Believe it or not, cocaine is a very common environmental contaminant - it is well known that it is present on many bank notes.
Cocaine is now so prevalent in society that one in 10 people who have never used the drug have traces on their hands, a study has found.
Researchers from the University of Surrey in the UK have created a definitive way to prove the difference between those using cocaine and heroin, and those exposed to the drugs due to environmental factors.
The study, published in the journal Clinical Chemistry, tested the fingerprints of 50 drug-free volunteers and 15 drug users who testified to taking either cocaine or heroin in the previous 24 hours.
Researchers tested fingerprints from the unwashed hands of the drug-free volunteers and, despite having no history of drug use, still found traces of class A drugs.
Around 13 per cent of fingerprints were found to contain cocaine and one per cent contained a metabolite of heroin.
By setting a "cut-off" level, researchers were able to distinguish between fingerprints that had environmental contaminants from those produced after genuine drug use - even after people washed their hands.
To test the possibility of transferring drugs through a handshake, drug-free volunteers were asked to shake hands with a drug user.
Fingerprints were then collected from the drug-free volunteers after contact.
Although cocaine and heroin can be transferred by shaking hands with a drug user, the cut-off level established allowed researchers to distinguish between drug use and secondary transfer.
"Believe it or not, cocaine is a very common environmental contaminant - it is well known that it is present on many bank notes," said Melanie Bailey, a lecturer at the University of Surrey.
"Even so, we were surprised that it was detected in so many of our fingerprint samples," said Bailey.
"We can give those tested the piece-of-mind of knowing that whatever the result of the test may be, it was not affected by their everyday activities or shaking hands with someone that had taken drugs," she said.