A method of making an active gas by generating a glow discharge, non-thermal plasma, in a gas mixture of a carrier gas and a more readily ionisable gas. The gas mixture is exposed to water vapour at or downstream from the generator to form the active gas. The gas mixture includes helium as the carrier gas and up to 40% by volume of at least one noble gas such as argon, krypton, or xenon as the more readily ionisable gas. The gas mixture is ejected at a temperature between 5° C. to 42° C. The active gas may be used for oral treatment such as cosmetic whitening of teeth, medical or non-clinical cleaning of teeth or for cleaning laundry or dishwashing items.
