PBT Assessment - REACh

PART R11 – PBT ASSESSMENT
bioaccumulation potential is warranted. This may include critical review of its bioaccumulation
potential according to (Q)SARs and bioaccumulation models taking into account its potential for
uptake and metabolism (EC, 2003). The result of such an assessment may be so uncertain that
further bioconcentration or bioaccumulation testing may have to be undertaken to determine
whether the substance is B or vB.
Experimental testing to determine the BCF
The standard test to study the BCF in fish is the OECD 305 bioconcentration test guideline (OECD,
1996). In this guideline BCF is experimentally estimated using a flow through exposure regime
with an initial uptake phase of up to 28 days followed by a depuration phase in clean water. The
BCF can be estimated from the ratio Cf/Cw (Cf: concentration of test chemical in fish at steady
state; Cw: concentration of test chemical in the exposure phase (water) or Ku/Kd (Ku: rate constant
for uptake and Kd: rate constant for depuration; provided that first order – one compartment
kinetics apply). In cases where substances meet the screening criterion for B or vB, it is probable
that these substances are very hydrophobic and have a very low aqueous solubility. Due to these
properties it can be very difficult to test them in aqueous exposure systems such as the OECD 305
test. Alternatively, a recently developed dietary test (Anonymous, 2004) could be used to determine
bioaccumulation potential through food or to derive data to estimate a BCF. However, many studies
to determine the BCF of hydrophobic substances have been performed following aqueous exposure.
The interpretation of such studies must be done with care. Many such studies were conducted
following earlier versions of the OECD 305 test guideline, and may include the following possible
artefacts or shortcomings:
• Difficulties in measuring the ‘true’ aqueous concentration due to sorption of the substances to
particulate and dissolved (organic) matter;
• Unstable concentration of the test substance in water and thus highly fluctuating exposure
conditions
• Adsorption of the test chemical to glass walls or other materials;
• Volatilisation.
• Testing at concentrations clearly above the water solubility of the test chemical, normally via
the inclusion of dispersants or vehicles which would lead to an underestimation of the BCF
• Determination of a BCF as the ratio between the concentration in fish and in water but under
non steady state conditions
It is important to realise that in many of the studies that have investigated relationships between
molecular dimensions and reduced uptake, i.e. based on ‘lower’ BCFs than expected, it was not
always possible to exclude occurrence of some of the above mentioned shortcomings or artefacts
and truly reduced uptake. Thus rules relating to molecular dimensions or mass proposed in the past
and claiming reduced uptake should be critically reviewed.
Some studies have proposed a reduced uptake based on experimental bioconcentration studies. The
reduced uptake then usually refers to reduced uptake via the fish gills. This does not imply that
there will be reduced or no uptake possible via the gut uptake, i.e. from food, where other uptake
mechanisms may play a role. The extent to which those additional uptake mechanisms play a role in
bioaccumulation, however, is inadequately quantified for fish and aquatic invertebrates. There is
evidence, however, for certain highly persistent and hydrophobic chemicals that significantly
accumulate via the food, even for gill breathing organisms, but particularly for predatory fish higher
in the food chain.
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