The astonishing, alarming secrets our sweat can reveal about us

When I deposited my index fingerprint on a laboratory slide so that Simona Francese could analyse it, I felt as if I was giving her the password to my body's secrets. Most forensic scientists examine a fingerprint's pattern but Francese, a forensic scientist from Sheffield Hallam University, analyses the chemicals left behind in those whirls and swirls. Her aim is to develop techniques that will allow her to extract identifying information about people at a crime scene from the sweaty residues they leave behind.

Fingerprints are inked with sweat, a body fluid that holds revealing information about our health and our vices. Our sweat glands source perspiration from the watery parts of blood, and any chemicals flowing about your circulatory system can, in principle, leak out of your sweat pores.

Sweat holds markers of disease as well as a potpourri of other chemicals that hint at our more private selves

When she examined my fingerprint’s chemistry using a technique called mass spectrometry, Francese easily found evidence of my morning coffee, thanks to the caffeine circulating in my blood. Had I spiked my latte with a shot of whisky or snorted a line of cocaine as a breakfast chaser, Francese could have detected that, too. In fact, in collaboration with law enforcement, she has previously tested her technique on a stalker’s fingermark left behind on a window sill, and found chemical evidence that he had been indulging in alcohol and cocaine – something he had also admitted to law enforcement.

It's not just mind-altering substances that emerge in our sweat. One nurse in South Africa turned her sweat red thanks to an intense predilection for spicy tomato corn chips. Scientists matched the red pigment in her sweat to the chips' flavouring, immortalising her fondness for Nik Naks in medical literature.

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Researchers are also working on ways to distinguish vegans from meat eaters, based on chemicals left behind in sweaty fingerprints, as well as to distinguish biological sex and age. “To people trained in chemistry, it’s obvious that fingerprints aren’t just inanimate objects,” Francese says. “There is organic and inorganic matter there to be discovered.”

Sweat also holds markers of disease – certain cancers, for example – as well as a potpourri of other chemicals that hint at our more private selves, such as stress hormones.

But it’s not just forensic scientists who are curious about the sweat we leave behind throughout the day. Although fingerprints are the most minuscule of sweaty marks, many of us leave oodles of perspiration behind on spin bikes, yoga mats, T-shirts, bike helmets – you name it. In this era of personal measurement, we may soon be able to use it to learn more about our own inner workings.

Engineers are designing adhesive patches, for example, that are embedded with electronics to capture and analyse sweat, extracting information that could then be sent to smartphones. Such technology could also be built into a smartwatch that would analyse sweat emerging from skin and in contact with the device.

Smartwatches of the future may monitor your sweat for alcohol, and send you an alert when it’s wise to take a taxi home. Cars may eventually feature a fingerprint pad that requires drivers to assess intoxicant levels before the engine is permitted to start. Coaches might choose to monitor athletes, to improve training regimes – aerobic or anaerobic – based on lactate levels calculated by analysing sweat produced during a workout. Or, during an important team match, a player releasing biomarkers of stress or fatigue in their sweat might be replaced with someone fresher.

If our smartphones and smartwatches become able to monitor our sweaty secrets, what happens if there is a hack to an app?

Some sweat-patch developers aspire to accurately track glucose – an unexpectedly challenging goal, in part because skin bacteria eat glucose as soon as it hits the surface, disrupting precise measurement. Yet the hope is that one day people with diabetes will not have to rely on needles to faithfully track glucose levels.

Yet sweat monitoring, like most technological innovations, has progressive and dystopian potential. Being able to measure a person’s innermost secrets from a fingerprint could make it disturbingly simple for health insurers to identify some pre-existing conditions or for employers to do snap drug tests. Fingerprint analysis by law enforcement also creates potential for conflict with civil liberties. In the US, law enforcement can surreptitiously collect DNA from a wayward hair or from the saliva on a discarded coffee cup. In the UK, DNA is sampled from individuals arrested for a criminal offence. Although it is likely to be many years before fingerprint chemical analysis reaches the mainstream of forensic analysis, collecting a suspect’s fingerprint would be much easier than acquiring a DNA sample.

There are also tech privacy protocols to consider. If our smartphones and smartwatches become able to monitor our sweaty secrets, what happens if there is a hack to an app, or if our information is unwittingly shared with third parties?

Although many people bemoan their sweat's odour and the wet patches it leaves behind, the secrets it can reveal about human biology are fascinating. As we scroll through the list of chemicals Francese has detected in my single index fingerprint, I am overwhelmed by how much my own body gives away, and left thinking about how we might one day use the bounty of information flowing out of our pores – particularly on a hot summer's day. – Guardian

The Joy of Sweat: The Strange Science of Perspiration by Sarah Everts is published by WW Norton