Researchers looked at 234 bank notes from 17 cities in the U.S. and found that 90 percent had small traces of the illegal drug.
Bills from larger cities, such as Baltimore, Boston and Detroit, were among those with the highest average cocaine levels. Salt Lake City had the lowest.
Scientists analyzed only $1 bills from Washington and found that most had tiny amounts of cocaine.
Yuegang Zuo, a professor at the University of Massachusetts in Dartmouth, led the study. The findings were presented Sunday at the American Chemical Society's fall meeting in Washington.
Except for Washington, Zuo said he and his colleagues examined a range of denominations, from $1 to $100.
The U.S. Bureau of Engraving and Printing, which makes the country's paper currency, said nothing in the process would taint the paper with the drug.
"When it leaves here, it does not have any cocaine in it," Claudia Dickens, a spokeswoman for the bureau, said Tuesday.
The researchers didn't look at the same number of bank notes from each city. In all, they analyzed money from 30 places in five countries - the U.S., Canada, Brazil, China and Japan.
The bits of cocaine on most bills was so small that consumers shouldn't have health or legal concerns over handling paper money, Zuo said. Some drug amounts ranged from several thousand times smaller than a grain of sand to about 50 grains of sand.
Money can become contaminated with cocaine during drug deals, or when users snort the substance through rolled bills. It can then spread to other cash when banks process the money.
Zuo said his research shows an increase in contaminated U.S. cash. In a similar study two years ago, he found that 67 percent of bills had traces of cocaine.
Of the 27 bills analyzed from Canada, 85 percent had traces of cocaine. Eight of the 10 bank notes from Brazil were contaminated. Only a few of the 16 bills from Japan had the substance, and a little more than 20 of the 112 bank notes from China had bits of cocaine.
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