There are natural radioactive impurities in Zirconium which may be eliminated using modern purification processes. 
Once the impurities have been removed, Zirconia is considered to be a safe material. It is obviously important for the purification process which eliminates the radioactive impurities present to take place according to ISO standards. 

CO.N.CE.P.T. uses high-quality Japanese powders, processed using the world’s leading-edge technologies, and, in any case, conducts the strictest controls when the raw materials arrive, including on radioactivity values. 
Many scientific studies and publications confirm that levels of radioactivity of Zirconia (ZrO₂) are very low and use of this material in the medical field may therefore be considered safe and biocompatible. 
The maximum value of Becquerel (Bq) within which a material is considered safe for human health, set by ISO standards, is 0.2 Bq/g or 200 Bq/kg. 
Zyttria products manufactured by CO.N.CE.P.T. have radioactivity values of <0,0011 Bq/g (Technology and Nuclear Institute - Unit of Radiological Protection and Safety - Sacavém), i.e. much lower than the maximum permitted value.