Molecular size cutoff criteria for screening bioaccumulation potential: Fact or fiction?

Abstract

It has been asserted that, when screening chemicals for bioaccumulation potential, molecular size cutoff criteria (or indicators) can be applied above which no, or limited, bioaccumulation is expected. The suggested molecular size values have increased over time as more measurements have become available. Most of the proposed criteria have been derived from unevaluated fish bioconcentration factor (BCF) data, and less than 5% of existing organic substances have measured BCFs. We critically review the proposed criteria, first by considering other factors that may also contribute to reduced bioaccumulation for larger molecules, namely, reduced bioavailability in the water column, reduced rate of uptake corresponding to reduced diffusion rates, and the effects of biotransformation and growth dilution. An evaluated BCF and bioaccumulation factor (BAF) database for more than 700 substances and dietary uptake efficiency data are compared against proposed cutoff values. We examine errors associated with interpreting BCF data, particularly for developing molecular size criteria of bioaccumulation potential. Reduced bioaccumulation that is often associated with larger molecular size can be explained by factors other than molecular size, and there is evidence of absorption of molecules exceeding the proposed cutoff criteria. The available data do not support strict cutoff criteria, indicating that the proposed values are incorrect. Rather than assessing bioaccumulation using specific chemical properties in isolation, holistic methods that account for competing rates of uptake and elimination in an organism are recommended. An integrated testing strategy is suggested to improve knowledge of the absorption and bioaccumulation of large substances. Integr Environ Assess Manag 2010;6:210-224. ß 2009 SETAC

Equilibrium modeling: A pathway to understanding observed perfluorocarboxylic and perfluorosulfonic acid behavior

“…Consistent with the previously reported molecular size limitation of chemical uptake in tissues [33,47], perfluorotetradecanoic acid was excluded from this test. For the PFC(A)s, contrary to expectation, agreement is seen (Fig.…”

“…Because the K OW values of organic chemicals are seldom known to better than within an order of magnitude [51], agreement of better than an order of magnitude is generally considered excellent [9,48]. Although agreement would not be expected for perfluorotetradecanoic acid because of its size [31,47], as previously explained, it was also modeled. The BCF of perfluorotetradecanoic acid in the three tissues is calculated to be (1-5) Â 10 6 L/kg by Equation 1 but is measured to be (2.3-3.0) Â 10 4 L/kg, consistent with expectation for any neutral organic.…”

Polybrominated diphenyl ethers and polychlorinated biphenyls in freshwater fish from Taihu Lake, China: Their levels and the factors that influence biomagnification

“…Small molecules penetrate cell membranes more easily than large ones, which results in easier uptake as well as elimination, because passive diffusion controls the absorption of HOCs [41]. However, the membrane-penetrating capability of an HOC molecule will decrease with increased size, and a molecule penetrating membranes when the maximal molecular diameter is larger than 1.5 nm is difficult [17]. For the same reason, once a large molecule is absorbed by an organism, it will be difficult to eliminate it through membranes.…”

“…), no matter which TMFs of PBDEs and PCBs are used in a combined or separate way to analyze the influence of molecular size on biomagnification. Therefore, molecular volume seems better to analyze the influence of molecular size on biomagnification of PBDEs and PCBs than molecular weight, because it may reflect the essence of the processes of the uptake and elimination in biota, although several studies have used molecular weight to assess their influence on biomagnification of PBDEs [4,17,38].…”

“…Only a few studies have been performed in slightly PBDEcontaminated freshwater systems [6,16]. In addition, the influences of fish size and physical properties of HOCs (especially molecular size) on biomagnification are still not well understood [17,18], although factors such as the growth of species and physiochemical properties of chemicals (for example, molecular size and octanol-water partition coefficient [K OW ]) have been reported to affect biomagnification [7,10].…”