More than one in 10 people who have never used class A drugs may have traces of cocaine or heroin on their fingertips, forensic scientists say.

Researchers found tiny amounts of the illegal substances on 13% of volunteers who took part in a study after declaring they did not take the drugs.

Rather than suspecting dishonesty, the scientists believe the participants became contaminated from banknotes, tables and other surfaces.

“We were quite surprised. We thought there might well be cocaine knocking around in the environment, but we did not expect to pick up these drugs at the fingerprint level,” said lead author Melanie Bailey, a lecturer in forensic analysis.



“The people we took samples from were at the university, so it is possible they knew others who were taking substances, or had been in an environment where these substances were taken,” she added. In 2011, an official investigation for the Home Office found that 11% of banknotes were contaminated with cocaine.

The Surrey team recruited drug users and non-users to help them develop a forensic fingerprint test for class A drugs. The test exploits the fact that while innocent people can have traces of illicit drugs on their skin, users secrete the drug and its breakdown products in their sweat.

For the study, scientists analysed fingerprint swabs from 50 people at the university who claimed not to use class A drugs. They next swabbed 15 patients seeking help at drug rehabilitation clinics who testified to using either cocaine or heroin in the previous 24 hours.

Writing in the journal Clinical Chemistry, the researchers describe how their test picked up marked differences in the levels of cocaine and heroin found on the fingertips of drug users compared with non-users. The best results came when people washed their hands, because this removed any environmental contamination and made the real drug users stand out. After hand-washing, the test identified 87.5% of cocaine and 100% of heroin users.

The forensic test could now find its way into prisons and workplaces to crack down on illegal drug use. Because the test requires a person’s fingerprint, it is hard to fake and comes with built-in personal identification, Bailey said.