Huge light-induced rubidium atom desorption has been observed from porous silica samples placed inside Pyrex cells kept at room temperature. The desorption rate and the maximum vapour density change have been measured as a function of the desorbing light intensity. Their values are comparable with those recently obtained in Pyrex cells coated with organic films. A linear dependence of the desorption rate and a square-root dependence of the maximum density increase on light intensity indicate that diffusion inside the glass pores is a key mechanism. The application of this effect to develop light-controlled atomic dispensers compatible with high vacuum environment is discussed.