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More on the Interpretation of Mold Air Samples

test results; mold air samples; confusing

Are you staring at the results of mold air samples 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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579 thoughts on “More on the Interpretation of Mold Air Samples

    Hi Dylan,

    We had a new HVAC system installed in our remodeled home in February of 2020 in our attic. We used the A/C during the summer on and off and started to smell a musty/moldy odor in October when we ran the A/C again. We had two HVAC techs including the installer come to take a look and they did not see any leaks or visible mold. They were stumped, but one cleaned the ducts and the other cleaned the evaporator coil though they saw nothing there. We had a mold inspection done last week because I’ve had some severe allergy related health issues including severe food intolerance. Overall, there were no signs of mold, but we asked the inspector to test one of the air vents given the smell from October, which by the way, had no smell when the AC was run for the mold inspection, and the test of the vent came back positive for the following mold:

    Aspergillus/Penicillium 9 (raw count) 400 (count m3)
    Chaetomium 1 (raw count) 40 (count m3)
    Cladosporium 1 (raw count) 40 (count m3)
    Stachybotrys/Memnoniella 23 (raw count) 940 (count m3)
    Total Fungi 34 (raw count) 1420 (count m3)

    I am deeply concerned about this. The head of the HVAC company thinks this may be caused by undersized ductwork b/c it otherwise doesn’t make sense how we have mold this soon after a new install especially black mold. I’m prepared to have all the ductwork abated, remediated and re-installed. We have stopped using the heater as of today so that further spores don’t spread. My questions are the following:

    1. I’m curious if the mold count in our home may be more given that this is the count from just one vent? Should I assume this?
    2. Should I have the general air in the home tested?
    3. How can I determine if the condenser, furnace, evaporator coil, heat pump and boiler need to be replaced and remediated as well?
    4. Does our upholstered furniture or clothing need to be tossed?
    5. Should we invest in an air purifier or humidifier?
    6. Should we leave the home until the remediation takes place?
    7. Should I get a second mold test or additional samples taken?
    8. Should the insulation in the attic be changed?

    I am in over my head and don’t want to end up doing a remediation and going through all this expense to have this not work. Thank you so much in advance for any help you can provide!


    Those numbers certainly indicate some amount of mold issue in your home. However, I suspect the HVAC isn’t the location of the mold growth. HVAC equipment very rarely is the cause of mold growth. The materials used are not great substrates for mold growth, the one exception is when the ducts themselves have been flooded. I suspect there is mold growth elsewhere, and the HVAC is simply acting as a “highway”, a pathway that is pushing the mold into other areas of the house.

    The Chaetomium and Stachybotrys molds found in the air sample are water damage indicators, meaning they grow on building materials that have had some significant moisture. Typically, these mold types are found on drywall and wood materials that have been wet for some time. I haven’t seen these types of mold growing directly on HVAC surfaces before (although it technically may be possible under the right circumstances.

    At this time, I would say you want to get a detailed mold assessment done in the property. Find someone who will do a detailed visual assessment and use moisture tools like an infrared camera and moisture meter to look for hidden moisture. Your next steps really will be determined once you fully understand what moisture issues you are dealing with.

    To answer some of your questions:
    1 & 2, get a mold assessment done by a professional to better understand what is going on in your home.
    3, these things would only need to be cleaned or replaced if you see visible mold on them, it seems like that isn’t the case based on what the HVAC professionals have said.
    4, See answer to 1 & 2, the results you sent aren’t sky high, but that is only the levels from a single HVAC sample.
    5, These are things to consider after you have the mold situation figured out. The biggest thing in addressing mold is correcting the moisture issues that are allowing for the mold to grow. I suspect you mean a dehumidifier, as you want to maintain RH% in your house under 60% during warm months, and no higher than around 35% during freezing outdoor temperatures.

    Thank you so much for your time and expertise, Dylan. It is incredibly appreciated. We had a second mold assessment done on Thursday. Our bedroom and basement air were sample and two swabs were taken. The inspector found evidence of an old leak from before our remodel under our master bathroom from the crawl space, so he sampled that. He also found evidence of a leak in our attic from the roof from a recent rain last week due to our contractor not properly sealing a new vent flue, but it was a minor leak and I don’t believe this is the cause as it hasn’t rained until last week. We’ve had that roof patched to avoid further issue however our general inspection report during escrow did show evidence of prior leaks in the attic from the roof, but it was noted that it was not wet at the time of inspection. Could the old crawl space leak and the old attic leaks be the source? I also know there was a leak in the front of our home from a pipe where we connect our hose to water plants at the time of sale that we fixed early on, but it did cause moisture intrusion under the home around that area (we’re on a raised foundation). Could this also be the cause? My last question is whether it’s possible that there was mold in walls that were disturbed during demolition that caused spores to become active in the home during reno?

    Hi Dylan. We got our second set of labs back and there was no stachy in our bedroom air, but there were 3 raw counts of stachy found in the basement and in the outside air outside of our crawl space. The swab from the crawl space didn’t show stachy nor did the swab in the attic. I’m now thinking there must be something in the crawl space that’s causing this. There is a little bit of debris from the demolished bathroom that used to have a leak. Not sure if the debris has stachy on it. I’m also not sure if it’s due to a moisture intrusion in the crawl space. Could stachy go from the crawl space into the ductwork? Thanks.

    It seems like you are getting closer to an answer.

    Do you know how the lab analyzed the swabs? If the swabs were cultured, it is very hard to recover Stachy or Chaetomium in a cultured swab test. Also, it is important to remember that surface sampling is only testing the small area that was swabbed, not the whole space. I could take a swab a few inches from mold growth and have a result with no mold. This means there could still be some hidden growth someplace in the crawlspace or the basement. Lastly, it could be that there was some mold growth in the past that was removed, but the air was never cleaned. Residual mold spores can persist in the air after a shoddy remediation job for many months.

    HVAC ducts act as a pathway, so it is possible mold from other areas of the home could enter the ductwork and the air in unaffected rooms. Especially if you have ducts that run through the crawlspace.

    I’m not sure about the swabs. Here’s what was listed on the lab report for the swabs: EMSL Method MICRO-SOP-200. The crawl space swab showed aspergillus/penecillium – rare, epicoccum – rare, unidentifiable spores – rare, hyphal fragment – rare, fibrous particulate – low. The attic swab showed cladasporium – rare, hyphal fragment – low, and fibrous particulate medium. That leak area is dry now and my husband threw out the tainted piece of insulation there. The basement air sample showed the following raw counts: 1 altemaria, 3 ascospores, 1 chaetomium, 54 cladosporium, 1 epioccocum 15 aspergillus/penicillium, 17 basidiospores, 3 myxomycetes and 3 stachy with total fungi being 98 and 12 hyphal fragment. The outside air taken by the crawl space entry (basement is an enclose part of the crawl space but finished) showed the following raw counts: 3 altemaria, 1 ascospores, 167 cladosporium, 15 aspergillus/penicillium, 33 basidiospores, 6 myxomycetes with total fungi being 238 and 5 hyphal fragment . I had a third mold inspector come today which is pretty pitiful since all this money spent should go toward remediation. The prior two inspectors suggested we just sanitize the HVAC system but I don’t believe this issue started in the HVAC ductwork. All other parts of the HVAC look clean to the eye and upon inspection. Today’s inspector seemed to be the best out of the three. He popped off baseboards in the basement and saw evidence of mold and termites from prior moisture intrusion and on some lower wall areas showed moisture damage. His screwdriver went right through one of the wood panels on the basement wall. The basement was not remodeled when we remodeled the home last year b/c we couldn’t afford it. He seems to think it’s coming from the basement. He also sees a lot of old water intrusion marks in the attic and tested for fungus up there. All of our ducting is in the attic and was installed new up there during remodel. He thinks we need to water proof our crawl space because there was a lot of previous water intrusion (we have a raised foundation) and he suggested we have debris from remodeling removed from the crawl space along with whole home sanitization and the HVAC system. I would also install an air scrubber and UV light in the HVAC going forward along with HEPA filters. We may just need to gut the basement at this point. It’s on our remodel to do list, but we weren’t planning to do it any time soon. I’m not sure it makes financial sense to remediate it. We’re still awaiting his lab results then need to come up with a game plan. I’m sick from the mold exposure and my fear is these remediation tactics won’t be fullproof. Is my home salvageable? Any thoughts on this?


    None of the lab results that you have shared with me indicate a major issue that can not be remediated. Feel free to share the results from this most recent round of tests and I will let you know my thoughts.

    Hi Dylan,

    I’m certified by NAMP and used to complete remediation work years ago. A lot of the knowledge I used to have has faded since I dont do this type of work anymore. I do still however have my sampling equipment and recently ran a sample for a client. One interior and one exterior. The only elevated counts where as follows…
    Basidospores Int- 890 – Ext- – 100
    Cladosporium Int- 550 – Ext – 460
    Myxomycetes Int – 550 – Ext – 200
    I’m in the midwest and we are freezing at night and thawing in the day. No snow cover or precipitation for at least a week prior to the samples taken. The resident is hardly home so his house is closed up quite a bit. My thinking is that naturally, the exterior spore levels are going to come back low during this time of the year and it has been pretty mild during the days latley. My thinking is this… a lot of the interior spore count could be lingering from the previous weeks of warm weather and trapped whereas the exterior has pretty much cleared out due to freezing temps and no precipitation. I feel like these are decent results for the interior I would love your take on this if you have a minute. Thanks in advanced!


    I would agree with your interpretation that these levels are just a reprenstation of past outdoor levels. Basidiospores and Myxomycetes are almost always from outdoor sources (and they are pretty low as it is at 890/550). Cladosporium can grow indoors, but these levels are pretty low and wouldn’t be a concern on their own.

    I was wondering if I should be concerned about a couple numbers on my mold test:
    Thanks for your help!


    Typically, we do not collect air samples in attic spaces. They can be very hard to interpret, but these levels don’t seem concerning to me.

    I am looking to purchase a home and need to be mindful of mold issues. The air quality test reflected elevated levels for the Pen/Asp group. I am wondering, is the a high level in comparison to most basements? Is it considered to be concerning? Any feedback you can offer will be GREATLY appreciated!

    Pen/Asp group Control – Outdoors – Raw Count = 4; Spores/cu.m = 213; Percent = 11.12%
    Pen/Asp group Center of Basement – Raw Count =21; Spores/cu.m = 1,120; Percent = 100.00%


    It seems like there may be some slight moisture issues in the basement, we can see results like this in spaces that have some chronic humidity issues. I would say ~30% of basements that we assess might have a slightly elevated level like this, and the most common cause is humidity. You want to maintain humidity below 60% in the summer time and usually below 35% in the winter (this changes depending on how cold the weather gets in your climate).

    Thanks for your time. How does this look? We are purchasing a new home.
    3 rooms:
    19 raw, 253m3
    12 raw, 160m3
    2 raw, 27m3
    Outside air: 26, 347m3

    The first air test picked up a single spore of Stachy in two rooms, 40m3, but that was all gone second repeat test.

    God bless you for your help.

    These numbers look acceptable to me. We can see Asp/pen in a range from 0-1000 naturally occurring, and these were all below the outdoor sample at the time.

    Since you retested and the Stachy did not pop back up, I would say that first indication was an anomaly and not likely a concern.

    So, my second air testing company invited me to the lab to review the slides from the first company, which was really cool. I had to go pick them up from the first tester due to chain of custody, and bring them in, reexamined the stachy slides. There were actually two spores per slide for stachy, upstairs and downstairs. The other lab had missed the second spore in each.

    What do you make of this? I have heard stachy comes and goes in air samples. I am super concerned because we have adopted 8 kids with special needs and i want to protect their health.


    Getting to check out slides under the scope is a cool experience!

    Spores can be missed because the laboratory typically only looks at a portion of the slide under the scope. Two raw count Stachy spores in each of the samples is still a pretty minimal result (some industry experts don’t confirm a “positive” result until there are 10 spores), and with the follow-up sample having no Stachy it is a good sign. Typically if you have a mold issue with Stachybotrys, there are other concerns on the air samples, which I do not see on your samples.

    Hi there. Thank you for such a useful site. Just had an air analysis on 1st and 2nd floor of home. Everything else tested came back pretty low/normal range except for the Basidiospores on the first floor were 5,147 cubic m. Outside control was 333 cubic m and upstairs count was 293 cubic m. I had noticed the inspector left the nearby front door open for a long time before testing (was closed for actual test) but wondering if it could have been a factor or if it’s something else. Would appreciate any thoughts. Thank you!

    Basidiospores are fungal spores that come from mushrooms. Almost always basidiospores are from outdoor air, so unless you have had major water issues and are actually seeing mushrooms growing in your home (or if you have a lot of houseplants), I would assume that the basidiospores are just naturally occurring from the outdoor air.

    Thanks for your thoughts on this. No visible mushrooms or houseplants. Can indoor numbers be that much higher than outside if it’s just blowing in? It is in a wooded area very close to forest service land.

    Yes, for spore types like basidiospores, the most likely cause would be air naturally moving into the home from open doors and other air gaps to the outdoors. The outdoor sample is just a snapshot in time, outdoor mold levels can vary drastically depending on factors such as wind speed/direction, precipitation, temperature, etc. We can see natural basidiospores levels outdoors over 20-30,000 at times.

    Can you please advise on the following results on 2nd floor air sample:
    Cladosporium 389 Raw Count/8590 Count/m3 &75.5% of Total
    Stachybotrys/Memnoniella 3 Raw Count/70 Count/m3 & 0.6 of Total sample
    Cheiromycella 9 Raw count / 200 Count/m3
    Aspergillus/penc 20 Raw Count/ 400 Count/M3
    Yeast 34 Raw count/ 750 Count/M3 & 6.6% of sample
    Insect fragment 5 Raw count/ 100 Count M3
    Total Fungi 516 Raw Count/ 11357 Count/m3

    These come back as Elevated or slightly elevated.
    It’s floor had some of this but lower.
    Told yeast was high because the seller bakes in kitchen on 1st floor and there is a small kitchenette on 2nd floor. It is a single family home built in 1920 and has furnace in attic for 2nd story.


    Without an outdoor reference sample (read the blog above), it is hard for me to give a detailed interpretation. It does seem that the Cladosporium levels are slightly elevated (but it is possible this level is from the outdoors). What sticks out more is the Stachybotrys/Memnoniella. These are water damage indicator molds, meaning they really only occur in homes that have some amount of water damage and mold growth.

    I don’t buy the explanation that the yeast is from baking. Unless the 1st floor is a commercial bakery, I would not expect baking to aerosolize enough yeast for it to be detectable on the 2nd floor.

    Hi Dylan,

    Thanks so much for the work you’re doing here! We’re looking to purchase a home and just got back the air test results. Can you tell me if these are concerning? The Penicillium/Aspergillus numbers scare me.

    Penicillium/Aspergillus types
    Outside sample 290 spores/m3
    11 raw count
    Inside sample 3100 spores/m3
    118 raw count

    Cladosporium species spores
    Outside sample 930 spores/m3
    35 raw count
    Inside sample 210 spores/m3
    8 raw count

    Outside sample 4300spores/m3
    161 raw count
    Inside sample 560 spores/m3
    21 raw count


    I will say the Asp/Pen levels are slightly elevated. This may mean there has been some sort of minor moisture issue in the past, possibly a damp basement. Asp/Pen levels can be in the 10,000’s, and even over 100,000 in homes with major mold issues so relatively speaking these levels aren’t off the charts but it is indicative of some sort of moisture/mold problem in the home.

    Thanks for the feedback Dylan. The living room where the sample was taken from has a bedroom next to it with a full bath. There was a toilet seal leak in there for a while which caused the tile floor to decay. The subfloor and tiles were replaced two months ago. One thing to add is that the entire carpet in that living space is 30 years old. Is it possible the spores from the water leak settled in the carpet and were disturbed when the inspector walked on the carpet to setup his air sampling equipment? If I replace the entire carpet, will this cause the asper/pen levels to come down? Can an old dirty carpet cause these numbers to be elevated?

    Also wanted to mention that there is crawl space underneath the living area and it has a HVAC and water heater in the crawl space. Most of the dirt has a vapor barrier but some dirt is exposed.

    I think your carpet and the HVAC in a crawlspace without a full barrier are your most likely suspects, those are very good observations!


    To give proper interpretation I would need more information, most importantly there should always be an outdoor reference sample collected to compare. This seems slightly elevated, but without additional context, I can’t go beyond that.

    Hi There! Thanks so much for the work you’re doing here! We’re looking to purchase a home and just got back the air test results. The seller had disclosed that there was a leak due to damaged flashing around the chimney, but it was completely repaired in the basement and living room (drywall, insulation, flashing, wood beams, siding & outdoor deck). The repairs were done about a month prior to our air test but the home has been closed and unused since – the sellers had a remediation company visit who advised that we should add hepa filters and do a top to bottom cleaning of the home but nothing more. I wonder if these levels are in line with recommendation or if it will need a deeper remediation. The living room area has carpet which im guessing could be contributing to the issue.

    [Raw Count / (Count/m3) / % of Total]
    outdoor sample:
    Aspergillus/Penicillium 12 / 250 / 9.6%
    Cladosporium 24 / 510/ 19.5%
    total: 129 / 2610

    unfinished basement:
    Aspergillus/Penicillium: 14 / 590 / 35.8%
    Cladosporium: 18 / 760 / 46.1%
    total: 62 / 2660

    living room:
    Aspergillus/Penicillium: 28 / 840 / 31.6%
    Cladosporium: 32 / 1400 / 52.6%
    total: 62 / 2660

    These numbers are relatively low, higher than the outdoor sample but still within the possible range of outdoor levels. We can see asp/pen in the outdoors up to about 1000 spores/m3 or so. I do not think this is evidence that more remediation is needed, I think you hypothesis about the carpet is a good one. I would have some additional HEPA air cleaning and also have the carpets cleaned with HEPA vacuums. You probably don’t need a top down cleaning but you could for extra piece of mind if you wanted.

    Hi Dylan,

    It would be greatly appreciated if you could help us interpret our results from a recent mold analysis from a air samples. We seem to have a Penicillium/Aspergillus problem, but we’ve not be able to find a source. The inspector/tester was unable to find a source but pointed out a few areas of potential. The inspector said the following, “Due to the abnormal levels of Penicillium /Aspergillus it
    is my recommendation that all porous materials such as carpet and fabric be removed and discarded.” Does that seem normal?

    Thank you for your time.

    (from the report) Source of Mold Growth: Previous leaks, high humidity

    (raw ct. spores/m3 %)

    Family Room
    Ascospores — 4 160 2
    Basidiospores — 22 880 9
    Cladosporium — 83 3320 35
    Penicillium/Aspergillus — 123 4920 52
    Smuts/Periconia/Myxomy — 3 120 1

    Ascospores — 21 840 9
    Basidiospores — 80 3200 33
    Cladosporium — 123 4920 51
    Curvularia 1 40 <1
    Epicoccum 1 40 <1
    Cercospora 1 40 <1
    Penicillium/Aspergillus — 3 120 1
    Smuts/Periconia/Myxomy — 2 80 <1

    Master Bedroom
    Alternaria — 1 40 <1
    Ascospores — 1 40 <1
    Basidiospores — 15 600 14
    Cladosporium — 9 360 9
    Curvularia — 3 120 3
    Nigrospora — 1 40 <1
    Penicillium/Aspergillus — 73 2920 70
    Smuts/Periconia/Myxomy — 1 40 <1

    I've already begun remediation of a few of the likely sources of humidity and I've got a dehumidifier in the basement.

    Results like this can be tricky to track down the source. Unless you have carpets or other porous materials that you can confirm have been wet and have mold growth, you do not necessarily need to discard them. I would start with having a detailed cleaning done, cleaning all walls floors and ceilings. While this is going on I would also have air scrubbers running to filter airborne mold from the air.

    I live in a building, can these mold have me sick? Here are my scores, thank you for your help and support.

    Aspergillus penicillioides 7
    Aureobasidium pullulans 161
    Chaetomium globosum 17
    Cladosporium sphaerospermum 3
    Eurotium (Asp.) amstelodami 19
    Paecilomyces varioti 2
    Cladosporium cladosporioides1 23
    Epicoccum nigrum 41
    Mucor amphibiorum 16
    Penicillium chrysogenum 19

    Hertsmi scored a 4
    Aspergillus penicillioides 7
    Chaetomium globosum spore 17 weighting a 4

    Hi Dylan!
    I would VERY much appreciate your take on just one result that was positive using Home Mold Lab/IAHA LAC. A reading of Asp/Pen 53 indoors (hyphal fragment) with 320 being the outdoor reading. I recently tested positive (urine) for mold, and am going through DMSA chelation (for 3 varieties of aspergillus as well as other ‘stuff’ (i.e., lead). I believe my mold exposure was prior to my current residence. Is there any reason to suspect that my current indoor reading (as low as it is) could be a possible contribution to ongoing health issues? Thanks in advance, Dylan! I so appreciate your time :^)


    That level is very low, we can see Asp/Pen in the outdoor air at 1000 spores/m3 at times! Homes with major Asp/Pen problems can be over 100,000 spores/m3, so your 53 seems normal.