More on the Interpretation of Mold Air Samples

Are you staring at the results of a mold test and struggling to figure out what it all means?  Let me try to help you.


The most common method used for mold air testing is uses a spore trap cassette. Because there is always a background amount of airborne mold spores, a key step in collecting air samples is to have a “reference” sample collected from the outdoor air at the time of the indoor testing. Since there are no guidelines on acceptable mold levels in the air, an outdoor sample is the best tool for interpreting the results of the indoor samples. Without the outdoor sample, you are limited in what you can gather from the results of the indoor samples.


There are various D.I.Y. air tests you can pick up at your local hardware store requiring you to place out a petri dish for several hours. Most of those tests are not very helpful in giving you any sort of usable data to evaluate the air in your home and as a general suggestion should be avoided.


Interpreting mold levels can be tricky. If you would like feedback on your mold air sampling results, please comment below and I will do my best to give any feedback I can in my free time. The views expressed in the blog post and comments are my own, and not necessarily those of Indoor Science. If you need a more immediate response regarding your results you can book a 30-minute phone consultation for $98 by clicking here.

Dylan McIntosh

Dylan McIntosh

Dylan McIntosh is a Senior Project Manager that performs indoor air quality assessments, industrial hygiene testing, and laboratory mold analysis. Mr. McIntosh holds a Bachelors of Science degree in Biology from the University of Illinois - Springfield. Dylan is an ACAC Council-Certified Microbial Investigator (CMI) and an Pan American Aerobiology Certification Board (PAACB) Certified Spore Analyst. In his words… “Throughout my life, I always had a dream of becoming an astronaut. That dream hasn’t worked out (yet) so I started a career in the next best thing, indoor air quality! In my free time I enjoy outdoor activities with my dog, cooking, and being involved with A Special Wish - Chicago; a local charity.”

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374 thoughts on “More on the Interpretation of Mold Air Samples

    Good morning Dylan, I would greatly appreciate your input on these samples and whether I should be concerned and/or take any corrective action. Thank you. Tom Greto (I hope this email formatted the table as I laid it out; one can never be sure with these feedback forms.)

    Room/Area Description Sample Number Results
    Spore Density or Concentration Predominant Genus

    Air Samples

    Exterior -Air 1-2869 3648 2348 spores/m3 Basidiospores (1867)

    Mechanical room-Air 2-2869 3660 627 spores/ m3 Chaetomium (587)
    Stachybotrys (13)

    Basement-Air 3-2869 3656 220 spores/m3 Aspergillus/Penicillium (120)
    Chaetomium (33)
    Stachybotrys (13)


    Both the mechanical room and the basement have water damage indicating mold (both Stachybotrys and Chaetomium). I would look into what possible moisture issues are affecting the space and what needs to be done to address it as well as mold remediation.

    Hi…I work in a school and had air quality testing done in my office — which is very small space – maybe 10×10 with the following results reported:
    Ascospores: 1/40/4.6
    Aspergillus/Penicillium: 1/10/1.1
    Basidiospores: 1/40/4.6
    Chaetomium: 16/680/78.2
    Cladosporium: 3/100/11.5
    Total Fungi: 22/870/100
    Hyphal Fragment: 8/300
    Analyt. Sensitivity 600x: 42
    Analyt. Sensitivity 300x: 13

    Should I be concerned about the high levels of Chaetomium? I also had my own air purifier running in there before the same was collected – do you think that could have skewed the results? Thank you SO much for your time.

    I would be concerned about the Chaetomium levels, that is indicative of a moisture and mold issue. Running an air purifier could have an effect on the results, the results could be skewed low depending on how recent to the testing it was running.

    So thankful for the wealth of information on your blog. I myself am in the midst of selling a home and have recently had a test performed. I had a burst pipe in the room in question over 18 months ago but it was repaired and I THOUGHT dried appropriately. I have had 3 companies out to give estimates based on my test results and they have all said extremely differently things including, “let’s rip it all out and see what we are dealing with” to “well, your issue may be a foundation issue and even though you’re on a slab, your mold might be underneath your hardwoods – we can’t know until we rip it all up” and “you don’t need remediation.” I’m so confused. Results below:

    Acospores / 32 …1280 … 25
    Basidiospores / 55 …2200…43
    Cladosporium / 34…1360…26
    Epicoccum / 1…40…<1
    Cercospora / 2…80…2
    Rusts / 1…40…<1
    Smuts/Periconia/Myxomy / 2…80…2
    Zygomycetes / 2…80…2
    background debris: 3
    sample volume: 25
    total spores: 129/5160

    Acospores / 3…120…43
    Basidiospores / 2…80…29
    Cladosporium / 1…40…14
    ***STACHYBOTRYS / 1…40…14
    background debris: 3
    sample volume: 25
    total spores: 7/280

    I know the Stachybotrys is only present indoors so I have an issue somewhere but there is no visual mold is present based on mold inspector's report. An "elevated" moisture reading was noted on a vanity cabinet in the room but I am wondering if it's possible to tell from this test if I'm dealing with a relatively minor remediation or if there's a bigger issue at hand. I know you don't have a crystal ball but these tests are so hard to interpret. Thanks in advance for your time.

    When stachy results are this low, it is always difficult to interpret. I would not jump into any major remediation issues with only these results. I would investigate the moisture issue that was found, and maybe have some exploratory holes cut to investigate if there are mold issues inside the walls.


    Some labs differentiate spores which are found in clumps or chains, others do not. There is some logic behind the idea that spores which are clumped or chained together are closer to the source of growth than isolated spores, however, there is no actual research into this that I am aware of. I would pay much more mind to the levels of the molds, not if they are clumped or not.

    Hello, I am Real Estate agent and my client had mold air quality testing done. The following are the results of concern (higher inside than outside) Thank you very much for your time.
    Chaetomium INSIDE Raw =2, Count/m = 80, % of total = 15.1 OUTSIDE nothing
    Curvularia INSIDE Raw = 1*, Count/m = 10*, % of total = 1.9 OUTSIDE Raw = 1, Count /m = 40, %of total = 0.3
    Epicoccum INSIDE Raw =1, Count/m = 40, % of total = 7.5 OUTSIDE nothing
    Myxomycetes++ INSIDE Raw = 1, Count/m = 40, % of total = 7.5 OUTSIDE nothing
    Pithomyces++ INSIDE Raw = 2 Count/m = 80, % of total = 15.1 OUTSIDE Raw = 5, Count/m = 200, % of total =1.7
    Total Fungi INSIDE Raw = 13 Count/m = 530 %of total = 100 OYTSIDE Raw = 285, Count/m = 11610, %of total = 100

    Analyt. Sensitivity 600x INSIDE Count/ m = 41 OUTSIDE Count/m = 41
    Analyt. Sensitivity 300x INSIDE Count/ m = 13* OUTSIDE Count/m = 13*
    Skin Fragments (1-4) INSIDE Count/m = 2 OUTSIDE Count/m = 1
    Fibrous Particulate (1-4) INSIDE Count/m = 1 OUTSIDE Count/m = 1
    Background (1-5) INSIDE Count/m = 1 OUTSIDE Count/m = 1

    The only thing of note on the indoor sample is the presence of Chaetomium. The level is fairly low, however, this is a water damage indicator mold, meaning it is mainly only present in properties that have experienced some water damage.

    Thank you in advance for your time. We recently noticed wet carpet along one wall of our finished basement after very heavy rains. Pulling up the carpet and padding we noticed damage to the carpet tacks, indicating that this was likely not the first time there had been some water intrusion. My husband cut away the dry wall to look for the source and found a crack that was clearly the route for water entry, which is being repaired. There was also mold growth between the back of the drywall and the plastic sheeting separating the drywall from the insulation. He continued to cut away drywall and insulation until he had a clean margin. We had a mold inspector come just to be safe, and while he found no levels of moisture anywhere in the home that would support mold growth, no smell and didn’t find any visual evidence of remaining mold growth (he even reached up behind where we had cut the drywall away and looked/felt around and found nothing), he is recommending a full remediation of the basement because there was one spore of chaetonium and three hyphal fragments on the air sample. He said it could just be residual from when we cut the drywall to find the leak (and cut out everything moldy) but it’s an automatic test failure if chaetonium is present at all. He also took the control sample from our front sidewalk away from the house, but the basement has a walkout that backs up to a ravine with lots of vegetation and seems to be a better ‘control’? We want to follow the correct procedure but not overkill, appreciate your thoughts.

    Outside – Sample vol 25L
    Basidiospores 63 raw /2520 per m3 / 83%
    Cladosporium 1 / 40 / 1
    Curvularia 2 / 80 / 3
    Smuts/Periconia/Myxomy 2 / 80 / 3
    Zygomycetes 7 / 280 / 9
    Pithomyces 1 / 40 / 1
    Background debris 3
    Total spores – 76 / 3040

    Basement – Sample vol 25L
    Hyphal Fragments 3 raw / 120 per m3
    Ascospores 8 / 320 / 21
    Basidiospores 13 / 520 / 33
    Chaetomium 1 / 40 / 3
    Cladisporium 11 / 440 / 28
    Epicoccum 1 / 40 / 3
    Pennicillium/Aspergillus 4 / 160 / 10
    Pithomyces 1 / 40 / 3
    Background debris 3
    Total spores – 39 / 1560

    Different inspectors have different thresholds for what would be a “failure”. It is difficult to say if a single spore is evidence of additional areas of mold growth or if it is left over from your husband cutting out the moldy drywall. I would suggest retesting before doing any further remediation.

    The outdoor sample would not be much different in the back of the home or the front. Due to air movement outdoors, the mold in the air is influenced from all over, not just the immediate area. Also, Chaetomium would not typically show up on any outdoor sample, which is why the inspector noted that a single spore result stands out.

    Hello we just got mold testing done in our basement due to a water leak from a window. The company first recommended an air test and the results came back as:
    Aspergillus/penicillium: 13173 count/m3
    Chaetomium sp: 4800 count/m3 (mold company concerned about number)
    Cladosporium sp: 2080 count/m3
    Stachybotrys sp: 373 count/m3 (mold company concerned about number)
    Basidiospores 107 count/m3

    All other levels were under 100/m3

    I feel like they mold company is trying to get the most money out of us as possible. They didn’t do a surface test even though we knew where the water was coming in. They want us to spend a lot more money now to do some remediation and then another $400 airs test. I was just searching for help to try to understand this all. Thank you!

    I would echo their concerns about the Aspergillus, Chaetomium, and Stachybotrys levels. Those are all very high results.

    I am in the inspection period of a home purchase. I had a regular inspection done but I also did my own ERMI. Concerned now that I have results back……
    Aspergillus ochraceus = 215
    Penicillium corylophilum = 68
    which puts the house at a ERMI 10.4 = Q4 red. ack!

    Aspergillus penicillioides = 99 (4 points)
    Wallemia sebi (0 points)
    so this comes out to a 4 which is considered “fine”

    Why does the ERMI says Q4 (almost at the top and serious) but the HERTMSI says basically okay?

    Help! A confused potential homeowner. Thank you!

    This blog is to help interpret the results of air sampling for mold, not DNA based tests like ERMI. HERTSMI is only looking at 5 species of mold, where ERMI is looking into 36 species of mold.

    Hi. Thank you for the clarification. I am new to this! So, in your opinion, should I have done an air test instead of the ERMI?

    Hello. I had mold testing done in our rental house. Air analysis detected penicillium/Aspergillus with a raw count of 3,764 and 50,061 for cubic sq ft. Also detected was Cladosporium growth of 670,598 sq. Is this a dangerous toxic level for humans or pets. I have a compromised immune system due to cancer and haven’t felt good lately but noticed I feel better when not in house. Help me understand the levels as I feel my landlord was aware prior to me moving in 6 months ago and is acting like it’s ok and not to b concerned.


    These numbers are very high. I am not a medical professional, so I can not assess if any amount is “safe” or “harmful”, but I can say that both Asp/Pen and Chaetomium levels are highly elevated, and an indication of moisture and mold growth in the property.

    Amazing you provide such valuable feedback! Here’s my house results after water intrusion from toilet overflow. Thank you, thank you in advance. Medium Type: Air-O-Cell; Exposure: 15.00 l/min. for 5.00 min; Reporting limit: 53 spores/cu. m

    Outside: Raw Count: 29 Spores/cu. m: 1,550
    Bed 1: Raw Count: 1 Spores/cu. m: 53
    Bed 2: Raw Count: 1 Spores/cu. m: 53
    Outside: Raw Count: 6 Spores/cu. m: 320
    Bed 1: N/A
    Bed 2: Raw Count: 1 Spores/cu. m: 53
    Outside: Raw Count: 4 Spores/cu. m: 213
    Bed 1: Raw Count: 2 Spores/cu. m: 107
    Bed 2: N/A
    Hyphal Fragment
    Outside: N/A
    Bed 1: Raw Count: 1 Spores/cu. m: 53
    Bed 2: Raw Count: 1 Spores/cu. m: 53
    Outside: N/a
    Bed 1: N/A
    Bed 2: Raw Count: 2 Spores/cu. m: 107
    Outside: Raw Count: 1 Spores/cu. m: 53
    Bed 1: N/A
    Bed 2: Raw Count: 2 Spores/cu. m: 107

    The Chaetomium found in Bed 2 is a yellow flag to me. Chaetomium is an indicator of water damage and is a toxigenic fungi.

    Thank you. A mold inspection consultant provided a certificate of mold damage remediation specifying that the property did not contain evidence of mold damage (despite the report showing 2 spores of chaetomium found in the bedroom) – should that be sufficient? Or do I need to request written documentation from the consultant saying chaetomium was actually cleared? There was water damage from a toilet overflow into that bedroom that is said to have been remediated. Would not the chaetomium need to be cleared?


    Since there are no regulations or guidelines determining what a normal amount of mold indoors is, consultants have to use their experience and professional judgment when writing reports. In my professional opinion, I would not have written a post remediation verification letter with the Chaetomium present.

    I have had three mold tests done.

    The first showed 427/m3 Aspergillus/Penicillium in the HVAC system. We were told the problem was identified and fixed. The vent was cleaned and sterilized. Two weeks later, a second test was peformed, and it showed 147/m3 Aspergillus/Penicillium. On both days, the outdoor control sample contained NO DISCERNIBLE AMOUNT of Aspergillus/Penicillium. I was told that 147/m3 was an acceptable level and not a concern.

    22 days after the second test, a third test was conducted and showed 270/m3 Aspergillus/Penicillium. Again, there was NO DISCERNIBLE AMOUNT of Aspergillus/Penicillium in the outdoor control sample. I think we have a problem that has not been properly fixed as of yet.

    What I really need to know though is this: are any of these numbers an “acceptable” level really? What could be causing this in the HVAC system? The numbers I shared are from one room in particular that consistently reads high in these spores. Other rooms also have elevated levels of Aspergillus/Penicillium, but at lower levels like 27 in the living room on the day 427 was recorded in a bedroom during the first inspection, and 200 in the living room on the day 270 was recorded in the same bedroom. The living room and the bedroom are on different floors of the home. Is this something I need to worry about? Now that we know there IS mold, what do we do to find the source? I thought we already took care of it, but now it seems the problem was only temporarily abated. What do we do now?


    This is a very specialized situation, where someone like me could not say if it is acceptable or not. Depending on where in the HVAC system the samples were taken, the style of HVAC system and the filter used, and many other variables can play a role in how to interpret the results.