Shallow traps influence the radiation damage behaviour of lead tungstate scintillation crystals when exposed to continuous low dose rate radiation. Radiation initiates diffusion of Oxygen vacancies in the crystal giving rise to their aggregation if their density is high enough, which creates deeper traps than single vacancies. The situation may occur in high luminosity experiments where scintillation crystals are exposed for long periods of time to irradiation. The experimental results presented in this paper illustrate the importance of optimizing the crystals by reducing the concentration of the shallow traps and not only of the deep traps.