Where is Dioxin? Part 1
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.
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 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.
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.
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.
This article is Copyright 1994 by Lewis A. Shadoff. It may be freely redistributed in its entirety provided that this copyright notice is not removed. It may not be sold for profit or incorporated in commercial documents without the written permission of the copyright holder. While all information in this article is believed to be correct at the time of writing, this article is for educational purposes only and does not purport to provide legal advice.