For cocaine users, a rolled up $20 bill may be the most convenient tool for snorting the powder form of the drug. Or so it would seem from a new analysis of 234 banknotes from 18 U.S. cities that found cocaine on 90 percent of the bills tested.

Perhaps that's not surprising given that the U.S. Office of National Drug Control Policy reports that more than 2 million Americans used cocaine in 2007, which has been linked to ill effects ranging from debilitating addiction to heart attacks. The United Nations Office on Drugs and Crime, for its part, reported in the same year that 6 million Americans admit using cocaine annually, consuming a total of as much as 457 metric tons in a year.

"Cocaine is a powerfully addictive stimulant and one of the most commonly abused illicit drugs in the world," says chemist Yuegang Zuo of the University of Massachusetts Dartmouth, who conducted the tests and presented the findings today at the biannual meeting of the American Chemical Society, which is taking place in Washington, D.C. That city ranked highest in the survey—95 percent of the sampled bills there bore cocaine contamination—along with Baltimore, Boston and Detroit. Salt Lake City had the lowest average levels of contamination. "The examination of cocaine contamination on paper money can provide objective and timely epidemiological information about cocaine abuse in individual communities," Zuo argues.

What might be more surprising is the fact that the percentage of contaminated bills seems to be rising; just two years ago, Zuo did a similar study that found cocaine on only 67 percent of banknotes in Massachusetts. "It is too early to draw a conclusion about why," Zuo says. "The economic downturn may partly contribute to the jump."

But the U.S. Drug Enforcement Administration (DEA) notes that other measures, such as pretrial urine samples from defendants accused of crimes, show that drug use, at least in the D.C. area, has gone down slightly—only 29 percent of adult arrestees had traces of cocaine in their urine in the first six months of 2009, the lowest level since 1985. "We know that cocaine prices have gone up significantly in the last two years, which usually deters use of that drug," says special agent Melissa Bell of the Washington, D.C., division office of the DEA. "Junkies go on to use something cheaper."

Levels of cocaine ranged from .006 micrograms to more than 1,240 micrograms—the equivalent of 50 grains of sand—on U.S. bills, and $5, $10 and $20 bills on average carried more contamination than $1 or $100 bills.

Zuo and his colleagues also tested banknotes from Brazil, Canada, China and Japan, and found that Asians appear to use the drug less—only 20 percent of the 112 Chinese renminbi notes tested had traces, and only 12 percent of 16 Japanese yen notes tested bore the drug.

But Canadians seem to be just as fond and, perhaps, a bit sloppier in their consumption or dealing. More than 2,350 micrograms of cocaine were found on some of the Canadian bills, 85 percent of which had some level of contamination, while 80 percent of Brazilian reals also bore traces of the drug.

Whether this means drug use is on the rise or that ATMs and other bulk cash-handling machines—where one contaminated bill can spread powder to many others—are ever more ubiquitous cannot be discerned. "It is still difficult to tell quantitatively how much is due to primary contamination, such as during a drug deal or [use], and how much is due to secondary contamination, such as interaction between contaminated and uncontaminated bills," Zuo says. "Both may contribute ... [but] it seems clear that the banknotes containing 1,240 micrograms of cocaine were used directly during a drug deal or uptake [drug use]."

Previous studies, stretching as far back as 1987, have found varying levels of cocaine contamination, some even higher than the new finding. But Zuo is the first to analyze foreign banknotes for contamination and the first to employ a new method of gas chromatography, which detects the chemical signature of the drug without damaging the actual money, to do the analysis.

The finding might complicate an anti-drug dealing tactic used by the Federal Bureau of Investigation (FBI) and other enforcement agencies, Zuo says. In some cases, the FBI compares the levels of cocaine contamination on seized bills to levels found on bills in general circulation, treating this as evidence. "Sometimes [drug dealers] use these studies to try to get their money back when we seize it," Bell notes. But the DEA's drug-sniffing dogs are not actually detecting cocaine; they are sensing a chemical used in its manufacture that dissipates more quickly. "So they don't get their money back," Bell says.

Regardless, it would seem, according to this research, that C-notes are not as popular with drug dealers (or users) as perhaps popularly depicted. "You rarely see them breaking out the hundreds unless they're buying kilos," Bell adds. "The user on the street is going to be breaking out the five, ten or twenty."