Contamination can spread to bank notes not involved in the illicit drug culture because bills are processed in banks' currency-counting machines.

'To my surprise, we're finding more and more cocaine in bank notes,' said study leader Yuegang Zuo, of the University of Massachusetts in Dartmouth.

Zuo says that the high percentage of contaminated US currency observed in the current study represents nearly a 20 percent jump in comparison to a similar study he conducted two years ago. That earlier study indicated that 67 percent of bills in the US contained traces of cocaine.

'I'm not sure why we've seen this apparent increase, but it could be related to the economic downturn, with stressed people turning to cocaine,' Zuo said.

Such studies are useful, he noted, because the data can help law enforcement agencies and forensic specialists identify patterns of drug use in a community.

These findings were presented at the 238th National Meeting of the American Chemical Society.