It isn’t just the stuff you get up to on your phone that can reveal your dirtiest secrets, it’s also the chemicals, molecules, and microbes on them.

A new study in the Proceedings of the National Academy of Sciences has shown that researchers can identify a person’s lifestyle based on the molecule traces left on an everyday object. This includes their diet, if they’re on medication, their choice of soap or moisturizer, whether they’re ill, where they’ve been, and a handful of other personal habits.

“You can imagine a scenario where a crime scene investigator comes across a personal object – like a phone, pen or key – without fingerprints or DNA, or with prints or DNA not found in the database. They would have nothing to go on to determine who that belongs to,” senior author Pieter Dorrestein, a professor at UC San Diego School of Medicine, said in a statement. “So we thought – what if we take advantage of left-behind skin chemistry to tell us what kind of lifestyle this person has?”  

They put their highly sensitive technique of mass spectrometry to the test by gathering 39 participants and swapping four spots on each of their phones and eight spots on each of their hands. They then paired these results with a crowdsourced database of chemical structures from commercial products and medicines.

Although they couldn’t create one-to-one matches like fingerprints or DNA evidence, they created a personalized lifestyle “read-out” from each phone. This picked up on things such as if they had used anti-fungal skin creams, hair loss treatments, eye drops, or antidepressants. It even managed to tell if they had used sunscreen or insect repellent in the past few months.

"By analyzing the molecules they’ve left behind on their phones, we could tell if a person is likely female, uses high-end cosmetics, dyes her hair, drinks coffee, prefers beer over wine, likes spicy food, is being treated for depression, wears sunscreen and bug spray – and therefore likely spends a lot of time outdoors – all kinds of things,” added first author Amina Bouslimani.

Not only that, but the chemicals on the participants' hands could matched to their individual devices. “In 69 percent of the cases we could perfectly match up the chemical profile, the molecular profile, on the phone to the person that it belonged to,” said Dorrestein.

This technique has its obvious applications in forensics and police work, like detecting traces of firearms, explosives, or illegal drugs. However, the researchers also hope it could be used in medical and environmental studies. For example, it could test for a person’s exposure to environmental pollutants or chemical hazards. It could also assess how people are metabolizing medications during clinical trials.