The analysis of cultural heritage artefacts with ionizing radiation has rapidly increased in popularity over the last decades - in particular at synchrotron sources - leading to many fascinating new discoveries. However, the damage inflicted to the materials under study during the irradiation is still largely unknown. Organic samples, such as paper, are especially vulnerable to degradation by X-rays. In this work we quantify the radiation damage on a molecular level and show that the chemical reactions triggered promote cellulose degradation and continue to alter the material for several months after the irradiation took place.