Pollution is known to play an important role in optical performances of high power laser components. In this study, we propose a method to characterize the particulate pollution and to evaluate its impact on the optical component lifetime. We applied it at the Ligne d'Intégration Laser (LIL), which is a prototype installation of the future Laser Mega Joule facility. By systematic examination of each particle, we obtain, in number, size, and chemical composition, an overview of pollution deposited on optical surfaces. Then we deduce the origin of the principal pollutions. After irradiation of the polluted samples, with conditions close to the maximum of the LIL operation, no laser-induced damage is observed. In contrast, irradiation induces an important cleaning.