Police forces worldwide have been using fingerprints to place suspects at the scene of crimes for more than a century. More recently, DNA has been added to that mix. Often, however, fingerprints get smudged or hidden, even if no gloves are worn, and DNA may not be detectable. In these cases, someone's sweat may reveal their presence.
You may not like to think about it, but each square centimeter of skin contains 100 sweat glands per centimeter (650 per square inch). Even when not particularly hot and bothered, people leave sweat on anything they touch. “Each of our skin secretions are different and, therefore, unique to us. Like a fingerprint,” said Dr Jan Halámek of the University of Albany in a statement.
Our sweat contains an array of amino acids and metabolites. Although most are common to all people, the concentrations of each varies, so that a sample is quite distinctive. In Analytical Chemistry Halámek describes an attempt to see if concentrations of just three metabolites could be used to identify individuals.
If you were uncomfortable with the idea of sweat being left everywhere you might want to look away now, but one of the metabolites Halámek measured is urea, the key component of urine. The other two were lactate and glutamate. Using 25 samples collected from the forearms of volunteers, and another 25 artificially produced, Halámek showed it is possible to distinguish each sample based on the metabolite concentration.
“Currently, investigators tend to overlook the presence of sweat at crime scenes. Our paper is proving it has value,” Halámek said. “Without sufficient DNA evidence, which can take days or weeks of analysis, it can be difficult to determine how many people were present at a crime scene. We can quickly gather that information.” Metabolite analysis, Halámek told IFLScience, can take just 30-40 seconds.
There is still a long way to go before forensics teams have this tool in their armory. Halámek told IFLScience his team is working to overcome the challenge of “mixed sweat samples”, where two or more people touched the same surface.
Moreover, metabolite concentrations don't only reflect an individual's genes, like a fingerprint. Diet, health and recent exercise alter metabolite concentrations. So to keep a profile on file, as is done with DNA of people with convictions, would – at the very least – require a series of records to track the way an individual's sweat changes with time. Even without this, however, a sweat sample can reveal someone's sex and approximate age.
Halámek has previously proposed using sweat recognition for authentication to access electronic devices. He argues it would be harder to fake someone's sweat profile than to make a mold of their fingerprints to gain inappropriate access.