The concentration of the electric field strength in the neighborhood of micropores and cracks may lower the nominal external intensity for electric avalanche breakdown by a factor 2–100 depending on the geometry of the crack and the dielectric constant. The presence of absorbing inclusions at the edge of microcracks will often be the dominant mechanism giving the lowest surface damage threshold. Inclusions and cracks with characteristic dimensions less than about 10-6 cm will not lower the breakdown threshold appreciably.
N. Bloembergen, "Role of Cracks, Pores, and Absorbing Inclusions on Laser Induced Damage Threshold at Surfaces of Transparent Dielectrics," Appl. Opt. 12, 661-664 (1973)