User Guide to Thresholds and Classification - Environmental ...

337User Guide for Thresholds and Classificationsdescriptors inappropriate, for example, surface activity, which makes both the measurement and use of logK OW inappropriate.Difficult substancesSome chemical substances are difficult to test in aquatic systems and guidance has been developed toassist in testing these materials (OECD, 2000). This document is a good source of information on the typesof substances that are difficult to test for bioconcentration and the steps needed to ensure valid conclusionsfrom tests with these substances. Difficult to test substances may be poorly soluble, volatile, or subject torapid degradation due to such processes as phototransformation, hydrolysis, oxidation, or biotic degradation.To bioconcentrate organic compounds, a substance needs to be soluble in lipids, present in the water, andavailable for transfer across the fish gills. Properties that alter this availability will thus change the actualbioconcentration of a substance, when compared with the prediction. For example, readily biodegradablesubstances may only be present in the aquatic compartment for short periods. Similarly, volatility, andhydrolysis will reduce the concentration and the time during which a substance is available forbioconcentration. A further important parameter, which may reduce the actual exposure concentration of asubstance, is adsorption, either to particulate matter or to surfaces in general. There are a number ofsubstances, which have shown to be rapidly transformed in the organism, thus leading to a lower BCF valuethan expected. Substances that form micelles or aggregates may bioconcentrate to a lower extent thanwould be predicted from simple physico-chemical properties. This is also the case for hydrophobicsubstances that are contained in micelles formed as a consequence of the use of dispersants. Therefore, theuse of dispersants in bioaccumulation tests is discouraged.In general, for difficult to test substances, measured BCF and KOW values – based on the parent substance– are a prerequisite for the determination of the bioconcentration potential. Furthermore, properdocumentation of the test concentration is a prerequisite for the validation of the given BCF value.Poorly soluble and complex substancesSpecial attention should be paid to poorly soluble substances. Frequently the solubility of these substancesis recorded as less than the detection limit, which creates problems in interpreting the bioconcentrationpotential. For such substances the bioconcentration potential should be based on experimentaldetermination of log K OW or QSAR estimations of log K OW . When a multi-component substance is not fullysoluble in water, it is important to attempt to identify the components of the mixture as far as practicallypossible and to examine the possibility of determining its bioaccumulation potential using availableinformation on its components. When bioaccumulating components constitute a significant part of thecomplex substance (for example, more than 20% or for hazardous components an even lower content), thecomplex substance should be regarded as being bioaccumulating.High molecular weight substancesAbove certain molecular dimensions, the potential of a substance to bioconcentrate decreases. This ispossibly due to steric hindrance of the passage of the substance through gill membranes. It has beenJanuary 2012 EPA0109