Saidin is tainted. The taint causes the male channelers to go mad and then to begin rotting alive. Verin mentions this in The Great Hunt and Rand dreams that the flesh on his hands has rotted away and the bones are visible in The Shadow Rising. Saidar is not tainted. None of the descriptions of women channeling mention any type of impurities. Women channelers describe the embrace of saidar as the essence of joy itself. Rand describes saidin as making him won't to vomit every meal he has ever eaten. I'm not sure how saidin will be cleansed but it has something to do with bringing the two powers together in some way. The symble for the Aes Sedai of the Age of Legends is a yin-yang without the dots in the halves. The drop in the number of channelers is caused by the popullation droppping. This, plus the culling done by the Red Ajah and the fact that most Aes Sedai never marry or have children is the reason why numbers have been dropping in the White Tower. This also explains why there is an abundance of male channelers, men who have the dormant ability will continue to live a normal life and father children.