A much bigger problem is the cleaning of paper money. The researchers Nabil Lawandy and Andrei Smuk claim that 150,000 tons worth of paper money, unfit for recirculation, are removed from circulation every year at a cost of $10,000,000,000. The researchers have developed a method using supercritical CO2 at 60°C and 5000 psi said to effectively clean both paper and polymer banknotes.
The researcher's paper is entitled Supercritical Fluid Cleaning of Banknotes.
(Hat tip: Marginal Revolution).
I don't collect paper money. Do collectors typically clean it? I have seen a YouTube video (from Suriname the former Dutch Guiana!) showing obsolete banknotes being cleaned in a bucket. Three minutes in the presenter admits he's never done it before!