Approximately 80% of all paper money in the US contains traces of cocaine.
Why? It's not because 80% of US bills have been used to snort cocaine - it's because cocaine is a very fine powder, and one bill can contaminate an entire cash drawer. The concentrations of cocaine on each bill are very small, so don't worry about accidentally getting high. And unless you're a bank teller, don't worry about detectable amounts appearing on you.