How reliable are field-derived biomagnification factors and trophic magnification factors as indicators of bioaccumulation potential? Conclusions from a case study on per- and polyfluoroalkyl substances.

This review examines the usefulness of the metrics BMF (biomagnification factor) and TMF (trophic magnification factor), derived from field measurements of the levels of contaminants in naturally occurring biota, for characterizing the bioaccumulation potential ("B") of chemicals. Trophic magnification factor and BMF values greater than 1.0 are often considered to be the most conclusive indicators of B status, and the TMF criterion has been referred to as the "gold standard" for B categorization. Although not wishing to dispute the theoretical primacy of field-derived BMFs and TMFs as B metrics, we make the case that, in practice, the study-to-study (and even within-study) variability of the results is so great that they are of very restricted usefulness for assessing B status, at least in the case of the per- and polyfluoroalkyl substances (PFASs), on which we focus here. This conclusion is based on an analysis of the results of 24 peer-reviewed studies reporting field-derived BMFs or TMFs for 14 PFASs, for which BMF values often range over several orders of magnitude from <<1.0 to >>1.0, sometimes even in the same study. For TMFs, the range is a factor of approximately 20 for the most intensely studied PFASs (perfluorooctanoic acid [PFOA] and perfluorooctanesulfonic acid [PFOS]). We analyze the possible causes for such variability: To some extent it results from the differing ways in which the metrics are expressed, but most of the scatter is likely attributable to such factors as nonachievement of the tacitly assumed steady-state conditions, uncertainties in the feeding ecology, the impact of metabolism of precursor compounds, and so forth. As more trustworthy alternatives to field-derived BMFs and TMFs, we suggest the implementation of dietary BMF studies performed under strictly controlled conditions on aquatic, terrestrial, and avian species, as well as the consideration of measured elimination half-lives, which have been demonstrated to be directly related to BMF values.