The present invention relates to a gas purifier containing 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 the media 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.