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Environ Sci Technol. 2004 Jul 1;38(13):3659-66.

Toxic ratio as an indicator of the intrinsic toxicity in the assessment of persistent, bioaccumulative, and toxic chemicals.

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  • 1Swiss Federal Institute of Technology, Institute for Chemical and Bioengineering, CH-8093 Z├╝rich, Switzerland.


Persistence, bioconcentration, and toxicity (PBT) are important hazardous properties of organic chemicals. In PBT assessments, it is desirable that the three criteria P, B, and T are independent. However, this requirement is not fulfilled if an aqueous lethal concentration (LC50) is used as T indicator because LC50 includes both bioconcentration and intrinsic toxicity. Indicators for intrinsic toxicity such asthe internal lethal concentration (ILC) are independent of a chemical's bioconcentration potential. However, ILC50 data are scarce and difficult to measure. Therefore, the toxic ratio (TR) is proposed here as an alternative. TR is defined as the ratio of a chemical's LC50 estimated from a QSAR for baseline toxicity and the experimental LC50 value. TR can also be interpreted as a measure of the ILC relative to the ILC for baseline toxicity. A TR of 10 separates specifically toxic chemicals from baseline toxicants. With some 800 chemicals, the practicability of classifying chemicals in terms of TR is demonstrated. Employing TR as toxicity indicator leads to different T scores for 30% of the chemicals studied. The baseline toxicity of hydrophobic compounds with TR < 10 does not receive a high T score but is still indicated by a high B score. The toxicity of specifically toxic hydrophilic substances is given additional emphasis by high TR values. These classification changes require that the interpretation of the B and T dimensions in PBT assessments is redefined.

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