New study: 90 percent of U.S. bills carry traces of cocaine

Research presented this weekend reinforced previous findings that 90 percent of paper money circulating in U.S. cities contains traces of cocaine. 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.

"Imagine a bank teller who's working with cash-counting machine in the basement of the bank," Negrusz said. "Many of those bills, over 90 percent, are contaminated with cocaine. There is cocaine dust around the machines. These bank tellers breathe in cocaine. Cocaine gets into system, and you can test positive for cocaine. ... That's what's behind this whole thing that triggered testing money for drugs."

Compared with currency from Brazil, Canada, China and Japan, U.S. bills had the highest percentage of cocaine, with 90 percent of 234 bank notes contaminated. Canada followed with 85 percent and Brazil with 80 percent. China and Japan had the lowest, with 20 and 12 percent respectively.