Fatty acids, C16-18

Endpoint summary

Additional information

Fatty acids are ubiquitous and dynamic in the environment and are metabolised in water and soil by microorganisms. Fatty acids occur in the environment both naturally and anthropogenic uses. Microbial metabolism is the primary route of degradation in the environment while fatty acids are an integral part of the cell membranes of every living organism from bacteria and algae to higher plants and animals. Each of these organisms contain fatty acids as part of their food reserves and consume them to produce energy required for anabolic and catabolic metabolism. 

In water fatty acids are abiotically stable (OECD SIDS, 2009). Based on the ready biodegradability and molecular structure (aliphatic, mostly saturated carbon chains) hydrolysis is not a relevant degradation pathway and thus was not tested. Modelled data on the photodegradation in air are available for aliphatic fatty acids of C8-C22 carbon chain length. The data show a decreasing photodegradation half-life with increasing chain length. Unsaturated fatty acids undergo photolysis faster than saturated. The half-life declines with the number of double bounds. The calculated half-lives are in the range of 15.4 hours for octanoic acid (C8) to 5.7 hours for octadecanoic acid (C18) (OECD SIDS, 2009). Direct photolysis is not expected to contribute appreciably to the overall breakdown rate in water and soil, since the environmental degradation of these substances is predominantly of biotic nature.

The data set for biodegradation includes standard biodegradation studies as well as data obtained by valid QSAR models. As summarized in the category justification, the members of the fatty acid category demonstrates ready biodegradability. The vast majority of the experimental results revealed ready biodegradability which was supported by reliable QSAR predictions. The consistent positive test results over the whole category supersede single negative results. In conclusion, aliphatic fatty acids comprising C8 - C22 carbon chain length are judged to be readily biodegradable. This judgment is consistent with the hazard assessment presented in the OECD SIDS (2009) for the category “Aliphatic Acids Category” where aliphatic fatty acids with a carbon chain length in the range of C8 – C22 were described to be readily biodegradable.

Adsorption potential to sediment and soil is shown for fatty acids starting at a chain length of 12 and higher indicated by a Koc value of approximately 800 for lauric acid (C12). Accordingly, fatty acids with a shorter chain length partition mainly to the water phase. The members of the fatty acids category with chain length greater than 14 have a low potential of mobility in soil based on high Koc values and low water solubility. Volatilisation is not expected to be a significant transport process or dissipation route for fatty acids in the environment. The log Pow of fatty acids are in the range of 1.57 to 9.91. This suggests that some fatty acids may tend to bioconcentrate in the environment. 

A fish bioaccumulation study is available for a C12 fatty acid-sodium laurate which showed negligible evidence of bioaccumulation potential in fish tissues with an estimated BCF of 225 L/kg after 28 days exposure.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 considered that there will be no risk to aquatic organisms from bioconcentration/biomagnification of fatty acids. Similarly the range of log Koc values given suggests that some fatty acids may be expected to adsorb to sediment. It is considered that rapid biodegradation and the ubiquity of fatty acids will not have any environmental relevance. Therefore it is considered that there will be no risk to sediment dwelling organisms.