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What causes musty air in your home?

The causes of musty air at home and a five-step solution to reduce it
Updated on January 10, 2024
Written by
Jeanne Yacoubou
Jeanne is our newest air quality aficionado/writer. Still shocked by the arrival of Summer 2021 wildfire smoke from California to her Maryland (East Coast) home, she’s forever indebted to her HEPA filter-equipped air purifier.

Have you ever hiked into a forest on a rainy day, boots sinking deep into squishy leaf litter and dark, rich soil, noticing the peculiar woodsy and earthy smells? These are musty smells.

Musty scents are normal in a forest. Musty odors in your basement, however, are most certainly not normal.

In fact, indoor musty air signals the ongoing destruction of your home’s physical structure by mold and mildew. It is also harmful to your health because those pungent, earthy odors in your home are hallmark signs of fungus growth.

Mold and mildew thrive in warm, wet, and dark places like basements and crawl spaces that have been damaged by water leaks or flooding. Metabolic products of mold and mildew, called microbial volatile organic compounds (MVOCs), create musty air. High concentrations of some MVOCs are harmful to your health in enclosed spaces.

In this article, you find out everything you need to know about musty air in your home, including its causes and our 5-step solution to help you eliminate and prevent musty air in your home.

Musty air is distinctly pungent, similar to the smell of damp soil or a sponge that’s been wet for days. It’s unpleasant to breathe and may have a sharp tinge to it.

The United States Federal Emergency Management Agency (FEMA) suggests that musty air odors range from earthy to foul stench.

The causes of musty air at home

The chemicals giving off those musty smells in your home are generated from mold and mildew (types of fungus) that have taken up residence in your home — especially in warm, dark, and moist corners in basements, crawlspaces, attics, or bathrooms.

Specks, powders, or fuzzy mounds of mold and mildew in a kaleidoscope of colors are common on paper, old books, wood paneling, fabric, or insulation close to a water leak or where it’s very damp.

Mold and mildew flourish where you may least expect it to. Maybe on an apple core kicked under the sofa, dead skin cells mixed with dust mite cadavers nestled in your carpet, or an old pair of leather shoes in the attic close to the leak in your roof before you discovered it. In all these cases, the mold originally came from outside.

Normally, in your home, this is not a problem as long as there’s occasional ventilation and regular cleaning. And no water leaks or intrusion. Then, you manage to keep airborne microscopic mold spores (seeds) at low levels in your home. You don’t even know they’re coming in when you open a window for fresh air unless you are extremely allergic to mold.

For most people, it’s only when you actually smell musty air, or visually see mold growing in your home, that you’re in trouble.

Musty air is full of microscopic mold spores and hyphae (branches). Wherever the smell is most pronounced, you don’t have far to look to find mold colonies (mycelia) flourishing — like in a damp, dark corner of your home.

Breathing musty air is not merely the harmless annoyance it was thought to be just 40 years ago. More recent evidence proves that breathing moldy air is harmful to your health. Individuals inhaling moldy air may suffer allergic symptoms, including coughing, sneezing, itchy eyes, or runny nose.

More seriously, some of the negative health effects of inhaling musty air in people susceptible to mold allergies include:

  • Asthma
  • Sinusitis
  • Bronchopulmonary aspergillosis
  • Hypersensitivity pneumonitis
  • Skin or mucous membrane infections

What contaminants are present in musty air?

Musty air contains contaminants from mold and mildew. Both are types of fungus. Mold and mildew can be any color, although white, black, and green are most common. Their texture ranges from powdery to fuzzy or slimy.

Pungent odors in musty air come from up to 300 fungal volatile organic compounds (FVOCs) characterized by researchers so far, yet most of them are not even unique to fungi.

They’re known for their low odor thresholds. This means your nose will detect mustiness emanating from even a small amount of FVOCs.

Here are some examples of FVOCs commonly found in water-damaged houses and their characteristic odors at low concentrations.

ChemicalOdorConcentration (ppm)
3-methy-1-butanolSour, sharp or malty0.010–35
GeosminMusty, earthy0.0009 
Dimethyl disulfideRotting eggs, meat0.00003

The mold genera most commonly associated with musty smells include:

  • Aspergillus 
  • Penicillium 
  • Alternaria 
  • Cladosporium
  • Mucor
  • Ulocladium

There are volatile primary and secondary metabolites (breakdown products) produced by mold and mildew that contribute to mustiness. 

Primary metabolites from mold

Primary metabolites result from the normal growth processes of all living organisms, including fungi. They include water, carbon dioxide, and several volatile organic chemicals (VOCs).

Microbial VOCs (MVOCs), called primary metabolites, could be from several different chemical families, including aldehydes, alcohols, and ketones. 

Some of these compounds have distinctive smells – like fruity, grainy, malty, or mushroomy – that contribute to the musty air in your home.

Secondary metabolites from mold

Things get more interesting in the world of mold when you consider secondary metabolites (SM) and their unique odors. These breakdown products are not created during daily growth activities (like eating).

Rather, fungi have developed an entire arsenal of chemicals over their long evolution to protect themselves from destruction by competitor microbea — and even to get ahead in a mold-friendly environment like your damp basement.

Amazingly, many SMs are unique to a particular fungal species and specific to a precise phase in their life cycle.

Even more astounding is the fact that when living under environmental stress (nutrient scarcity, presence of competitors), the FVOC profile of a certain mold species differs from that under normal growth conditions.

Furthermore, SM profiles vary in fungi of the same species growing on different growth media — including paper, wood, or fabric.

Generally, SMs include members of the following carbon-based chemical classes: alcohols, aldehydes, carboxylic acids, esters, ethers, ketones, terpenes, and thiols (sulfur-containing compounds). 

Fungi aren’t known for producing SM in the lab where they can be studied. So, pinpointing the chemicals responsible for your basement’s mustiness requires sophisticated testing.

Scientists discovered that a type of alcohol, 1-octen-3-ol, was the most common FVOC in damp houses. It carries a distinctive “earthy, mushroomy” odor.

Other scientists focused on legacy wood preservatives noted that chloroanisoles and the chlorophenols they’ve derived from cause musty, moldy odors in moist places. But since there may be hundreds of FVOCs produced from multiple mold species in your indoor air, your home’s moldy indoor air is likely a one-of-a-kind, complex mesh of nuanced odors collectively referred to as musty.

Compound NameOdor
1-octen-3-olEarthy, “mushroomy”
HexanalFat, tallow, grass
BenzophenoneAlmond, burnt sugar
DecanalSoap, tallow, orange peel
Dimethyl disulphideCabbage, onion, putrid

Five steps to get rid of musty air

Here’s an easy, five-step solution to treat and prevent musty air in your home.

Step 1: Remove all moldy, mildewy materials in the damaged area. Make sure to follow FEMA guidelines and disinfect with Sporicidin (including HVAC parts). You may wish to hire a mold remediation specialist for this work, especially if your problem is extensive.

Step 2: Prevent any more water from entering the area. Fix water leaks, repair the roof, do basement waterproofing, grade soil away from your home, etc., to prevent any more water from entering and causing mold problems. Next, be sure to use mold-resistant paint, as this can help avoid mold developing in the future.

Step 3: Ventilate the area thoroughly. Open windows and doors on low-humidity days. Use fans and exhaust fans to circulate the air. Be aware that fans spread mold spores so make sure steps #1 and #2 are thorough and complete.

Step 4: Invest in a good dehumidifier. Install a dehumidifier (or two, depending on area size) and keep indoor relative humidity to 40% max. 

Step 5: Consider using an air purifier for mold spores. Look for an air purifier with a high-grade HEPA filter to trap airborne mold spores, and with activated carbon, zeolites, or mineral oxides to remove musty odors. 

Common questions about musty air indoors

Common in moldy basements or water-damaged homes, 1-octen-3-ol is one of the major fungal volatile organic compounds (FVOCs). All of its toxicity testing has occurred in its liquid form. So, the United States Food and Drug Administration approved 1-octen-3-ol in foods and perfumes. 

Likewise, the U.S. Environmental Protection Agency approved it in insect lures, saying “When released into air, octenol is not harmful to humans, to other non-target organisms, or to the environment. There is the potential for toxicity if ingested.”

However, much research on this chemical in gaseous form, including a study on human subjects, revealed adverse neurological effects at low concentrations (3 ppm). 

Some researchers warn that in a non-ventilated, damp basement exhibiting mold growth, octenol could build up and negatively affect people exposed to it. 

Some fungal secondary metabolites (SM), known as mycotoxins, are harmful to humans. Some of the most common ones are not volatile and include aflatoxins or versicolorins.

To date, there is not enough evidence to say whether volatile organic compounds (VOCs) from mold are mycotoxins, even though exposure to many of them results in health problems.

Until definitive evidence emerges, some researchers coined the term volatoxins to describe FVOCs.

Humidity encourages mold and mildew growth. With enhanced growth, you’ll have greater levels of fungal volatile organic chemicals (FVOCs) in your home making the indoor air even mustier.

A portable hygrometer will tell you what the relative humidity is in your home: 

  • If it’s above 40%, a dehumidifier will bring it down to a value that’s not as hospitable to mold growth. A dehumidifier will not clean the air, but it will remove the moist conditions in which mold thrives.
  • If you’re in a humid climate or prone to water intrusion, maintaining indoor humidity below 40% will help keep mold under control.

According to the U.S. Environmental Protection Agency, properly designed, hospital-grade UV-C lamps may eliminate some mold growing on HVAC surfaces such as coils or ducts. However, UV-C in portable air purifiers intended for indoor home use do not protect against mold. 

Wrapping up

Mold and mildew do important ecological work, returning nutrients from dead and decaying animals and plants to the soil, breaking down corpses and providing rich soil for growing food.

However, mold and mildew inside your home are unwelcome guests that may cause structural damage to your home or lead to adverse health impacts.

MVOCs generated by mold and mildew are responsible for musty air that could range from earthy to a foul stench depending on the mix of mold and mildew species present as well as on the amount of infestation. 

Eliminating and preventing indoor musty air caused by mold and mildew is possible with the 5-step solution outlined in this article.

A dehumidifier and air purifier are two important tools for successful and long-lasting control of mold and mildew in your home’s indoor air.

About the author

Jeanne Yacoubou

Jeanne is our newest air quality aficionado/writer. Still shocked by the arrival of Summer 2021 wildfire smoke from California to her Maryland (East Coast) home, she’s forever indebted to her HEPA filter-equipped air purifier.