Two Spanish chemists have found that U.S. dollar bills contained the highest traces of cocaine powder, indicating the popularity of the illegal drug.

The study by Sergio Armenta and Miguel de la Guardia from the University of Valencia published in the latest issue of the journal Trends in Analytical Chemistry also found that Spanish euro notes are the most contaminated with the drug among European currencies.

The chemists calculated that U.S. bills contain an average of between 2.9 and 28.8 micrograms of cocaine, with some 1996 bills containing more than 1,300 micrograms.

A microgram is one-millionth of a gram. One gram of powder can fill half of a tea bag.

Spanish notes, on the other hand, contained an average of 155 micrograms of cocaine.

Armenta and la Guardia explained that the bills were contaminated because the same fingers handle cocaine and cash during sales of the drug and many users sniff the powder using wrapped banknotes.