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Mold FAQs:

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Here is an extensive list of frequently-asked questions abut mold contamination and links to supporting in-depth articles about finding, cleaning, and evaluating mold in buildings, mold related illness, and preventing mold growth in buildings.

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Mold and Indoor Air Quality Frequently Asked Questions (FAQs)

These Mold Questions and Answers, or Mold FAQs address common mold-related topics such as crawl space mold, mold water damage, health problems related to mold, how to kill mold (this is actually a mistake, the proper approach is to remove mold not kill it), how to get rid of mold, indoor air quality problems caused by mold, and the health effects of mold exposure.

We receive more telephone calls and email queries about mold in buildings than we can return - for which we apologize. Our website is an attempt to offer some general advice to folks who have a concern about mold. However no one can offer sound diagnostic advice by email or over the telephone. If financial or health concerns are significant you should consult a qualified expert.

One of our correspondents, A.K. recently asked a set of great questions, which he was kind enough to write down - form the basis for a good part of this Mold FAQ. These frequently asked questions or FAQ's about mold investigation shown below are discussed here. For more in-depth and more organized discussion of how to decide if you have a mold problem, when to hire a professional, what to do about mold, and what investigation methods are valid and invalid, see the Mold Information Center at

Questions about Building Mold Contamination

How extensive an investigation and cleanup is really needed?

Our wife and we live in a condo in MD. Last fall when we had several tropical storms in a row, we began to smell a musty odor in one of our bedrooms. A spore trap air sample (non-viable methodology) was taken from the cavity of the external wall which showed Pen/Asp of 800 in the wall versus Pen/Asp of 311 outdoors.  The wall was then removed and visual and swab tests confirmed the presence of high levels of mold. The room was remediated - but we are still attempting to get the builder to identify the source of the moisture in that room.

What was done to determine the scope of cleanup needed and how confident are we that it was adequate? Without considerably more work than a few air or swab or culture tests we have no basis to decide what to do, even if those tests suggest that there is a problem. Was there a thorough visual inspection to identify probable leak sources? Did your investigator take a building history?

Were the mechanical systems examined for leaks? It is certainly possible that the original leak or moisture problem remains, which means you could have wasted the cost of the cleanup - if you have to do it again due to failure to fix the underlying cause. From your question alone we can't tell if you needed a more expert and more costly investigation. See when to hire an expert for more advice. Small areas of moldy stuff don't justify hiring anyone except in rare cases. Just clean up or remove moldy stuff which is less than 30 sq.ft. - but even in a small mold case, take more care if you have special health risks.

What should we make of wall vacuum test results done without other inspection and testing?

Vacuuming building cavities depending on how and when they are performed, can be like searching for a needle in a haystack while looking through a straw: if you find evidence of a problem you're lucky. But if you don't find evidence, when using a very limited-scope method, that doesn't mean that a problem is not there.

Short vacuum pump duration for microscopic examination (to avoid sample overload, e.g. 2-=3 minutes on an Air-O-Cell) does not move enough air to reliably find what may be in the wall cavity. An experiment done with Louis Relle in New Orleans LA demonstrated that wall-vac tests found less than 10% of large problem mold cavities that could be discovered by cutting drywall openings in a building.

Longer vacuum pump duration samples for viable sampling (2 hours into an MCE cassette for culturing) still may not move enough air to sample through a cavity, particularly if the cavity is insulated. Further, two-hour samples means that most-likely very few sample points were collected, making the inspection scope extremely limited and thus overall confidence in the accuracy of the picture of the building lower.

Remember that this approach relies on culture of the sample. Did you know that only about 10% of molds grow in any culture at all? You're 90% uncertain of the accuracy of the test at the outset. One can't be sure that the mold that grew in the culture represents the dominant problem mold or whether it's just a low-occurrence (in the building) spore that liked the media (in the culture). We like cultures for further genera/species identification of samples but we are nervous about relying on them to tell me if the building has a problem or not.

How should we interpret relatively low spore trap count results?

Spore trap air samples were taken from all of the exterior outlets - twelve in total. Two of them showed Pen/Asp of 1330 versus Pen/Asp of 267 outdoors. Is this a problem?

We can't say without more information about the inspection, tests, and test environment. Usually low indoor mold spore levels, certainly in the hundreds of spores/M3 of air, are not themselves a problem except for someone who has become hypersensitive to the particular material present. But a more thoughtful look at exactly what molds are present and the form in which they appear, might still be suggestive of an indoor mold colony presence even though at the moment of measurement the number was low. See Mold in Air: Quantitative Analysis - Spore Counts

Air samples are notoriously unreliable ( when they're used to disprove the presence of problem levels mold. In other words finding a comparatively high level of problem mold indoors is diagnostic. Finding a low level, on an initial screening investigation, is not so reliable, unless accompanied by other building historic, inspection, and testing data.

Air sample results that compare relatively close counts of indoor and outdoor mold, Pen/Asp for example, is a bit like comparing apples and oranges because the outdoor Pen/Asp might not be the same species as the Pen/Asp in the indoor sample, even if the counts are the same. An expert lab might pick this up simply by observing that the outdoor and indoor P/A spores are morphologically different, even if they're not speciated.

More apples and oranges: an indoor count that is elevated above outdoor is taken by some experts as indicating a possible problem. An indoor count whose absolute number is relatively low, say 300, may taken to represent an acceptably clean building compared with the levels we find in known-moldy buildings (often in the thousands).

But low indoor count of a few hundred spores/M3 of air might still indicate a probable indoor mold problem if your count data shows that, for example using Pen/Asp, the outdoor count of Pen/Asp was 7% of total spores (or particles) in the outdoor sample, while the indoor count of Pen/Asp was 60% of the total spores in the indoor sample.

The microbiology lab industry is under pressure from the legal community and other clients to produce tests that have numbers, even though the numbers are in truth, wildly inaccurate. While there is definitely an important place for air sampling, cultures, and vacuum samples, expert visual inspection, possibly invasive inspection, and qualitative testing is more reliable in accurately characterizing a mold risk and the extent of mold cleanup needed in buildings.

What does Viable and Non-Viable Spore Count mean to me?

This is mostly our opinion and many investigators take a different view, particularly some of our CIH friends. Viable spores mean that the lab counted the number of spores that were alive in the sense that they produced a colony when grown on a culture medium in the laboratory.

Non-viable spores or a non-viable spore count, counts a spore level without trying to see how many spores in the sample would grow on a culture. We have many concerns about the confusing shortcomings of culturing: in short, many molds that are viable won't grow in any culture, and besides, many molds may remain toxic or allergenic (as particles) even if they're dead or non-viable.

Finally, I'm not sure that what grows in the culture is in fact the same as what's the dominant problem in the building. Cultures form an important tool in the hands of an expert, but used in a simplistic manner they're simply giving you junk-science answers to a question involving possibly big costs and health risks. See Cultures to ™Test' for Mold at

How useful are moisture measurements for finding mold?

Measuring for moisture is a useful tool in evaluating a building - provided the moisture from a leak or vapor trap is still present! Like other investigative methods I've discussed, used alone or without more building knowledge, just looking for moisture would be unreliable - a building may have had a severe leak and thence a severe mold problem, but the water may have dried. The problem is still there to be found, but it won't be indicated by active moisture levels.

See Humidity: How Low Should You Keep Indoor Humidity to Avoid a Mold Problem and Condensation: Detecting and Correcting Attic Condensation and Preventing Mold and Ice Dam Leaks in Buildings

Why do a few investigators want to make test cuts into walls or ceilings? When are they appropriate?

Individual random test cuts into drywall or plaster walls or ceilings to look for mold

Random test cuts for mold or random wall vacuum samples, are probably unreliable. Even if it is known that there have been leaks into a structure, the path of flow of water through a building may be difficult to predict. I've disassembled finish surfaces in buildings to find that a very large amount of water flowed across a ceiling through only a single joist-bay, then down into a pair of wall stud bays, into the basement. A single random opening for water would have a low chance of finding this path. Just because you don't find mold in one spot doesn't mean that you will find it in another spot.

Obviously one does not want to do un-needed demolition, but in some circumstances this is the most-sure approach to a difficult area such as the case above. As a general rule, we don't cut an opening or do anything invasive or destructive without a good reason, and not without the property owner's ok.

Tactical test cuts for Mold

Tactical test cuts may be reasonably made if the investigator has found evidence to suggest a highly-suspicious spot. At a test cut big enough to see into a wall or ceiling, perhaps 4 square, we can inspect the reverse side of the finish material, the insulation in the cavity (if present) and the other wall cavity surfaces.

Extensive strip cuts to inspect for mold or other building damage

In very difficult cases where we have a very high suspicion of a hidden mold problem, but have been unable to trace external visual evidence and historical data to the actual problem source, we may call for long narrow strip cuts through drywall, across walls or ceilings.

Are condominium or multi-story building balcony leaks a common mold source?

Most of the exterior walls of our unit are under large balconies for the unit above ours - we are the last condominium of our type in the stack before a new floor plan begins above us. Referring back to the rectangle drawing, if you draw two diagonal lines connecting the short lines and the bottom long line to make large triangles in the lower left corner and lower right corner you will have a sense of where the balconies are located above us. We have been told that unless balcony construction is "perfect" it is a likely source of water penetration. Do you have any thoughts on this?

Balcony leaks in multi story buildings such as you describe are notorious - bad flashing on the balcony, bad drainage, or leaky doors and door thresholds. A friend in Chicago had to stack towels along her doors when the rain was blowing. If water leaks under parquet or into wall cavities from the balcony that could be a good mold cause.

If we smell mold, is mold present and is that a problem?

We have tended to smell the musty odors more when it is humid. Does mold smell more in humid weather because it is growing or is the smell more noticeable perhaps because of the windy/breezy conditions associated with storms? Or maybe its both or neither?

Most people have a pretty good idea of moldy or musty smell as associated with mold. If you smell mold or find it at important levels in screening samples of air, dust, or vacuumed surfaces, (by quantity or by particle type in samples) it is probably there. Humidity changes cause mold to behave differently - species dependent, including release or not of spores, and release or not of MVOC's which is what you 're probably smelling.Wind of course might pressurize some building walls differently, causing wall cavity contents to be noticed or not. I can't tell from smell alone whether the mold is a large or small area, nor can we tell by smell alone if it's allergenic or toxic.

See the next FAQ - Why do mold odors occur ™

Why do mold odors occur in our home following rain?

Recently we began to smell a musty/fishy odor coming from all of the electrical outlets on the exterior walls of our condo. Again, the odors were detected following a period of heavy rain. (We had occasionally smelled similar odors for fleeting moments but had done little to investigate until learning of potential mold/moisture problems in the unit above ours.)

The builder discovered a flashing leak that explains some of the musty odors on the floor above us and that there were leaks in the units next to us and above us before we lived in this condo. We thought that one of these leaks explained the confirmed mold in our bedroom, but recently have learned that leak was not really over the location of the mold.

Odors at exterior outlets sure sound as if there has been leakage into the wall and a probable mold colony. We need an expert visual inspection and possibly invasive sampling, combined with building history, to find and follow leak paths and high humidity cavities in order to inspect the most-likely mold reservoir targets in a building. The odors may be MVOC's which may be produced by some mold genera/species at varying levels as humidity, temperature, air pressure, and other variables change.

We don't know enough about MVOCs - some experts believe that these organics can be a health hazard. Since MVOC production by a mold colony varies as indoor weather and other conditions vary, we don't count on testing for the presence/absence of MVOCs as a mold screen.

What if the lab says that our mold samples have a lot of debris in them?

Each of these samples have background debris levels of 3+ and 4+. Is it really likely that these samples show no mold or only 4 spores or is it more likely that there is so much dust and debris on the sample that the mold (especially the smaller Pen/Asp spores) could not be detected? (I should note that these were the rooms in which we detected the strongest odors.)

We agree that a debris-loaded sample can obscure mold, especially small colorless spores like the Pen/Asp groups which are hard to see in the first place. High levels of debris in IAQ samples hurts you two ways: first, important particles in the sample may be hidden in the debris; second, if we're talking about a spore trap or impaction air sampler, which deposits particles onto a slide in a particle trace, once the trace area has got a lot of debris on it, other particles that enter the collector will simply bounce off and blow away in the collector's exhaust, thus under-representing the level of problem particles present - if they are present.

Absence of evidence is not evidence of absence, but probably the best investigative path is to follow the water path, making test cuts in walls and ceilings if needed (small, not messy, sealed afterwards) for actual inspection and better testing.  

What do Basidiomycetes mean in our indoor air samples?

Out of our twelve samples, two samples show not a single spore of mold and two others show only a Basidiospore count of 133 (4 spores in each sample of 30 liters). In the most recent tests that we had done the outdoor Basidiospore count was 3110.  Two of our samples had Basidiospore counts of 1,470.

Does your point about different species apply to Basidiospores as well? In other words, does a high level of Basidiospores in the wall indicate a problem even when the number is significantly lower than the outdoors? It seems that the outdoor counts of basidiospores were quite high on the days in question and we wondered whether the high indoor counts were really due to outdoor air or just appeared unproblematic because the outdoor sample was so concentrated on the date in question.

Basidiomycetes are common in outdoor air, and represent a very large group of spore genera/species. Certain basidiomycetes are very often present in our samples in the Northeast U.S. such as two of our good-looking favorites, Ganoderma applanatum or G. tsuge.

What's the Difference between "Mold" and "Mildew"?

A mycologist or a good text will tell you what substrates a particular mold has been known to grow on and which types of material it prefers.

For example, mildew is unlikely to be found ever growing on an indoor surface (except for a house plant), since mildew is a pair of sub-group of molds (powdery mildew and downy mildew) which grow only on living plants. You won't find mildew inside, current indoor mildewcide and mildew-resistant products notwithstanding.

We think it's just a less scary name that some folks like to use. Indoor samples that contain molds that are not believed to not grow on building materials are thus unlikely to point to a problem. Other indoor molds, including some of the Basidiomycetes, are water and rot indicators.


Photograph of paint failure details Paint lab Photograph of failed paint sample chip (C) Daniel Friedman Paint lab Photograph of sample of failed paint

Why did the test consultant collect samples at different volumes outdoors and indoors?

In the most recent tests that we had done the outdoor sample consisted of 90 liters and the indoor samples consisted of 30 liters. I know that the spores per cubic meter are adjusted to reflect the size of the sample, but still we are wondering if that kind of a sampling approach makes sense. I have seen at least one person question the competence of a tester when the outdoor sample was 150 liters and the indoor sample was 30 liters.

Whoever said that the difference in sample volumes is an error has shown you that they have an incomplete understanding of the science of test instruments, and certainly has not looked at samples in a lab. Indoors the overall airborne particle level is often higher than outside (unless you're next to a compost facility or a leaf blower).

We have to adjust the indoor sample size down to avoid overloading the sample (see our comments about too much debris in samples). True a shorter sample period is less representative, but I've already addressed this issue of "snapshots" - no short term air sample in a building can pretend to represent the widely ranging building conditions that occur just under normal use and occupancy. Turning on or off a fan, walking across a carpet, or opening windows and doors makes large changes in the airborne particle level over a very short time.

Should we collect wall cavity samples near where we suspect high mold, like at our electrical outlets?

When taking cavity air samples we were wondering whether testing near the electrical outlets would be a good or bad place to take the sample. For example, might testing near an electrical outlet contribute to the higher levels of background debris or somehow otherwise skew the sample. All of our samples were taken behind outlets and we had recently had several of the rooms painted (including the outlet covers) so we wondered if the removal and painting process had contributed to background debris and possibly skewed the results.

This is an interesting question, and I'm not sure of the best answer. If you're highly suspicious that there is a big mold problem in walls due to moisture getting in at the electrical outlets, I'd stop collecting air samples and look into the wall.

When a sample is collected using quiet conditions in a building, and without placing the collector right at a highly-suspect source, the investigator may be trying to tell you what you're being exposed to under "average" building conditions, perhaps for medical reasons. And I agree that such may be useful.

In our diagnostic field investigation work for mold and allergens indoors, I'm usually:

1) trying to decide if there is evidence a problem at all, and if so, if there is justification for more invasive inspection.

2) If the answer to these questions is "yes" I'm trying to find the problem source, find its extent, and thus write a remediation plan.

So while we may compare quiesced-building condition with active building condition (people activity, fans or HVAC on) in general I take a more aggressive approach when looking for presence/absence of a problem  - stirring up local dust and air, rather than trying to get just a "quiet" sample. 

We believe what's important is a consistent methodology so that one can compare buildings and also before-and-after cleanup conditions, and it's important to document the building conditions at the time of inspecting and testing (quiet, busy, HVAC on/off, etc) - otherwise interpreting results is a bit constrained by lack of information that makes a HUGE difference in the anticipated particle levels.

Might the mold being detected in our home just be left over from construction?

I have a feeling the builder may try to tell us that although the tests detected mold in two locations, the mold is left over from construction and that there is no current source of moisture. How likely or unlikely do you think that is, especially given the fact that our unit is directly beneath two large balconies as mentioned above.

IF a building has a lot of soft fuzzy surface like wall to wall carpet and you make it very moldy the fuzzy surface (or debris or clutter) will act as a dust particle reservoir and may re-release mold or allergens enough to bother some occupants.  IF a building is hard-surfaces without the carpet and clutter, ordinary activities of daily living, and ordinary cleaning, would be expected to dilute and in a practical sense remove temporary dust/mold debris from an event during construction. 

BUT IF there is a remaining mold reservoir in the building, and IF it's a genera/species which is readily airborne (such as Pen/Asp) you'll continue to get varying levels of it in air and surface dust and surface vacuum samples until you find and clean or remove the reservoir.

Is all black mold dangerous? Isn't some mold just cosmetic?

Not all black mold is dangerous - it might be unhealthy, or it might just be cosmetic - an expert needs to examine it microscopically to be sure.

Beware of spending to clean up cosmetic-only mold such as the black mold that may be found on new framing lumber - it arrived that way from the lumber yard (Ceratocystis and Ophistoma groups). But also beware: problematic and cosmetic mold may both be present, especially on wet or treated wood lumber.



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