Until Jim comes along with the right answer, I came across this info. It appears air/air is fine:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
HT2000
Process gases
by material
(Plasma/shield) Mild steel: O2/Air, O2/O2, Air/Air, N2/CO2
Stainless steel: Air/Air, N2/Air, N2/CO2, H35/N2
Aluminum: Air/Air, N2/Air, N2/CO2, H35/N2
HTH
Paul