Abstract: An atmospheric pressure plasma system (1) sharing electrodes (4) defining a plasma region (5) mounted in an enclosure housing (2). The enclosure housing has an open to atmosphere entry port assembly (10) and exit port assembly (11) for the continuous transfer of work-pieces through the plasma region (5). The embodiment illustrated is for precursor process gases having a relative density less than that of the ambient air so that the precursor gases rise in the enclosure housing (2) expelling the heavier ambient and exhaust gases. Where the gases have a relative density greater than ambient the port assemblies (10 and 11) are sited above the plasma region (5).