What is indoor Mold air  Testing?

aka nonviable spore trap sampling


When testing for airborne mold spores, samples are taken from "complaint" rooms for example  the basement, a bedroom, kitchen in short an area where there is suspect mold or some complaint with mold or heath concerns.  The testing inside typically has a background sample obtain, one or two additional air samples are acquired from outside the building being tested.  

Indoor spore trap Air SampleIndoor complain room sample

For comparison when you perform mold testing you can also acquire a background air sample from outside.  The exterior sample is the comparison sample as mold spores grow abundantly outside.  Airborne mold spores should, at most times, be higher outside than the same mold spores on the inside of the building when tested.  If you have a very high spore outside you would expect to find that spore inside, but at a lower concentration. 

Outdoor spore trap air sample

Outdoor or Background Air Sample


Does everyone who tests for mold know what they are doing?

Unfortunately many people who dabble in the mold world do not know what they are doing or how to interpret the mold testing data they have laboratory analyzed.  This can be confidently stated as our office gets constant phone calls from clients who had mold testing performed and the testing was never explained.   The knowledge of the mold sampler is as important as the laboratory testing data.   As you need to have first hand knowledge of the area tested ti interpret the analysis. 

environmental education training

The people taking on of our mold classes complained that the room had mold. Testing revealed a very low mold count, our mold instructor was just teaching a boring mold class which accounted for the tired look on the attendees faces.  Just kidding, we know so much we actually instruct on the topic.


All mold samples can have bias and it is the responsibility of the mold inspector to take into account the room sampled and how the contents or condition of the room could affect the sample.   In short if you sample a room with say old boxes you could see a higher spore in the sample that is associated with growth on cardboard.  The mold professional has to be aware and explain how mold that grow on an item can influence the sample results.  Typical room items that can have spores that can influence results include: 

  • Fireplaces Fire wood
  • House Plants
  • Old Books
  • Cardboard Boxes
  • Animals (pet dander)
  • Dust and dirt

As a general rule, total indoor airborne spore concentrations in a typical clean HVAC supplied dwelling are less than the “average” outside concentrations, hence the reason for the comparison of the outside air sample.  The airborne spores should also be less than approximately 1,500 cts/m3. Aspergillus /Penicillium and other hyaline spores are on average less than 700 cts/m3. Indicator fungi such as Stachybotrys, Chaetomium, 

Remember, there is always a likely exception to every rule or generalization, and because there is no direct relationship between the collected indoor and outdoor samples, performing a direct comparison with limited sampling is often and can be misleading. The range of expected variability when comparing limited data sets must also be considered.

Penicillium /Aspergillus – the most common mold species to appear in indoor air samples. The majority of the hundreds of sub-species are allergenic; only a few are toxic. This group of species only grows with the humidity in the air as its water source.

Cladosporium – the most common mold species and is considered to be an allergenic.

Curvularia – another common allergenic mold

Chaetomium – a common water marker that usually indicates wet paper and/or drywall.

Stachybotrys – the most common toxic mold species, but not all sub-species are toxic. These species need a direct water source to grow.

Memnoniella – a sister mold to Stachybotrys. The two species will grow together; also considered toxic.Mold spore species and levels differ within each state (if there are regulations determined at all) agreements are hard to come by with analysts and scientists. A comparison to an outdoor air sample is usually used as the rule of thumb. The following mold spore ranges use the spore/m3 number and not the raw count for each species when interpreted in a lab’s

Air Sample Report”.0-50 spores – these trace levels are not an issue. Even Stachybotrys is not considered an issue if the sample does not also contain water markers like Chaetomium and Fusarium or high levels of Penicillium/Aspergillus.

50-200 spores – still very low levels; the toxic mold species Stachybotrys and Memnoniella are the only species to be considered an issue at this level.

200-500 spores – the most common species (Penicillium/Aspergillus, Cladosporium and Curvularia) are not an issue and stay within the normal range.

500-1500 spores – sometimes the Penicillium/Aspergillus & Cladosporium levels are in this range and do not require remediation. If water intrusion or mold was not found during the, these levels can be caused by normal life in an enclosed environment.

1500-3000 spores – this point indicates that an issue may be apparent, unless a corresponding number in the outdoor sample exists. If water intrusion or mold issue wasn’t found, these levels can be achieved by a dusty home or A/C system. An inspection is warranted. 

3000-10,000 spores – without a corresponding number in the outdoor sample, some remediation is necessary. A perimeter clean-up is needed if a mold spore source has been identified. If water intrusion or mold issue wasn’t found, the home may need to be cleaned and the duct system should be evaluated.

10,000-25,000 spores – without a corresponding number in the outdoor sample, a mold spore source is usually identified and remediation is needed. If no water intrusion or mold issue was found, the duct system may need to be cleaned and a general cleaning of the residence.

25,000-75,000+ spores – a mold issue will be easy to identify. Clean up will be required and should be performed by a Professional Mold Remediator.

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Conditions under which indoor mold growth can occur include: Historical flooding without proper cleanup

  • Moisture intrusion occurring through sub-flooring, walls, windows, or roofs
  • Plumbing, water line leak, toilet overflows or sewer backups
  • Moisture condensation within HVAC systems - Persistent elevated relative humidity above 70%, and inadequate housekeeping

Mold and fungi require three basic criteria to colonize the inside of a building including

  1. A source of moisture
  2. A food source
  3. Lack of surface disturbance and/or air movement


Moisture sources in buildings occur most commonly through moisture intrusion in walls and foundations, or as condensation around windows or in HVAC systems. In some parts of country such as the Southeast U.S. for example, the relative humidity during certain times of the year is high enough to act as a significant moisture source alone. Indoor food sources for mold can be any organic material provided by a flood or sewer backup or materials present in the dwelling such as carpet backing, linoleum backing, drywall paper, ceiling panels, or the buildup of plant and/or skin cell fragments or debris on inorganic surfaces. Skin cell fragments are a significant food and colonizing source where a high occupancy exists, or adequate housekeeping is not performed. Molds colonize most readily where air disturbance is minimal. For this reason, mold colonization occurs most frequently in closed or concealed spaces such as closets, storerooms, basements, refrigeration units; or on the back or underside surface of furniture.


Experienced mold professionals