence of dioxins is intertwined with emotionally charged concerns about the risk to human health and the environment, and the concomitant involvement of environmental activist groups, chemical manufacturers and manufacturing groups, and government agencies.
I will take a more or less historical approach to the topic since advances in measurement technology and toxicology were very rapid during this period of time.
TRICHLOROPHENOL
As mentioned in "What is Dioxin", 2,3,7,8-TCDD was originally found to be associated with the trichlorophenol chemical production process. Initially, searches for this material in the environment were based on the manufacture and use of trichlorophenol and its derivatives (some pesticides).
An early environmental study was carried out by the USEPA called the Dioxin Implementation Plan which studied beef fat, milk, and liver tissue from cattle which grazed in areas where 2,4,5-T herbicide was used for rangeland brush control and from cattle where it was not used (for comparison). A few samples from farms in Missouri were found to be positive in this study.
However, in light of today's knowledge, it is not certain that the source of dioxins was 2,4,5-T. This area of Missouri was the site where illegal spreading of chemical process oil from trichlorophenol production was carried out on dirt roads and, in one case, in a horseback riding arena to control dust.
No discussion of the presence of dioxins would be complete without mention of Agent Orange, a herbicide containing 2,4,5-T used in Vietnam. The dioxin content of Agent Orange varied widely from sample to sample from less than 0.1 part per million (ppm) to over 60 ppm.
This material, diluted in kerosene, was sprayed over forested areas in Vietnam during the Vietnam Conflict. Various claims of adverse health effects due to exposure to Agent Orange by Vietnam veterans have been filed and are, to this day, being litigated.
COMBUSTION
Combustion processes, where the fuel contains both carbon and chlorine, produce a wide range of dioxin isomers. It has also been discovered that above a certain temperature, however, (800 deg.C, 1470 deg.F) dioxins are destroyed.
It has been concluded that there is a natural background level of dioxins in the environment due to low temperature combustion events such as forest fires. This discovery produced some concern that people were at risk if they lived or worked near municipal incinerators, coal-fired power plants, home fireplaces, wood burning stoves, etc.
Many of these possible sources have been studied, and the dioxin yield determined. As expected, low temperature combustion sources such as municipal incinerators and wood burning produce higher levels of dioxins than the high temperature processes such as coal-fired power plants. All the levels found are in the part per trillion range or below.
FISH
A wide range of dioxin isomers have been found in fish, with the isomers containing chlorines in the 2,3,7, and 8 positions (See "What is Dioxin".) predominating. It turns out that biological organisms selectively retain those isomers containing chlorine in the 2,3,7, and 8 positions, the others being eliminated at a much faster rate.
This was demonstrated in a study where fish were exposed to fly-ash containing a known composition of dioxins. Analysis of the fish showed the distribution of isomers in the fish to be altered from that of the source, with 2,3,7,8 isomers predominating even though they were not the predominant ones in the fly ash. Thus it is difficult to infer the source of contamination from the analysis of exposed species.
HUMANS
Two early studies tried to determine whether humans were accumulating dioxins due to the use of 2,4,5-T in forestry. One was by NIEHS (National Institute of Environmental Health Studies) and the other sponsored by The Dow Chemical Company. Human milk samples were analyzed from people living near areas where 2,4,5-T was used and from a control group.
The study of milk was chosen since it is a fat-rich substance (Dioxins dissolve in fat as opposed to water.) which does not require violation of the body (as, for example, blood samples would) and the consequent legal ramifications of obtaining samples. No detectable dioxins were found in any samples. The technology at that time was capable of detecting, at best, 1 part per trillion of dioxins in milk, and about 10 ppt in fat (because the samples are, necessarily, smaller).
With improvements in methodology for the analysis of dioxins (especially increased sensitivity), there is now a demonstrated background level of dioxins found in humans based on the analysis of fat samples. This level is in the low part per trillion range. Not all samples from humans have been positive, but a significant number have been positive in so-called control groups.
Where is Dioxin? Part 2
PENTACHLOROPHENOL
PETROLEUM
Part 1 discussed the presence of dioxins due to the use of 2,4,5-T herbicide and from combustion. This issue continues with a discussion of the presence of dioxins in pentachlorophenol and petroleum.
PENTACHLOROPHENOL
Pentachlorophenol (sometimes called simply penta, but not PCP which is something else) is made from an exhaustive chlorination of chlorophenol tars. The product is purified by dissolving the penta in water (by the addition of caustic to make the water-soluble salt) and the subsequent phase separation of non water soluble materials (tars). The water solution is then made acidic and the precipitated penta filtered and dried.
Pentachlorphenol has a wide range of dioxins in it (virtually every possible isomer) with the higher chlorinated analogs predominating. The level of total dioxins in pentachlorophenol is in the mid to high part per million range, and in the separated tars in the percent range. It should be noted that the major portion of dioxin in both is octachlorodibenzo-p-dioxin which is about as toxic as table salt
The major use of pentachlorophenol is as a wood preservative. It is used commercially in the preservation of telephone poles, fence posts, and board lumber used in contact with the ground. It is also available
The treating of telephone poles with pentachlorophenol is done in large vessels under pressure to aid the penetration of the wood. It is common for workers to come in contact with the treating solution and wood, causing visible evidence of toxification (chloracne). This may be the most common vector for human industrial exposure to dioxins.
It is now forbidden to use pentachlorophenol treated wood as the construction material for animal feed storage bins. This has, in the past, been a source for the exposure of cattle to dioxins by transfer of dioxins in treated wood to the feed and hence humans through the consumption of meat and milk from those exposed animals.
Pentachlorophenol has also been used as a bactericide in many products. These have included paint, cosmetics, ink, fabric dyes, and other substances used in the home. This may be (may have been?) the most common route for exposure of the general population to above background levels of dioxins.
Cow hides have been preserved using penta in South America. Contaminated gelatin extracted from those hides was imported to the US and used to make capsules to hold medication. This caused a recall of that medication and restrictions on the sources of gelatin for capsules.
It should be noted that a Canadian study found that penta was one of the most significant sources of dioxins in that country.
PETROLEUM
Since dioxins are formed as a natural product of combustion, fires during the period when the plant life source of petroleum was alive would have produced traces of dioxins just as they do today.
Those dioxins which fell on the plant life would have been incorporated in the crude oil. In the absence of studies to determine it, I would estimate that the total level of dioxins in crude oil from such a source to be less than 1 part per trillion and probably in the part per quadrillion range or less.
Crude oil undergoes extensive processing to make the final products such as gasoline, heating oil, and kerosene. Where do the dioxins, if any, end up in this complex process? The possibilities are that they are unaffected by the process, are destroyed in the process, are formed in the process, or are changed in the process. Since dioxins are not very volatile, if any remain I would expect them to end up in a "heavies" of some type. It is also possible that they are concentrated wherever they end up.
No studies have been done, as far as I know, to determine any of this. Based on a knowledge of chemistry and the properties of dioxins, an educated guess could be made as to the most likely scenario(s), not by me, however, I am no petroleum chemistry expert.
When petroleum products are burned, the level of dioxins formed would depend on the availability of chlorine and the temperature. Even very low levels of dioxin emissions from automobiles, home heating, and power generation would be a significant portion of the total dioxins produced throughout the world due to the large quantities of petroleum involved.
I do not know of any definitive studies which have been carried out on these combustion products to determine the dioxin levels produced. Reassuringly, no detectable dioxins were found in a study of emissions from a coal-fired power plant.
It is fair to state that the "jury is still out" on the question dioxins in petroleum products and process streams and of petroleum combustion as a source of dioxins until definitive studies are done.
By Lewis A. Shadoff. |
