What's That Musty Odour

Damp odours, musty odours. Sometimes it feels like buildings are just meant to have that odour because it is so commonplace. We sometimes associate basements with “naturally” having a musty odour. Entering a home that has been shut for a while, the assumption is that that odour is on account of the “stale air.”  But what if there is more to it than just being a nuisance odour?

That thing that you are smelling is called mVOCs (Microbial Volatile Organic Compounds). According to the US Environmental Protection Agency (EPA), “Some compounds produced by molds have strong smells and are volatile and quickly released into the air... Because mVOCs often have strong or unpleasant odors, they can be the source of the “moldy odor” or musty smell frequently associated with mold growth.” This musty odour is actually your first signal that mould is actively growing indoors.

Yin et al, in the article, “Diversity and functions of fungal VOCs with special reference to the multiple bioactivities of the mushroom alcohol” state: “In recent years, fungal VOCs have received increasing attention because it has become apparent that they are bioactive and can play the role of chemical molecular signals in many environments [...] Moreover, they may cause health problems in humans when encountered at high concentrations in water-damaged (and subsequently mold-contaminated) schools, workplaces and homes [...]” They go on to state that over 300 VOCs ‘have been detected from the chemical profiles of microscopic and macroscopic fungi. Any given species of fungus may instantaneously release dozens of different volatiles as combination of alcohols, ketones, esters, small alkenes thiols, monoterpenes, cyathene diterpenes, and sesquiterpenes [...]’ (1012).

It is easy to fall under the false premise that despite there being a musty odour, having no visible growth, or having gained results from surface or air samples that do not reveal a mould issue, means that it is okay to continue to occupy the space without concern. But there is what we call “hidden mould” – mould growth occurring behind walls, in plenums and crevices where they are difficult to detect using traditional sampling methods. In the article, “Dominant microbial volatile organic compounds in 23 US homes,” Timothy J. Ryan and Catherine Beaucham highlight this key characteristic of mVOCS – “their ability to diffuse from enclose spaces, from behind vinyl wallpaper or barriers, or off heating, ventilation, air conditioning (HVAC) filters free of visible contamination...” (978).

Just as spores and mycotoxins, exposure to mVOCS do have harmful effects on human health. Saghir et al note the following in the article. “Role of Microbial Volatile Compounds (mVOCS) inntoxicity from molds-infested buildings: a case report”: “Some of the health effects of mVOCs include their impact on sensory organs (irritation of skin, eyes, nose, and throat), respiratory tract (allergies, asthma, bronchitis), central nervous system (headaches, dizziness, brain fog, fatigue, focusing issues, and mood changes); they also interfere with immune function, cellular health, and cause general malaise (nausea, lethargy)” (2).

It is highly recommended that occupants do not wait to actually see mould growth to get an assessment done. Small indicators, particularly the hint of that musty odour should raise concerns that can lead to early detection which could make a world of difference in costs of corrective action. Be proactive!

References:

Ryan, Timothy J. and Beaucham, Catherine. “Dominant microbial volatile organic compounds in 23 US homes.” Chemosphere vol. 90. 977-985, (2013).

Saghir et al. “Role of Microbial Volatile Compounds (mVOCS) inntoxicity from molds-infested buildings: a case report.” Archives of Clinical Toxicology vol. 8(1). 1-5, (2026).

United States Environmental Protection Agency. “What does mold smell like?” EPA, 2025, https://www.epa.gov/mold/what-does-mold-smell

Yin, Guohua et al. “Diversity and functions of fungal VOCs with special reference to the multiple bioactivities of the mushroom alcohol.” Mycology vol. 16,3 1011-1022. 2 Feb. 2025, doi:10.1080/21501203.2025.2453717