Chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclic aromatic compounds with similar chemical and physical properties. There are 75 congeners of chlorinated dibenzo-p-dioxins. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most widely studied of these compounds. TCDD and chemically similar compounds are collectively called dioxins; TCDD serves as the reference compound for this class of compounds (EPA 1994b,d), but it represents a small portion of incineration emissions of PCDDs and PCDFs.

Sources

Historical records of the concentrations in sediment cores show that in the Great Lakes area the levels of dioxins started to rise greatly in the mid 1930s (Czuczwa and Hites 1986), a time corresponding to the growth in the production of chlorinated organic chemicals. The levels in the sediments began to decline in the 1970s, as particulate emission controls began to be imposed. Although such observations are consistent with combustion sources being a major source (particularly incinerators without heat recovery which were phased out in the 1970s), unidentified sources still might be dominant, because attempts at mass balances suggest that the observed deposition rates are greater than can be accounted for by known sources (Brzuzy and Hites 1996).

In addition, there are substantial differences between the homologue distribution of dioxins found in the environment and those emitted by incinerators—differences that cannot be explained by models of the environmental fate of dioxins from combustion sources. Other sources which may be important include the burning of wood treated with pentachlorophenols, secondary copper smelting, fireplaces, and motor vehicles.

Behavior in the Environment, Pathways, and Exposures

The 2,3,7,8 chlorinated dioxin and furan congeners appear to be resistant to natural degradation, bioaccumulate in many organisms and, possibly, biomagnify to the highest levels of the food chain (ATSDR 1998a). The dominant transformation processes affecting their fate are surface photolysis and gas-phase diffusion or volatilization with subsequent photolysis (Yanders et al. 1989). Models (ATSDR 1998a) of the behavior of TCDD show that it is transported primarily through the air and distributed regionally, and that it accumulates primarily in soils. The decomposition rates, both photochemical and bacterial, decrease with extent of chlorination so that the higher chlorinated PCDDs persist longer.

ATSDR (1998a) reports that most of the measurements of CDDs in air tend to be very close to current detection limits. CDDs are found at the greatest concentrations in urban air with octachlorinated dioxin (OCDD) being the most prevalent congener (up to 0.100 ppq), heptachlorinated dioxins (HpCDDs) being



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