I was just wondering given that 75% of US currency is tainted by drug traces we can safely assume it was used in illegal transactions. So if we compare this to

I was just wondering given that 75% of US currency is tainted by drug traces we can safely assume it was used in illegal transactions. So if we compare this to "dirty" bit-coins how do you determine if a coin is dirty. How recent does this transaction have to be for the legitimate organization to refuse it because if a bit-coin is considered dirty forever after being involved in a dirty transaction that wouldn't work. I'm probably missing some intricacies of how the block-chain works since you can do transactions of any size not necessarily 1 bit-coin.