But it's our money is already full of animal grease.
I mean...take a drunkard getting kicked out the club. She/He gets a greasy mystery kebab of pizza dripping with who knows what oil. She/He pays the taxi driver with a pile of notes out from his pocket...their are covered in grease and other such fun....the bank note is made of cotton paper. It's gonna soak up a little surely!
At least with the polymer ones you can soak them in mild disinfectant after a hard night's taxi-ing before passing them on ;). Can't do that wil the paper ones for long at all.