Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

for the binding of free-metal ion and other metal complexes to the site of toxic action in an
organism; and it also considers competition between metal species and other cations (Paquin
et al., 2002). The GSIM is fundamentally similar to the FIAM in that it accounts for competition
between metal ions and hardness cations at the physiological active gill sites, but whereas the
FIAM is largely conceptual, the GSIM was used in interpreting toxicity test results for individual
metals and metal mixtures (Pagenkopf, 1983). The BLM was adapted from the GSIM and uses
the biotic ligand, rather than the fish gill as the site of toxic action (Di Toro et al., 2001; Paquin
et al., 2002). This approach, therefore, considers that the external fish gill surface contains
receptor sites for metal binding (Schwartz et al., 2004) and that acute toxicity is associated with
the binding of metals to defined sites (biotic ligands) on or within the organism (Paquin et al.,
2002). The model is predicated on the theory that mortality (or other toxic effects) occurs when
the concentration of metal bound to biotic ligand exceeds a threshold concentration (Di Toro
et al., 2001; Paquin et al., 2002). Free metal cations “out compete” other cations and bind to the
limited number of active receptor sites on the gill surface, which may ultimately result in
suffocation and/or disruption of ionoregulatory mechanisms in the fish, leading to death (Di Toro
et al., 2001; Paquin et al., 2002). Because the BLM uses the biotic ligand (not the fish gill) as
the site of action, the model can be applied to other aquatic organisms, such as crustaceans,
where the site of action is directly exposed to the aqueous environment (Di Toro et al., 2001).
Although the BLM is currently being considered as a tool for regulating metals on a sitespecific
basis, there are potential limitations in using the BLM to regulate metals in surface
waters that should be understood in developing air quality criteria for lead. For example, BLMs
developed to-date have focused on acute mortality/immobilization endpoints for fish and
invertebrates. Chronic exposures are typically of greatest regulatory concern, but chronic BLMs
to date have received limited attention (De Schamphelaere and Janssen, 2004). In addition,
BLMs account for uptake of dissolved metal, but dietary metals have also been shown to
contribute to uptake by aquatic biota and, in some cases, increased toxicity. Besser et al. (2005)
observed that chronic (42-day) Pb toxicity to the amphipod Hyalella azteca was greater from a
combined aqueous and dietary exposure than from a water-only exposure and the authors
conclude that estimates of chronic toxicity thresholds for Pb should consider both aqueous and
dietary exposure routes. The feasibility of incorporating dietary metals into BLMs is under
investigation. Another important issue that must be addressed in developing and applying a
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