The beam-gas scattering lifetime of the Pohang light source (PLS) storage ring was studied. To eliminate the dominant Touschek effect for measurements of the beam-gas scattering lifetime, two methods were used separately: vacuum pressure increase by helium injection and very low electron beam density. Then two vertical beam scrapers were used to investigate the dependence of the Coulomb lifetime on the acceptance limiting aperture size. It was possible to obtain the separate ranges of Coulomb and bremsstrahlung lifetimes in the pressure range of normal operation. The results indicate that the vacuum is still an important factor for PLS, especially when small Sap undulators are installed.