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A comprehensive investigation of the effect of Ar, H_{2}, air, and CO on the saturated breakdown field (E_{sat}) of molybdenum, tungsten, and copper has been performed at intensive breakdown conditioning. A significant decrease in E_{sat} is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of ∼10^{-5}   mbar, the decrease in E_{sat} is found to be up to 50% and ∼30% compared to UHV conditions for molybdenum and tungsten, respectively. In addition, a ∼30% decrease is found when molybdenum is conditioned with a ∼10^{-5}   mbar CO pressure. Surface analysis measurements and breakdown conditioning in O_{2} ambience imply that the origin of the decrease in E_{sat} is closely linked to oxide formation on the electrode surfaces. Ex situ heat treatment of molybdenum is shown to improve the breakdown characteristics drastically.