Chances are there is cocaine in the wallets of citizens in several countries.

Researchers looked at 234 bank-notes from 17 cities in the United States (US) and found that 90 per cent 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 analysed 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 meeting in Washington.

Banknotes

Except for Washington, Zuo said he and his colleagues examined a range of denominations from $1 to $100.

The researchers did not look at the same number of banknotes from each city. In all, they analysed money from 30 places in five countries - the US, Canada, Brazil, China and Japan.

The bits of cocaine on most bills was so small that consumers should not 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 US cash. In a similar study two years ago, he found that 67 per cent of bills had traces of cocaine.

Of the 27 bills analysed from Canada, 85 per cent had traces of cocaine. Eight of the 10 banknotes 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.