电气设备，Pollution Degree，是什么含义 ？英语不懂

Pollution Degree
Pollution Degree (of environmental conditions) – A conventional number based on the amount of conductive or hygroscopic dust, ionized gas or salt, and on the relative humidity and its frequency of occurrence resulting in hygroscopic absorption or condensation of moisture leading to reduction in dielectric strength and/or surface resistivity of the insulating materials of devices and components.