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

APPENDIX IX
present in low concentrations compared to the non-specific acting components. Nevertheless,
these specific acting constituents should on a case-by-case basis be taken into account in the
environmental risk assessment at least in a qualitative way.
QSARs
The identification of the blocks when applying the HBM may be dependent on the use of QSARs
for the estimation of physico-chemical properties (e.g. log Kow, water solubility, melting point
and vapour pressure) and degradation rates (e.g. photodegradation and hydrolysis rates), when
measured values are not available. There are reasonably well accepted methods for the
generation of these data using readily available data bases, or QSARs. There are no widely
accepted QSARs for biodegradation, but it is considered adequate, at least for screening, if
experimentally determined rate constants for the “blocks” of interest are not available, to use
QSAR estimates for block identification, according the principles laid down in Chapter 4 on the
Use of QSARs.
The use of QSARs is well established for predicting the acute toxicity of simple hydrocarbons,
and can be used to supplement the available ecotoxicity data. Whilst the accuracy of QSARs for
more complex hydrocarbons and for chronic toxicity may need further consideration, they
provide an adequate default where experimental data are not available (in particular where the
values are found not to be key to the outcome of the risk assessment).
The minimum data-set available for each priority petroleum substances, is usually not sufficient
for risk assessment using the HBM, because it will usually comprise tests conducted with the
whole petroleum substance. Since in the HBM process individual hydrocarbons are blocked
together on the basis of their environmental fate and ecotoxicological properties, additional data
on these hydrocarbons are also required. These may be measured data, but it is foreseen that
values derived from QSARs will be helpful for filling datagaps in the establishment of blocks.
When the overall risk assessment for the petroleum substance is undertaken (with the
PEC/PNEC ratios for the blocks calculated and summed), those blocks contributing most to the
overall PEC/PNEC ratio can be identified. It should be noted that any decision on the final
outcome of the risk assessment when the overall PEC/PNEC ratio is close to or greater than one,
will need to be based on measured (rather than QSAR) data. Hence, for each block (unless the
contribution of the particular block is found to be irrelevant to the outcome of the risk
assessment), representative measured base-set data should be available. These data could be on
any component of the block, since by definition, blocks are comprised of hydrocarbons with
similar fate and ecotoxicological properties. Data on some individual hydrocarbons suitable for
this purpose, are already available as the IUCLID database shows.
For “block” identification, QSARs for short (algae, daphnids and fish) and long-term (daphnids
and fish) toxicity are given in Chapter 4 on the use of QSARs. These QSARs can be used for
chemicals with a non-specific mode of action, i.e. for most petroleum substance components.
Considering the assessment factors presented in the TGD (see Section 3.3.1 of the main text) a
factor of 10 on the QSAR derived long-term NOEC is proposed. More guidance on the use of
QSARs in general can be found in Chapter 4.
“Blocks” which do not exhibit acute toxicity
There will be a number of “blocks” for which no acute toxicity is indicated at the limit of water
solubility. Adema (1986, 1991) found no short-term toxicity for n-decane or higher homologues
and for alkylbenzenes with a carbon number higher than 14. This does not necessarily mean that
315