Hundreds of chemicals stream from our bodies every second, releasing into the air easily as they have high vapour pressures. This means they boil and turn into gases at room temperature – and give clues about who we are, and how healthy we are.
Since ancient Greek times, we’ve known that we smell differently when we are unwell.
While we rely on blood analysis today, ancient Greek physicians used smell to diagnose maladies, writes Aoife Morrin in The Conversation. If they took a whiff of your breath and described it as fetor hepaticus (meaning bad liver), it meant you could be headed for liver failure.
If a person’s whiff were sweet or fruity, physicians thought this meant that sugars in the digestive system were not being broken down, and that person probably had diabetes. Science has since shown the ancient Greeks were right – liver failure and diabetes and many other diseases, including infectious diseases, give your breath a distinctive smell.
In 1971, Nobel Laureate chemist Linus Pauling counted 250 different gaseous chemicals in breath. These gaseous chemicals are called volatile organic compounds, or VOCs.
Since Pauling’s discovery, other scientists have discovered hundreds more VOCs in our breath. Many of these have distinctive odours, but some have no odour that our noses can perceive.
Scientists believe that whether or not a VOC has an odour we can detect, they can reveal information about how healthy someone is.
A Scottish man’s Parkinson’s disease onset was identified by his wife, retired nurse Joy Milner, after she was convinced that his smell had changed, years before he was diagnosed in 2005.
This discovery has led to research programmes involving Joy Milner to identify the precise smell of this disease.
Dogs can sniff out more diseases than humans because of their more sophisticated olfactory talents. But technological techniques, like analytical tool mass spectrometry, pick up even more subtle changes in VOC profiles that are being linked to gut, skin and respiratory diseases as well as neurological diseases like Parkinson’s.
Researchers believe that one day, some diseases will be diagnosed simply by breathing into a device.
Where do VOCs come from?
Breath is not the only source of VOCs in the body. They are also emitted from skin, urine and faeces.
VOCs from skin are the result of millions of skin glands removing metabolic waste from the body, as well as waste generated by bacteria and other microbes that live on our skin. Sweating produces extra nutrients for these bacteria to metabolise, which can result in particularly odorous VOCs.
Odour from sweat only makes up a fraction of the scents from VOCs though.
Our skin and also our gut microbiomes are made up from a delicate balance of these microbes. Scientists think they influence our health, but we don’t yet understand a lot about how this relationship works.
Unlike the gut, the skin is relatively easy to study – skin samples can be collected from living humans without having to go deep into the body. Scientists think skin VOCs can offer insights into how the microbiome’s bacteria and the human body work together to maintain our health and protect us from disease.
In my team’s laboratory, we are investigating whether the skin VOC signature can reveal different attributes of the person it belongs to. These signals in skin VOC signatures are probably how dogs distinguish between people by smell.
We are at a relatively early stage in this research area but we have shown that you can tell males from females based on the acidity of the VOCs from the skin. We use mass spectrometry to see this, as the average human nose is not sophisticated enough to detect these VOCs.
We can also predict a person’s age with reasonable accuracy to within a few years from their skin VOC profile. This is not surprising, considering that oxidative stress in our bodies increases as we age.
Oxidative stress happens when your antioxidant levels are low, and causes irreversible damage to our cells and organs. Our recent research found by-products of this oxidative damage in skin VOC profiles.
Not only are these VOCs responsible for personal scent, they are used by plants, insects and animals as a communication channel.
Plants are in a constant VOC dialogue with other organisms, including pollinators, herbivores, other plants and their natural enemies like harmful bacteria and insects.
VOCs used for this back and forth dialogue are known as pheromones.
Science and love pheromones
In the animal kingdom, there is good evidence VOCs can act as aphrodisiacs. Mice, for example have microbes, which contribute to a particularly smelly compound called trimethylamine, allowing mice to verify the species of a potential mate. Pigs and elephants also have sex pheromones.
It is possible that humans also produce VOCs for attracting the perfect mate. Scientists have yet to fully decode skin – or other VOCs that are released from our bodies. But evidence for human love pheromones so far is controversial at best.
One theory suggests they were lost about 23m years ago when primates developed full colour vision and started relying on their enhanced vision to choose a mate.
However, we believe that whether human pheromones exist or not, skin VOCs can reveal who and how we are, in terms of things like ageing, nutrition and fitness, fertility and even stress levels. This signature probably contains markers we can use to monitor our health and diagnose disease.
Morrin is Associate Professor of Analytical Chemistry, Dublin City University, Ireland.
Study details
Predicting Chronological Age via the Skin Volatile Profile
Melissa Finnegan, Shane Fitzgerald, Aoife Morrin, et al.
Published in Journal of the American Society of Mass Spectrometry on 7 February 2024
Abstract
Skin volatile emissions offer a non-invasive insight into metabolic activity within the body as well as the skin microbiome and specific volatile compounds have been shown to correlate with age, albeit only in a few small studies. Building on this, here skin volatiles were collected and analysed in a healthy participant study (n = 60) using a robust headspace-solid phase micro-extraction (HS-SPME) gas chromatography–mass spectrometry (GC-MS) workflow. Following processing, 18 identified compounds were deemed suitable for this study. These were classified according to gender influences and their correlations with age were investigated. Finally, 6 volatiles (of both endogenous and exogenous origin) were identified as significantly changing in abundance with participant age (p < 0.1). The potential origins of these dysregulations are discussed. Multiple linear regression (MLR) analysis was employed to model age based on these significant volatiles as independent variables, along with gender. Our analysis shows that skin volatiles show a strong predictive ability for age (explained variance of 68%), stronger than other biochemical measures collected in this study (skin surface pH, water content) which are understood to vary with chronological age. Overall, this work provides new insights into the impact of aging on the skin volatile profiles which comprises both endogenously and exogenously derived volatile compounds. It goes toward demonstrating the biological significance of skin volatiles and will help pave the way for more rigorous consideration of the healthy “baseline” skin volatile profile in volatilomics-based health diagnostics development going forward.
ACS Publications article – Predicting Chronological Age via the Skin Volatile Profile (Open access)
See more from MedicalBrief archives:
Simple ‘sniff test’ reliably predicts recovery of severely brain injured patients
UK clinical trial launched into development of cancer-indicating breath test
Not to be sniffed at: Dogs diagnose malaria to better-than WHO standards