What causes the musty mould smell? MVOCs explained
That musty, earthy mould smell comes from MVOCs, microbial volatile organic compounds. These are gases like geosmin, 1-octen-3-ol (mushroom alcohol) and 2-MIB that mould gives off as it feeds and grows. Because they are byproducts of active metabolism, a lingering musty smell usually points to live, growing mould and hidden moisture, not just old dormant spores.
We had a sealed-up Queensland house through a wet season, and the thing I remember most is the smell. You would wipe a spot of mould off the wall, the surface would look clean, and a day or two later that same damp, earthy note was back in the same corner. Same corner, same smell. It took me a while to understand why. The smell is not the stain. It is a gas the mould is still making, and cleaning the surface does nothing about the moisture feeding it underneath.
This page is about the cause of that smell, the actual chemistry, in plain English. If you want the practical side, I have linked a companion piece on what to do further down.
What are MVOCs (microbial volatile organic compounds)?
MVOCs are low-weight gases produced during the normal metabolism of fungi and bacteria. They are, in the words of one study, "released during growth of fungi, mold and other microbes" (a peer-reviewed study of mould MVOCs). Mould is a living thing, and like anything alive it gives off waste gases as it eats. Those gases are what your nose reads as mould.
The musty smell is not one chemical. It is a blend. Across all microbial species, researchers have catalogued more than a thousand different MVOCs, though any one damp room usually gives off no more than a few dozen. A small handful of those compounds do most of the smelling. Three matter most:
- Geosmin, the damp-earth-after-rain smell.
- 1-octen-3-ol, often called mushroom alcohol, the earthy-mushroom note.
- 2-MIB (2-methylisoborneol), a second strong earthy-musty odorant.
What is geosmin, and why does it smell like damp earth?
Geosmin is the compound behind the smell of wet soil after rain, and it is the same molecule that gives active mould its deep, damp-basement note. It is made by a range of microbes, "including most Streptomyces and several species of cyanobacteria, myxobacteria, and fungi" (the Nature Chemical Biology paper on geosmin). Chemically it is a bicyclic terpene alcohol, built by the microbe from the same terpene pathway that makes many plant scents, folded into shape by a single enzyme. When you smell that heavy, damp note in a wardrobe or a bathroom, geosmin is usually doing the heavy lifting.
What is 1-octen-3-ol (mushroom alcohol)?
If geosmin is the damp-earth note, 1-octen-3-ol is the mushroom note. It is one of the most common MVOCs found in damp houses with fungal growth, and it smells earthy and mushroomy (the same indoor-MVOC research). What makes it useful as a warning sign is where it comes from. Fungi build 1-octen-3-ol by breaking down linoleic acid, a fat in the material they are growing on. The fat is oxygenated and then split to release the compound (research on how fungi make 1-octen-3-ol). That detail matters, because it ties the smell directly to a mould that is actively feeding. No feeding, no fresh supply.
What is 2-MIB (2-methylisoborneol)?
2-MIB, short for 2-methylisoborneol, is the third of the classic earthy-musty odorants. It is produced by the same broad group of microbes as geosmin, and like geosmin it carries a strong earthy, mouldy smell even in trace amounts (a study of five earthy-musty odorants). Water utilities know 2-MIB and geosmin well, because both are noticeable in drinking water at concentrations far too low to detect any other way. In a home they add to the same musty signature.
Why are humans so sensitive to the mould smell?
Our noses are extraordinarily good at detecting geosmin. The odour threshold sits at roughly 10 nanograms per litre or less in water (the earthy-musty odorant analysis), and other work puts it as low as 4 to 10 nanograms per litre (the Leibniz Institute report on the human geosmin receptor). That is among the lowest odour thresholds known, which is the whole reason a small patch of active mould is smellable long before it is visible. In 2024 researchers went a step further and identified the specific human smell receptor for geosmin, called OR11A1, a receptor tuned narrowly to this one earthy compound. As of 2026 the specific nose receptor for it has been identified. We are, in a real sense, built to notice it.
What does active mould smell like versus old, dormant mould?
Because MVOCs are made while the mould is alive and feeding, the strength of the smell tracks how active the growth is. A strong, persistent musty smell usually means a live colony with a moisture supply. Faint or absent smell can mean the growth has dried out and gone quiet, or that what is left is settled spores waiting for water. Here is the practical difference:
| Signal | Active, growing mould | Old or dormant spores |
|---|---|---|
| MVOCs being produced | Yes, continuously, as the mould feeds | Little to none once growth stops |
| Typical smell | Strong, persistent, musty and earthy | Faint or absent |
| Moisture | Usually present, the growth needs water | Not necessarily present |
| What it tells you | A live colony and a moisture source, often hidden | Settled spores waiting for water |
This is also why a smell that keeps coming back after you clean is worth taking seriously. The stain wipes off. The gas returns because the colony, and the damp feeding it, are still there. If that is where you have landed, our guide on what to do about a musty mould smell covers the practical steps.
Why can I smell mould when I cannot see it?
MVOCs are gases, so they travel. They diffuse out through wall cavities, under floors and behind cabinetry while the mould itself stays out of sight. The US EPA puts it plainly: "You may suspect hidden mold if a building smells moldy, but you cannot see the source" (the US EPA guide to mould, moisture and your home). A musty smell with no visible mould is a common sign that growth is happening somewhere hidden, usually wherever the moisture is quietly collecting.
Does a musty smell always mean there is mould?
Not always, and it is worth being honest about that. Some of the same compounds, including 1-octen-3-ol and various alcohols and ketones, are not made only by microbes. They can also come off building materials, cooking, plants and furnishings. So a musty smell is a strong practical indicator of active mould and hidden damp, but it is not proof on its own. The sensible move is to treat the smell as a prompt to go looking, find the moisture source and confirm the growth, rather than assume.
One more nuance is worth knowing. Some researchers have proposed an indoor 1-octen-3-ol level of around 10 micrograms per cubic metre as a rough indicator of microbial contamination (the MVOC identification study). That is a research marker used in studies, not a legal exposure limit, so treat any single number quoted online with that in mind.
The smell is a moisture problem wearing a disguise
Strip it all back and the smell points at one thing: water. Mould "needs moisture and nutrients to grow," as the NSW Health mould factsheet states, and the moisture is the part you can actually control. The food source, dust, timber, plaster and fabric, is everywhere in a normal home and cannot be removed. The water can. That is why wiping a surface clean does nothing lasting if the corner stays damp, and why the smell returns in the exact same spot. For the full picture of how growth gets established and why moisture is the lever, I have written up how mould works and why moisture is the root cause.
Frequently asked questions
What is the musty smell in my house?
It is a mix of MVOCs, microbial volatile organic compounds, given off by mould and related microbes as they grow. The main earthy-musty ones are geosmin, 1-octen-3-ol (mushroom alcohol) and 2-MIB. Your nose is picking up gases, not the visible stain.
Does a musty smell mean the mould is still growing?
Usually. MVOCs are released while mould is actively feeding, so a persistent musty smell generally points to a live colony and a moisture source rather than just old spores. It is not a guarantee, because the same compounds can occasionally come from other sources, so it is worth confirming the moisture.
Can you smell mould behind walls?
Yes. MVOC gases diffuse through cavities and floors, which is why a room can smell mouldy with nothing visible. The EPA notes you may suspect hidden mould when a building smells mouldy but you cannot see the source.
What is geosmin?
Geosmin is the earthy compound behind the smell of wet soil and damp basements. It is produced by soil microbes such as Streptomyces and by some fungi, and humans can detect it at extraordinarily low concentrations.
Related guides
- Winter condensation and mould on walls in Australia
- How to stop mould in a humid Queensland home
- What actually stops that musty mould smell
By Ben, founder of Aurala Naturals. Last updated July 2026.
