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


    Appreciate your willingness to share your expertise. Recently we had our home inspected, no visible water damage or high moisture reading in wall sections tested were found, and air samples were taken outside, and in 3 bedrooms along with a surface sample of some mold/staining on a few of the pieces of framing lumber in the attic. We live in central Texas where warm/hot humid weather is the norm. Any red flags in you opinion?

    Alternaria – raw count – 3 spores/m3 – 120
    Ascospores – raw count – 5 spores/m3 – 200
    Basidiospores – raw count – 29 spores/m3 – 1160
    Bipolaris/Drechslera – raw count – 3 spores/m3 – 120
    Cladosporium – raw count – 188 spores/m3 – 7520
    Penicillum/Aspergillus – raw count – 30 spores/m3 – 1200
    Smuts/Periconia/Myxomy – raw count – 2 spores/m3 – 80

    Bedroom 1 – Upper level* Attic was accessed from to different access points in this room prior to test.
    Alternaria – raw count – 1 spores/m3 – 40
    Ascospores – raw count – 6 spores/m3 – 240
    Basidiospores – raw count – 36 spores/m3 – 1440
    Bipolaris/Drechslera – raw count – 0 spores/m3 – 0
    Cladosporium – raw count – 40 spores/m3 – 1600
    Penicillum/Aspergillus – raw count – 44 spores/m3 – 1760
    Smuts/Periconia/Myxomy – raw count – 2 spores/m3 – 80

    Bedroom 2 – Upper level
    Alternaria – raw count – 0 spores/m3 – 0
    Ascospores – raw count – 1 spores/m3 – 40
    Basidiospores – raw count – 33 spores/m3 – 1320
    Bipolaris/Drechslera – raw count – 0 spores/m3 – 0
    Cladosporium – raw count – 16 spores/m3 – 640
    Penicillum/Aspergillus – raw count – 37 spores/m3 – 1480
    Smuts/Periconia/Myxomy – raw count – 0 spores/m3 – 0

    Bedroom 3 – Main level
    Alternaria – raw count – 1 spores/m3 – 40
    Alternaria – raw count – 0 spores/m3 – 0
    Basidiospores – raw count – 21 spores/m3 – 840
    Bipolaris/Drechslera – raw count – 0 spores/m3 – 0
    Cladosporium – raw count – 10 spores/m3 – 400
    Penicillum/Aspergillus – raw count – 20 spores/m3 – 800
    Smuts/Periconia/Myxomy – raw count – 0 spores/m3 – 0

    Surface sample
    Hyphal Fragments – L or 101-1,000 spores
    Cladosporium – VL or 20-100 spores
    Penicillum/Aspergillus – Scattered spores or 1-20 spores.


    The only thing slightly concerning is the Aspergillus/Penicillium levels in bedroom 1 and bedroom 2. They are slightly elevated, but not to the point where I think there is a major issue in these areas. Maybe a good, detailed cleaning of these areas with a hepa vacuum will lower those levels below the background amount.

    I am currently renting an apartment. I’ve been here for about 2 and a half years, and I’ve had several health issues that seem to be getting worse. I never considered mold a possible culprit, but I recently got ground water in my living area, all along the back wall, where the floor meets the plaster, and realized when the water wasn’t getting in, it was sitting somewhere. My landlords agreed to have a mold inspection done, and these were the results of present spores-
    Ascospores – Exterior raw 102 ext cubic meter 4080 31% Interior raw 3 Int cubic meter 120 2%
    Basidiospores – Exterior raw 180 ext cubic meter 7200 55% Interior raw 68 Int cubic meter 2720 48%
    Cladosporium – Exterior raw 10 ext cubic meter 400 3% Interior raw 9 Int cubic meter 360 6%
    Penicillium/Aspergillus – exterior raw 38 ext cubic meter 1520 12% Interior raw 62 Int cubic meter 2480 44%

    There is mildew present on two window panes. No physical sample was taken. There was also no sample taken from the sink or shower in the bathroom (very dark, hard to clean mildew). Additionally, water has gotten into the apartment before, before I moved in, and my landlord’s said it had been taken care of. This is a 350 sq. ft. unit, backing and the back wall is about 2 feet underground. Should I be worried??


    The levels of Penicillium/Aspergillus is elevated indoors. With the history of moisture in the property, I would be interested in seeing what is inside the wall cavity in the area that has been damp. Also, mildew technically does not grow in the indoor environment, what those stains on the windows and shower are is actually mold growth. Some people (especially uneducated landlords) will call growth mildew because they are afraid to say the “mold” word. Mold in the shower is hard to keep a handle on once the discoloration starts since the area is inherently exposed to routine moisture. Growth around windows is an indication of condensation issues in the apartment. This happens when the indoor air is humid and meets a cold surface such as a window in winter.

    Our family owns a 100 year old building. Our tenant insisted on having a mold test done. The spore trap results showed that the spore counts were elevated compared to the outside.

    Outside baseline spore trap (non-viable methodology):
    Basidiospores: raw ct- 2, spores/m3 110
    Cladosporium: raw ct 41, spores/m3 2,200
    smuts,myxomycetes, periconea: raw ct. 2 , spores/m3 27

    Bedroom 1:
    Penicillium/asperigillus: raw ct 25, spores/m3 2,800
    Cladosporium: raw ct 51, spores/m3 5,700
    Ascopores: raw ct 1, spores/m3 110
    Basidiospores: raw ct 5, spores/m3 560
    Chaetomium: raw ct 3 spores/m3 40

    Bedroom 2:
    Penicillium/aspergillus: raw ct 21, spores/m3 2,300
    Cladosporium: raw ct 32, spores/m3 3,600
    Ascospores: raw ct 1, spores/m3 110
    Chaematomium: raw ct 5, spores/m3 67

    Report did not show presence of other molds. Humidity readings taken on day of test was 52%. Air sample in bedroom 2 was taken near a closed window that reportedly show surface mold growth.

    We are concerned because the mold investigator wants 5 more air tests around the house to determine if there is wide spread “contamination.” I question the additional testing, because they want air samples taken directly next to the windows which show visible signs mold. These windows/blinds are also caked in dust. My guess is that the results will show elevated levels of spores. Also per EPA and CDC, if there are visible signs of mold, there is no need for more air testing.

    There is also wall to wall carpet in the flat which has been in the unit since the tenant moved in 25 years ago. Carpet shows staining and dirt. Also, tenant has had a large dog in the unit for at least 9 years. In the past, there have also been some minor leaks in the house which were repaired.

    He is recommending that the bedrooms be professionally remediated ( carpets ripped out and windows replaced) and depending on the results of the other tests, my guess is he will want the rest of the house remediated (living rm, dining rm, kitchen). The only visible mold is around the window sills, frames, and glass. He also mentions presence of interior condensation on the windows and frames. These are old single pane windows, and they are recommending that these be replaced with double panes.

    My questions:
    1. Do these levels indicate a need for professional remediation? Is he going overboard with his recommendation?
    2. Do we need additional testing for the rest of the flat/apartment? Should we just assume the rest of the house needs professional remediation?
    3. Could the elevated spore count come from the carpet? (no samples were taken)

    I would prefer not to do additional testing as I know this is going to cost us a lot of money (paying tenants to relocate, cost of new windows, cost of remediation, cost of new carpet, etc.)

    Grateful for any advice you could give us.


    Also, I wanted to add – A mold investigator hired by the tenant also ran a test a few weeks earlier, and the results were completely different. His air test showed

    Outdoor sample:
    Cladosporium- raw ct – 22, spores/m3 800
    Penciliium/Aspergillus- raw ct – 0, spores/m3 0
    Other Badiospores – raw ct – 7 , spores/m3 280

    Bedroom1 :
    Cladosporium – raw ct 0, spores/m3 0
    Penicillium/Aspergillus – raw ct 54, spores/m3 2,200

    Bedroom 2:
    Cladosporium – raw ct – 2, spores/m3 80
    other Badiospores – raw ct – 2, spores/m3 120
    Peniciliums/Aspergillus, – raw ct – 32, spores/m3 1300

    All other molds were negligible or zero. We are not sure why there is such a large discrepancy other that they may have sampled it under different conditions or even locations in the room.

    These results are elevated, and the presence of condensation is concerning If you have condensation on the interior surface of windows, you can also be getting condensation in the wall cavity around the window as well. This can lead to mold growth beyond what you are seeing in the window sill. Instead of taking a bunch of air samples around windows with visible mold growth, I would suggest having some small holes cut into the wall near the windows to check for hidden mold growth.

    If the carpet has been wet from leaks or flooding, it could have mold growth. Carpet also acts as a reservoir for mold. This means as elevated levels of mold float around in the air, they will settle onto surfaces. When the mold settles into carpet it can remain trapped in the carpet for long periods of time if they are not cleaned. Over time large amounts of settled mold can become trapped in the carpet.

    Thank you for your response. We live in the fog belt of California so moisture is a constant problem. Do you feel with these elevated counts that professional remediation is warranted?



    I am not a medical professional, so I can not assess if the numbers are healthy or unhealthy. Without knowing the outdoor levels at the time of the assessment, my interpretation is very limited. It seems like these levels are elevated, but I can not say to what extent they are without the outdoor sample results.

    Desr Dylan,
    Thanks for helping people make sense of these lab reports. Its very kind of you to do so.
    I live in Hawaii. Live in an old small home surrounded by a lot of vegetation. Several years ago we had roof leaks that were repaired. Just this past year we began having some roof leaks into the home again and it took the land lord several months to fix. The indoor ceiling is primarily dry wall over the ceiling rafters with little to no crawl spaces to investigate for mold. Clear and evident water discoloration is visible on the drywall in some places. Mold inspectors came in and read high levels of moisture in the lower corners of the ceiling drywall on many parts of the ceiling of the home.
    After the roof was reshingled the landlord had a mold remediation come in. They didn’t remove any of the drywall, but just fogged the whole house for 24 hours with some “special” mold killing enzyme. After the “fogging” the mold sits ample rest they took reported low mold spore counts. I didn’t trust this would work so inrequested another air test 6 weeks after.
    The following is from that test. I know some of these numbers are high, but how concerned should I be;

    Aspergillus/penicillium sp;
    exterior – 1700 spores/m3. RC 18
    Living room – 27000 spores/m3 RC 277
    Bedroom – 4400 sp/m3 RC 46

    Cladosporium sp.
    Exterior -9300 sp/m3 RC 97
    Living room – 3000 sp/m3 RC 31
    Bedroom – 16000 sp/m3 RC 170

    Hyaline sores, acremonium sp.
    Exterior – NA
    Living room – NA
    Bedroom – 12000 sp/m3

    Myxomycetes/ustilago/periconia sp.
    Exterior – NA
    Living room -190 sp/m3 RC 2
    Bedroom – 96 so/m3 RC 1

    Fusarium sp.
    Exterior – 290 sp/m3 RC 3
    Living room – NA
    Bedroom -480 sp/m3 RC 5

    Exterior – 14000 sp/m3 RC 142
    Living room – 31000 sp/m3 RC 324
    Bedroom – 35000 sp/m3 RC 369


    Moldy and water damaged materials need to be physically removed to be properly remediated. While chemicals and fogging agents may “kill” the mold, it does not remove the mold from the indoor environment. Even dead mold can cause health issues, so the only correct way to treat mold is to physically remove it from the space. As shown in your lab results, the indoor levels are still elevated.

    I am currently staying in a rental that has some obvious mold growth. Some existed when we moved in, some grew rather quickly after storm surge due to Hurricane Michael. There is an amazing amount of demo to be done and my landlord’s insurance company is not in a hurry. Below are our results from our recent mold test. I’m trying to determine whether or not it is safe to stay before and during demo/remediation. Thank you for your help in advance.

    Upstairs Bedroom
    Air Sample
    Asp/Pen Raw 81/ m3 1080
    Chaetonium Raw 1/ m3 13

    Downstairs Bedroom
    Air Sample
    Asp/Pen Raw 61875(est due to high count) / m3 825000
    Chaetonium 0/0
    Stachy Raw 13 / m3 173

    Air Sample
    Asp/Pen Raw 17 / m3 225
    Chaetonium 0/0
    Stachy 0/0

    Asp/Pen Raw 30615 / cm2 189860

    That is not enough information to give any feedback. I would suggest looking through some of the comments to see the kind of information others send over for review.

    Hi Dylan,
    Are levels of pen/asp at 5400 raw ct and 9,700,000 per m3 high and if so what would be your recommendation? I was not provided any outside numbers???
    Also the report says:
    Hyphae fragments 30
    Skin cells 67-330
    Pollen <67
    Background debris 3
    Limit of detection 67
    Sample volume (liters) 15

    I have no idea what any of this means
    Thank you


    That is higher than I have ever seen for Asp/Pen levels. Even without outdoor numbers, I can say those are highly elevated. There are some serious mold issues inside the area the sample was collected from.