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Appendix 3. CONCAWE Position Paper on the
Environmental Assessment of Metabolites derived from
Petroleum Substances for PBT purposes
Summary
In conducting a PBT assessment of hydrocarbon substances, the Hydrocarbon Block
(HCB) Method is used (EC, 2003) together with predictive tools for assessing the
primary half-life. In the following paper the consequences of this for assessing major
metabolites is discussed. The review shows that all hydrocarbons must degrade (under
oxic conditions) by first forming ketone, aldehyde and subsequently carboxylate and
hydroxyl substituents. A further assessment demonstrates that for all the major classes of
hydrocarbons, the major metabolites formed are in all cases less toxic, less persistent and
less bioaccumulative than the parent molecule. Consequently it can be concluded that for
PBT assessment purposes, the metabolites of hydrocarbons are not required to be further
assessed.
Introduction
Petroleum substances typically contain hydrocarbons that exhibit large differences in
physico-chemical and fate properties. These properties define the pattern of emissions
and differential environmental distribution of the constituent hydrocarbons, and
consequently it is not possible to define a unique predicted exposure concentration (PEC)
for a petroleum substance. Furthermore, it is not possible to directly apply current risk
assessment guidance developed for individual substances to complex petroleum
substances. To provide a sound technical basis to assess environmental exposure and
risks of petroleum substances, CONCAWE devised the Hydrocarbon Block Method in
which constituent hydrocarbons with similar properties are treated as pseudo-components
or "blocks" for which PECs and predicted no effect concentrations (PNECs) can be
determined (CONCAWE, 1996). Risks are then assessed by summing the PEC/PNEC
ratios of the constituent blocks. While this conceptual approach has been adopted by the
EU as regulatory guidance (EC, 2003) experience in applying this method was limited.
Recent studies demonstrate the utility of the HCB method to gasoline (MacLeod et al.
2004, McGrath et al. 2004; Foster et al. 2005) and further work has been on-going to
support the practical implementation of the HCB methodology to higher boiling
petroleum substances that pose an environmental hazard. Utilising the basic approach
described in these papers, the generic approach (as described in Comber et al, 2006) has
been developed and is in the process of being written up.
One of the issues that arise in such risk assessments, whether of single substances or
hydrocarbon blocks, is the environmental impact of any metabolites that are formed. In
order to assess this issue, the first task is to understand the range of hydrocarbon
structures and thus the type of structures that would be involved in generating
metabolites. There are four major classes of hydrocarbons to be addressed:
paraffins/alkanes; iso-paraffins; naphthenics (cyclic alkanes) and aromatic structures,
which will also include polyaromatic compounds. Within these basic classes there is an
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