The use of a purging gas flow as a means of protecting the cryogenic optics in space infrared telescopes from condensible gaseous contaminants has been studied. The significance of both the purge/contaminant differential scattering cross section and the purge gas density distribution is analyzed. It is concluded that it is important to use the correct form of the differential scattering cross section as well as accurate purge gas density distributions when estimating the effectiveness of purging flows for contaminant protection. Experimental verification of the techniques that are used here to calculate purge gas density distributions are presented. Complete contaminant rejection from the interior of a telescope is unlikely, since for realistic purge gas densities most of the contaminants prevented from reaching the optical surfaces will condense on the telescope's internal optical baffling. Specific examples of contaminant shielding for two modes of purge gas injection are presented for the Orris I telescope. The effectiveness of helium and neon as purge gases is compared.