We have investigated systematically the effects of various gas adsorbates (H-2, N-2, O-2, and H2O) on the electronic structures and the field emission properties of open edges of single-walled carbon nanotubes by density functional calculations. All of the molecules, except N-2, dissociate and chemisorb on open nanotube edges with large adsorption energies. The Fermi levels are moved toward the valence (conduction) bands for O-2 (H-2, H2O) adsorption induced by the Mulliken charge transfer on the tube edge. The Fermi level shift for N-2 adsorption is negligible. Adsorption of H2O enhances the field emission current, whereas H-2 adsorption does not affect the field emission current much because of the absence of the density of states near the Fermi level. The correlation of the electronic structures and the field emission current is further discussed.