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ENVIRONMENTAL EXPOSURE ASSESSMENT
2.3.6 Abiotic and biotic degradation rates
In this section, the following processes are described:
• hydrolysis in surface water;
• photolysis in surface water and in the atmosphere;
• biodegradation in the sewage treatment plant;
• biodegradation in the environmental compartments (surface water, soil, sediment).
Transport and transformation (“fate”) describe the distribution of a substance in the environment,
or in organisms, and its changes with time (in concentration, chemical form, etc.), thus including
both biotic and abiotic transformation processes. In general, the assessment of degradation
processes should be based on data, which reflect the environmental conditions as realistically as
possible. Data from studies where degradation rates are measured under conditions that simulate
the conditions in various environmental compartments are preferred. The applicability of such
data should, however, be judged in the light of any other degradation data including results from
screening tests. Most emphasis is put on the simulation test results but in the absence of simulation
test data, degradation rates and half-lives have to be estimated from screening test data.
For substances where a range of degradation data is available, a “weight of evidence” approach
should be employed. When more than one simulation test result is available, a suitable half-life
in the higher end of the observed range should be selected taking into account the realism,
relevance, quality and documentation of the studies in relation to environmental conditions.
When more than one screening test result is available, positive test results should be considered
valid, irrespective of negative results, when the scientific quality is good and the test conditions
are well documented, i.e. guideline criteria are fulfilled, including the use of non-adapted
inoculum (cf. OECD, 2001c). The results of screening tests may be negative due to toxic effects
of the test substance, whereas simulation tests employing a low concentration of the test
substance may give a more realistic estimate of the degradation in the environment. By using all
available degradability test data in this way, it is possible to establish a comprehensive
evaluation of the degradability of the substance.
In this section, methods for derivation of degradation rate constants are described for abiotic
degradation (hydrolysis and photolysis) and biotic degradation (in soil, sediment, water, and
sewage treatment). For hydrolysis and photolysis, only primary degradation is measured. In
general, risk assessment focuses on the parent compound. Nevertheless, if stable degradation
products are formed, the risk assessment should include these. It is possible that the rate of
reaction is such that only the products need to be considered, or in intermediate cases both the
substance and the degradation products will require consideration. It is important to have
information about which chemical species were responsible for any effects that were observed in
the aquatic toxicity studies.
Where substances degrade by complex interaction mechanisms, for example abiotic degradation
followed by biodegradation, and where there are no internationally recognised protocols for
simulation tests, the use of relevant field data could be considered provided that the kinetics of
full mineralisation or formation of possible metabolites have been determined.
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