Interactions of polychlorinated dibenzo-p-dioxin, dibenzofuran, and biphenyl congeners for producing rainbow trout early life stage mortality.

Fish-specific toxic equivalency factors (TEFs), which relate the toxic potency of polyhalogenated aromatic hydrocarbons (PHAHs) to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) based on the endpoint of early life stage mortality, have been used in assessing the risk to fish early life stage survival of complex mixtures of PHAHs in feral fish eggs. Use of TEFs assumes that PHAH congeners act additively. However, this has not been unequivocally determined. Isobolograms and a probit model were used to assess the validity of the additivity assumption by determining the significance of interactions between pairs of polychlorinated dibenzo-p-dioxin (PCDD), dibenzofuran (PCDF), and biphenyl (PCB) congeners when injected into newly fertilized rainbow trout eggs in ratios bracketing those found in feral lake trout eggs from the Great Lakes. The majority of congener pairs tested acted additively in causing rainbow trout early life stage mortality: [1,2,3,7,8- pentachlorodibenzo-p-dioxin (1,2,3,7,8-PCDD)/TCDD]; [2,3,4,7,8-pentachlorodibenzofuran (2,3,4,7,8-PCDF)/1,2,3,7,8-PCDD]; (2,3,4,7,8-PCDF/TCDD), (2,3,7,8-tetrachlorodibenzofuran/2,3,4,7,8-PCDF); [3,3',4,4'-tetrachlorobiphenyl (PCB 77)/3,3',4,4',5-pentachlorobiphenyl (PCB 126)]; [2,3,3',4,4'-pentachlorobiphenyl (PCB 105)/TCDD]; (2,2',4,4',5,5'-hexachlorobiphenyl/TCDD); (PCB 105/PCB 126); and (2,3',4,4',5-pentachlorobiphenyl/PCB 126). The only pairs showing evidence of a statistically significant interaction that deviated from additivity were (TCDD/PCB 77) and (TCDD/PCB 126). Taken together, these results suggest that the use of fish-specific TEFs to determine TCDD equivalents contributed by individual congeners in a fish egg sample and then adding these TCDD equivalents to determine the total amount contributed by all congeners may not exactly predict the mortality risk posed to fish early life stages by the mixture of TCDD-like congeners in the eggs. However, the relatively small deviations from additivity in the rainbow trout sac fry mortality test (1- to 4-fold) are less than traditional uncertainty factors used in noncancer risk assessments (10-fold/factor) and are not sufficient to warrant a change away from the additivity assumption in assessing the risk to fish early life stage mortality posed by TCDD and related compounds in eggs.