It is not just DNA that people scatter to the wind as they go about their business. They shed a whole range of other chemicals as well, in their breath, their urine, their faeces and their sweat. Collectively, these molecules are referred to as metabolites….

The most common way of analysing metabolite content is gas chromatography-mass spectrometry. This technique sorts molecules by their weight, producing a pattern of peaks that correspond to different substances….There are, however, a lot of information sources out there, in the form of publicly available metabolite databases. The databases themselves are getting better, too…. A study just published by Feliciano Priego-Capote at University of Cordoba, in Spain, for example, shows it is possible to extract much meaningful information from even a dried-up drop of sweat. “The day is coming soon”, observes Cecil Lewis, a molecular anthropologist at University of Oklahoma, who is studying the matter, “when it will be possible to swab a person’s desk, steering wheel or phone and determine a wide range of incredibly private things about them….

The police may be tempted to push the boundaries as well. The fourth amendment to America’s constitution protects against unwarranted searches and seizure of evidence. This means it is hard to force someone to give a sample. But if obtaining such merely requires taking a swab of a surface in a public place—perhaps a keyboard someone has just used—the 4th amendment is unlikely to apply.

That is not necessarily wrong, if it means more criminals are caught and convicted. But it needs to be thought about carefully, because many metabolites are sticky. Cocaine is a case in point. Studies have shown that as many as two-thirds of the dollar bills in circulation in America carry traces of this substance, which might thus end up on the fingertips of the innocent, as well as the guilty.

Excerpts from Metabolites and You, Economist, Feb. 15, 2019