The invention relates to a gas purifier comprising a filtering media having a photocatalytic action, a system for illuminating said media with UV radiation, a time-delay means or a volatile organic compound analyzer, and a means for automatically adjusting the speed of the gas passing through it or for adjusting the intensity of the UV illumination, said adjusting being carried out as a function of the time determined by the time-delay means or as a function of the content of a volatile organic compound analyzed by the analyzer. The invention also relates to a process for the purification of gas using a purifier comprising a filtering media having a photocatalytic action and a system for illuminating said media with UV radiation, according to which, when the concentration of a compound in the gas is greater than a value V1, the operating capacity of the purifier is lower than its capacity when the concentration of the compound in the gas is less than the value V2, V2 being less than or equal to V1. The invention also relates to a process for the purification of gas using a purifier comprising a filtering media having a photocatalytic action, a system for illuminating said media with UV radiation and a time-delay means controlling the capacity of the purifier.