Money can be contaminated with cocaine during drug deals or if a user snorts with a bill. But not all bills are involved in drug use; they can get contaminated inside currency-counting machines at the bank.
"When the machine gets contaminated, it transfers the cocaine to the other bank notes," Zuo said. These bills have fewer remnants of cocaine. Some of the dollars in his experiment had .006 micrograms, which is several thousands of times smaller than a single grain of sand.
This information could be useful when dealing with a case in which there is a claim that there are trace amounts of controlled substances found on an item of evidentiary value.