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In the course of its average 20 months in circulation, U.S. currency gets whisked into ATMs, clutched, touched and traded perhaps thousands of times at coffee shops, convenience stores and newsstands. And every touch to every bill brings specks of dirt, food, germs or even drug residue.

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

"When I was a young kid, my mom told me the dirtiest thing in the world is money," said the researcher, Yuegang Zuo, professor of chemistry and biochemistry at the University of Massachusetts Dartmouth. "Mom is always right."

Scientists say the amount of cocaine found on bills is not enough to cause health risks.

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 ... >>>