2,3,7,8-Tetrachlorodibenzodioxin TCDD

Some hquid defoamers are preemulsified relatives of paste defoamers. In addition to the fatty components mentioned above, kerosene [8008-20-6] or an organic cosolvent such as 2-propanol have been used to enhance stabiUty of the oil—water emulsion and the solubiUty of the defoamer s active ingredients. These cosolvents are used less frequently as concerns increase about volatile organic emissions (VOCs) from the paper machine. Additionally, the use of ultrapure mineral oil in defoamers has become commonplace. Concern about the creation of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) in the pulping process has led to the discovery of unchlorinated precursor molecules, especially in recycled mineral oil and other organic cosolvents used in defoamer formulations (28). In 1995 the mineral oil that is used is essentially free of dibenzodioxin and dibenzofuran. In addition, owing to both the concern about these oils and the fluctuating cost of raw materials, the trend in paper machine defoamers is toward water-based defoamers (29). 

When pushed to the limit by overriding human health concerns, residue chemists have achieved detection limits of Ippt (Ingkg ) or even into the low ppqr (1 pg kg ) range. An example at the 1 ppt level is provided by methods for 2,3,7,8-tetrachlorodibenzodioxin (TCDD) in milk and TCDD in adipose tissue. Eor relatively clean matrices such as water and air, preconcentration on solid-phase adsorbents followed by GC or gas chromatography/mass spectrometry (GC/MS) can provide detection limits of 1 ng m and less for air (examples in Majewski and Capel ) and 1 ngL and less for water (examples in Larson et A summary of units of weight and concentration used to express residue data is given in Table 1. 

Dioxin is a common name for dibenzo-l,4-dioxane, which is 1,4-dioxane fused with two benzene rings. The name dioxin is often used incorrectly in the news media for 2,3,7,8-tetrachlorodibenzodioxin (TCDD), a toxic contaminant in the synthesis of the herbicide called 2,4,5-T or Agent Orange. Surprisingly, TCDD has been in the environment for many millions of years because it is also formed in forest fires. Most dioxins are toxic and carcinogenic (cause cancer) because they associate with DNA and cause a misreading of the genetic code. 

Whereas three or more or//io-chlorines block rotation about the ring-connecting bond, congeners with none, or only one, or//io-chlorine can relatively easily achieve a planar configuration (colloquially referred to as coplanars ) and may behave as isosteres (compounds with similar shape, functionality, and polarity) to 2,3,7,8 — tetrachlorodibenzodioxin (TCDD). These bind significantly to the dioxin receptor, and meas-... 

Up to eight positions on the ring structures of both PCDDs and PCDFs can be chlorinated yielding different isomers with the same numbers of C, H, and Cl atoms in the molecule. The toxicity of these congeners depend on the position of the Cl atoms in the molecule. 2,3,7,8-Tetrachlorodibenzodioxin (TCDD) is considered the most toxic of the congeners. 

Polychlorinated dibenzodioxins and furans (PCDD/Fs) are persistent, bioaccumulative toxic by-products of incomplete combustion—whether manmade or natural—and certain chemical processes. 2,3,7,8-tetrachlorodibenzodioxin (TCDD) is the most toxic of the family of 210 compounds. 

Naphthalene (NAPH), pyrene (PYR), and 2,3,7,8-tetrachlorodibenzodioxin (TCDD) were chosen as typical air contaminants. For purposes of human exposure and risk assessments, the concentrations in films at 25°C and 1 atm total pressure assuming air concentrations are at the saturated vapor levels. Choose any realistic parameters needed and justify any key assumptions. 

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