Although solid insulation dielectric strength is important, the usual bottleneck for high voltage is creepage distance, such as margins between wire and core along the layers of insulation, or margins between lead joints and frame along the leads and coil sides. (Reuben Lee)

For design purposes, the specific knowledge about the creepage distance of the insulator for the contaminated environment is important. This can be done easily if the mathematical relationship between the creepage distance and Salt Deposit Density (SDD) is known. (IEEE Xplore)

The higher the pollution degree, the worse the environment. For obvious reasons, the creepage distance required for safety will need to be greater to account for increased pollution. (Lamothe Approvals Inc.)