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.”

Leave a Reply

Your email address will not be published. Required fields are marked *

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

    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.