Half-life, PCDDs

Their mobility may be increased by the simultaneous presence of organic solvents such as mineral oil. The half-life of TCDD in soil has been reported as 10-12 years, whereas photochemical degradation seems to be considerably faster but with a large variation that might be explained by experimental differences (solvents used, etc.). Highly chlorinated PCDD/PCDFs seem to be more resistant to degradation than those with just a few chlorine atoms. 

Dioxins (PCDDs) and furans (PCDFs) are polyhalogenated aromatic hydrocarbons of high toxicity. There are a total of 210 different congeners 75 dioxin congeners and 135 furan congeners, of which 17 are potentially toxic. Dioxins and furans are now found prevalent in air, water, and soil in almost all natural environments. PCDD/Fs are strongly bound to organic matter, where half-life in soil has been estimated at 10-20 years (Ryan et al., 1987). PCDD/Fs enter the environment primarily as unintentional byproducts of combustion and chemical processes. 

Sinkkonen, S., Passivirta, J. (2000) Degradation half-life times of PCDDs. PCDFs and PCBs for environmental fate modeling. Chemosphere 40, 943-949. 

Of the other congeners, several have been studied. An elimination half-life of 29.5 days was estimated for 1,2,3,7,8-PCDD in Sprague-Dawley rats following a single oral exposure (Wacker et al. 1986). 

OCDD was more persistent in Fischer 344 rats with an estimated elimination half-life of 3-5 months following 10 daily oral doses (Bimbaum and Couture 1988). These congeners were excreted primarily in the feces following biliary elimination as metabolites (1,2,3,7,8-PCDD, at least three phenolic metabolites) or parent compound. A 13-week dosing study in which Sprague-Dawley rats were administered various mixtures of CDDs estimated liver half-lives of 14.5, 29.3, 45.6, and 100 days for... 

Table I records the molar extinction coefficient (e ) at various wavelengths (A), of solutions of PCDDs 1-4. Water-acetonitrile (2 3 v/v) was used as a solvent for compounds 1-3 whereas, in the case of Og-CDD 4, the solvent was neat acetonitrile. These spectral data were used in the prediction of the direct sunlight photolysis rates, i.e., kgp and corresponding half-life (L3/2)sp) pollutants 1-4...

Just as the model selection can vary widely, so can the assumptions that go into assessing exposures. Factors such as environmental half life of the agent in question (PCB, PCBF, PCDD s inside buildings) the opportunity for exposure to contaminated surfaces, inhalation rates, airborne concentrations based on actual air measurements or mathematical estimation, etc. all can significantly affect the exposure side of the equation and thus affect risks. 

TCDD in areas of intense sunlight has been demonstrated to be significant (4). However, most risk assessments done to date place a half life from 5-10 years on PCB/PCDD/PCDF s found inside buildings or in areas on nondirect sunlight. 

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