The effect of pollution is related to that of the dry bands which cause partial discharges and corona effects. These types of electrical stress cause surface erosion and localised damage to the collar material protecting the ZnO blocks [94]. The partial discharges cause a drop in oxygen content of the gas surrounding ZnO blocks together with simultaneous formation of corrosive gases and this was found to accelerate ZnO degradation [95]. Moreover, a white deposit of silicone, whose origin is not explained, is observed [96] in contaminated arresters where corona activity is present.