The paper describes experimental work carried out at the Shiobara Laboratory on the performance of salt-contaminated insulators in both natural and artificial humid conditions. The purpose was to analyse, in relation to humidity, one type of flashover mechanism caused by salinity deposited on a porcelain surface and absorbed moisture in humid conditions. For natural humid conditions, dew, fog and drizzle were available. Based on experience of natural humid conditions, an artificial test with steam fog permitting the gradual formation of humid conditions, was developed. This method produced a flashover voltage of the insulators similar to that in natural conditions. An antifog-type insulator showed the best performance for the insulators that have similar flashover voltage. However, flashover voltages of the two long-term tests showed a great difference from that of the conventional short-term test. A hypothesis of ‘danger degree for flashover’, is assumed in order to explain the essential differences between the pollution- test methods and results. The danger-degree concept is a combination of two factors: transfer of salinity and humidity