Toxic equivalency factors of polybrominated dibenzo-p-dioxin, dibenzofuran, biphenyl, and polyhalogenated diphenyl ether congeners based on rainbow trout early life stage mortality.

Polybrominated and polychlorinated biphenyls (PBBs/PCBs), dibenzo-p-dioxins (PBDDs/PCDDs), dibenzofurans (PBDFs/PCDFs), and diphenyl ethers (PBDEs/PCDEs) are persistent, lipophilic environmental contaminants that may pose a risk to fish early life stage survival. To determine this potential risk, a rainbow trout early life stage mortality bioassay was used in which the potency of individual polybrominated chemicals was compared to the potency of the most potent polychlorinated chemical in these classes, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Following injection of newly fertilized rainbow trout eggs, fish-specific toxic equivalency factors (TEFs) were calculated as the molar ratio of TCDD LD50 to brominated compound LD50. Signs of toxicity were identical to those produced by polychlorinated TCDD-like chemicals and included yolk sac edema, pericardial edema, multifocal hemorrhages, reduced growth, and craniofacial malformations. Polybrominated dibenzo-p-dioxins, dibenzofurans, and biphenyls exhibited decreased potency with increased bromine substitution. Only 2,3,7,8-TBDD was more potent than 2,3,7,8-TCDD, whereas other polybrominated dibenzo-p-dioxins were equipotent or less potent than identically substituted polychlorinated dibenzo-p-dioxins in this assay. Although two PBDF congeners were equipotent to identically substituted PCDFs, 2,3,7,8-TBDF was 9-fold more potent than 2,3,7,8-TCDF. Both 3,3',4,4'-TBB and 3,3',4,4',5,5'-HxBB were 10-fold more potent than identically substituted polychlorinated biphenyls. The halogenated diphenyl ethers and di-ortho polybrominated biphenyls were inactive in this assay. Thus, in this in vivo assay the polybrominated and polychlorinated TCDD-like chemicals were not always equally potent. To assess the risk posed by mixtures of these chemicals to feral fish populations, fish-specific TEFs for both polybrominated and polychlorinated chemicals should be used.