The metabolism of novel flame retardants and the internal exposure and toxicity of their major metabolites in fauna - a review

The worldwide production and usage of novel flame retardants increase their exposure to non-human fauna. Animals can accumulate and metabolize these novel flame retardants including novel halogenated flame retardants (NHFRs) and organophosphate flame retardants (OPFRs), which is of considerable significance to their internal exposure and final toxicities. In this review, recent studies on the metabolic pathways and kinetics of the two classes of novel flame retardants and the internal exposure and toxicity of their major metabolites are summarized. The results showed that the metabolic pathways of OPFRs were similar among various animals, while the metabolism kinetics (or toxicokinetics) were variable among species. O-dealkylation, hydroxylation and phase II conjunction were the most likely pathways for OPFRs. NHFRs might be metabolized through the pathways of debromination, hydroxylation, dealkylation, and phase II conjunction. We also suggested that di-alkyl phosphates (DAPs) and hydroxylated OPFRs (OH-OPFRs) were the predominant metabolites in the animal body. DAPs, 2,3,4,5-tetrabromobenzoic acid (TBBA) and 2-ethylhexyl tetrabromophthalate (TBMEHP) have relatively higher internal exposure levels in fauna, which might attribute to their high conversion rate and stability in the body. The metabolism of OPFRs and NHFRs in non-human animals may eliminate their acute toxicity but not their chronic toxicities (especially for endocrine-disrupting effects), which suggests attention should also be paid to the major metabolites. Based on the issues mentioned above, we proposed that the metabolic processes in multitrophic organisms, the transfer of major metabolites across the food web, and the co-exposure of the novel flame retardants and their metabolites in fauna are worth studying in the future.