2-Propenoic acid, tris(1-methylethyl)silyl ester

Toxicity to aquatic algae and cyanobacteria

Description of key information

Toxicity to algae: 72 h ErC₅₀ 0.166-0.548 mg/l (measured as silanol hydrolysis product tris(1-methylethyl)silanol) (OECD 201). The ErC₅₀ is equivalent to 0.32 - 1.0 mg/l when expressed in terms of the parent substance.

NOEC (72 h) tris(1-methylethyl)silanol: 0.00191 mg/l.

Key value for chemical safety assessment

EC50 for freshwater algae:

0.166 mg/L

EC10 or NOEC for freshwater algae:

0.002 mg/L

Additional information

A 72 hour EC₅₀ value of 0.166 - 0.548 mg/l and NOEC of 0.00191 mg/l have been determined for the effects of the hydrolysis products of the registration substance on growth rate of Desmodesmus subspicatus. In view of the test media preparation method and static exposure regime it is likely that the test organisms were exposed predominantly to the hydrolysis products of the tested substance. The results are expressed relative to measured initial concentrations of the silanol hydrolysis product tris(1-methylethyl)silanol.

A 72 hour EC₅₀ value of 1.3 mg/l and NOEC of 0.31 mg/l have been determined for the effects of the registered substance on growth rate of Selenastrum capricornutum (new name: Pseudokirchneriella subcapitata). The results are expressed relative to nominal concentrations of tri(isopropyl)silyl acrylate (TIPSA). When expressed relative to equivalent mean measured concentrations of tris(1-methylethyl)silanol (TIPSOH) the corresponding values are 0.61 and 0.17 mg/l respectively.

It is possible to calculate mean measured concentrations of the tri(isopropyl)silyl acrylate from the measured concentrations of tris(1-methylethyl)silanol. However, the Netherlands CA, who reviewed the submitted SNIF dossier for the registered substance, amended the dossier and interpreted the algal study based on nominal concentrations of 2-propenoic acid in solution. The current dossier reflects the assessment derived by the Netherlands CA.

Acrylic acid is toxic to algae based on evidence presented in the EU Risk Assessment Report (ECB, 2002). Effects on growth rate from two tests are reported in that assessment and give ErC₅₀ values ranging between 0.13 – 0.21 mg/l, and ErC₁₀ values of 0.03 mg/l from both tests. 

Calculation of the concentration of acrylic acid that would have been present in the tri(isopropyl)silyl acrylate test medium at the algal EC₅₀ values of 0.32-1.0 and 0.82 mg/l (parent) shows this to be 0.10 – 0.32 and 0.26 mg/l respectively in the two available studies. These concentrations are consistent with toxicity being caused by the acrylic acid, and correspond with the ErC₅₀ value for acrylic acid of 0.13 mg/l reported in the EU Risk Assessment Report (RAR) (ECB, 2002). The predicted EC₅₀ value with TIPSOH for algae, based on non-polar narcosis, is approximately 3 mg/l, and ChV value approximately 1 mg/l, more than 10-fold higher than the measured concentration (Mayo-Bean, ECOSAR v2.0, October 2017). It is therefore likely that exposure to the acrylic acid hydrolysis product will have resulted in at least some (and possibly all) of the growth rate effects observed in both algal tests.

The evidence suggests that using the algal study for derivation of PNEC for TIPSOH would be too conservative. Acrylic acid has a specific toxicity to algae so is expected to be less toxic than TIPSOH to fish and invertebrates. The PNEC is therefore based on the data with aquatic invertebrates.