CAS RN: 1746-01-6

Exposure Summary

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is formed from the incineration of wastes and from the production of bleached wood pulp and paper. It also occurs as a contaminant in the manufacture of various pesticides and, as a result, has been directly released into the environment during the use of these pesticides. TCDD is found in combustion emissions of various sources, including power plants which use coal or oil, wood burning, and home heating systems. TCDD is also formed in automobile and diesel exhaust. It is formed during the synthesis and combustion of polyvinylchloride. If released to air, a vapor pressure of 1.5X10-9 mm Hg at 25 deg C indicates TCDD will exist solely in the particulate phase in the atmosphere. Particulate-phase TCDD will be removed from the atmosphere by wet and dry deposition. If released to soil, TCDD is expected to have no mobility based upon a mean Koc value of 2.45X10+7. Volatilization from moist soil surfaces may be an important fate process based upon an estimated Henry's Law constant of 3.2X10-6 atm-cu m/mole. However, adsorption to soil is expected to attenuate volatilization. TCDD was photo-degraded 99.9% during and immediately after soil application in test site studies, indicating this is the most probable route of degradation. TCDD is not expected to volatilize from dry soil surfaces based upon its vapor pressure. Based on laboratory studies using aquatic sediments and lake water, TCDD is expected to be recalcitrant to microbial attack. If released into water, TCDD is expected to adsorb to suspended solids and sediment based upon the Koc. Volatilization from water surfaces is expected to be an important fate process based upon this compound's estimated Henry's Law constant. Estimated volatilization half-lives for a model river and model lake are 21 and 161 days, respectively. However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 5.1X10+4 years if adsorption is considered. The photolysis half-life for TCDD in natural waters from marshes, ponds and lakes was 3.5-5.9 hours. BCF values of 1585 to 5.1X10+6 for fish suggest bioconcentration in aquatic organisms is very high. TCDD does not hydrolyze in environmental waters and is not expected to react with photochemically produced singlet oxygen or peroxy radicals in water. Occupational exposure to TCDD occurs through inhalation and dermal contact to fire fighters and cleanup workers involved with PCB transformer fires, to workers involved with incineration operations, and to workers handling pesticides, hexachlorophene, trichlorophenol, or other compounds which may contain small impurities of TCDD. Monitoring data indicate that the general population is exposed to TCDD primarily through the ingestion of food. The estimated average daily intake of TCDD is 34.8 ng/day for the US population. Foods from animal origins (e.g., milk and dairy products, beef, fish, and eggs) comprise 95% of the estimated total daily exposure to TCDD. Secondary exposures occur via inhalation of ambient air and dermal contact with consumer products containing TCDD as an impurity. TCDD is found in human milk, blood and tissues. However these concentrations in the general population appear to be decreasing with time. (SRC)
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