We have applied density functional theory to investigate different types of carbon nanotubes (armchair (4,4)CNT and zig-zag (7,0)CNT) as sensors of some pollutant gas molecules, especially CO, CO₂, NO and NO₂. We show, for the first time, that the adsorption of pollutant gas molecules on carbon nanotubes are improved by introducing the monovacancy defects on the surfaces of (7,0)CNT. The adsorption energies, the optimal adsorption positions and the orientation of these gas molecules on the surfaces of carbon nanotubes are studied. It is found that the most adsorbed pollutant gas is NO molecule on (7,0)CNT.