Mold Testing Services
Mold Testing -- Sioux Falls, South Dakota
Testing for Mold
Visible Mold
Why Test for Visible Mold? Lab analysis of visible mold has a number of useful purposes. It is also typically inexpensive and easy to do. Many labs only charge about $35 per test. Tests for visible mold are useful to document that mold is actually present. In some cases, spotting may actually not be mold at all (soot,etc – see Checking for Mold – not mold section). Documentation can have many uses. A landlord may refuse to acknowledge mold is present in a building or that it is mold at all. Before a clean-up, mold testing can help determine the types of mold present while, after a clean-up, tests can be performed to document that visible mold is no longer present (although, usually when no visible mold is obvious, there is often not a need for tests).

Testing can determine what type of mold is present. Some molds are considered worse than others. Some molds can also indicate very damp to wet conditions while other molds may only require limited dampness to grow. Some molds are very common and may not necessarily pose a concern. Problem wet molds include Chaetomium and Stachybotrys (black molds) their presence is often associated with a water problem. Aspergillus and Penicillum are problem molds that may require only dampness for growth. A common, often non-problem, mold found on many indoor surfaces, including OSB, is Cladosporium. Small amounts of this mold are often considered less of a concern than others.

Testing can sometimes verify active mold growth. Visible mold can occur on surfaces many months or years after it has stopped growing. However, microscope testing showing the presence of hyphae (root-like structures) may indicate active growth is still occurring. Brightly colored and/or soft fuzzy growth are often indicative of active growth. Many molds turn dark or black over time when growth has stopped.

Testing (visual size of colonies) may help indicate the time of growth. Under ideal conditions in a petri plat, visible mold typically takes 3-4 days to become obvious to the naked eye (less time under very warm, wet conditions). As a rule of thumb, any visible mold on a surface has usually taken at least a few days to occur. When a wall is opened up the next day after a water leak and found to have mold, this is almost certainly pre-existing mold. It is much more difficult to tell how old large amounts of visible mold are. In many cases, it may take a few weeks or more for extensive growth to occur on the walls and ceiling of a building.

How to Collect a Visible Mold Tape Test.
Only a SMALL sample (thumb-nail size) is required for lab analysis. TAPE tests can be taken using clear scotch tape and taking a one inch sample. Before taking the sample, fold ½ inch edge over to make a tab so you can hold the tape without sticking to it. Press the tape lightly against the surface. Avoid touching the sticky side of the sample with your fingers. Different colored or patterned mold typically require separate tapes. USE ONLY CLEAR SCOTCH TAPE. Do NOT use frosted tape or heavy duty clear packing tape. DO NOT fold the tape over against itself. Press the sticky side of the tape against a glass slide or the inside of a small ziplock bag.  For BULK samples, use a knife or scissors to collect a small thumb-nailed sized sample to be bagged.

Whenever possible, it is very helpful to take digital PHOTOS while collecting the samples. These can be e-mailed or included with the samples. It helps to label the sample area with a written note on the wall (paper, masking tape etc) being sampled such as test#1, N wall basement, 3-4-10. Take both close-up and further away photos.

Chain of Custody (COC).
When collecting visible mold tape tests, each sample should include information as to who took the sample, the date sampled, type material and where taken. For example: sampler John Smith, my house, N wall basement – dark spots on drywall. Contact information including name, phone number, e-mail, and mailing address. MTS or other labs provide COC forms on request

Visible Mold Tests - Limitations.
A tape sample of visible mold is typically INSUFFICIENT to provide an adequate assessment of concerns or health risks (unless large amounts are present). An adequate assessment often requires a site visit by a mold professional and AIRBORNE mold tests. MTS is often sent a mold tape and then asked to confirm if a health risk is present. This is obviously insufficient information for us to make any kind of assessment. Small amounts of visible mold are present in many building and often pose no concern.

Interpreting Visible Mold Test Lab Analyses.
  Because tape tests are typically taken from mold colonies, they can often be identified more accurately under the microscope than can airborne mold tests.  For visible mold, lab analyses are often reported in two different ways. The first is qualitative, the second is quantitative. Qualitative tests report the type of mold present and sometimes offer an approximation as to the amount present – such as light, moderate or heavy.

Quantitative testing assumes visible mold is collected from a specific square area and then the number of mold spores are counted per area. Most visible mold spots typically have millions of mold spores over one square inch or so. This means most quantitative tests for visible mold will show hundreds of thousands to millions of spores per square inch or square centimeter. This is often frightening to the homeowner and may give the false implication that a severe health risk exists. MTS has seen other consultants inform clients that their tests showed enormous number of spores from visible mold spotting whereas tests of “clean” surfaces would often show very small number of spores, implying a serious concern.  In fact, any visible mold growth typically has large numbers of spores. As long as the area of visible mold is not extensive (or in the breathing zone), it may not be a concern.  MTS does not recommend this form of lab testing. Comparing mold spore counts from visible mold to areas where no visible mold occurs often has very little usefulness.

Airborne Mold - Basics
The main health concern from mold is AIRBORNE EXPOSURE. Airborne mold spores (and associated smaller “pieces” of mold) are too small for the human eye to see. For persons allergic to mold, small levels of airborne mold may be enough to cause symptoms while non-allergic persons in the same building may have no symptoms at all.

In many cases, a room may show no obvious visible mold and yet still have very high levels of airborne mold spores. For this reason, LAB ANALYSIS of airborne mold is required. Any adequate assessment of mold in a building should include testing for airborne mold. Simply because a building shows no obvious visible mold or water damage does NOT mean that a mold concern (high airborne levels) may not still exist (or that lots of visible mold may not occur hidden inside wall cavities, etc).

This typically involves collection of airborne mold spores onto a collection device. The mold spores are then identified under a microscope. The TYPE of mold and the NUMBER of mold spores are determined. Air samples are often taken at a measured flow rate for a specified period of time. The total air flow and number of spores are often then converted to mold spores per cubic meter (spores/m3).

Types and Numbers of Mold Spores
The type of mold present in the air is very important. By “type”, we mean the mold is identified to a major grouping (genus, etc) or even down to a specific species. In general, for airborne tests, molds may be grouped into: common outdoor molds, common indoor molds, problem indoor molds, and problem indoor wet molds. Different types of mold can be used to determine between a problem and non-problem building.

Airborne molds are present in the air everywhere both inside and outside building all year round. In a normal (non-problem) building, the airborne mold spores (at least in the Midwest) are usually dominated in warmer weather by the common (often non-problem) mold Cladosporium. That is, Cladosporium is often the most common mold spore detected in the air (both inside and outside). As long as the numbers are not too high, this often indicates a “normal” (non-problem) condition.

In contrast, certain other types of molds, especially Aspergillus-Penicillium, when dominant (especially at high levels) usually indicate a mold concern exists. That is, these are “problem mold” indicators. Aspergillus-Penicillium spores are very small and often grouped together when airborne spores are counted under the microscope. They may require only slight dampness to grow. They are also very sensitive to disturbance (easily become airborne) and may float through the air for many hours or more. Slight levels of Aspergillus-Penicillium are not uncommon in buildings, especially in basements, and may not always indicate a major concern. But, extremely high levels pose an immediate concern.

Another important mold are the “wet problem molds”. These molds usually require very damp to wet conditions to grow. These molds often have larger or sticky spores that tend to settle quickly from the air. These include especially Chaetomium and Stachybotrys. These types of mold are usually NOT found in airborne mold tests of non-problem buildings. High levels of these molds usually indicates a likely mold concern (and also often indicates very damp conditions and recent disturbances). High levels of these molds often pose an immediate health concern.


Seasonal and Climatic Variation. Very large variations in the airborne mold levels occur. Numbers are typically much higher in the warmer months and often quite low in the winter (especially when snow cover is present). Airborne mold levels can rise dramatically in damp weather and immediately after a rain. Slight winds may increase mold levels while strong winds may cause either a decrease in the levels or, more rarely, cause a dramatic increase in levels. MTS has seen outside levels vary from <100 total spores/m3 (winter, snow cover) to >500,000 total spores/m3 (damp summer). Very large differences in outside levels can even occur hourly during a day.

While these outside levels vary greatly, indoor levels are surprisingly insensitive to these changes (unless there are open windows or doors). There is a general increase in indoor levels in warmer weather compared to colder weather but this is typically not very large. Indoor levels are usually much smaller than outside levels in warm weather. However, indoor levels are actually often less than outside levels in cold weather. Generally speaking, in warm weather, inside levels should be lower than outside levels.

Any disturbance that can stir up dust often causes a large increase in airborne mold spores. Seemingly very minor disturbances can often causing surprisingly large increases in levels. Simply opening or closing a door or walking across a room may cause a large increase. That is, airborne mold tests are very sensitive to any disturbances. Windy days sometimes cause an increase in indoor airborne mold levels (regardless of outside mold levels).

Settled Mold.
In some situations, such as in a vacant building, after a clean-up  or remodeling, or in rarely used guest rooms or basements, high levels of airborne mold that were stirred up at one time may have had  time to settle out on surfaces, especially into carpeting. Standard (passive, undisturbed) airborne mold tests in these situations may detect only low or normal airborne mold levels. However, under disturbed conditions, this same room may show very high airborne mold levels. But, in general, in occupied rooms under normal use, disturbed testing is not needed.

Mechanical HVAC Filtration.
While there are many exceptions, in general, buildings with mechanical filtration often have lower airborne mold levels than those with natural ventilation (or without filters). The better the filtration, the lower the airborne mold levels. Because office buildings (especially medical facilities) often have better filtration (often 2-4 inch pleated filters or better) than do houses, offices tend to have lower airborne mold levels than do houses.  Slightly higher levels in a house versus an office does not necessarily mean there is a mold concern in the house.

Basements & Crawlspaces.
Generally speaking, basements and crawlspaces tend to be damper than the rest of a building and often have much higher airborne mold levels than upper floors. This is especially true of older farm house cellars.  However, while elevated levels may occur in a basement, the main floor (and upper floors) often still have normal airborne mold levels. As for crawlspaces, very high airborne levels can occur and yet not pose any concern to the rest of the house (as long as airborne mold from the crawlspace cannot reach the rest of the house).

Variations in A Building.
Very large variations can occur from room to room in a building and from floor to floor. MTS has worked at many sites where very large differences occurred in basement rooms in a house and even within a large room. There is a common misperception by many that airborne mold in a building must become evenly distributed among all the rooms. This is absolutely false.


When collecting airborne mold tests, questions include:

What is the reason for the testing?
Where should tests be collected
What are the number of tests needed?
Should this include disturbed tests?
What can the client afford?
What is the time frame?

Reasons for testing include: Is there a health concern? Is a building being sold? Has visible mold and musty odors been found in a building? What rooms in a building are affected or need to be cleaned? Was an adequate clean-up performed? Cleaning firms and banks often insist on documentation after a mold clean-up to demonstrate a concern no longer exists.

It must be emphasized that commercial mold testing is different from research mold testing. In commercial testing, the client (such as a homeowner or renter) often has a very limited budget and needs immediate results. There are many limitations to typical commercial testing.

In research testing, costs and time are not limiting and large numbers of different types of tests can be taken. In commercial testing, it is impractical to test every room in a building doing duplicate testing with different forms of mold testing. Rather, the fewest tests needed are typically chosen.  Where such limited testing indicates major problems, further follow-up testing may be justified.  By necessity, commercial testing is often quite limited and typically includes only a few rooms in a house or office. Commercial testing, unless requested, is not meant to provide an intensive assessment of every room. Rather, it is meant to provide enough information, for the costs allowed, to determine if a concern exists or that adequate clean-up, etc. has been done. For a $500 fee, a client should not expect a $10,000 assessment.

Testing should typically include both problem (or suspected) rooms and non-problem rooms (often called controls). In warm weather, an outside test should always be taken as well. At a minimum, at least 2-3 tests should be taken from problem areas and 1-2 tests from non-problem areas.  In many buildings, at least 2 tests should be taken from the basement and/or crawlspace.

Typically, airborne mold tests should be taken at the start of an assessment (after a brief walk-thru to decide where to test). Opening up walls, taking visible tape samples, pulling up carpets, etc should wait until AFTER first completing airborne mold tests. Minimize any disturbances before sampling. Windows should be closed during the visit (and preferably since the night before). Ventilation systems should be operated normally (in some cases, sampling before and immediately after turning on the ventilation may be useful).  Ideally, photos and a site sketch should document where the tests were taken.

Standard (passive or undisturbed) Tests. These are typically taken in the breathing zone about 3.5 ft to 5 ft above the floor. This should be away from any nearby walls or blowing air (any windows should be closed). Efforts should be made to avoid stirring up dust and mold (minimizing disturbances). A stand is useful. Each test should be taken the same way. The pump flow rate should be checked with a calibration device at the start of each site visit. An unused sampling cassette should be inserted into the sampling pump during the calibration. Follow recommended procedures for sampling. For airborne total mold cassettes (Allergenco, Air-O-Cell), typical flow rate is 15 liters/minute for 5 minutes.

Disturbed (Aggressive) Tests. It is usually preferable NOT to stir up dust or mold when taking airborne mold tests. However, in some situations, lots of airborne mold may have settled out onto surfaces, especially carpeting, and may not be detected by normal (undisturbed) tests. Buildings left vacant for weeks or more, especially with the HVAC shut off, unused basements, etc. may be worth performing disturbed tests on (after first doing normal undisturbed tests). Where a clean-up was performed and persons are experiencing health symptoms after the clean-up may be a candidate for one or more disturbed tests.

There are many ways to perform disturbed tests. Simply jumping up and down on a floor, shutting doors hard, vacuuming, slapping a throw rug against the floor may be enough to cause a large increase in airborne mold levels. Alternatively, a fan or blower, especially a strong blower such as a leaf blower can be used to stir up dust and mold.

Disturbed tests must be done with caution and are NOT appropriate for every situation. In occupied buildings, especially where there are infants, elderly, persons with asthma, etc, disturbed testing should often not be done.  However, in vacant building or temporarily unoccupied rooms, this can sometimes be done. In some cases, the room in question can be isolated and placed under containment and/or negative air pressure before trying disturbed tests.  Basements often show very large increases in airborne mold after disturbed testing. However, it should be emphasized, the experience of MTS and others suggest that LIVING mold is often more of a health concern than dead or dormant mold. Stirring up dust in a warehouse or basement may cause a large increase in airborne mold levels but may not always indicate an immediate health concern.

Disturbed tests can provide greatly varying results and may be difficult to interpret. Generally speaking, almost any normal room will show a large increase in airborne mold levels (often >1000-5000 spores/m3) when disturbed. However, a room showing a very large increase (>10,000  to 50,000 spores/m3 may indicate that lots of mold has settled out onto surfaces.

In-wall tests. In some cases, airborne mold tests can be taken inside wall cavities. This can be done by inserting a small tube (Wall-Chek,etc) connected to the sampling device through a small (often pencil sized) hole into a wall. This hole can sometimes be made in inconspicuous locations such as behind baseboards or behind the faceplate over wall outlets. Another method is to remove a faceplate around an outlet, make some small holes and place a cone over the outlet to collect an air sample. There are many limitations to doing in-wall sampling. The wall cavities are often stuffed with insulation which may block off the sampling tube. Any work to make holes in the wall may stir up mold and dust, especially drywall dust, and may clog up the sampling device.

Interpretation of results from in-wall tests must be done with caution. The lab results can vary enormously and usually do not have a good correlation with the airborne mold levels in rooms (inhalation exposure). A lab tests showing no mold may have been taken while the in-wall tube was plugged over with insulation even though the wall cavity was actually very moldy. On the other hand, cutting into walls often stirs up very high levels of mold (which may not relate to room levels).

Ceiling plenum tests. In some cases, airborne tests are taken inside the space inside ceilings (called the plenum) between the suspended ceiling tiles and the roof (or floor above). This should be done lifting up a ceiling tile as little as possible to insert the sampling device or tube through the tile into the plenum. Lifting up ceiling tiles often stirs up dust and settled mold and may not be representative of actual airborne mold levels inside the ceiling plenum.

HVAC System tests.

Carpet tests. And surfaces – swabs??


The five most common reasons for airborne mold testing are: (1)  Pre-Purchase Assessments; (2) Damage Assessments ; (3) Health Concerns (Unknown Cause); (4) Clean-Up Verification and; (5) Baseline testing.

Pre-Purchase Assessments. One of the most common reasons to perform airborne mold testing is to provide information for the sale of a property. Visual inspection alone may fail to detect a serious mold concern.

Damage Assessments. When water or mold damage has occurred to a building, airborne mold testing can help determine which rooms or floors may have been affected and which areas may or may not need clean-up.

Health Concerns (Unknown Cause).  In houses and offices where unknown health symptoms occur, one of the first concerns is often mold. Airborne mold testing can help determine if mold really does play a role in the symptoms. Note: many health symptoms are NOT related to mold.

Clean-Up Verification. Water or mold clean-ups often stir up very high airborne mold levels. A very common form of testing, sometimes called clearance testing,  is to provide documentation that an airborne mold concern no longer exists after a clean-up. Commercial cleaning firms typically request third party testing to provide documentation. Building owners, buyers, bankers, etc. also often insist on such documentation.

Baseline testing. This is testing performed, often on annual basis, to develop information as to normal airborne mold levels in a building. This can be very useful if a mold concern does develop. Unfortunately, only a few of the larger companies typically do such testing.


The most commonly used form of airborne mold testing by consultants is called Total or Non-viable testing. These tests collect all (or almost all) of the mold spores present in the air. This includes both living (viable) and dead mold spores. They typically rely on a calibrated air pump and a sticky collection medium.  An air sample is drawn through the collection device. The collector (often a glass slide) is then removed and analyzed under a microscope.

Because these samples do not have to be cultured, they can be analyzed immediately after collection. Lab results are often available 1-2 days after collection (same day results are sometimes possible). Quick results are extremely important when persons must decide if there is a health concern, whether expensive cleaning equipment must be left on site, etc. Another important benefit is that these tests are relatively inexpensive.

Total mold testing is very useful in that almost all of the mold spores whether living or dead are collected. By contrast, the viable (culturable) method often detects only a fraction of the total molds. Total mold testing can reliably detect many forms of molds, such as Stachybotrys, that are not readily collected using viable tests. 

The primary drawback of Total testing  is that many mold spores are difficult to identify exactly without seeing their growth structures formed during culture. The small spores of Aspergillus and Penicillium spores are typically grouped together as Aspergillus-Penicillium. Nevertheless, most molds can be grouped into major categories. This is usually quite sufficient for most purposes.  However, in certain cases, it may be helpful to have viable testing done for more exact identification. 

There is another important consideration in that the total tests do not distinguish between living and dead molds. There is some evidence that living molds pose a greater allergenic concern than do dead or dormant molds. Dead molds still pose health concerns. That is, in some situations, high levels of airborne mold derived from stirred up dust may pose less of a concern than high levels of mold from living mold spores.

Unfortunately, the reliability of total airborne mold tests depends on a number of factors which may affect the accuracy of the results. These include the professionalism of the sampler, the number of samples taken, whether and disturbances are involved and the professionalism of the lab analyst in analyzing the sample.  Some studies sending split samples to different labs have shown large differences among the labs. Nevertheless, at least qualitatively, total testing provides a fast and fairly inexpensive means to assess airborne mold levels in buildings. That is, while one can argue over some numbers, very large numbers of total airborne mold typically indicate a concern while very low levels do not.

Viable testing is the collection and culture of living mold. Various growth media are placed in collection plates (usually petri plates) and airborne samples are collected onto the growth media. The Andersen sampler is one common collection device. The samples are then cultured (incubated usually at a warm temperature) for about 3-5 days until mold colonies form and are sufficiently developed to identify the growth structures of specific molds.

Viable tests offer the most accurate assessment of the types (and sometimes the numbers) of some of the living mold present. However, there are many important limitations. These tests typically only detect a fraction of all of the molds present and may not detect certain types of mold at all. The type of culture media determines which molds are most likely to grow. Many molds present in the air may not grow on the media chosen. Some molds rapidly overgrow the collection plates. Considerable variability can occur even when using duplicate petri plates. Viable testing is also easily overwhelmed by high levels of airborne mold and typically greatly underreports high levels of mold. 

Adequate assessment usually requires at least two types of culture media (media for damper molds – such as MEA and media for drier molds – such as DG18). The requirement for multiple media and culture means that viable tests are often more expensive and take much longer to analyze than do total mold tests. Typical lab turn-around time is often a week or more.

One important benefit of viable testing is that this was the first form of airborne mold testing developed and it has been in use over many years. Researchers tend to use viable mold testing much more often than total testing. For this reason, there is a large literature on viable tests (more than for total tests).

In the experience of MTS, viable tests often greatly underestimate the total airborne mold present. They often miss problems that total testing finds. On the other hand, in some cases (the minority), viable testing has helped to define a problem that total testing did not.  Ideally, when time and costs permits, it is preferable to use both total and viable testing. Total testing may be considered a more reliable “screening” tool to get the big picture while viable tests can help to pin down details.

Another form of viable (culturable) airborne mold testing are home test kits. These are also called settle plates. These are low cost kits sold to homeowners and others for about $10-15 each. They include a growth media and petri plate with a cover. The plate is filled with the media and then left uncovered in a chosen area (usually for about an hour). The cover to the plate is then put over the petri plate and it is incubated (typically in a paper bag) for a few days. The individual colonies are then counted. Or, the plate (or lifts from it) are submitted to a lab for analysis.

There are severe limitations to these tests. The growth media will only allow certain forms of mold to grow. Depending on where the plates are exposed, some molds will settle out of the air onto the plates. Many problem molds, such as the smaller Aspergillus-Penicillium spores or the heavier Stachybotrys spores may not be detected during such a test. The lack of a calibrated air flow also means any test results must be more qualitative than quantitative.

It is preferable to collect a number of these tests in more than one place, including problem and non-problem areas and an outside test (in warmer weather). The samples can then be visually compared to each other after a few days (must be exposed for same time periods).  Mold will show up as individual colonies. These tests are often frightening and misleading to homeowners who are typically dismayed to find any mold colonies at all on the plates.  The fact is that most plates (as would exposed damp bread or cheese etc) will show at least a few colonies after being exposed to the air. However, large numbers of colonies (often >20-30), especially blue-green (Penicillium) colonies may indicate a possible airborne mold concern. These tests are much less reliable than standard airborne mold tests. However, for persons with limited financial resources, they may sometimes be a useful indicator.

TESTING FOR MOLD -  other tests MVOC, human test - allergens


References (IN PROGRESS.)

Prezant, Bradley, Weekes, D. and Miller, D., Editors, 2008, Recognition, Evaluation, and Control of Indoor Mold,  Am, Inst. Industrial Hygiene, Fairfax, VA (Chapter 17 – mold guidelines). Useful reference.

Yang, Chin and Heinsohn,Patricia, 2007, Sampling and Analysis of Indoor Micro-organisms, Wiley-Interscience, 273p. (Excellent reference for mold sampling)