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The Problems Caused by False Negative Mold Tests

by Catherine
air testing for mold collage

How to Get an Accurate Diagnosis When Air Testing for Mold

Recently, during some email correspondence with a fellow mold sufferer, I was reminded of a problem that afflicts many people trying to find out answers about mold in their indoor environments: The prevalence of false “negatives” or “safe” spore count results in certain air testing methods for mold. It is a problem that I am intimately acquainted with, because my family also got an “all clear” verdict on our extremely toxic home from the first mold inspector that we hired.

In this person’s case, the mold inspection and mold testing were also performed by a professional. The inspection only consisted of the collection of spore-trap air samples that were sent off to a lab for analysis. No “problematic” molds or spore count levels were identified in any of the samples collected. Spore trap air sampling was also the testing method used in our home that didn’t indicate our very big mold problem. It is a testing method that typically compares indoor air samples to outdoor air samples. Types of molds and number of spores are analyzed by microscopy to determine whether or not a mold problem exists with the indoor air. (I think determinations made on indoor environments that are solely based on outdoor mold comparisons is faulty logic. You can read my post HERE about why.) The home was then “cleared” or declared “safe” and she and her husband put mold out of their minds and continued living there.

As a result, they became progressively more ill. But, since mold was ruled out by professional air testing, for a time, they quit pursuing the mold avenue as a possible cause for their health problems. In desperation, the wife began some correspondence with me and decided to hire another inspector for a second opinion. This time, I was able to help her connect with a certified home and environmental inspector who understands the connection between environmental toxicants, like mold, and health. His inspections include an occupant interview, visual inspection, home history, multiple testing methods, moisture readings, and much more. His inspection did NOT find the home to be safe. He found exceedingly high levels of mold throughout the home and in the ceiling and wall cavities. The results of his testing were so alarming that she and her husband moved out of the home immediately and are now staying with family, so that they can heal and decide how to best handle the situation. While the second inspector’s findings were not happy ones, they finally gave some reasons behind the couple’s failing health ever since moving into the home.

How Can Toxic Mold Be Missed?

It is definitely scary how testing done by two different professionals in the same home can yield such different results and information. But there is another layer to her story: Even the seasoned, incredibly competent second inspector did not find alarmingly elevated mold counts in his air samples. Put simply, the dangerous levels of mold were present in the home the whole time—for both inspections–but didn’t appear in ANY of the air samples. Of course, the second inspector found the mold, because his methodology was much more comprehensive, but what if it hadn’t been? Then two inspections may have occurred, but the homeowners wouldn’t have known how truly sick their living environment really was.

To give you a more complete picture, here is one of her recent emails to me:

Dear Catherine,

Remember how I told you that our first mold inspector who did the air sampling found nothing? Well, Jeff found indoor mold dispersed almost EVERYWHERE (aspergillus, penicillium, Chaetomium, plus a fourth brown-colored mold species he could not identify.)

The funny thing is that the mold only showed up on his tape samples and bulk samples taken from inside the walls and ceilings—not in his air samples either. Weird, huh? Even Jeff was puzzled as to how there could be so many mold spores growing and/or deposited all over the house, yet so little showing up in the air samples.

We do not have indoor air conditioning or heating. We rarely use fans, because the electrical wiring in our house is ancient and sketchy—I was even doing all of our laundry with a paddle in an old-fashioned wash basin, rinsing things in a claw-foot tub, and hanging things outside to dry!

All that is to say—there were no electrical fans, HVAC systems, or anything disturbing the air in our home to generate any airflow. I can only assume that is why the mold spores didn’t show up in any significant concentrations in air samples? Maybe there just wasn’t enough airflow to make the spores airborne at the time of testing? Jeff says he agrees with my theory, but he still insists that it is highly unusual for there to be SO much mold present in a home, yet so little showing up in the air samples. He said that aspergillus spores, in particular, form fragile chains that are easily broken and dispersed into the air, even with the most minor disturbances. He admits that he is still scratching his head as to the discrepancy between his tape samples and the air samples. Ah, well. Do you have any insights?

I think I reread her email 20 times before writing my response, because I wanted to make sure that I understood all of the variables at play during testing before making any conjectures about where things went wrong. She definitely didn’t need one more person who is supposed to know about mold telling her that they didn’t have any answers. I mean, here is this couple, being proactive, doing the right things, even hiring professionals to help them, but getting “clear” air samples in their contaminated home. How do I explain that?

I have many theories, backed by evidence, of course, which I will get to later, but false negative air tests can plague families looking for real answers. What is going on? If the mold is there, these air tests, especially the professional air testing methods should find it, right? Unfortunately, this isn’t always the case.

To begin my explanation, here is my email response to her:

The sad reality is that your experience with the negative air test results is not rare. I have so much personal knowledge of this, I could probably write a book! I have actually seen incorrect air tests so frequently that I have written to heads of the restoration industry, environmental inspection industry, and the Indoor Air Quality Association about my findings, research and observations of this problem. I wasn’t just writing to share my family’s experience with false negative air testing results in our mold-toxic home, but to share things I’ve learned with all of the air testing I do with mold plates. I have tested how things like the seasons or indoor ceiling fans, etc., can affect mold counts in the same room just minutes apart. None of these variables, as far as what I have seen from mold inspection training manuals, seems to be quantified or taken into consideration during testing. I truly think that inspector education needs to improve, because countless variables influence the test results and can mean the difference between “passing” or a “failing” a home. With such variability in results, people should be properly educated to not rely upon one testing method to determine their home’s overall safety.

I guess what I am saying is that air testing is FLAWED at best. It represents a miniscule sample from a miniscule moment in time. An air test doesn’t take a home’s history, geo location, elevation, air flow or lack thereof, or its inhabitants’ symptoms into consideration. An air test can be a valuable tool, but only when used in context and with other home inspection tools and tests. This is why the expertise of the person hired to do the inspection and testing is almost MORE important than the testing itself. A good mold inspector, like Jeff, knows the limitations of all of their testing tools. They rely as much on what they see and feel as what the tests reveal. There is only a danger in using air testing when it is used as the sole way to discern and determine the safety of a home.

IAQ Deck about air testing for mold

An informed and skilled inspector should perform a thorough visual inspection, occupant interview, and take tape samples, dust samples, air samples, moisture readings, and particle counts. Without taking the time to go through those steps, it may be difficult for an inspector to know if a home has a mold problem. I do not know what the industry standards are on the skills or learning required to become a certified mold inspector, but from my own experience, and from hearing from countless people like you, it seems that these basic principles are not being taught. Most of the really excellent inspectors out there, like Jeff, are the way they are because they have their own personal stories with environmental illness or mold, or they have a real thirst for learning and like the detective work inherent in figuring out what is going on inside a home that is making its occupants sick.

You probably already know this from reading the blog, but we spent about $1,000 on our first mold inspection. That inspection only consisted of air sampling and testing. He told us, based upon the lab reports from that testing, that our house was perfectly safe. He told us that the mold we did have in our home was no different and not at higher levels than the fungal ecology right outside of our front door. He was incorrect, because he did not look further than some air samples, but we did not know any better, and, quite frankly, neither did he. Thank goodness I kept digging, and found a more knowledgeable person to help us. 

Now for what I think occurred in your situation: I have found that if the indoor humidity is high, above 55% or higher, dust and mold spores are inhibited from becoming airborne. Because of your geo location and the still conditions inside your home, I can only suppose that the humidity was quite high during both air tests for mold and prevented both inspectors from getting good samples. You never mentioned this in any of our correspondence, but was the interior of the home painted  prior to your moving in? It is also worth mentioning that mold is often painted over. This seals it into building materials where moisture can find it from inside of the wall. If this is the case, it would require some serious disruption to become airborne or to show up in testing.

Anyway, this is much more than you wanted to know, I am sure. Please take care. I hope things are looking up and your health is improving. I am so thankful Jeff was able to help.



So, after that cautionary tale, how can you prevent the same thing from happening to you?

Go Into It Knowing That Any Indoor Air Sample is a Blip in Time

The first thing to remember is that an air test, just like any other way of testing for mold in an indoor environment (dust and tape, and TAP testing) is a blip in time. Essentially a moment is captured, cultured and/or analyzed from an environment. Even with multiple samples taken, this can be a problematic foundation from which to base judgement. An air sample, whether on a petri dish, or in a vacuum cassette, grabs a momentary sample from an ever-changing environment. If site conditions aren’t controlled and taken into account, how do you know if your sample is viable? The answer is, you don’t.  A number of factors inside a home are extremely variable and affect indoor mold counts. Individual air samples can show tremendous variation from minute to minute. This should make “ok” airborne mold test results a thing to always view with skepticism when looked at alone. They are just a piece of a bigger puzzle and should be regarded as such. Many more factors must be taken into consideration when evaluating a living space for mold.  (Of course, gathering multiple samples and utilizing a variety of sampling and testing methods addresses some of this.)

Factors That Can Affect Airborne Mold Spore Counts on Tests

To give you more information, and to dive deeper into how some specific variables affect mold spore counts on actual test results, I have created the list below. Each potential source of variation is described to help you view your environment and your test results more like a detective. I have also indicated how each of these variables affect results, so you can manipulate them for the best testing outcomes.

1.)    Still indoor air conditions – If no doors or windows are open, no fans are running, and no occupants are present, lower airborne mold counts are produced. Try to make your testing environment as close to “normal, daily life” as possible to get the best results.

2.)    The numbers of occupants present indoors during testing – A room with just one person present will produce higher spore and particle counts. Thus, the more people present in a home and coming and going during testing, the more spores that tend to appear in testing results. You definitely don’t want your home to be Grand Central Station, but, if an inspector is alone in an empty house, realize that his air test results are going to likely be lower than what is actually present.

3.)    The type of mold contamination in a home – Certain molds appear in airborne test results more than others. For example, Stachybotrys is a large and not easily airborne mold. It is also very sticky. These spores are not normally found in indoor air, even if a large mold reservoir is present, unless the reservoir is being disturbed. Some of the only cases when I have seen Stachybotrys on diagnostic test plate results are cases when demolition or renovations have recently occurred. Stachy is one of the more toxic molds, but it is often missed in air tests. On the other hand, Aspergillus sp. are smaller mold spores and are easily made airborne. (This can still vary, though, depending on indoor humidity levels, temperature, and the growth stage of the mold.) Air tests tend to find these spores more easily. Thus, if you know you have water damage and have visible mold somewhere, your visual evidence is more important that what your air test says.

4.)    The outdoor mold counts at the time of testing – I do realize that spore trap testing done by a professional does take outdoor mold counts into account. There are problems inherent with that methodology as well. (Read my post about that HERE.) The main problem is that the way these counts are compared is flawed. The labs compare genera, but not usually species. For example, the same genera could appear both outdoors and indoors (say Aspergillus), but the species could be completely different (one indicative of water damage and another not), which makes such a comparison meaningless. Also, unless you live in a sealed bubble, it is impossible to prevent outdoor mold spores from entering your indoor environment. The trick here, in my opinion, is to see patterns in your indoor testing and areas of concentration that do not “jive” with the outdoor fungal ecology. A good example of what I mean here is that during seasonal pollen and mold blooms outdoors, you should expect to see higher counts indoors. But in the winter, during a rain, or when there is snow cover on the ground, it should be expected that the indoor counts would also be much lower, with not as many typical outdoor molds appearing in lab results. In other words, indoor conditions should “make sense.” If they don’t, look deeper.

5.)    Carpeting – A room with wall-to-wall carpeting will yield a higher mold spore count than an uncarpeted room, even if all other conditions are equal. Thus, realize that your high counts in the carpeted room could be from the actual carpet and not from water damage. This would be an instance where placing testing equipment or your plates high in the room could be helpful. Also, testing the carpet separately and under a microscope and comparing that to an air sample, might show if the carpet itself is the problem.

6.)    The height at which the air sample is collected – Many mold inspectors place their air samplers at chest or head height, because collecting air at that level best represents the particles that are inhaled by the home’s occupants. spore trap air testing setupThe problem is that mechanical disturbance of any kind will cause large variations in a room’s dust level. Dust contains mold spores, and when disturbed, both can become airborne. The size and heaviness of mold spores determines how easily and how long they will stay airborne. Sampling too high, or sometimes too low, can miss quite a bit. One great example of this is an air sample taken in the same room, where the test plate placed on top of the coffee table came back clean, whereas the test plate placed under the coffee table came back with spores too numerous to count. It was the coffee table that was contaminated, but if the lower sample had been the only one taken, it could’ve led to a massive and unnecessary remediation. It is always best to test at multiple sites and levels in the same room. This can give you a better idea of the overall mold picture.

7.)    The aggressiveness of the methods used for air sampling – The person gathering the sample should NOT be passive. This is especially pertinent during clearance testing post remediation. Waving a paper over tabletops, stirring a room with movement or the use of a fan, or rapping on ductwork prior to gathering a sample are going to give you a better picture of the actual indoor environment. If an inspector is there alone, gathering samples methodically from an unoccupied home, you are likely to not find hidden mold problems.

8.)    A mold reservoir in close proximity – Some rooms have higher airborne spore counts because of their close proximity to a moldy space. The natural stack effect (air being pulled from the lowest post up through a home or building, so that it can escape out of the top) makes this particularly true. For example, in a home with a large mold reservoir in the basement, the kitchen showed extremely high airborne mold counts, because the interior door to access the basement was located in the kitchen. Not only was the stack effect at play here, but each time the basement door was opened, the kitchen was flooded with mold spores. The kitchen did not actually have any water damage or leaks, but became a highly-contaminated space due to its close proximity to the moldy basement. The basement needed major remediation, but the kitchen also needed to be carefully treated and cleaned to keep the occupants safe and healthy. In this case, testing in the basement and the kitchen would’ve given a very good picture of what was happening.

9.)    Open Windows – This is one of those confounding factors: On the one hand, open windows can create considerably higher indoor mold counts than when the windows are shut. But, on the other hand, in some situations, an open window can also decrease indoor mold counts. Depending on the pressure inside your home (the use of fans, HVAC, outdoor temperature and weather all affect this) interior air can either be drawn outside or outside air can be forced in. In cases where there is a great deal of positive pressure, opening a window forces interior air out of the home. This would result in lowered airborne mold counts. If a home has a lot of negative pressure, though, opening a window causes the outdoor air to flow inside. This would raise your airborne spore counts. This is why I always encourage people to keep windows closed during testing. It helps to eliminate this problem.

10.) The use of fans and HVAC systems – Mechanical disturbances of any kind, but especially those caused by a consistent blowing action, will obviously increase the amount of dust and particulates in the air. Thus, mold counts will be higher than when fans and HVAC is off. My suggestion is to always leave HVAC systems on for testing. You want mold spores that may be present in your HVAC system and ductwork to appear in the testing. I also suggest either cutting a sample from your HVAC filter to send into the lab for analysis to pair with your air test, or TAP testing a filter onto a mold plate. Doing this in addition to the air testing will help you to better determine the source of any problematic mold. For fans, if you normally have them on, I say, leave them on. If not, leave them off. You just don’t want the room to be completely still.

11.)  Indoor Humidity Levels – Humidity is also a confusing variable, because, while high indoor humidity fosters mold growth, in inhibits air movement. Thus, the dryer it is inside when you are testing, the more particulates and mold spores you are going to be able to capture for analysis. My advice here is to do the best you can to create dryer indoor conditions when testing. You can do this by taking some of the previous advice of using fans and your HVAC system. You can also purchase an inexpensive hygrometer to measure the indoor humidity prior to testing.hygrometer for humidity monitoring

12.)  Air Filtration and Purification Devices – In my experience, I have seen that the active use of these devices during air testing actually makes no significant difference in test results when a home is moldy. I know that sounds counterintuitive, but the fact is that any air purification device has a fan. The fan disturbs the air, so that the device can filter particulates from the air. But, if a home has a significant mold problem, an air purification device is not going to solve it. It will help to filter and remove some spores, dust and particulates in the immediate space where it is placed, but it cannot fix a leak, stop water intrusion, decrease humidity below 50%, or stop the water issue that is behind the mold growth. Thus, don’t worry about the device “ruining” your results. Rather, take test samples in your rooms with purifiers. If you have really high counts in those rooms too, you really need to do some deeper digging.

With So Many Variables and Limitations, Why Air Test for Mold at All?

I know you were wondering this, so I thought I would go ahead and address it.

1.)    Any screening mechanism gives you information that you did not have before. I am a big advocate for using inexpensive mold plates to test and monitor your environment and things for mold. (Read my post on why I test HERE.) The more you test, the more you know. You can also start to see how trends and changes in your environment are affecting your health. I have found this information to be incredibly useful and valuable for my continued recovery. Also, just like you cannot rely on air tests to give you all of the information, other screening mechanisms, like visual inspections become much more valuable when paired with air test results.

2.)    Air testing for mold is a great way to cross-check remediation efforts and cleaning for mold efforts in your home. If an area has been deliberately cleaned or remediated in anyway, or if you have removed a contaminated object from your home and want to know if the problem is taken care of, air testing can provide valuable information about the efficacy of the work or process.

The bottom line is YOU DON’T KNOW UNLESS YOU TEST.

Before I close this post, I want to leave you with positive action steps, so that you can be on your way, armed with knowledge and competence.

Best Practices for Using Air Testing for Mold to Your Advantage

1.)    Test frequently.

2.)    Take TAP, tape and dust samples too.

3.)    Open interior doors during testing. This prevents a condensation problem or a leak in a closet wall from going undetected in the results.

4.)    Close windows to the outside during testing.

5.)    Allow overhead fans and your HVAC system to run normally during testing.

6.)    Leave interior lights on during testing.

7.)    Avoid vacuuming prior to testing.

8.)    Run tests your ductwork and mechanical systemsThis should include visual inspections, tape samples, and air samples.

9.)    Don’t ignore the obvious. If you SEE or SMELL mold, it should not need to come up on an air test for you to recognize that there is a problem. Any water/moisture issue leading to that mold growth needs to be addressed to preserve your health.

Overwhelmed? Don’t be. If you have questions or comments, write to me below or email me at catherine@moldfreeliving.com.
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April - 12:21 pm

Hi, was wondering how to find a company that will do the more thorough testing? We had the air test done and it was fine, even had a second pot and they looked for moisture in the home. But family symptoms continue to decline. When I called around, everyone seemed to do the same basic air test. Thanks for sharing this!

Catherine - 7:41 pm

Hi, April,
Yesweinspect.com is a great resource for testing. So is https://environmentalanalytics.net/about-us/ Both would be able to guide you in thorough testing practices to find the source of the issue.

Irma Hernandez - 8:02 pm

Hi Catherine,
We hired a professional company 1 week ago and they did not detect anything on swab samples but found high air mold levels. We had just done a demolition for renovations when we noticed black on studs and frame.
I’m confused on how nothing was detected on swap samples but air sample were high.
Also we feel we need a second opinion because when I called the company we hired they could not tell me why there are no specific mold counts or levels. It just says present, high, medium and low.
Is there someone you recommend for mold testing who has more experience and is knowledgeable with mold toxicity?
We live in California. Thank you.

Ps I did order the plates you mentioned. We tested the garage and are waiting for results

Catherine - 3:03 pm


Here is a great resource on testing and it will also explain why there are testing discrepancies. There must be many different kinds of testing done to get the best picture of what is going on in the home. https://iseai.org/mp-files/mold-testing-101-iseai.pdf/ I would use the ACAC.org website to find an indoor environmental professional near you. Look for ACAC candidates with CIEC and CMC certifications. For remediation, look for ACAC candidates with CMRS and CMR certifications.

Patrick - 6:22 pm

Hi Catherine,
I am at a loss for what to do next. I started getting sick around the time we got a new house, married, first kid and new used car. My symptoms are sinus swelling and fatigue. Later on dizziness came. First thing I noticed in the house was high humidity. Humidity would get up to about 60% in spring and fall. I got a stand alone dehumidifier for master bathroom. I went to many doctors, etc and nothing was ever found. I did end up getting 2 nasal surgeries but fatigue persists and swelling returns. I ended up getting an Austin air purifier, didn’t seem to help any. I kept getting sicker. I did an ERMI and it came in not too high at 4, HERTSMI was also 4. Had 2-3 mold inspections done and nothing major was found.
I got another dehumidifier for other bathroom and another Austin Air. Many air samples all low. Probably the biggest thing ever found was cladosporium mold found in 2 of the supply boots growing on the fiberglass board. Those were replaced along with the HVAC unit. New unit controls humidity much better. About 2-3 years ago my 8yo daughter developed bruxism and some nasal congestion. I also have a history of bruxism. About a year ago my 5yo developed mood issues and reduced appetite. About 3-4 months ago my son was getting tummy issues and all they could find was strep. That lasted about 2-3 weeks and went away but has come back. My 8yo has been sick off and on the past 2 months with nausea. Also some hair loss. Wife also has hairloss but might be related to thyroid. She has high TSH and her mother had thyroid issues too.
I’ve taken 2 different urine mycotox tests and they were completely negative. I’m unsure what to do next. Maybe it’s my car? Maybe in house but can’t find it? We seem to have symptoms of VOC illness. Maybe it’s from MVOCs? Should I test car for mold via agar plate? Hire a much more expensive mold inspector? Do VOC testing?

Catherine - 9:31 pm

ERMI scores and air testing will not tell you everything. What molds were present on your ERMI? What cleaning of the home was done after the HVAC was replaced? What about ductwork? Was that replaced? What about wall-to-wall carpet? Was that present while the HVAC was contaminated? Has it been replaced? Do you have a below-grade basement or a crawlspace? I would also look into heavy metals? Has your water been tested? Are you drinking tap water? While your symptoms do sound like mycotoxins, they could also be other things. I will say that the tell-tale signs of the sinus issues do have me feeling that something in the home is causing your sickness. My best recommendation would be to do environmental mycotoxin testing the home and to also have your actual ERMI test evaluationed by someone who looks at the molds present and not the score. The score often leads people to think a home is safe when it is not. If your mycotoxin score for the home is high, you need to understand where the mold is actively growing. The home will also need a level of cleaning that is specific to the removal of mycotoxins.

Amber Rush - 2:54 am

Question– What makes you this happen? stachybotrys on diagnostic test plate results are cases when demolition or renovations have recently occurred– I did the ERMI testing and had a 2 for this mold and we just finished renovations about 2 years ago.

Catherine - 1:29 pm

Hi, Amber,
The ERMI test is different from an air test. The ERMI is a dust or vacuum sample, so the same logic does not apply. If you had Stachy appear on your ERMI, that can mean that you have hidden water damage or a mold reservoir somewhere. It could also mean that your renovations disturbed mold growth that has now gotten into your living spaces. Whatever the case, you should probably hire an environmental inspector to come see if they can find the source, so that you can fix it and do a thorough cleaning to prevent sickness.
Stachybotrys spores are heavy and do not remain airborne long. That makes them somewhat difficult to capture in air or spore trap samples. I was referring to situations in which a renovation was done and the demo uncovered water damage and mold growth. Sometimes opening that mold up sets spores free into the rest of the home–spores. I hope that helps and makes more sense.


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