Thin films of PbTe of different thicknesses have been prepared on glass substrates at room temperature by vacuum deposition. It is found that the electrical resistivity of the air-exposed films is much higher (by about 2 to 3 orders of magnitude) than that of the as-grown (unexposed) thin films. The electrical resistivity temperature behaviours of both the air-exposed and as-grown (unexposed) thin films of PbTe are different but both show hysteresis behaviour during successive heating-cooling cycles. These observations can be explained by considering that the desorption of absorbed gas molecules (mainly oxygen) and creation of defects at higher temperatures during heating influence the electrical conduction. Further, the time factor involved in gas desorption-adsorption can cause the observed hysteresis in temperature-dependent conduction behaviour. The as-grown (unexposed) thin-film conductivity exhibits the expected reciprocal thickness dependence due to the thickness effect, but the air-exposed film conductivity does not. This can be explained to be due to the complete masking of the thickness effect by the gas adsorption effect in air-exposed film conductivity. The reciprocal thickness dependence observed in the case of unexposed film conductivity has been explained by the 'effective mean free path' model. The low value of the 'grain boundary' mean free path obtained by the analysis points to the fact that in polycrystalline films, grain boundary scattering is extensive and controls the film conductivity.