Scientists have discovered that both drugs have become so widely used that 13% of people have traces of them on their fingers, without realizing it. That means that a little over one in ten people are unknowingly walking around with drugs on their hands.

The finding of cocaine and heroin study were recently published in the journal Clinical Chemistry.

In it, researchers tested the unwashed hands of 50 drug-free volunteers as well as 15 drug users who admitted to using the drugs in the past 24 hours.

The purpose of the study was to determine if scientists could create a fingerprint test that allowed them to differentiate between drug users and those who may have gotten drugs transferred to their hands through something like a handshake. While the finger test already exists as an alternative to traditional blood and urine tests, it needs improvement to avoid false positives.

“Believe it or not, cocaine is a very common environmental contaminant – it is well known that it is present on many bank notes. Even so, we were surprised that it was detected in so many of our fingerprint samples,” Dr. Melanie Bailey, lecturer in forensic analysis at the University of Surrey, said in an interview with Phys. “By establishing a threshold for significance on a fingerprint test, 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.”

The study was co-funded by Intelligent Fingerprinting, which makes a drug test that uses the sweat from fingerprints to test for cocaine, cannabis, and opiate use.