We present a photoluminescence (PL) fatigue study at liquid helium temperatures in mesoporous and microporous silicon (PSi) layers impregnated in air. In contrast to PL measurements at room temperature, at low temperatures no recovery of the PL is observed. The results show that the infrared (IR) and slow-luminescent visible (S) band display different behaviors under prolonged illumination. In the S band, similar dependencies in both kinds of specimens are observed with much stronger fatigue strength on the high energy side, revealing that this band is possibly composed of two different kinds of luminescent species. This idea is further supported by the measured change of the luminescence intensity and of the non-exponential PL decay at different photon energies. By analogy with the well-known photo-induced effect in CdSSe microcrystallites, photo-ionization may be responsible for the PL fatigue.