Apparatus comprising: - a fumehood; and - an air treatment device for purging unwanted substances from the exhaust air of the fumehood, the air treatment device comprising: - a non-thermal plasma reactor stage for producing air byproducts comprising O, N, OH·and O 3 and introducing those air byproducts into the exhaust air of the fumehood so as to treat the exhaust air of the fumehood, wherein the non-thermal plasma reactor stage comprises a plurality of non-thermal plasma reactor units, wherein the plurality of non-thermal plasma reactor units are arranged substantially adjacent to one another and substantially parallel to one another in an array having a cross-section which defines the effective working area of the nonthermal plasma reactor stage, and wherein each non-thermal plasma reactor unit comprises (i) either a wire electrode being disposed coaxial with, and internal to, a cylinder electrode, with or without a dielectric tube between both electrodes (100,105), (ii) or a wire electrode and a plate electrode, with the wire electrode being spaced from, or mounted to, the plate electrode by a dielectric plate, (iii) or a first plate electrode and a second plate electrode, with the first plate electrode being separated from the second plate electrode by a dielectric plate; and - a catalyst stage downstream of the non-thermal plasma reactor stage for further treating the air downstream of the non-themal plasma reactor stage. - a fumehood; and - an air treatment device for purging unwanted substances from the exhaust air of the fumehood, the air treatment device comprising: - a non-thermal plasma reactor stage for producing air byproducts comprising O, N, OH·and O 3 and introducing those air byproducts into the exhaust air of the fumehood so as to treat the exhaust air of the fumehood, wherein the non-thermal plasma reactor stage comprises a plurality of non-thermal plasma reactor units, wherein the plurality of non-thermal plasma reactor units are arranged substantially adjacent to one another and substantially parallel to one another in an array having a cross-section which defines the effective working area of the nonthermal plasma reactor stage, and wherein each non-thermal plasma reactor unit comprises (i) either a wire electrode being disposed coaxial with, and internal to, a cylinder electrode, with or without a dielectric tube between both electrodes (100,105), (ii) or a wire electrode and a plate electrode, with the wire electrode being spaced from, or mounted to, the plate electrode by a dielectric plate, (iii) or a first plate electrode and a second plate electrode, with the first plate electrode being separated from the second plate electrode by a dielectric plate; and - a catalyst stage downstream of the non-thermal plasma reactor stage for further treating the air downstream of the non-themal plasma reactor stage. - a non-thermal plasma reactor stage for producing air byproducts comprising O, N, OH·and O 3 and introducing those air byproducts into the exhaust air of the fumehood so as to treat the exhaust air of the fumehood, wherein the non-thermal plasma reactor stage comprises a plurality of non-thermal plasma reactor units, wherein the plurality of non-thermal plasma reactor units are arranged substantially adjacent to one another and substantially parallel to one another in an array having a cross-section which defines the effective working area of the nonthermal plasma reactor stage, and wherein each non-thermal plasma reactor unit comprises (i) either a wire electrode being disposed coaxial with, and internal to, a cylinder electrode, with or without a dielectric tube between both electrodes (100,105), (ii) or a wire electrode and a plate electrode, with the wire electrode being spaced from, or mounted to, the plate electrode by a dielectric plate, (iii) or a first plate electrode and a second plate electrode, with the first plate electrode being separated from the second plate electrode by a dielectric plate; and (i) either a wire electrode being disposed coaxial with, and internal to, a cylinder electrode, with or without a dielectric tube between both electrodes (100,105), (ii) or a wire electrode and a plate electrode, with the wire electrode being spaced from, or mounted to, the plate electrode by a dielectric plate, (iii) or a first plate electrode and a second plate electrode, with the first plate electrode being separated from the second plate electrode by a dielectric plate; and - a catalyst stage downstream of the non-thermal plasma reactor stage for further treating the air downstream of the non-themal plasma reactor stage.