AbstractThe critical body residue (CBR), estimated from aquatic toxicity QSARs and bioconcentration‐logKowrelationships, appears to be relatively constant, at about 4 mmol L−1of fish, for the acute toxicity of a variety of hydrophobic narcotic organic chemicals examined by the U.S. Environmental Protection Agency (Duluth, MN) in tests with the fathead minnow. However, for hydrophilic chemicals (logKow<1.5) the bulk of the toxicant is in the water phase rather than the organic/lipid phase of the organism, so the whole‐body residues in these cases should be similar to the LC50 water concentration. Over the logKowrange of – 1.5 to 6, acutely toxic whole‐body residues for narcotics can be approximated by the QSAR‐derived equation: CBR (mM) = 2.5 mM + 50/Kow. Estimates obtained by this method are in reasonable agreement with the limited literature data available for acutely toxic whole‐body residues of hydrophobic narcotic organic chemicals. Elimination half‐lives estimated from nonlinear curve fitting to time‐toxicity information were relatively constant for the Duluth bioassay data at approximately 3 h. Despite the relatively high variability of this type of kinetics data, the literature information for small aquatic organisms, from both toxicity‐and bioconcentration‐based tests, was in a similar range. It appears that QSARs created with raw aquatic bioassay data occur primarily as a result of the influence of chemical‐physical properties on the partitioning process. LogKowappears to have little to do with the inherent potency of the neutral, narcotic org
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