A. Chlorine dioxide is usually used by beauticians during unlicensed treatments. As such you have no standardisation controls as to the strength or other addititives. As the treatment  has already been carried out the first hurdle of possible harms has been cleared. In general the procedure carried out is classified as dentistry and if you complained to the GDC (general dental council) or your local trading standards it is likely that they would be stopped from trading.
Some mouthwashes use chlorine dioxide in a safe formulation eg REtardex.