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To investigate the effect of gas composition on the sensitivity of Bacillus pumilus spores to gas plasmas.Inert gas plasmas, oxygen-based plasmas and various moisturized air plasmas were used to inactivate B. pumilus spores in low gas pressure of 50 Pa. Although the treatment temperature did not exceed 55°C when exciting these plasmas, spore survival varied widely depending on the composition of the gas feed. Higher spore mortality was acquired by inert gases of low molecular weight except for helium. The highest spore mortality (4·54log reduction) was obtained when air with a 0·05 molar fraction of water vapour was used as the plasma carrier gas.Water molecules in the plasma carrier gas play a significant role in inactivation of B. pumilus spores.This strong inactivation may occur through hydroxyl free radicals generated from the moisturized air plasma.