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EC number: 915-093-1 | CAS number: -
No aquatic algae toxicity data of sufficient quality are available for the RM of W2C and WC (target substance). However, aquatic algae toxicity data are available for tungsten carbide (source substance), which will be used for reading across. Due to similar water solubility and lower toxicity for the target substance compared to the source substance, the resulting read across from the source substance to the target substance is appropriate. In addition, read across is appropriate because the classification and labelling is similar for the source substance than the target substance, the PBT/vPvB profile is the same, and the dose descriptors are, or are expected to be, lower for the source substance. For more details, refer to the read-across category approach included in the Category section of this IUCLID submission and/or as an Annex in the CSR.
No aquatic algae and cyanobacteria toxicity to aquatic invertebrates data of sufficient quality are available for the RM of W2C + WC (target substance). However, aquatic algae and cyanobacterias data are available for sodium tungstate (source substance), which will be used for reading across. Due to lower water solubility and lower toxicity for the target substance compared to the source substance, the resulting read across from the source substance to the target substance is appropriate as a conservative estimate of potential toxicity for this endpoint. In addition, read across is appropriate because the classification and labelling is more protective for the source substance than the target substance, the PBT/vPvB profile is the same, and the dose descriptors are, or are expected to be, lower for the source substance. For more details, refer to the read-across category approach included in the Category section of this IUCLID submission and/or as an Annex in the CSR.
In a 72-hr algal growth inbibition test with tungsten carbide, no toxic effects at the highest concentration tested (1 mg/L). Therefore, the ErC50 could not be ascertained and read-across to sodium tungstate was used for this endpoint. Two growth inhibition tests using sodium tungstate, determined an ErC50 of 52.9 mg sodium tungstate (31.0 mg tungsten/L), an ErC10 of 5.76 sodium tungstate (3.38 mg tungsten/L), and a NOEC of 0.812 sodium tungstate (0.476 mg tungsten/L). The ErC10 from this study was considered to be more appropriate and will be carried forward to the classification and risk characterization.
Due to lower transformation/dissolution results for the RM of W2C + WC (the target substance) than sodium tungstate (the source substance), the resulting toxicity potential would also be expected to be lower, so read-across is appropriate. In addition, read-across is justified because the classification and labelling is the less severe for the target substance and the PBT/vPvB profile is the same. Finally, the dose descriptor for the target substance is expected to be sufficiently lower than the source chemical, and read-across to the source chemical is adequately protective. For more details refer to the attached description of the read across approach.
Two toxicity tests were performed with sodium tungstate using two different concentration ranges in order to bracket the desired test endpoints (EC values and NOEC values) for the various growth parameters (yield and growth rate).
In the first study with sodium tungstate, conducted at a concentration range of 10.9 to 168 mg/L (mean measured values), statistically significant reductions in both yield and growth rate were observed at all test concentrations, with inhibition of growth rate ranging from 23% at the lowest test concentration to 65% at the highest test concentration. The NOEC could not be determined, and the EC50 based upon growth rate was estimated to be 52.9 mg sodium tungstate/L (mean measured concentration). In the second study, conducted at a concentration range of 0.35 to 17.7 mg/L (mean measured values), inhibition of growth rate ranged from 0% at the second lowest concentration (0.81 mg/L) to 24% at the highest concentration. The EC50 for growth rate was thus greater than 17.7 mg/L. The highest test concentration in which growth rate was not statistically significantly less than that in the control, i.e., the NOEC, was 0.812 mg/L. This NOEC, however, must be considered as an artifact of the unusually low coefficient of variation that occurred in this test (0, 1 or 2% CV at all test concentrations). The next highest test concentration to the NOEC, 1.79 mg/L, exhibited only a 4% inhibition of growth rate, and only 8% inhibition occurred at 3.83 mg/L. These small effects are almost certainly not biologically significant, despite being statistically significant due to the unusual precision of the test. As stated in the ECHA guidance R10 (ECHA, 2008), when the statistical “power is high, it may occur that biologically unimportant differences are statistically significant“. There has been a recommendation within OECD to phase out the use of the NOEC, which also has the disadvantage of being highly dependent upon the selection of test concentrations and does not allow for the estimate of a confidence interval. An alternative, and according to ECHA, preferred method of expressing chronic toxicity is the use of the EC10. The advantage of the use of a regression-based endpoint such as the EC10 is that information from the entire concentration-effect relationship is used and confidence intervals can be calculated.
The EC10, as determined from the results of the second test with sodium tungstate, was 5.76 mg sodium tungstate/L with 95% confidence limits of 4.94 and 6.58 mg/L. This value is considered to be more appropriate for estimating a chronic toxicity than the NOEC for this endpoint.
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