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Category name:

Justifications and discussions

Category definition:
The Long Chain Aliphatic Esters category contains monoesters of saturated and unsaturated natural fatty acids (palmitic, stearic and oleic acid) and monoalcohols. Fatty acids linear, saturated and unsaturated with a carbon chain length from C16-18 and C18’ and fatty alcohols, linear, saturated and unsaturated with a carbon chain length from C10 to C18 and C18’ are included in the category. The category comprises mono-constituent substances as well as mixtures (UVCBs) on condition that mixtures fulfil category requirements.
Category order description:
The category applies to all systemic and local mammalian toxicity endpoints as well as ecotoxic and environmental fate endpoints (biodegradation).
Category rationale:
The Long Chain Aliphatic Esters category contains monoesters of saturated and unsaturated natural fatty acids (palmitic, stearic and oleic acid) and monoalcohols. The monoesters are produced from reaction of fatty acid with alcohols. The carbon atom number of the alcohols ranges from C10-C18. Structural similarities of the category substances are reflected in similar physico-chemical properties and mode of action. Long Chain Aliphatic Esters have a common metabolic fate that involves stepwise hydrolysis to the carboxylic acid and carboxylic alcohols. The fatty acids are naturally occurring and feed into physiological pathways like citric acid cycle, sugar synthesis and lipid synthesis. Fatty alcohols can be further metabolized or excreted directly via urine and are expected to have a low order of toxicity.

Physico-chemical properties
Fatty acid esters in the category are esters of long chain alkyl acids (≥ C16) and long chain alkyl alcohols (≥ C10). Depending on the alkyl chain length and saturation, the substances in the category are under ambient conditions solids or liquids.
The molecular weights range from 422.74 to 536.97. The melting temperature is in the range from 69 °C to 7 °C for liquids and 54.3 °C for solids so far available. The shorter the chains the lower the melting point. Additionally, unsaturated components are characterized with lower melting points than saturated ones.
According to Blake et al. (J. Chem. Eng. Data, 1961, 6, 87-98), esters of long chain acids with β hydrogen atoms in the alcohol moiety (i.e. alcohols with C3, e.g. propanol) decompose in the range between 262 and 283 °C. Since for longer chains the boiling temperature is higher, esters of fatty acids esterified with alcohols ≥ C3 and having a molecular weight exceeding 300 amu have a boiling point > 300 °C and decompose before boiling. The vapour pressure of category members is very low. Valid calculated values are below 0.001 Pa at 20 °C. Partition coefficient log Kow exceeds 3 (< 10) for all category members. The positive correlation with the overall number of CH2 units is observed.
Very low water solubility (< 1 mg/L) characterizes all Long Chain Aliphatic Esters in the category.

Environmental fate and pathways
All substances in the category for which tests are available are readily biodegradable and therefore hydrolysis is not a relevant degradation pathway in the environment. The potential for adsorption to soil and sediment is high. Hence, these compartments are expected to be the main targets for environmental distribution. However, due to the ready biodegradability they will not persist in these compartments. Evaporation from the water phase into the air is negligible due to the low vapour pressure. However, if emitted into the air the category members are susceptible to indirect photodegradation (DT50 < 24 h). A BCF was calculated to be 3.162 – 28.88 L/kg wet weight (BCFBAF v3.00, regression based estimate) which indicates that the potential to accumulate in aquatic organisms is possible but low. When including biotransformation rate constants the BCF was 0.89 – 0.93 L/kg (Arnot-Gobas estimate, including biotransformation, upper trophic). If taken up into fish, the category members are expected to be rapidly metabolised and excreted.

Acute study results show that toxicity of Long Chain Aliphatic Esters for mammal and aquatic organisms is negligible (no effect up to the water solubility limit for fish, daphnia and algae). Their metabolites fatty acids and fatty alcohols feed into such physiological pathways as citric acid cycle, sugar synthesis and lipid synthesis.

Human toxicity
Grouping of Long Chain Aliphatic Esters is justified as they are metabolized to physiological metabolites (naturally occurring fatty acids) and to an alcohol component. Figure 1 presents the metabolic fate of fatty acids and fatty alcohols (exemplarily with isodecanol). Long chain fatty acids (LCFA) are incorporated into chylomicrons and enter the lymphatic system. LCFA are re-esterified to triacylglycerols and either metabolized for energy or stored in adipose tissue. The long chain alcohols can be further metabolized to the corresponding aldehydes and fatty acids or excreted as glucuronic acid conjugates. Based on the available data on mammalian toxicity, all members of the category Long Chain Aliphatic Esters were found to be not harmful in any aspect, as confirmed by studies on acute toxicity, irritation, skin sensitisation, oral repeated dose toxicity and genotoxicity.