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Mold Smell -  Everything you need to know:

1.)  The human nose is incredibly sensitive to mold smells.  

Some organic compounds, such as Geosmin, are detectable at levels as low as 5 parts per trillion.  For comparison, a cutting edge VOC meter won't register anything beyond parts per billion.  The high sensitivity of the human nose has served us well, for example, alerting us to poor drinking water.  Some musty odors are considered pleasant to most people, including the odor stemming from freshly turned soil, which comes from Streptomyces.

moldy smell from moldy wall

Perhaps worst of all, Geosmin is a common problem among winemakers and can wreak havoc among young wines, such as Beaujolais.  As can be imagined, the wines take on an aroma described as "wet earth".  Not good.

2.)  Moldy smells are a poor indicator of the actual presence of mold spores.

A strong musty odor in a building ( a basement for example), is a helpful clue but is not an absolute indication of mold growth.  This can cause no shortage of frustration when attempting to track down the culprit.  I've collected many samples in buildings with strong musty odors, only to find perfectly normal levels of mold spores. 

The alternative is often true as well.  A building can have no detectable odors, and yet the air sampling will indicate a highly elevated number of mold spores.   

mold smell from bacteria

Why does this occur?  The mold smell and the physical mold growth are two entirely distinct items.  Mold can growth and propagate without a significant production of odor causing compounds.  This can lead to false negatives, i.e. I don't smell any musty odor, so there must not be a mold problem.  

Complicating the matter even more, false positives are possible as well.  Because other non-fungal growth, such as bacteria, can cause odors as well, an absolute correlation between a mold smell and mold is not possible.  

3.)  Musty odors & health problems - connection?

The latest research has failed to show a correlation between an MVOC and an allergic response (Hokkaido University Graduate School of Medicine).  However, the research surrounding this issue is fairly recent and further studies may find a connection.  A minor correlation was noted between MVOCs and sinus infections. 

The question of health problems caused by musty odors is difficult to answer.  For those with mold allergies, a mold smell can quickly bring to mind previous negative health reactions.  This alone can be a powerful stimulus and cause a psychosomatic response.  Until further research is performed, musty odors that occur in the absence of excessive mold spore counts should not be interpreted as a health concern.  However, like many other nuisances, this is not to suggest ignoring the problem.  Learn more about proper mold remediation techniques. 

4.)  Must odors can come from a variety of sources other than mold growth:

  • Plumbing.  Bacterial growth in plumbing can cause musty odors that are nearly indistinguishable from odors due to mold growth.
  • Laundry:  Alright folks, this should be obvious.  Damp, unwashed laundry can create musty odors with actual mold growth.  However, certain front load washers can foster mold growth if water becomes trapped in the door area. 
  • Crawl space.  Damp crawl spaces are a very common sources of mold musty odor.  Often the solution is as simple as installing a vapor barrier.  If the moisture content within the soil is significant, consider addressing the drainage or installing a sump pump. 

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