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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 a Certified Industrial Hygienist (CIH) 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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473 thoughts on “More on the Interpretation of Mold Air Samples

    Hi – We are currently purchasing a home and have used two companies to do a mold test for comparison and I wanted to get your thoughts.

    Company 1:
    Test – Air-O-Cell
    Outside Sample
    Alternaria – Count 2 – Spores 16
    Aspergilus/Penicillium – Count 4 – Spores 32
    Basidiospores – Count 16 – Spores 128
    Chaetomium – Count 1- Spores 8
    Cladosporium – Count 15 – Spores 120
    Nigrospora – Count 2 – Spores 16
    Rusts – Count 5 – Spores 40
    Smuts/Myxomycetes – Count 1 – Spores 8
    Stachybotrys 1 – Spores 8
    Total Count 47 – Spores 376

    Inside Living Area
    Aspergilus/Penicillium – Count 23 – Spores 184
    Basidiospores – Count 12 – Spores 96
    Chaetomium – Count 1 – Spores 8
    Cladosporium – Count 13 – Spores 104
    Oidium/Persospora – Count 1 – Spores 8
    Rusts – Count 2 – Spores 16
    Totals – Count 42 – Spores 336

    Company 2
    Test Micro-5
    Outside Sample
    Alternaria – Count 3 – Spores 120
    Ascospores – Count 3 – Spores 120
    Basidiospores – Count 113 – Spores 4520
    Cladosporium – Count 84 – Spores 3360
    Epicoccum – Count 1 – Spores 40
    Penicillium/Aspergillus – Count 2 – Spores 80
    Total – Count 206 – Spores 8240

    Inside Living Area
    Alternaria – Count 1 – Spores 40
    Basidiospores – Count 27 – Count 1080
    Cladosporium – Count 52 – Spores 2080
    Penicillium/Aspergillus – Count 80 – Spores 3200
    Stachybotrys – Count 1 – Spores 40
    Total – Count – 162 Spores 6440

    I’m located in Arizona. I wanted to know your thoughts of these results and why there seems to be such a big difference between results. Thank you!

    I can not say why the numbers are so different, except that maybe they were collected from different areas indoors?

    The first company has some very interesting outdoor results. Chaetomium and Stachybotrys are almost never found in outdoor samples, and I can say I have never seen both of them together on an outdoor sample like this.

    The Asp/Pen levels from the second company are pretty high, they also found Stachybotrys which is a water damage indicator. I would say that there is possibly some history of moisture issues in the property.

    I hope you are doing well. We are currently in the process of purchasing a new home and mold remediation has been completed several times. Does anything about the numbers below concern you?
    Cladosporium Spores: Raw 8, 53 Spores/m3 (same as outside)
    Penicillium/Aspergillus group spores: Raw 51, 340 Spores/m3 (outside 6.7/m3)
    Aureobasidium spores: Raw 15, 100 Spores/m3 (outside 20/m3)
    Epicoccum Spores: Raw 3, 20 spores/m3 (outside 13/m3)
    Nigrospora spores Raw 1, 6.7 spores/m3
    smuts/periconia/myxomycetes: Raw 5, 33 spores/m3 (outside 20 spores/m3)

    Our total spores/m3 is 550 vs outside of 120.

    Thank you!


    In my opinion, these numbers look okay. The Penicillium/Aspergillus levels are slightly higher than outdoors but below the level that I typically see in areas with a major issue.

    Thanks for doing this!
    We just tested for mold after having flooding last year (downstairs was gutted, aired out, and rebuilt). The company wants us to fog and get an air scrubber (about $1200 total). Do you agree?

    Ascospores 1 Raw Count 30 Count/M3 11.1% of Total
    Aspergillus/Penicillium 3 Raw Count 90 Count/M3 33.3% of Total *This is the one they were concerned about
    Basidiospores 2 Raw Count 60 Count/M3 22.2% of Total
    Cladosporium 1 Raw Count 30 Count/M3 11.1% of Total
    Myxomycetes++ 2 Raw Count 60 Count/M3 22.2% of Total

    Aspergillus/Penicillium 1 Raw Count 30 Count/M3 50% of Total
    Cladosporium 1 Raw Count 30 Count/M3 50% of Total

    Thanks again!

    Hi Dylan,

    Hope you can help interpret an indoor air quality report performed on my son’s home which had extensive water damage to the first floor and basement of his ranch home. A malfunctioning dishwasher ran for approximately 5 hours while no one was home. Part of the report included a chart entitled “Total Fungal Genera Chart (Spores / m3)” at the top and numbers at the bottom starting at 0 and increasing by 5 up to 35. The rooms/areas tested were listed on the left with a colored bar chart and an associated number (indicating the number of spores per cubic meter??). He has lived in the home for 3 years. His wife is currently pregnant and they are concerned about the test results. The results are as follows:

    Master Bedroom
    Basidiospores 13
    Aspergillus / Penicillium 27

    Hallway at Bedrooms
    Cladosporium 20
    Basidiospores 7

    Living Room
    Ascospores 7

    Kitchen / Dining room
    Myxomycetes 7
    Cladosporium 13

    Basement SE Area
    Ascospores 13

    Basement SW Area
    Cladosporium 13

    Basement NE Area (Seems like a major issue here? Location of water meter)
    Chaetomium 7
    Bipolaris / Drechslera 7

    Basement HVAC Room
    Basidiospores 13
    Ascospores 7

    Myxomycetes 13
    Cladosporium 33
    Basidiospores 27
    Aspergillus / Penicillium 13
    Ascospores 13

    Cladosporium 7
    Basidiospores 33
    Aspergillus / Penicillium 13
    Ascospores 20

    An information you can provide regarding the results is greatly appreciated.

    Warm regards,



    The sample in the basement is slightly concerning, Chaetomium is a water damage indicating mold, and is not present in buildings normally. The result is very low, so it could be a false positive or a very small amount of mold in this area. To me, I would further investigate this area.

    Hi Dylan. Have a multi-level home in Escrow and the home inspector’s mold test came back as follows. Home has radiant in-floor heating, so no HVAC ducting or AC. It is in Utah, snow on the ground, low humidity. There is a crawlspace. Spoke with a mold guy he recommended this morning and that person read the report and said it was ‘alarming.’ He is going to visit the property today to poke around. Home Inspector did not see any mold visually. No odors. Sample was taken outside and several samples in various indoor rooms as follows. Numbers are spore count followed by m3.

    Alternaria 1/13
    Smuts/Myxomycetes/Periconia/Rusts: 1/13

    SW Lower BR:
    Ascospores: 5/67
    Aspergillus/Penicillium: 147/1960
    Basidiospores: 13/173
    Chaetomium: 5/67
    Cladosporum: 9120
    Curvularia: 1/13
    Smuts/et al: 68/907
    Stachybotrys: 680
    Total: 254/3387

    North Lower Level Bedroom:

    Ascospores: 3/40
    Asperillus/Penicillium: 41/547
    Basidiospores: 3/40
    Chaetomium: 1/13
    Cladosporum: 2/27
    Smuts et al: 25/333
    Stachybotrys: 5/67
    Total: 80/1067

    Lower Level Family Room

    Ascospores: 7/93
    Aspergillus/Penicillium: 189/7530
    Basidiospores: 5/67
    Chaetomium: 8/107
    Cladosporum: 25/333
    Smuts et al: 61/813
    Stachybotrys: 4/53
    Total: 299/3987

    Laundry Room (mid level)

    Ascospores: 1/13
    Aspergillus/Penicillium: 22/293
    Basidiospores: 2/27
    Cladosporium: 4/53
    Smuts et al: 9/120
    Stachybotrys: 2/27
    Total: 40/533

    Master Bedroom (top level of home)

    Basidiospores: 1/13
    Cladosporum: 4/53
    Smuts et al: 8/107
    Stachybotris: 1/13
    Total: 14/187

    Would this be considered ‘alarming’ in your opinion? Hoping to keep this deal together but there seems to be so much conflicting info out there.



    I would say these levels are concerning. There are high levels of Aspergillus/Penicillium indoors, combined with water damage indicators such as Chaetomium and Stachybotrys in the samples. These results tell me there is either ongoing or past severe moisture issues in the property.

    Hi! We are currently selling our 6 year old home home in an area that has had several new homes with moisture and mold issues. A potential buyer had air quality testing done, which showed:
    Aspergillus penicillin raw count 17 (370/m3)
    Chaetomium raw count 1 (20/m3)
    Outdoor control showed 0 aspergillus & chaetomium.
    Buyer pulled their offer based on the presence of chaetomium. Home inspection by mold inspection company was unable to locate any areas of concern via thermal imaging and moisture readings of drywall and baseboards in finished basement.
    Should we be concerned? We are unsure of how to proceed.


    These levels are very low, and to me would not be setting off major alarms. A single spore of Chaetomium could, however unlikely, enter the home from the outdoors. Another possibility is it could have been on the mold inspector’s clothes or equipment before they entered your home. If an experienced inspector did not find any mold or moisture concerns in your home, it is likely that the Chaetomium was just an anomaly, not an indication of mold growth in the home.

    Hi we recently had an air sample test taken in our home after finding mold in our upstairs bathroom subflooring. Which we had removed/ replaced with new subflooring. The test was done 2 days after it was replaced. The question now is do we need to Fogg to kill off the ramaing mold spores in the air? It seems like a low level to me are these numbers of any concern?

    Sample 1. — Indoor (2nd Floor)
    Pen/Asp group — 267 Spores/cu. mHyphal Fragment — 53 Spores/cu. mAscospores — 53 Spores/cu. m

    Sample 2. — Outdoor
    No Fungal Spores Observed — N/A Spores/cu. m Dust / Debris: Very LowOpaque Particles: Very Low

    Fogging is not a recommended step in any remediation project. You need to physically remove mold from the air, not just kill it. These levels are within the expected background levels, and wouldn’t require any additional measures. If you wanted to, you could have an air scrubber run in the space for a day or two, but I would not recommend fogging.

    Hi, I am wondering if these lab results are concerning? Its difficult to tell from the report and a little frustrating. Thank you for your help!

    Raw Count Spores
    Basidiospores 18 960
    Cladosporium 8 427
    Pen/Asp group 8 427
    Ascospores 1 53
    Bispora 1 53
    Mitospores 1 53
    Smuts/Periconia/Myxomycetes 1 53
    Hyphal Fragment 3 160


    To effectively interpret lab results there needs to be an outdoor control sample collected at the time of the indoor samples. Without that reference sample, I can not reliably interpret lab results. I can say there are no major red flags on this sample, but there could be minor issues. Again, the finer details can not be interpreted without that outdoor sample.

    Had water leak and thus in 2 bedrooms had “scores” of 2,200 spores/m3 and 1,300 spores/m3 re Chaetomium. Had mold remediation work done (replaced floors/ripped out sheetrock etc) now scores of 120 spores/m3 and 430 spores/m3-do I still need to do further work here or at ok levels. Outside reading basically zero.
    Also at same time first test done living area was ok now on second test had score of 1200 spores/ms outside reading 400 for Penicillium/aspergillus-is this an issue now and not sure how living area only had score of 170 before around a month earlier (note could dust cause this as house not lived in a lot over last month)


    I would suggest having the remediation company perform additional air scrubbing and do a detailed cleaning of the area using a HEPA vacuum. These numbers seem like they are just residual spores in the air from the remediation that needs to be cleaned. Ideally, the Chaetomium levels should be zero indoors.


    I’m looking at purchasing a newly renovated home and have results from outdoor air and 2 indoor samples. I’m particularly looking for some help interpreting hyphal fragments being detected (particularly the Living Room result) – what do these indicate? I am also curious if you see anything else that you’d deem noteworthy and potentially indicative of a problem. Thanks in advance!

    All samples 150L, Air-O-Cell analysis (Methods MICRO-SOP-201, ASTM D7391)

    Outdoor air
    Raw Count / (Count/m^3) / %
    Basidiospores – 5 / 100 / 71.4%
    Cladosporium – 1 / 20 / 14.3%
    Myxomycetes – 1 / 20 / 14.3%
    Total – 7 / 140 / 100%
    Hyphal Fragment – 1 / 7 (found at 300X)

    Living Room
    Aspergillus/Penicillium – 5 / 100 / 14.5%
    Basidiospores – 25 / 510 / 73.9%
    Cladosporium – 4 / 80 / 11.6%
    Total – 34 / 690 / 100%
    Hyphal Fragment – 4 / 80 / — (found at 300X)

    Finished Basement (shares wall with a crawlspace)
    Ascospores – 1 / 20 / 12.5%
    Asperigillus/Penicillium – 2 / 40 / 25%
    Basidiospores – 4 / 80 / 50%
    Cladosporium – 1 / 20 / 12.5
    Total – 8 / 160 / 100%
    Hyphal Fragment – 1 / 7 / — (found at 300X)


    I don’t see any concerns in these results. Hyphal fragments are growth structures for mold. Lab reports note them more as background information, these levels don’t indicate any issues.

    We received our pre-remediation test results back and a handful of mold organisms were found: Alternaria (spore estimate very heavy and mycelial estimate many), Ascospores (27/m3 & 40% of total), Chaetomium (27/m3 and 40% of total), and Cladosporium (13/m3 and 20% of total). The remediation company indicated that he would highly recommend fogging the entire residence since Chaetomium is mycotoxin-producing and that filtration media cannot clean the air because the particles are too small.

    In my minimal research online I’ve read concerning things about like “people who are exposed to Chaetomium may be predisposed to permanent neurological damage of the myelin sheath” and “can also cause permanent DNA damage” and “Chaetomium is the only mold that inhibits cell replication”. In light of this information, I’m reaching out to you as a second opinion on what we should consider in our next steps. I’d like to approach this from an informed standpoint as i’m concerned not just about the mold, but also the fogging chemicals as i have 2yo and am 8 months pregnant.

    Thanks for your help!

    Fogging is not a proper method for mold remediation. The “Holy Bible” of mold remediation, IICRC S520 does not recommend fogging in any capacity. The mold needs to be physically removed from the environment, not just killed by a chemical. I would find a remediation company that will follow S520 to properly take care of the mold.

    Again, no industry guidelines recommend fogging. Fogging will only kill the mold, but even dead mold poses health concerns, therefore the mold needs to be physically removed from the environment.

    Hi Dylan. I have a three-season-room that is entirely made of wood – no insulation or sheetrock. It was an old deck they enclosed. The air sample was declared ELEVATED with Penicillium/Aspergillus identified as disproportionally higher when compared to the control sample. The control identified No Spores/m³ while the air sample of the Back Sun Room area noted 920 Spores/m³. I spent a lot of money on this report and wanted to discuss abatement with an honest non-bias expert such as yourself. Would the fact that one of the windows in that room has been not quite sealed up and the house being empty for a year make a difference? Also we are about a block from the Long Island Sound which is about 25% salt water (I think) and across the street from a large pond. Just wondering what you think we need to do and how bad this actually is. Our inspector seems like someone who just does this on the side so not super helpful. Thank you!!!

    Those results are slightly elevated, and unless you are seeing visable mold growth or water intrusion there probably isn’t a need for remediation. Just do a detailed cleaning of the room and the numbers should go to a more normal level.

    I know you’ve mentioned foggers aren’t recommended for remediation but after cleaning should we maybe do that for thoroughness?

    If the cleaning is done properly there is no need for fogging. No industry standard uses fogging as a step in remediation, so I would not recommend it for any reason in a mold project.

    You should read the EPA’s guide on mold remediation in schools and large buildings as well as IICRC S520. These topics are getting away from the purpose of the blog post, I wish you well in your cleaning/remediation efforts!

    Can you help us understand this? Daughter with 10 months moving in she has been diagnosed with asthma. Have all the counts and comparisons.

    Hello, I would just like to know if my ERMI test results should be concerning or if investing in an air purifier would do the job. A little background info: I live in a very small 300 sq ft. apartment and have had worsening symptoms of fatigue, digestive issues, and headaches (thus the doctor’s recommendation to get this done). On top of this, mold exposure actually showed up pretty high on my blood test. I’ve been suspicious of my AC/Heating unit since moving in. It’s a big unit–over a meter long and projects about a foot inside under the window. It smells horrible and the fans inside are caked with what looks like mud, but I’m pretty sure is years worth of mold/mildew buildup. Judging from the ERMI results, is it plausible that this could be from the AC unit alone? I have no visible mold anywhere else.
    Aspergillus penicillioides – 1300
    Aspergillus versicolor – 180
    Cladosporium sphaerospermum – 100
    Eurotium (asp. Amstelodami – 2000
    Stachybotrys chartarum – 1
    Wallemia sebi – 46
    Ccladosporium cladosporioides 1 – 290
    Cladosorium cladosporioides 2 – 20

    Everything else not included above was below 40.


    This blog is meant for help with laboratory air sample analysis, not ERMI. ERMI is very tricky to interpret and is not the ideal way to assess for ongoing mold issues. ERMI looks at dust, which means it is looking into a historical perspective of the home, and can not be used to say if the home has current mold issues reliably. The EPA (who developed the ERMI test) has not cleared the ERMI test to be used in this way.