Spanish banknotes carry heaviest cocaine traces

"Traces of the drug are found not only on notes that have been in direct contact with it, but on nearly all the notes in circulation," said Miguel de la Guardia, a co-author of the study and a professor in the Analytical Chemistry Department of the university, according to Spanish daily 20 Minutos.

Germany, by comparison, had traces of cocaine five times lower than those in Spain. Irish euros came in lower still, bearing an average concentration of just 0.576 microgrammes of the drug.

The study, published this week in the latest issue of Trends in Analytical Chemistry, a scientific journal, also reviewed earlier studies on traces of cocaine found on various paper currencies around the world.

United States dollars generally have the highest concentration of cocaine of any currency in the world. A 2006 study also found that 94 percent of Spanish notes were contaminated with cocaine traces.

Data has showed that between 40 and 51 percent UK pound notes are contaminated, while just six percent of Swiss francs have cocaine traces above one nanogramme per note.

The high rate of contamination does not mean that almost all Spaniards are using cocaine, the scientists were quick to underscore. Notes that have not been in direct contact with the drug become "cross-contaminated" from contact with other notes, and by money counting machines in banks.

But cocaine "has become entrenched in Spanish society, and is mortgaging the neuronal development of a whole generation," said Mr de la Guardia.

The Iberian Peninsula is Europe's main entry point for most drugs trafficked across the continent, according to a report published in July by the United Nations Office on Drugs and Crime.

Of the two countries on the peninsula, Spain is used most by cocaine smugglers, the report said.

In 2006, some 50 tonnes of the drug were seized in the country, said the report, mainly at ports in Galicia, Andalucia, Barcelona and Valencia. By comparison, the country that saw the next largest tonnage of cocaine seized in 2006 was the Netherlands, where some 11 tonnes were intercepted.