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Environmental fate & pathways

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If a substance has a low potential for bioaccumulation and/or a low potential to cross biological membranes then one is not required to conduct studies for bioaccumulation. This is the case for the dimerised fatty acids, and in this particular case, Fatty acids, C16-18 and C18-unsatd., branched and linear, CAS No. 68955-98-6 no tests on bioaccumulation are available, 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* therefore one can only expect to have a very low water concentration and consequently a reduced bioavailability, thus exposure to the aquatic environment would be extremely low. This would automatically reduce the uptake of the chemical through the aquatic environment. 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). Also CIR (1987) and Iwaoka and Perkins (1976) 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, 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.

*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.

 

References:

CIR (1987) Final report on the safety assessment of oleic acid, lauric acid, palmitic acid, myristic acid, stearic acid.J. of the Am. Coll. of Toxicol.6(3):321-401.

Iwaoka W. T., Perkins E. G. (1976). Nutritional effects of the cyclic monomers of methyl linolenate in the rat. Lipids 11:349-353

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.