IStock Photo 9949972 © Baris Onal

Someone has been saying yes to drugs, and the proof is likely in your wallet. According to a study released in August by scientists from the University of Massachusetts Dartmouth, up to 90% of paper money in the US carries traces of cocaine.

The researchers analyzed 234 bills, in denominations from $1 to $100, from 17 cities in the United States. The odds that a US banknote contains trace amounts of cocaine are 1 in 1.11 (90%).

Of the cities in the study, Washington, DC had the dubious honor of topping the list, with 95% of its paper currency carrying minute amounts of cocaine. Boston, Detroit, and Baltimore were runners-up, while money from Salt Lake City had the lowest level of contamination.

Bills from Brazil, China, Japan, and Canada were also tested. Like its American counterpart, Canadian money showed an average cocaine contamination levels around 85%, while 80% of Brazilian, 20% of Chinese, and 12% of Japanese banknotes were tainted.

This isn’t the first time drugs have shown up on money. Lead study author Yuegang Zuo conducted a smaller study two years ago that found cocaine on 67% of bills from Massachusetts, while a 2001 study by a separate group found cocaine on 92% of 50 US banknotes. And a 2004 study found that compounds from marijuana were present in 10.3% of American and 22.5% of foreign bills tested.

So how are all these drugs getting onto the Benjamins? Some probably comes from direct contact with narcotics during use—for example, snorting a line of powdered cocaine through a rolled-up twenty. The odds that a person in the US over the age of 12 has ever tried cocaine are 1 in 6.9, while 1 in 43.48 has used cocaine in a given year, based on 2007 statistics.

In the recent study, the least contaminated bills that still contained cocaine had just 0.006 micrograms of the drug, but some bills held more than 200,000 times more, clocking in at 1,240 micrograms, comparable to the mass of about 50 grains of sand. It’s likely that the more contaminated bills were directly involved in drug use or sales and later transferred the powder to other bills in banks’ currency-counting machines or ATMs.

But don’t worry—experts say that even the most contaminated bills contain far too little cocaine to accidentally get you high or make you fail a drug test.