Time-resolved photoluminescence investigation on SiO2 nanoparticles was carried out in controlled atmosphere, with the aim to discern the effects induced on the typical blue luminescence band by high power UV Nd:YAG laser photons (4.66 eV) and by some selected molecular species of the air (O2, N2, CO2, H2O). These factors ultimately determine both the brightness and photostability of the emitting defect, so as to limit the unique and attracting potentialities offered by this system in many applicative fields. Here it is highlighted that the effects due to photons and molecules, singularly considered, are not additive, the radiation being more dramatic in reducing the emission efficiency. Moreover, by analyzing the kinetics to convert the defects in a non-luminescent configuration both by the direct (photon-defect) and indirect (photon-molecule-defect) interactions, the threshold bleaching fluence is derived, ranging …