wim,
“Polluted” is another way of saying “stagnate.”
It’s about circulation. Because in vishnu mudra, the ring-finger and thumb are on opposite sides of the sagittal plane of the hand (or opposite sides of the hand/body), it supports the body with alternately opposing energy across that division, complimentary to the breathing pattern. Contrary to that, because the thumb and index finger are on the same side of that division (or same side of the hand/body) support from the mudra comes from only one direction, it does not alternate in coordination and so energy stagnates or gets “polluted.”
You will notice that by holding the left hand in chin mudra during anuloma viloma, the index and thumb effectively come together to send energy in one direction, lending strength and support to the spine and right arm to effectively help hold it up. Sahweet!
But also, what swami Sivananda meant by “polluted,” is that you use that finger for picking?..nose, ears, and other assorted…thangs? Sometimes you get just a short answer and it should be good enough. Short answer here is…it’s funky.
I hope that helps.
peace,
siva