A device 1 for delivering a partially-ionised first stream of gas, comprises a generator 24 of non-thermal plasma having a plasma generating chamber 25 defining a flow path therethrough for the first stream of gas and communicating downstream of the chamber with at least one first outlet 30 from the device for the partially ionised first stream of gas. The device 1 additionally comprises at least one second outlet 34 from the device for a second stream of gas, the configuration of outlets 30 and 34 enabling the second stream to shield the first stream downstream of the said first outlet 30. Interaction of the first stream of gas with the surrounding atmosphere is thus kept down.