We present the results of an experimental study of the structural effects induced by Lyman-α UV photons in crystalline water ice at low temperature carried out by using in situ infrared spectroscopy. We studied the evolution, under UV irradiation, of the 3 micron band profile. We found that an UV dose of few eV per water ice molecule is enough to fully amorphize the structure at the investigated temperature of 16 K. The evolution of the 3 micron feature during UV irradiation is compared with that observed for fast ion irradiation carried out in the same experimental set-up. Both photons and fast ions are present in space, they can induce variations in the profile of the 3 micron band of crystalline water ice. Thus these effects must be considered when one attempts to deduce, from the profile of the observed ice band, information on the temperature history of the responsible dust.