An apparatus for treating a gas stream includes a plasma abatement device that has a reaction chamber and a plasma torch for generating a plasma stream for injection into the chamber for treating the gas stream. A first inlet conveys a gas stream into the plasma abatement device for treatment, and a second inlet, in a normal condition of the apparatus, is in flow communication with a source of reagent for conveying a reagent into the plasma device for improving the efficiency of the treatment. In a back-up condition of the apparatus, the second inlet is in flow communication with a gas stream source for conveying a gas stream into the device for treatment.