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Toxicity to terrestrial plants

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The exposure assessment shows that the terrestrial environment can be exposed to ammonia and trimethylsilanol, associated with normal use, and predicted environmental concentrations are determined. Testing for toxicity to terrestrial organisms is not considered necessary because: Ammonia is assessed separately to trimethylsilanol. Data are available for the toxicity of ammonia to terrestrial plants. Concentrations of ammonia in soil resulting from use of 1,1,1,3,3,3-hexamethyldisilazane are minimal compared to the natural background and other anthropogenic sources (see Section 9 of the CSR).  

Ammonia: The following results for terrestrial toxicity of ammonia are sourced from the OECD HPV Chemical Programme, SIDS Dossier Ammonia (CAS 7664 -41 -7) 2007. The effects of ammonia gas were tested on terrestrial plant species. 4 hour LOECs for damage to leaf area ranged from 3 ppm to 40 ppm. A 1 hour LC100 of 40 ppm was recorded for tomato and sunflower plants, where complete injury was caused. High concentration tests from 4 to 8 minutes duration resulted in EC50s for foliar necrosis of 250 ppm NH3 (tomatoes) to 1000 ppm NH3 (buckwheat and tobacco). A 4 hour exposure to 1000 ppm NH3 killed moist spring rye seeds, whereas radish seeds were still viable after 16 hours. 250 ppm NH3 for 16 hours reduced rye seed germination by half, but had no effect on radish seeds. Treatment of barley roots with 1.6x10-3 M unionised ammonia reduced their respiration 23 percent and 3.3x10-3 M unionised ammonia caused a 78 percent inhibition after 4 hours. The respiration of barley roots treated with 1x10-3 to 3x10-3 M unionised ammonia was reduced by 46 to 62 percent within 4 hours.  


Published data are available for toxicity of trimethylsilanol to terrestrial plants (Côté-Beaulieu et al, 2009). The plants were dosed at 1.7 mM aqueous solutions of silicone ortho- monomethyl- dimethyl- and trimethylsilicic acids, which correspond to silicic acid, methylsilanetriol, dimethylsilanediol and trimethylsilanol respectively. In the study, wheat plants (Triticum aestivum L.) were exposed with Hoagland solutions containing the test substances in peat based systems and in hydroponic systems. During the hydroponic exposures, plants were infected with fungus mildew.

Effects were observed and the LOEC is given as 1.7 mM. When multiplying this result by the molecular weight of trimethylsilanol, this gives a NOEC of <153 mg/l.

However, the study was not conducted according to any guideline, and there is no indication it was conducted according to GLP. The methods used were not in accordance with any standard guideline, exposure was to aqueous solutions and silica deposits were also detected in the shoots of the controls in the peat-based studies. It is difficult to assess the significance of the results for the standard REACH endpoints. Therefore, the study is considered reliability 4 and the data are not considered to be relevant for the risk assessment.

In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in a long or short-term terrestrial toxicity to invertebrates/higher plants study because, as indicated in guidance R.7.11.6 (ECHA 2017), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio is below 1 and therefore the risk is already adequately controlled and further testing is not justifiable.  The substance is highly water soluble and is not readily biodegradable, but has low bioavailability and low potential for adsorption (based on log Kow <3 (1.19) and log Koc 1.6). Low toxicity was observed in short-term aquatic tests, and there is no reason to expect any specific mechanism of toxicity beyond narcosis. Therefore, the occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would be considered unlikely.  Trimethylsilanol is classed as soil hazard category 3 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R7.c, 2017) based on potential for high persistence (DT50 >180 days), lack of ready biodegradability and low toxicity to aquatic organisms (EC/LC50 not <1 mg/l). In this situation, a screening approach is applied: a confirmatory long-term terrestrial test is usually appropriate, in addition to the equilibrium partitioning approach with an extra factor of ten in order to determine whether further full tests are necessary. In the event that terrestrial invertebrate and plant studies need to be conducted, the definitive terrestrial risk characterisation would use a PNECsoil based on the lower of the two test results with an assessment factor of 50 (unless soil microorganism data are available as well, in which case, the assessment factor would be 10).  The confirmatory test is to be conducted with the most sensitive organism group based on short-term aquatic testing. For this substance, invertebrates were the most sensitive organism, indicating a preference to conduct a confirmatory test with terrestrial invertebrates. The PNECscreen(EQPM) for trimethylsilanol is derived from the short-term test results with invertebrates and has a value of 0.12 mg/kg dwt. For the purpose of the screening assessment comparison only, an extra factor of ten is applied (PECx10/PNECscreen(EQPM)). Based on the exposure assessment of 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3), PECx10/PNECscreen(EQPM) for trimethylsilanol does not exceed a value of 9.58. Full testing in both terrestrial plants and invertebrates would result in a new value of PNECsoil. This value could only be more conservative than the value of PNECscreen(EQPM), in the situation that standard testing in terrestrial plants or invertebrates exhibited a dose response with a NOEC/EC10 ≤ 6 mg/kg dw. There is no basis to expect such toxicity for trimethylsilanol based on the absence of significant toxicity observed in aquatic tests. In the case of trimethylsilanol, the registrants consider that a long-term terrestrial study is unlikely to affect the outcomes of the chemical safety assessment. As such the registrants propose that further testing (including the confirmatory study) is not necessary. Overall, it is concluded that the risk characterisation conclusion is sufficiently conservative and therefore further in vivo testing is not considered necessary. Details on how the PNEC and the risk characterisation ratio have been derived can be found in IUCLID Section 6.0, CSR Section 7, and Chapters 9 and 10 of the Chemical Safety Report, respectively.