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The Dimerised Fatty Acids and its Derivatives category covers C16 - C18 unsaturated fatty acids derived monomers, dimers and trimers, as well as their hydrogenated products in different proportions and in accordance with their corresponding production and purification processes. They are all prepared by the dimerisation (and in some cases also isomeristation) of C16 - C18 unsaturated fatty acids. As UVCB substances derived from natural sources, members of this category are chemically similar as they are all essentially a complex mixture of C16 - C18 unsaturated and saturated, branched and linear fatty acids, their monomers, dimers and trimers with varying structural geometric isomers. In the category family, all substances have an overlap in regard to their composition. With reference to information of existing category, the category of Dimerised Fatty Acids and Its Derivatives is based on similarities in physicochemical and toxicological properties and 2 sub-categories were further defined on the basis of their environmental fate and environmental toxicity. The first sub-category covers three monomeric (by-) products of the dimerization process (readily biodegradable substances).

Sub-category 1: predominantly monomers

ID No.


Common Name

Chemical Name



Monomer acid

Fatty acids, C16-18 and C18-unsaturated, branched and linear,



Hydrogenated monomer acid

Octadecanoic acid, branched and linear”




Isooctadecanoic acid

Derived from the same starting substance, all substances in this category have a homologous composition of fatty acids with a C16 - C18 carbon chain in diverse forms, that is susceptible to oxidation of metabolic process. The similarity of category members is justified on basis of scope of variability and overlapping of composition, representative molecular structure, physico-chemical properties, tox-, ecotoxicological profiles and supported by various QSAR methods, which are described in greater detail within the category justification. There is no convincing evidence that any one of these chemicals might lie out of the overall profile of this category or sub-category, respectively. The key points that the members share are:


- Common origin of C16-18 unsaturated fatty acids

- Similar/overlapping structural features

- Similar metabolic pathways

- Similar physico-chemical properties, (for example the log Koc is > 4 < 5, the log Kow is judged to be > 4, and a poor water solubility for all members)

- Common properties for environmental fate & eco-toxicologcial profile of the two sub-categories (ready biodegradability, no toxicological effects up to the water solubility limit for aquatic organisms)

- Common levels and mode of human health related effects


Since the group concept is applied to the category member Dimerised Fatty Acids and it Derivatives (sub-category 1 predominantly monomers). Data will be generated from a representative sub-category member to avoid unnecessary animal testing. Additionally, once the group concept is applied, substances will be classified and labelled on this basis. The selection of Fatty acids, C16-18 and C18-unsaturated, branched and linear (CAS No. 68955-98-6) as representative test substance for the sub-category 1 “monomers” was an iterative process of combination of different aspects:

The water solubility of Fatty Acids, C16-18 and C-18, unsaturated, branched and linear, in its entirety, as a complex mixture (the sum solubility of all the components of the test material) estimated to be 15.0 mg/L at 20°C. The water solubility of Fatty Acids, C16-18 and C-18, unsaturated, branched and linear based on its 2 major constituents is 0.6 mg/L at 20°C. A higher solubility would imply that if exposure were to occur this substance would not only be adsorbed to soil particles (logKoc >5) but also concentrations may exist within the soil pore water also. As an earthworm tests allows uptake via all possible routes, via surface contact, soil particle ingestion and the porewater, Fatty Acids, C16-18 and C-18, unsaturated, branched and linear was determined as the most bio-available substance of the 3 sub-category members.


The only difference between this member of the sub-category and all other members is the degree of saturation or hydrogenation, respectively. In fact, all substances are closely similar. Based on an evaluation conducted using the OECD Toolbox, the degree of saturation is estimated to not have any effect on the toxicity profile of C18 fatty acids. The profiles for C18 fatty acids containing 0, 1 or 2 double bonds are the same in terms of lack of structural alerts, toxic hazard classification by Cramer, aquatic toxicity mode of action by OASIS, aquatic toxicity classification by ECOSAR, etc. For the unsaturated fatty acids additional organic functional groups are indicated (allyl, alkene), but these are not coupled to any structural alerts for protein binding. Experimental data on the long-term toxicity of 16/18C fatty acids to daphnia seem to indicate that unsaturated fatty acids even have slightly higher toxic potential compared to the corresponding saturated fatty acids (MOE, 2008; MOE 2003, entered under IUCLID section 5.1.4). This observation could possibly be explained by the fact that additional steps are required for the β-oxidation process, when double bonds are present, and more energy is thus needed for the metabolism of unsaturated fatty acids (Berg, Tymoczko and Stryer, 2002). Based on this information, Fatty Acids, C16-18 and C-18, Unsaturated, Branched and Linear can be considered as a worst case read-across substance. Additionally, the production volume of the unsaturated form is much higher than the other members. Therefore one can assume that the risk of any direct exposure to the soil would be highest here due to large production volumes. Whereas, Octadecanoic acid branched and linear, is more or less to be considered as an intermediate for the production of Isooctadecanoic acid, therefore one would expect a limited exposure to the terrestrial environment. Isooctadecanoic acid also has a lower expected production volume and a limited direct exposure to soil, although it cannot be excluded as a possible use.

Based on all aspects discussed above, it was decided to test Fatty Acids, C16-18 and C-18, Unsaturated, Branched and Linear as representative member of the sub-category 1 “predominantly monomers”.

A chronic earthworm study was conducted according to OECD guideline 222 with the read-across substance Fatty acids, C16-18 and C18-unsaturated, branched and linear, “Monomer acid” (CAS No. 68955-98-6). As no effects occurred in the this study, the members of the subcategory 1 predominantly monomers, are considered as not harmful to terrestrial organisms and no further testing is needed.

As the test substance is not classified as toxic or harmful, the substance is not considered to cause toxic effects in predators. Additionally, the uptake and bioaccumulation potential is low and thus the test substance is considered to cause no hazard to predators. Thus, a study with birds is not needed due to animal welfare reasons and no PNEC oral is derived from data on mammalian toxicity.


Berg, J.M., Tymoczko, J.L. and Stryer, L. (2002) Biochemistry, 5th edition, W.H. Freeman and Company

Ministry of the Environment, Government of Japan (2003). Daphnia magna, reproduction test by oleic acid. Food Research Laboratories. Report No. 14053. 2003-03-31.

Ministry of the Environment, Government of Japan (2008). Daphnia, reproduction toxicity test for palmitic acid. Mitsubishi Chemical Safety Institute Ltd.,Laboratory. Report No. A050381. 2008-02-14.