PARENT SESSION

5B a - RA: Ranking and Chemical Specific
Hall 6
8:30 AM - 12:30 PM, Tuesday, 29 April 2003
Co-chair: Loonen, H.¹, McCarty, L.², ^{1 2}

(TU6/4) Toxic ratio: an indicator of the inherent toxicity for PBT classification.

Maeder, Valérie¹, Scheringer, Martin¹, Escher, Beate², Hungerbühler, Konrad¹, ¹ Swiss Federal Institute of Technology (ETHZ), Zurich, Switzerland² Swiss Federal Institute for Environmental Sciences and Technology (EAWAG), Dübendorf, Switzerland

ABSTRACT- Persistent, bioaccumulative, and toxic (PBT) chemicals are a particularly problematic substance class. Several PBT identification methods exist, but there is still need for improvement, particularly for the toxicity indicator. When assessing chemicals according to the three criteria P, B, and T, the independence of the criteria is an important condition. This condition is not fulfilled if toxicity test results such as the LC50, based on external exposure, are used as T indicator. Indeed, the LC50 integrates both bioaccumulation and toxicity. Using indicators for inherent toxicity like critical body residues or internal effect concentrations would remedy that inconsistency. However, such data are not readily available and experimental methods are arduous and have not been internationally standardized. Therefore, we propose to use the toxic ratio (TR) as alternative. It is conceptually based on inherent toxicity, but can be derived from LC50 values. The TR is defined as the ratio of LC50 estimated from a QSAR for baseline toxicity and the experimental LC50 value. With some 300 chemicals the practicability of this approach is demonstrated. A TR of 10 is taken as threshold value separating specifically toxic chemicals from baseline toxicants. A comparison of both indicators (TR and LC50) shows that employing TR as toxicity indicator would lead to a reclassification of 30% of the chemicals studied. Particularly, the hydrophobic baseline toxicants, penalized twice in a PBT assessment when T is based on external exposure, would be reclassified as non-toxic. On the other hand, the hydrophilic and specifically toxic chemicals, overlooked by an exposure-based indicator, would be identified as toxic.

Key words: inherent toxicity, PBT assessment, critical body burden, toxic ratio