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Since no data are available on yttrium zirconium oxide, the aquatic toxicity of the substance is evaluated using data for zirconium dioxide (or other zirconium compounds) and yttrium oxide. Both zirconium dioxide and yttrium oxide were found not to be toxic (acutely) to fish and aquatic invertebrates. The available reliable studies were performed according to internationally accepted guidelines and did not report any acute adverse effects (mortality in fish, immobilization in daphnids) at the limit test concentration of 100 mg/L. Based on this information, yttrium zirconium oxide can also be concluded not to be toxic to fish and aquatic invertebrates.

Adverse effects on algal growth were observed for several zirconium substances (e.g., zirconium basic carbonate, zirconium dichloride oxide, and a reaction mass of zirconium dioxide and cerium dioxide). However, phosphate monitoring indicated that the observed effects are concurrent with phosphate depletion and hence are due to phosphate deprivation. No evidence has been found for direct toxic effects in algae upon exposure to zirconium compounds. No data has been added to the dossier on yttrium oxide or other yttrium substances because phosphate complexing behaviour is also known for rare earth elements such as yttrium and Visual Minteq (v3.0) modelling confirmed that whenever phosphate in the algal test medium is in excess of yttrium, all yttrium will be lost from the aqueous solution (hence no exposure), whereas whenever yttrium is in excess, all phosphate is depleted from the test medium, and only phosphate deprivation effects will be observed. Because the phosphate complexing behaviour of zirconium and yttrium represents a technical difficulty which cannot be resolved (e.g. by phosphate dosing during the tests), it is not considered possible to obtain meaningful results from algal growth inhibition experiments with yttrium or zirconium compounds. The phosphate deprivation effect may affect algae in the environment very locally at point discharges but this kind of effect is not considered relevant for entire aquatic ecosystems.

Refinement of the hazard assessment through inclusion or generation of long-term toxicity data (fish, aquatic invertebrates) is not considered necessary because the substance is not classified for any hazard, because the available acute toxicity data with zirconium dioxide (or other zirconium compounds) and yttrium oxide did not reveal any relevant adverse effects, and because yttrium zirconium oxide is anticipated to have a limited water solubility.

Finally, toxicity to micro-organisms can be waived because of the limited water solubility of the substance and the known complexation behaviour of zirconium and yttrium (i.e., strong complexation with phosphate and pH-dependent complexation with carbonate and hydroxide). Micro-organisms in a STP are anticipated not to be exposed to yttrium zirconium oxide due to this behaviour and limited water solubility.

Based on the available information yttrium zirconium oxide can be concluded to be non-hazardous to aquatic organisms.