Association between contaminant tissue residues and effects in aquatic organisms

Associations between tissue residues and toxicity to aquatic organisms were examined to evaluate the applicability of the critical body residue (CBR) approach across different chemical classes. Chemical classes and mode of action categories evaluated included narcotics (polar and nonpolar), excitatory agents, AChE inhibitors, reactives/irritants, CNS seizure agents, aryl hydrocarbon (Ah) receptor agonists, and inorganic metals and organometals. This evaluation indicated that empirical data do not support broad application of the CBR concept across chemical classes. This conclusion is particularly important for polar and nonpolar narcotics because the CBR concept was specifically developed for these chemical classes. The variability observed in tissue residues between chemicals within a given mode-of-action class appears to be generally of the same order of magnitude as the variability of aqueous measures of toxicity such as LC50 values (Table 3; Fig. 10). This observation suggests that either (a) the reported tissue residues were dependent on the aqueous dosing regime; (b) the tissue measurements do not accurately reflect the internal dose at target organs with substantially greater precision than water exposure measurements; or (c) many of the same sources of variability associated with aqueous exposures, such as chemical structure, individual species sensitivity, biotransformation processes, and lipid content, also apply to tissue-based measures of exposure. An additional source of uncertainty of CBRs is whether a chemical has been correctly assigned to a mode of action category. [figure: see text] The CBR approach outlined by McCarty (1986, 1987) and McCarty et al. (1993) underlines an important concept in aquatic toxicology, i.e., that internal chemical dose is the true measure of toxicity for many chemicals rather than imputed dose based on aqueous exposure. Nevertheless, without more refined and accurate examination of that actual internal dose and without additional consideration of differences in sensitivity between species, differences in toxic potency between chemicals, and differences in toxicity of environmentally modified or biotransformed compounds, the CBR approach may not offer practical advantages over conventional media-based exposure assessment.