technical guidance documents - Institute for Health and Consumer ...

APPENDIX III
not be available and expert judgement will be necessary to decide whether the exposure of
the aquatic organisms is adequately described. Such non-measured concentrations are
normally described as 'nominal' concentrations and refer to the level at which it was
intended that exposure would occur. Such concentrations may be acceptable if the test
substance:
- is sufficient soluble in test water, i.e. the test concentrations are below the water
solubility;
- is relatively stable in test water;
- has a low absorbance to the system delivery and exposure apparatus;
- is non-volatile.
For the interpretation of data that were generated by using solubilisers the altered
bioavailability (enhancement/reduction) has to be considered. For many substances,
including poorly water soluble substances, volatile substances and substances that hydrolyse
or adsorb on surfaces, nominal concentrations are often not appropriate and additional
information may be necessary in order to verify the actual exposure concentrations. In some
cases, the choice of a semi-static or flow-through system (fish test) may allow a presumption
of a stable exposure concentration. In general, the more likely it is that the physical chemical
properties of the substance would lead to a loss of concentration over the course of the test,
the more important it becomes to verify the concentration by direct analysis of the test water
at suitable points throughout the test. Where the exposure concentration can not be
determined with confidence, the test should be regarded as ' not-valid' for the purposes or
risk assessment;
• The environmental conditions which exist during the test should be recorded and be both
stable and appropriate. Significant variations in the environmental conditions such as pH,
temperature, water hardness, oxygen levels and light regime can induce undue stress within
the test organisms and hence false levels of toxicity. Absence of information on these
parameters would suggest that the test system was not well described although would not
necessary invalidate the data if other quality criteria are met;
• The L(E)C50 would normally be determined on a statistical basis from the effects observed
over a range of concentrations. It is important, therefore, that sufficient organisms are tested
at each concentration level and sufficient concentration levels are chosen so as to allow a
statistically valid derivation to be made of the appropriate effect concentration. In the
absence of this details, a clear indication of the method used to calculate the effect (or no
effect) concentration may be sufficient. Limit tests would not normally be acceptable expect
as a means of demonstrating no toxic effects;
• At issue is whether the duration of a standard toxicity test(s) is long enough for the
compound to reach steady state and elicit a toxic response (Hawker and Connell, 1985;
Connell, 1990; Kristensen and Tyle 1990). For many organic non-metabolizable
compounds, the time to reach respectively 80% and 95% of the steady state concentration is
depending on lipophilicity of the compound (OECD, 1994b).
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