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Classification & Labelling & PBT assessment

PBT assessment

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PBT assessment: overall result

PBT status:
the substance is not PBT / vPvB

The PBT Assessment for Fatty acids, C16-18 and C18-unsaturated, branched and linear, “Monomer acid” (CAS No. 68955-98-6) is based on the criteria set out in the “Guidance on information requirements and chemical safety assessment, Chapter R.11: PBT Assessment” (ECHA, 2012).


Fatty acids, C16-18 and C18-unsaturated, branched and linear, “Monomer acid” is readily biodegradable in one study according to OECD 301B (67% (CO2 evolution) after 28 d). Thus, the test substance does not meet the screening criterion for persistency and it is not considered to be persistent (P) or very persistent (vP).


No tests on bioaccumulation are available for Fatty acids, C16-18 and C18-unsatd., branched and linear “Monomer acid”, nevertheless the potential for bioaccumulation is expected to be low.

First of all, Fatty acids, C16-18 and C18-unsatd., branched and linear, has a poor water solubility. Although the result of the water solubility study based on the sum solubility of all the components of the test material was 15 mg/L, it was only 0.6 mg/L based on the two major constituents. Also, media preparation trials for the chronic daphnia study showed that the solubility of the substance lies below 5 mg/L. Therefore one can only expect to have a very low aqueous concentration and consequently a reduced bioavailability, and exposure to the aquatic environment would be extremely low. In addition the biotic degradation of the substance would also contribute to a reduced concentration in the aquatic environment. As ECHAs R.7c Endpoint specific guidance states that readily biodegradable substances are most likely to be rapidly metabolised in organisms.

Due to the potential of these substances to absorb, one may assume that if uptake will occur, it can be attributed to the pathway of ingestion of soil or sediment. From the toxicokinetic behaviour of monomeric acids in mammals it can be assumed that unsaturated monomeric C16-C18 fatty acids are more readily absorbed than saturated fatty acids like octadecanoic and isooctadecanoic acid but less than fatty acids with shorter chain length. Fatty acids occur naturally in all aquatic organisms and are ubiquitous in the aquatic environment, where fatty acids are predominantly readily biodegraded in an aerobic environment by microorganisms. Microbial metabolism is the primary route of degradation in aquatic environment.  As nutritional energy source, fatty acids are absorbed by different uptake mechanisms in mammals depending on the chain length. Long chain fatty acids (>C12) are absorbed into the walls of the intestine villi and assembled into triglycerides, which then are transported in the blood stream via lipoprotein particles (chylomicrons). In the body, fatty acids are metabolised by various routes to provide energy. Besides this, fatty acids are stored as lipids in adipose tissue and as precursors for signalling molecules and even long chain fatty. In addition fatty acids are an integral part of the cell membranes of every living organism from bacteria and algae to higher plants. Fatty acids are known to be easily metabolised. The rate of metabolism of fatty acids was considered to vary in proportion to their water solubility (Lloyd, 1957). In case of absorption fatty acids will undergo rapid metabolism and excretion (either in the expired CO2 or as hydroxylated or conjugated metabolite in the urine in the case of cyclic fatty acids) as they feed into physiological pathways like the citric acid cycle, sugar synthesis, and lipid synthesis. As fatty acids are naturally stored in the form of triacylglycerols primarily within fat tissue until they are used for energy production (fat storage tactic), it is therefore concluded that there will be no risk to organisms from bioconcentration/biomagnification of fatty acids within the food chain.

In conclusion, the range of log Kow values given suggests that the dimerised fatty acids may be expected to have tendency of a higher bioaccumulation, however, this only indicates intrinsic potential of the substance, and does not truly reflect its behaviour in the environment. For instance, one must consider the biodegradation of the substance, and also its degradation within living organism, for example, metabolism. Although readily biodegradability does not preclude a bioaccumulation potential, one can assume that it will certainly decrease the bioavailability of the substance. Furthermore, as fatty acids are the end products of carbohydrate metabolism in living organisms muscle tissues, an evaluation of anthropogenic distribution of fatty acids based on the concentrations determined in the organs and tissues of aquatic organisms may over estimated. Hence, if uptake does occur the metabolism and excretion (or rather elimination/depuration) of the these readily biodegradable fatty acids is so effective that Fatty acids, C16-18 and C18-unsatd., branched and linear does not pose a risk to organisms in regard to bioaccumulation/biomagnification.


Long-term toxicity testing with Daphnia magna resulted in a NOEC of ≥ 5 mg/L for Fatty acids, C16-18 and C18-unsaturated, branched and linear, “Monomer acid” (OECD 211). Moreover, the test substance is not classified according to Directive 67/548/EEC and Regulation (EC) No. 1272/2008 (2nd ATP). Thus, the criteria set out in Annex XIII of Regulation (EC) No. 1907/2006 are not met and the test substance is not considered to meet the T criterion.


Lloyd, L. E. and Crampton, E. W. (1957). The relation between certain characteristics of fats and oils and their apparent digestibility by young pigs, young guinea-pigs and pups. J. Anita. Sci. 16:377.

Likely routes of exposure:

Because the substance does not fulfill the PBT and vPvB criteria, no emission characterization needs to be performed.