2,3,7,8-Tetrachlorodibenzofuran. 2,3,7,8-Tetrachlorodibenzofuran (TCDF) is a polychlorinated dibenzofuran with a chemical formula of C12H4Cl4O. TCDF is part of the chlorinated benzofuran (CDF) family that contains between 1 and 8 chlorine atoms attached to the parent dibenzofuran ring system. The CDF family includes 135 compounds, of which only a few have been studied.

TCDF was discovered in the mid-20th century along with other CDFs. It was found to be an unwanted by-product in the manufacturing of chlorinated compounds; it is not commercially used or produced itself. TCDF is directly released into the environment via emissions of waste incineration, fires with PCB transformers, vehicle exhausts using leaded fuel, and bleaching of industrial products.

TCDF and other CDFs are known to have a lasting impact on the environment. TCDF can exist as both a gas and particles in the atmosphere, which results in deposition in the soil. It has an estimated half-life of around 60 days in the soil and atmosphere. This molecule can easily accumulate in the food chain, leading to potential human exposure. TCDF is listed as a hazardous air pollutant in the Clean Air Act of the 1990s. Monitoring programmes are established to keep track of the levels on TCDF in various environmental compartments to avoid damaging ecosystems.

Synthesis
TCDF is not purposefully manufactured. It is formed as a by-product of industrial manufacturing processes which place chlorine atoms in favorable reaction conditions, which include alkaline mediums, temperatures over 150 °C, and exposure to UV light, or radical forming substances. In the production of chlorinated pesticides CDFs are formed as byproducts by intermolecular condensation of ortho-chlorophenols. Intramolecular cyclization reactions of predibenzofurans also result in CDFs. In thermal waste treatment processes CDFs are produced by combustion and pyrolysis of organochlorine or non-chlorinated organic compounds in the presence of chlorides. The recycling of metal cables also leads to the formation of CDFs as chlorine is formed by burning the insulator polyvinylchloride which causes new CDFs. From PCBs, the production of CDFs can happen by the loss of two ortho clorines, loss of ortho as well as chlorine, loss of an ortho hydrogen and chlorine with an additional shift of chlorine from the 2 to 3 position, or loss of two ortho hydrogens.

Available forms
TCDF is not made industrially and there are no commercial uses for it. TCDF is released into the air as vapour after burning of hazardous waste, fires involving PCB mixtures, and a byproduct of bleaching pulp. Some of this vapour remains in the air as particles but some of it gets deposited into the soil as well as water. Most of the exposure and available forms is through the air and the consumption of high-fat meats that were exposed to the 2,3,7,8-Tetrachloro-dibenzofuran.

Toxicity
TCDF is one of the most toxic congeners of the polychlorinated dibenzofurans. Its toxicity is associated with fatty liver disease. Mainly the toxicity of TCDF has been recorded through testing of breast milk, adipose tissue, and blood serum. It is at this moment not yet classifiable as to its carcinogenicity to humans, classified therefore as type 3 of the IARC (International Agency for Research on Cancer). Toxicity levels and baselines are not yet known in humans but are being tested on animals.

Effects on animals
The recorded effects on animals are most commonly investigated in the house mouse as well as male guinea pigs. Exposure to TCDF has led to problems in multiple organs within mice such as its liver, jejunum, cecum, small intestine, large intestine. In general, the liver is the main target of TCDF, which leads to induced hepatic lipogenesis. When testing on female mice, the effects on embryos were recorded, showing that TCDF has the strongest effect on fetal kidneys and leads to 100% of fetuses to be teratogenic at toxicity levels which are not fatal for the mother. The exposure to TCDF leads to a decrease in glucose homeostasis leading to an abundance of glucose in the mouse body. It also promotes the increment in bile acid metabolic processes which leads to an increase in multiple acids such as deoxycholic acid, glycocholic acid, taurochenodeoxycholic acid, tauromuricholic acid, lithocholic acid, chenodeoxycholic acid, taurolithocholic acid, and an abundance of bile acids and salts. As mentioned previously, it affects the liver as it results in a positive regulation of lipid biosynthetic processes leading to an abundance of unsaturated fatty acids. The baseline toxicity levels for TCDF are known for the mouse, the guinea pig and monkey, being at  Guinea pig (Hartley, male, 3-4 wk old) oral 5-10 ug/kg, LD50 Mouse (C57B1/6, male, 6 wk old) oral >6,000 ug/kg, and LD50 monkey (Macaca mulatta, female, 2.0-3.7 kg) oral 1,000 ug/k.

References

Bibliography 
Ioannou, Y. M., Birnbaum, L. S., & Matthews, H. B. (1983). Toxicity and distribution of 2,3,7,8-tetrachlorodibenzofuran in male guinea pigs. Journal of toxicology and environmental health, 12(4-6), 541–553. 
Matsumura, F. (1995). Mechanism of action of dioxin-type chemicals, pesticides, and other xenobiotics affecting nutritional indexes. The American Journal of Clinical Nutrition, 61(3). 
Pope, C. N., & Liu, J. (202AD). Chapter 10 - Microbiome in toxicity and its modulation. In An Introduction to interdisciplinary toxicology: From molecules to man (pp. 127–138). essay, Academic Press.
Tai, H. L., Mcreynolds, J. H., Goldstein, J. A., Eugster, H. P., Sengstag, C., Alworth, W. L., & Olson, J. R. (1993). Cytochrome-p4501a1 mediates the metabolism of 2,3,7,8-tetrachlorodibenzofuran in the rat and human. Toxicology and Applied Pharmacology, 123(1), 34–42. 

Chloroarenes
Dibenzofurans