The Smell of Life

Sulfur-rich smells are easy to recognize—think of the ocean at low tide, a bit of garlic, or even bad breath. While not always pleasant, they’re often tied to life processes. On Earth, compounds like dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) are made by microbes, plants, and even our own bodies. For example, DMS is a byproduct of marine algae, and both DMS and DMDS are linked to the metabolism of sulfur-containing foods like garlic and onions.   

Interestingly, these same compounds can also show up when organic matter breaks down. Over time, the mix of smells changes—starting with more sulfur notes like DMDS and shifting toward other compounds like ketones and acids. But in the early stages, it’s sulfur that dominates the scent, reinforcing that these molecules are deeply tied to life, not just what comes after it.

They even show up in everyday biology—DMS contributes to halitosis (bad breath), and sulfur compounds in urine can reveal what you’ve eaten recently, like garlic or leeks. And while they can be a nuisance in places like sewers due to their strong odor and reactivity, these volatile sulfur compounds are powerful chemical clues that life is (or was) at work.

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Reported Sensory Thresholds for Sulfur Compounds

Compound Structure Sensory Description Range (ppb) hydrogen sulfide H2S rotten egg, sewage-like 0.9 - 1.5 ethyl mercaptan CH3CH2SH burnt match, sulfidy, earthy 1.1 - 1.8 methanethiol, methyl mercaptan CH3SH skunk, flatulence, rotten cabbage, burnt rubber 1.5 diethyl sulfide CH3CH2SCH2CH3 rubbery 0.9 - 1.3 DMS, dimethyl sulfide CH3SCH3 ocean, canned corn, cooked cabbage, asparagus 17- 25 diethyl disulfide CH3CH2SSCH2CH3 garlic, burnt rubber 3.6 - 4.3 DDMS, dimethyl disulfide CH3SSCH3 vegetables, cabbage, onion-like at high levels 9.8 - 10.2 carbon disulfide CS2 sweet, ethereal, slightly green, sulfidy 5

Imagine cracking open a clam at low tide or walking through a marsh at dusk. The faint, tangy smell of sulfur in the air? That’s dimethyl sulfide (DMS), a molecule born of life. It’s a scent tied to oceans, microbes, and biology itself.

Now picture that same signature—those familiar chemical traces—not wafting from Earth’s shoreline but drifting through the atmosphere of a distant world. That’s exactly what a team of astronomers, led by Nikku Madhusudhan at the University of Cambridge, believe they may have found.

Using the James Webb Space Telescope (JWST), they detected not only dimethyl sulfide (DMS) but also dimethyl disulfide (DMDS) in the atmosphere of exoplanet K2-18b orbiting a star 124 light-years away. On Earth, these molecules are exclusively produced by living organisms, especially marine phytoplankton and sulfur-reducing microbes.

Is this the first scent of alien life?

K2-18b has long intrigued scientists. Discovered in 2015 and confirmed to host water vapor in its atmosphere by 2019, it lies in the habitable zone of its star—a region where liquid water could exist. The planet is a sub-Neptune, about 8 times the mass of Earth, likely hosting a vast ocean beneath a hydrogen-rich sky.

When JWST's near-infrared instrument first picked up hints of DMS, the signal was tantalizing but faint. Now, using its mid-infrared camera, a much stronger signal has emerged—not just for DMS, but for DMDS, a closely related molecule. Both are complex sulfur-containing compounds known to be byproducts of living metabolic processes—especially those involving the breakdown of dimethylsulfoniopropionate (DMSP), an osmolyte made by marine algae.

On Earth, the sulfur cycle involves a complex web of microbial transformations, particularly in anoxic oceanic zones. Phytoplankton produce DMSP as a way to handle osmotic stress; when grazed by zooplankton or lysed by viruses, DMSP is broken down into DMS. Other microbes metabolize sulfur compounds into DMDS, H₂S, and others.

If such a cycle—or something like it—exists on K2-18b, it would suggest a complex biosphere, not just isolated organisms.

But here's the rub: abiotic pathways for these molecules must be explored and excluded. Could volcanic activity, UV-driven chemistry, or some exotic atmospheric process generate DMS or DMDS in a hydrogen-rich atmosphere? Theoretical chemists are scrambling for answers.

So, caution remains the astronomer's motto. The team stresses that while the signal is the strongest yet, non-biological explanations must be thoroughly ruled out before claiming even the possibility of life.

A molecule that, on Earth, rises from algae-covered oceans, has now risen from the atmosphere of a distant world. Whether this is truly life, or an undiscovered quirk of chemistry, remains to be seen.

But for the first time, astronomy is starting to smell like biology.

REFERENCES

Srila W, Sripilai K, Binlateh T, Thammanichanon P, Tiskratok W, Noisa P, Jitprasertwong P. Relationship Between the Salivary Microbiome and Oral Malodor Metabolites in Older Thai Individuals with Periodontitis and the Cytotoxic Effects of Malodor Compounds on Human Oral Squamous Carcinoma (HSC-4) Cells. Dentistry Journal. 2025 Jan 16;13(1):36.

Dekeirsschieter J, Stefanuto PH, Brasseur C, Haubruge E, Focant JF. Enhanced characterization of the smell of death by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GCxGC-TOFMS). PLoS One. 2012;7(6):e39005. doi: 10.1371/journal.pone.0039005. Epub 2012 Jun 18. PMID: 22723918; PMCID: PMC3377612.

Madhusudhan, Nikku; et al. (March 2020). "The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b". The Astrophysical Journal Letters. 891 (1). L7. arXiv:2002.11115. doi:10.3847/2041-8213/ab7229

Schmidt SP, MacDonald RJ, Tsai SM, Radica M, Wang LC, Ahrer EM, Bell TJ, Fisher C, Thorngren DP, Wogan N, May EM. A Comprehensive Reanalysis of K2-18 b's JWST NIRISS+ NIRSpec Transmission Spectrum. arXiv preprint arXiv:2501.18477. 2025 Jan 30. arXiv:2501.18477 [astro-ph.EP]  https://doi.org/10.48550/arXiv.2501.18477

Ma J, Han Y, Ge J, Wen L, Ma C, Qi Y, Volmer DA. Comprehensive Two‐Dimensional Gas Chromatography–Mass Spectrometry for the Analysis of Atmospheric Particulate Matter. Rapid Communications in Mass Spectrometry. 2025 Jul 15;39(13):e10034.

The Breathprint of COVID-19

Bad breath in those infected with COVID-19 might be the least of their problems. But studying it helps in understanding the mechanisms of this deadly respiratory disease and developing diagnostic tests. 

Dozens of confirmed cases of halitosis owing to active infection by SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) have been reported in the literature (Patel & Woolley, 2020; Riad et al, 2020)

Possible explanations were decreased salivatory flow due to angiotensin‐converting enzyme 2 receptor-mediated alterations in the tongue, a greater risk of bad breath for mouth breathers who are also more prone to halitosis and increased attention to odor when wearing face masks. Another likely explanation is bacterial co‐infections arising from the novel coronavirus.

DNA analyses of microbial communities in the respiratory tract of those infected with SARS‐CoV‐2 frequently detect abnormally high bacterial reads of Prevotella, Streptococci, Treponema, Veillonella and Fusobacteria, known to emit malodorous volatile sulfur compounds and volatile fatty acids (VFAs). In addition to odor, VFAs could impair T- and B-cell proliferation responses and cytokine production.

What molecules could we expect to find in a person infected with the novel coronavirus? Lamote and colleagues review dozens of (often overlapping) molecules detected in other infections. Among those are aliphatic alcohols, branched hydrocarbons, alkane derivatives, terpenes, dimethyl sulfide and other sulfur and nitrogen-containing compounds. Three aldehydes (octanal, nonanal, and heptanal) drew special attention as candidate biomarkers in pediatric SARS-Cov-2 infection (Berna et al., 2020). These three biomarkers demonstrated 100% sensitivity and 66.6% specificity. Analysis of breath in two groups of adults with median ages 40 and 60 identified aldehydes (ethanal, octanal), ketones (acetone, butanone), and methanol that discriminated COVID-19 from other conditions. Aldehyde Heptanal had significant predictive power for severity of the disease.

It has been shown that properly trained dogs  are able to detect an olfactory signature of SARS-CoV-2 infection with a specificity greater than 90%. Several clinical trials have been initiated to study biomarkers of COVID-19 in breath by e-nose and other technologies. Two studies have been already completed and one paper reported successful detection using Aeronose (Wintjens et al, 2020) with 86% sensitivity and negative predictive value of 92%. Gas Chromatography-Ion Mobility Spectrometry allowed differentiation of patients with definite diagnosis of Covid-19 from non-Covid-19 with about 80% accuracy and 82.4%/75% to 90%/80% sensitivity/specificity. 

REFERENCES

Patel J, Woolley J. Necrotizing periodontal disease: Oral manifestation of COVID‐19. Oral diseases. 2020 Jun 7.

Riad A, Kassem I, Hockova B, Badrah M, Klugar M. Halitosis in COVID-19 patients. Special care in dentistry: official publication of the American Association of Hospital Dentists, the Academy of Dentistry for the Handicapped, and the American Society for Geriatric Dentistry. 2020 Nov.29

Lamote K, Janssens E, Schillebeeckx E, Lapperre TS, De Winter BY, Van Meerbeeck JP. The scent of COVID-19: viral (semi-) volatiles as fast diagnostic biomarkers?. Journal of breath research. 2020 Jun 29.

Berna AZ, Akaho EH, Harris RM, Congdon M, Korn E, Neher S, Farrej MM, Burns J, John AO. Breath biomarkers of pediatric SARS-CoV-2 infection: a pilot study. medRxiv. 2020 Dec. 7

Ruszkiewicz DM, Sanders D, O'Brien R, Hempel F, Reed MJ, Riepe AC, Bailie K, Brodrick E, Darnley K, Ellerkmann R, Mueller O. Diagnosis of COVID-19 by analysis of breath with gas chromatography-ion mobility spectrometry-a feasibility study. EClinicalMedicine. 2020 Oct 24:100609.

Wintjens AG, Hintzen KF, Engelen SM, Lubbers T, Savelkoul PH, Wesseling G, van der Palen JA, Bouvy ND. Applying the electronic nose for pre-operative SARS-CoV-2 screening. Surgical endoscopy. 2020 Dec 2:1-8.

Chemicals in food affecting body odor

Volatile compounds (complex organic and simple like hydrogen sulfide and ammonia), together with sugars and acids, are the main chemicals determining the characteristic aroma of food, as well as odors related to human body.

The bad smells are generally the result of a combination of odorous sulfur compounds and ammonia.

Volatile sulfur compounds are produced through bacterial metabolism of sulfur amino acids such as cysteine and methionine. High sulfur content in food is another source.

Choline  - a quaternary saturated amine - can lead to increases in the amount of trimethylamine responsible for sweet and sickly, fish-like smell.

How to estimate the amount of choline, sulfur and sulfur-containing aminoacids in your food?
You can do it easily with Aurametrix.
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