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The rate and extent to which chromium (III) oxide produces soluble (bio)available ionic and other chromium-bearing species in environmental media is limited. Further, the poor solubility of chromium (III) oxide is expected to determine its behaviour and fate in the environment, and subsequently its potential for ecotoxicity. However, the fate and toxicity of chromium (III) oxide in the environment is evaluated by assessing the fate of its ecotoxicologically relevant moiety, the chromium (III) ion, and read-across to data available for other chromium (III) substances is applied.

Regarding the acute (short-term) aquatic toxicity, the poorly soluble substance chromium (III) oxide is evaluated by comparing the dissolvedchromiumion level resulting from the T/Dp after 7 d at a loading rate of 1 mg/L with the acute ecotoxicity reference values (ERVs) as determined for the (soluble) chromium ion. The ERV is based on the lowest acute EC50/LC50 for algae, invertebrates and fish. The lowest reliable acute effect concentration determined for the (soluble) chromium (III) ion is the LC50 of 3.7 mg Cr(III)/L observed in a 48-h acute toxicity test on the effects of chromium (III) chloride on freshwater invertebrate Ceriodaphnia dubia (Baral et al., 2005). Dissolved chromium concentrations of <0.01 µg/L in the T/Dp after 7 days at pH 6 (i.e. the pH that maximizes the dissolution) are significantly lower than the short-term ERV derived (i.e. the lowest EC/LC50 of 3.7 mg Cr(III)/L for C. dubia). Hence, the substance chromium (III) oxide is not sufficiently soluble to cause short-term toxicity at the level of the acute ERV (expressed as the LC50). A reliable study on the short-term toxicity to fish with the poorly soluble chromium (III) oxide by Caspers (1992) provides further evidence on the low potential for acute toxicity since effects were not observed at the highest test concentration of 10,000 mg/L corresponding to an LC50 > 1.0 microg dissolved Cr/L (> 2.9 microg dissolved chromium (III) oxide /L).

In accordance with Figure IV.4 “Classification strategy for determining acute aquatic hazard for metal compounds” of ECHA Guidance on the Application of the CLP Criteria (Version 5.0, July 2017) and section 4.1.2.10.2. of Regulation (EC) No 1272/2008, the substance chromium (III) oxide is poorly soluble and does not meet classification criteria for acute (short-term) aquatic hazard.

Regarding the long-term aquatic toxicity, chromium (III) oxide is evaluated by comparing the dissolved chromium (III) ion level resulting from the T/Dp at a loading rate of 1 mg/L after 28 d with the lowest chronic ERV as determined for the (soluble) chromium (III) ion. The ERV is based on the lowest long-term NOEC/EC10 value for algae, invertebrates and fish. The lowest reliable chronic effect concentration determined for the (soluble) chromium (III) ion is the 72-h ErC10 of 2 microg Cr(III)/L observed for the effects of chromium (III) hydroxide sulphate on the growth rate of freshwater algae Desmodesmus subspicatus (Neuhahn, 2010). Dissolved chromium concentrations of <0.01 microg/L in the T/Dp after 28 days at pH 6 (i.e. the pH that maximizes the dissolution) are significantly lower than the long-term ERV derived (i.e. the lowest NOEC/EC10 value of 2 microg Cr(III)/L for Desmodesmus subspicatus). Hence, the substance chromium (III) oxide is not sufficiently soluble to cause chronic toxicity at the level of the chronic ERV (expressed as the ErC10).

In accordance with Figure IV.5 „Classification strategy for determining long-term aquatic hazard for metal compounds “of ECHA Guidance on the Application of the CLP Criteria (Version 5.0, July 2017) and section 4.1.2.10.2. of Regulation (EC) No 1272/2008, the substance chromium (III) oxide is poorly soluble and does not meet classification criteria for chronic (long-term) aquatic hazard.

The conclusion on a lack of an aquatic hazard potential of chromium (III) oxide is in accordance with a Tier-2 assessment (using T/Dp data) performed with MECLAS version 5.2 (http://www.meclas.eu/ accessed on 06.03.2020). The Metals Classification Tool (MECLAS) is a web enabled classification tool always updated to the latest classification guidance, legal ruling, information on toxicity references and available self-classifications. In sum, it can safely be assumed that chromium (III) oxide does not meet classification criteria of an acute or long-term aquatic hazard of Regulation (EC) No 1272/2008.

Since inhibition of respiration of microorganisms in activated sewage sludge was not observed for a soluble chromium (III) substance by Caspers (1988), a similar lack of a toxicity potential can safely be assumed for chromium (III) oxide.

Further, taking into account acute and chronic saltwater toxicity data of chromium (III) as summarised in the EU RAR on chromates (ECB, 2005) and speciation in environmental media, toxicity to saltwater organisms is not likely.

Regarding its potential for aquatic toxicity, chromium (III) oxide does not classify as “harmful”, “toxic or very toxic to aquatic life” or “may cause long lasting harmful effects to aquatic life”. Chromium (III) oxide is also not expected to be an unclassified hazard to the aquatic environment in accordance with ECHA guidance on IR & CSA, Part B: Hazard Assessment (V. 2.1, December 2011).

Based on the poor solubility of chromium (III) oxide in environmental media, it is concluded that chromium (III) oxide is not acutely and chronically toxic to aquatic freshwater organisms. Based on the poor solubility of chromium (III) oxide in environmental media, available saltwater and freshwater toxicity data, chromium (III) oxide is also not expected to be acutely or chronically toxic to marine organisms. Thus, chromium (III) oxide is not a classified or non-classified acute and chronic hazard to aquatic organisms.

Additional information

Chromium (III) oxide is poorly soluble in environmental media. Based on ECHA’s Guidance on the Application of the CLP Criteria (Version 5.0, July 2017), “the hazard classification schemes for metals and metal compounds are limited to the acute and long-term hazards posed by metals and metal compounds when they are available (i.e. exist as dissolved metal ions, for example, as M+ when present as M-NO3), and do not take into account exposures to metals and metal compounds that are not dissolved in the water column but may still be bioavailable, such as metals in food…”.

For poorly soluble metals and inorganic metal compounds that cannot be easily tested in aquatic toxicity test, the following is stated: ”where the compound is sufficiently poorly soluble that the levels dissolved following normal attempts at solubilisation do not exceed the available L(E)C50, it is the rate and extent of transformation, which must be considered.

According to ECHA’s Guidance on IR & CSA, Appendix R.7.13-2 (2008), “in some cases, the metal compound will be only poorly soluble and sufficiently stable to not rapidly transform to a water soluble form. Under these circumstances, the substance itself should be assessed taking into account its specific partitioning characteristics. For the aquatic environment, it can be assumed as a first estimate that the substance will dissolve up to its water solubility limit, and that this fraction will be the bioavailable form. Refinement of the assessment can be done by taking into account kinetics of the dissolution… In case ecotoxicity data are lacking for a specific metal or metal compound, read-across ecotoxicity data from other inorganic compounds of the same metal could be considered. The basic assumption using this approach is that it is the bioavailable metal fraction that is causing the effects (e.g. free metal ion or other specific metal species complexes (e.g. CuOH+). Ecotoxicity data of simple soluble metal salts can be combined on condition that the metal ion is responsible for the effects observed for the metal salts considered (e.g. CuSO4, CuCl2). Appropriate standard ecotoxicity effects data (acute, chronic) measured for such soluble metal salt are thus combined by expressing the effects data (NOECs and PNEC) as dissolved (bioavailable) metal ion concentration (μg Me/L)”.

ECHA’s Guidance on the Application of the CLP Criteria (Version 5.0, July 2017) further states that “A decision on whether or not the substance is classified will be made by comparing aquatic toxicity data and solubility data.

The solubility of chromium (III) oxide in environmental media is low. In a short-term transformation/dissolution test with a loading of 100 mg/L, a dissolved Cr concentration of 0.11 microg/L Cr was determined at pH 6 after 7 days whereas dissolved Cr concentrations were below the LOD (< 1 microg/L) at pH 8 (Hedberg and Wallinder, 2012). Transformation/dissolution at a loading of 1 mg/L resulted in dissolved chromium concentrations below the LOD (< 0.01 microg/L) at pH 6 after 7 days and 28 days. Furthermore, chromium (VI) could not be detected during the test (< 0.01 microg/L). In sum, chromium (III) oxide can be considered environmentally and biologically inert during short- and long-term exposure.

Please refer to the endpoint summary “Cr_Aquatic toxicity” for further details of the aquatic toxicity of soluble chromium (III) substances.