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EC number: 215-231-4
CAS number: 1314-35-8
All results are expressed in terms of nominal concentrations.
Considering the low aqueous solubility of the test substance (< 1 mg/L
WO3 under exposure conditions) no specific analysis was established.
No aquatic algae and cyanobacteria toxicity data of sufficient quality
are available for tungsten trioxide (target substance). However, aquatic
algae and cyanobacteria toxicity data are available for sodium tungstate
(source substance), which are used for read-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 description in the Category section of this IUCLID
submission or Annex 3 of the CSR.
In a 72-hr algal growth inhibition test with tungsten trioxide, there
were no toxic effects observed at the saturation concentration under
exposure conditions (1 mg tungsten trioxide/L). Therefore, the EC50
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 (based on
growth rate) of 0.812 sodium tungstate (0.476 mg tungsten/L). The ErC10
from this study was considered to be more appropriate and are carried
forward to the classification and risk characterization.
Due to similar or lower transformation/dissolution results for tungsten
trioxide (the target substance) than sodium tungstate (the source
substance), the resulting toxicity potential would also be expected to
be similar or lower, so read-across is appropriate. In addition,
read-across is justified because the classification and labeling is the
same or less severe for the target substance PBT/vPvB profile is the
same. Finally, the dose descriptors are, or are expected to be,
sufficiently similar or higher for the target substance, and read-across
to the source substance is adequately protective. For more details,
refer to the read-across category approach description in the Category
section of this IUCLID submission or Annex 3 of the CSR.
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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