Extrapolation of PBBs Environmental Transformation Mechanisms and Toxicity Risks of Byproducts

Polybrominated biphenyls (PBBs) are commonly used flame retardants that pose severe risks to humans. However, there is a lack of systematic research on the transformation process and biological toxicities of PBBs in the environment, which is not conducive to the prevention and control of pollution risks of PBBs. Therefore, the transformation pathways (i.e., photodegradation, microbial degradation, combustion oxidation, and in vivo metabolism) of PBBs and previously designed PBB substitutes were deduced first. Then the potential rodent carcinogenicity, rodent toxicity, mutagenicity, developmental toxicity, skin and eye irritation, skin sensitization, and aquatic toxicity of the transformation products were evaluated using the toxicokinetics (TOPKAT) model. Finally, 3D quantitative structure activity relationship (3D-QSAR) models were constructed to assess the human toxicity (i.e., carcinogenicity, developmental toxicity, hepatotoxicity, epigenetic toxicity, neurotoxicity, and immunotoxicity) of PBBs, PBBs substitutes, and their transformation products. Results showed that the transformation products of PBBs and their substitutes exhibit high toxicity risks (i.e., potential carcinogenicity, mutagenicity, and developmental toxicity) to organisms. The D3-A1 molecule had the highest carcinogenic risk probability at 0.826. The dihydroxy metabolite 2,2'-OH-PBB-80 of the PBB-80 molecule presented the highest potential developmental toxicity risk (toxicity probability 0.713). Polybrominated dibenzofuran (PBDF) showed the strongest skin irritation (probability 0.995). The combustion oxidation products of PBBs exhibited higher potential ecological and human health risks than other transformation products. Among potential toxicity risks to humans, the developmental toxicity of the transformation products of PBBs and their substitutes was theoretically significant, with characterization values ranging from 70.53 to 100.87. This is the first study to comprehensively evaluate the ecological and human health risks of PBBs and their transformation products by combining the inference of transformation pathways with the prediction of transformation product toxicities, providing theoretical support for the design of environmentally friendly PBB substitutes in future studies.