“Dioxins: An Overview and History” Hites, R. A. Environ. Sci. Technol. 2010, ASAP. DOI 10.1021/es1013664
One of my main reasons for drinking the green chemistry kool-aid is that I believe it allows me to still do cool chemistry as well as to contribute to building a more sustainable civilization (at least that’s the idea). The focus, in my eyes, is on the positive things you can do with chemistry. However, as the Spanish-American philosopher George Santayana first said, “Those who cannot remember the past are condemned to repeat it.” With that in mind, this post is about one of the most well-studied mistakes in the history of the chemical enterprise, polychlorinated dibenzo-p-dioxins (PCDDs), often referred to collectively in the literature as dioxins. In Ronald Hites’ recent ES&T feature article on dioxins, he tells the story of the most toxic of the 75 dioxin congeners, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD).
The most sinister aspect of dioxins, from a chemists’ perspective, is the fact that they were never intentionally produced as marketable products. Again returning to the most toxic dioxin, it is formed as a byproduct in the synthesis and/or use of 2,4,5-trichlorophenol, as shown below.
Due to the extreme toxicity of 2,3,7,8-TCDD (not to mention its classification by IARC as a “known human carcinogen” and the fact that it has been shown in animal models to be an endocrine disruptor), even ppm amounts of this stuff can be enough to cause some real trouble. Hites describes a number of examples where this has happened, and I will briefly summarize two here.
Chick Edema Disease. The first step in making leather is curing the animal hide, to prevent decomposition. Large amounts of sodium chloride will do the trick, though our good friend 2,4,5-trichlorophenol began being used for this purpose around 1950. After the hide is cured, the excess fat is scraped off, and along with the fat comes the extremely lipophilic 2,3,7,8-TCDD that was a contaminant in the aformentioned phenol. This fat was then saponified, and the resulting fatty acids were purified by distillation. The “purified” fatty acids were still contaminated with enough 2,3,7,8-TCDD that when they were added to chicken feed, it still resulted in the death of millions of chickens in the eastern and midwestern US in 1957.
Agent Orange. This herbicide is composed of equal amounts of the chlorinated aryl ethers known as 24-D and 245-T. The latter compound is synthesized from 2,4,5-trichlorophenol.
Agent Orange was sprayed by the US military on south Vietnam from 1965-1971, with the intent of killing food crops used by the north Vietnamese and defoliating the perimeters of US military bases. If you haven’t already guessed, agent orange was contaminated with somewhere around 3 ppm 2,3,7,8-TCDD. This may not sound like much, but in the high-stakes world of industrial chemistry, the spraying of 4.5 x 107 L of Agent Orange means that around 150 kg of 2,3,7,8-TCDD was added to the environment of south Vietnam. No epidemiological study of the Vietnamese people has been carried out, though the Institute of Medicine has concluded that there is “sufficient evidence of an association” between herbicide exposure and the incidence of a number of diseases experienced by US veterans of the Vietnam war.
Moral of the Story. So many issues to discuss, so little time (or expertise for that matter). I don’t want to get into a discussion of the moral implications of producing a carcinogen-laced herbicide to destroy the food supply of an entire nation, but as a chemist, there are a few take-away messages from this dioxins story. Firstly, and perhaps obviously, industrial-scale chemical production is much more dangerous than lab-scale production. However, there were many chemists at many lab benches running many reactions in 100 mL round bottom flasks that had the chance to change this story. My second and final take-away message from this story is that the 10th of the 12 principles of green chemistry, “Design for Degradation”, could have lessened and possibly eliminated much of the harm caused by dioxins. Halogenated arenes are notorious for being environmentally persistent pollutants, in part because that mother nature finds that pesky halogen so difficult to remove (see this book, mainly chapter 16). For 2,3,7,8-TCDD and the rest of its PCDD congeners, the half life for their removal from the environment is on the order of decades to centuries.
Finally, it should be mentioned that the largest source of environmental dioxin contamination is thought to be the combustion of chlorine-containing organic compounds (mainly in municipal waste, see here). Additionally, as if this story weren’t twisted enough, recent research has found that triclosan can be converted in the environment to many of the dioxin congeners.
Here are a number of helpful online resources for learning more:
IARC Agents, Groups 1-4.
“Environmental Behavior of Chlorinated Dioxins and Furans” Hites, R. A. Acc. Chem. Res. 1990, 23, 194.
“Degradation half-life times of PCDDs, PCDFs, and PCBs for environmental fate modeling” Sinkkonen, S.; Paasivirta, J. Chemosphere 2000, 40, 943.