Paper money contains high traces of cocaine, regardless of whether or not the paper money came into direct contact with the drug. And U.S. bills take the top spot, covered in the greatest amount of the illegal powder, while Spanish notes are the most highly contaminated in Europe, a new study finds.

The findings, detailed in the latest issue of the journal Trends in Analytical Chemistry, reflect the popularity of the illicit drug, the researchers say.

"These findings should not be surprising, because cocaine and other drugs are traded using cash, which is handled by the same fingers that directly touch the drugs or wrappings," chemists Sergio Armenta and Miguel de la Guardia from the University of Valencia in Spain write. "Moreover, many cocaine users use a wrapped banknote to sniff this drug, so inducing direct cocaine contamination of the banknotes."

Armenta and de la Guardia analyzed Spanish notes for cocaine traces, finding they contained an average of 155 micrograms of cocaine. (A gram of cocaine would fill about half a tea bag. A microgram is one-millionth of that amount.)

They also reviewed previous research focusing on cocaine concentrations found in different currencies around the world.

German Euros contained levels of cocaine that were five times lower than the Spanish ones. For Irish bank notes, one statistic suggested that of 48 notes studied the highest concentration found was 0.5 micrograms.

The chemists found U.S. bills contained an average of between 2.9 and 28.8 micrograms of cocaine depending on the year and city, with a maximum of more than 1,300 micrograms found on some 1996 bills.

One study based on 356 notes showed just 6 percent of Swiss francs were contaminated with cocaine at levels above one nanogram per note, where a nanogram is one-thousandth of a microgram. Some data suggest, the researchers found, that between 40 percent and about 50 percent of British pounds were contaminated with cocaine at levels of about 0.0011 micrograms per note.

It turns out, money really is dirty, and not just with drug traces. One past study revealed 94 percent of $1 bills collected from a community in western Ohio contained disease-causing or potentially disease-causing bacteria. The study, published in 2002 in the Southern Medical Journal, was led by Peter Ender, chief of infectious diseases at Wright-Patterson Medical Center in Ohio.

That's not too surprising, as $1 bills stay in circulation for an average of 21 months, according to the U.S. Bureau of Engraving and Printing, during which time they get handled by plenty of people. For larger bills, the life span is even longer, with $20 bills lasting about 24 months and $50 bills staying in circulation for 55 months.

When you handle coins, stuff also gets transferred to your hands, though it's mainly iron atoms (iron is one of the metals in change). Another research study revealed iron atoms from coins cause oils on your skin to break down, producing a "metallic" odor.