In the largest study to date of cocaine contamination in currency, scientists report that cocaine is present in up to 90 percent of paper money in the United States, particularly in large cities.  Scientists found traces of cocaine in 95 percent of the banknotes analyzed from Washington, D.C., alone.  The results were presented at the 238th National Meeting of the American Chemical Society held in August 2009 in Washington, D.C.

Money can be contaminated with cocaine if a user snorts it with a bill. But not all bills are involved in drug use — they can also get contaminated inside currency counting machines at a bank and other ways.

This is noteworthy for criminal defense practitioners, since a positive alert to the presence of cocaine residue on currency by a trained drug detection dog is often cited by law enforcement as evidence of drug dealing.