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Let’s start with a quiz.  Which house has a higher mold spore count?

The room on the left is a bathroom with significant mold growth on the walls and ceilings.  This was due to the lack of a bathroom fan.  The photo on the right is a basement with no visible mold growth, but a damp rug and a strong musty odor.  Mold testing was performed in both areas.

Comparison of two rooms prior to mold testing.

The results from mold testing?

Results of mold testing comparison

Why such weird results?

Despite the heavy staining from the mold growth in the bathroom (left), the flat growth profile did not lead to substantial spreading of spores.  The high spore count in the basement (right) was due to moisture trapped in the rug.  Despite the fact the area rug was only 6′ across, the constant exposure to vapor emissions from the concrete slab led to a tremendous amount of mold growth.  Amazingly, very little mold growth was visible.  This is common with carpeting, as the mold grows within the pad and carpet fibers without much visual evidence.  These are both good examples of when mold testing is helpful.

When should you test your house for mold?

Before any sampling is conducted, you must first identify the potential results.  This may sound obvious, but many inspection companies charge thousands of dollars for sampling without ever identifying a real benefit.  They may provide their clients with pages and pages of data, but no actionable information.   Before any sampling is collected, you must first identify potential actions you’ll take based on the results.

1.  You don’t see visible mold, but you have high humidity

If your home suffers from condensation on the windows, this is often an indication of poor air flow and high humidity.  High humidity is one of the main causes of mold growth.  In these scenarios, it is reasonable to suspect that mold growth may be occurring in the home and therefore, testing is a worthwhile option.  This is especially true if you see condensation on a modern, double pane window.

Condensation on window before mold growth.

2.  You’ve found visible mold growth, but it’s unclear what materials have been affected

Let’s say you own a condo and you find mold growth on an exterior wall behind a bed.  Your inspector concludes that the mold growth is due to condensation from a poorly insulated wall.  The extent of the actual growth is obvious, but the extent of the settled mold spores is not.  During mold growth’s cycle, thousands, even millions of spores can be released into the air.

Testing air in bedroom with mold behind bed

The challenge is that the number of released spores can vary dramatically from one project to the next.  Therefore, you cannot look at a site of visible mold growth and readily determine how many spores have been released.  This means that you also cannot determine what other materials may have been affected.

In these situations, an airborne mold test can be extremely helpful in determining the spread of those mold spores.  In the case of the mold growth behind the bed, the air sample will tell you if the carpeting, bedding, and other materials have been affected by the mold growth.

3.  Your family suffers from health problems commonly related to mold exposure

Mold is a common trigger for individuals with allergies and asthma.  If your symptoms seem to worsen at home, mold testing can be an effective way to determine if mold spores are the culprit.  A similar connection is found with sinus infections.   The EPA has excellent resources here.  You can also learn more here.

4.  You smell musty odors

Musty odors are often, but not always, associated with mold growth.  Because of this fact, air sampling is a great technique for verifying if elevated mold spore counts are truly present.  Mold growth releases odors during certain portions of its life cycle.  This release of odors is based on a variety of complex factors and is not a reliable indicator of active growth.  For example, you may have a building with a strong musty odor and very little active mold growth.  The EPA discusses it here.

Alternatively, you will often encounter projects with no odors at all, that in fact have very high mold spore counts.  The only way to accurately determine the presence of mold spores in these cases is to collect an air mold sample.

5.  Remediation has been performed and you need proof it’s clean

Airborne mold testing is commonly used for both internal and 3rd party clearance testing.  When a mold remediation project is completed, air testing is performed to verify that the mold spore counts have been reduced to normal levels.

Post remediation mold sampling photo.

When is mold testing not appropriate?

Areas outside the conditioned air space.

Airborne mold testing doesn’t make much sense for crawlspaces and attics.  Those areas are directly linked to the outside air and therefore not a controlled environment.  If the test results come back high, it may simply be due to the outside environment.  Additionally, in the unconditioned areas, your primary concern is the building structure, not the air quality.

Because you do not breathe the air in the attic or crawlspace, an air quality test in these areas is of little value.  In a crawlspace this is complicated even further by the natural mold growth that occurs on the damp soil.  Because of these factors, in attics and crawlspaces, a tape lift sample or visual observations is superior.

Open jobsite

Similar to an unconditioned space, if a project is open to the outside air, it is not a good candidate for air testing.  This is often the case during new construction.  This can also occur during a renovation.  For example, if the side of a house was removed for a remodel project, any testing collected inside the home would be of limited value.  To accurately test the air, a temporary wall must be constructed first.  Due to cost of building a temporary wall, it is often better to focus on visual observations and skip the air testing.

Mold growth on new construction home. No testing needed.

Obvious & extreme levels of mold

In cases where the mold growth is severe, mold testing often provides little value.  For example, if the entire interior of a home requires demolition, air sampling will not provide any additional value.  A simple visual observation will indicate to us that all surface materials will require removal, regardless of the findings of air sampling.

Extreme mold growth in a home. No testing necessary.

Types of mold testing

  • Air sampling
  • Direct sampling
  • Bulk sampling

Within each of these categories, there are two primary types of results provided by the sampling.  These are ‘quantitative’ and ‘qualitative’.  Quantitative sampling attempts to measure the quantity of mold spores in a given area.

The most common type of quantitative sampling is non-viable air sampling.  The results of this sampling are provided in a numerical format and compared against averages.  This provides the client with a sense of the severity of the mold problem.

Qualitative sampling is much simpler.  It seeks to verify if mold is present.  This type of sampling is less common, because mold growth can typically be verified visually, without laboratory sampling.  Qualitative sampling is useful when mold growth may be confused with similar looking substances, such as efflorescence.  A tape lift sample or bulk sample is the preferred method.

Air sampling – this is where it gets tricky


Air sampling is the most common lab test used in indoor environmental evaluations.  There is a good reason for this.  Air sampling is the most accurate method for determining exposure.  While a tape lift may accurately determine if mold growth is present on a particular surface, it will not tell you whether that mold growth has actually affected the indoor air quality.  Only an air sample can accomplish this.

Types of mold testing devices

As we discussed earlier, the primary exposure route for mold growth is through inhalation.   Therefore, testing the air for elevated levels of mold spores is the most accurate way to determine if the occupants are exposed to excess mold.

There are 2 main types of air samples.  Non-viable and viable image.  Both samples operate by running air through an impactor device.  The air is drawn into the device and impacted on a sticky substance.  The air hits the surfaces and flows around the sides while the mold spores stick to the surface.

Viable mold testing

Viable samplers, such as the Anderson Impactor, impact the spores on a petri dish.  The petri dish contains a food source for the mold growth.  Over time the mold grows, allowing the laboratory to identify the type of mold.

Non-viable mold testing

Non-viable testing is typically performed with a slit impaction sampler.  A popular example is the Zefon Air-O-Cell cassette. These are small plastic cassettes with a narrow opening and a sticky surface inside.  The mold spores are drawn through the slit and impact the sticky surface.  After the samples are collected, the lab opens the cassette and reviews the slid surface with a microscope.  The laboratory can identify the number of spores and the genus, but not the species.

Non viable most testing kit

Comparison of airborne mold testing techniques

Mold testing graph comparing different techniques

Viable or culturable samples have a lengthy analysis time, often 10-14 days.  This is the amount of time required for the mold spores to begin to grow.  Non-viable samples can be processed immediately, even the same day if necessary.

Viable samples can provide a species level analysis.  This can be important when attempting to draw a correlation between mold growing in a particular area and elevated mold spores in the air.  While a non-viable sample can provide a likely correlation, only a viable sample can prove the connection.  For this reason, when there’s a chance a sample will be used in a court case, a viable sample is recommended.

Photo of mold lab data

Notice how Penicillium and Aspergillus are grouped together

Viable samples are important when testing areas with severely immunocompromised patients.   These people are susceptible to infections from mold.  And because only viable spores can cause infections, testing for viability is critical.

One major drawback of viable sampling is the limited number of mold spores that can be measured.  Only spores capable of reproducing will be measured.  With non-viable testing, all spores are counted. Non-viable spores may still be able to produce allergies, and therefore a viable sample may miss a critical mold exposure problem.

Due to the processing time required for viable sampling, the cost is much higher, often 4-6 times as expensive.  In most situations, the fast turn around time and lower cost of the non-viable sample makes it the best choice.

Interpreting mold testing laboratory results

You’ve got the data, now what?  This is where the frustrations begin.

Here’s a typical mold testing scenario:

A homeowner hires an inspector to sample their house for mold.  They receive the results and are told everything is fine, the mold spore counts are well within the expected range.  But they’re a little unsure of the result, so they hire another firm to get a second opinion.  This inspector collects more mold samples, using the exact same techniques.  After a couple days the homeowner receives the results.  The spore counts are very similar, but the second inspector tells them to evacuate immediately – their home is full of dangerous levels of mold.

Who’s right?  Why the wild discrepancy?

The problem is due to a lack of understanding of how to interpret the results of mold testing.   Unfortunately, there are no agreed upon standards for interpreting this type of sampling. The EPA doesn’t have a guideline, nor does any other federal agency.  Other guidelines exist, but they vary tremendously in their conclusions.

Why interpreting mold testing data is so difficult

The challenges related to mold testing are frustrating, but understandable.  Mold spores are ubiquitous.  Unless you’re in a hermetically sealed clean room, there are hundreds or thousands of mold spores floating in the air.  This isn’t the main challenge though.  Mold testing can easily count the number of spores in a given quantity of air.

The problem is the lack of uniformity.  Let’s say you collect a sample, then run another one an hour later.  It would not be surprising at all to see a 100% increase in the number of spores.

Did the room suddenly become infested with mold?  No.  Perhaps the wind shifted slightly outside.  Perhaps spores were pulled into the room when the door was opened.  Merely walking on a carpeted surface can stir up hundreds of spores.

Comparison of mold testing samples

This leads us to our first rule:

Mold testing can only evaluate large increases (or decreases) from the baseline

A 10%, 15% even 20% increase or decrease over the control is statistically meaningless.  We’re looking for large changes – typically an order of magnitude.  Technically you can derive meaningful conclusions from small changes.  However, to achieve this you have to collect a large number of samples.  In most cases this is impractical and cost prohibitive.

In general there are 3 approaches, the Inside Comparison Method, the Outside Comparison Method and the Fixed Standard method.

Inside comparison method

The inside comparison method of mold testing collects a sample in an unaffected portion of the home.  This technique is useful if the entire home is relatively dirty and unkempt.  In some cases, the entire house may have elevated levels of mold spores because of humidity problems unrelated to the area you’re testing.  This must be identified in the beginning or the remediation contractor may have a hard time passing a clearance test.

If this pre-sample isn’t collected, the remediation project might fail, simply because the mold spores from the rest of the home is contaminating the containment zone.

The outside comparison method

This technique follows the same principle, but instead of collecting the comparison sample from an adjacent area of the home, the sample is collected from the outside.  This method is widely used and works well in most circumstances.  However, keep in mind, if it has recently snowed, the outside control sample may be extremely low.  In these cases, even the smallest number of mold spores would cause a failure.  A Fixed Standard approach would works better in these situations.

Example of mold testing report

Fixed standard method

Instead of comparing the mold spores in the containment to the remainder of the home, the mold spore counts are compared to a pre-determined level of acceptable mold spores.   For example, some standards apply this method by creating a threshold for specific types of mold.  I.e the spore counts for penicillium/aspergillus, stachybotrys and chaetomium cannot individually exceed 500, and the total of all spores must remain below 2,000.

individual types of mold spores 500 spores per cubic meter as the cutoff.  Others reference the total number of mold spores, regardless of the type.

Often the best approach is to combine several standards.  For example, if any of the following conditions are met, the mold spore levels are considered elevated:  1.)  The total spore counts level of all species exceeds 1,500 spores per cubic meter or 2.) Any indicator species, such as Stachybotrys or Chaetomium is greater than 300 spores per cubic meter.

Putting it all together with real world examples

Let’s compare two real projects and with unusual sampling results.  The first project involved a basement with a strong musty odor.  The odor was limited to just the basement storage area.  Numerous items were stores in the basement but no visible mold growth was observed. The client also did not recall any recent flooding events or moisture problems in the basement.

Basement with musty odor

During the inspection, a small area rug was moved, exposing an area of discolored concrete.  Based on this observations, moisture meter readings were taken from the rug, which revealed high amounts of moisture.  The rug was also wet to the touch. After removing the rug, the concrete was tested using the comparison technique.  This confirmed the initial hypothesis.  The discolored area of concrete was in fact wet.  Based on these results, what would you expect to find in an air test?

Moldy carpet before testing

Before we answer that question, let’s look at another scenario:

A client was attempting to install built in book shelves and after removing a piece of sheet rock found mold growth on the exterior sheathing.  The client subsequently removed other areas of sheet rock and found mold growth in many other exterior walls .  Throughout the entire home, mold growth was found on the sheathing.  Moisture meter testing indicated dry conditions throughout and it was determined that the mold growth was due to rain water saturation during the original construction of the home.

Mold growing in walls. Air mold test taken.

Air quality testing was performed to determine if elevated levels of mold spores were present. When comparing this scenario to the basement we looked at earlier with the small area rug, which do you think would have the higher mold spore count?

Let’s look at the data

The first set of data is from the house with mold inside the walls.  You’d expect a house with mold throughout all the walls to result in compromised indoor air quality, right?  Look at the data.  The house is quite clean.  The mold spore counts are in line with a clean, well-kept home.  Why?  The mold growth was several years old.  Because it was not actively propagating, the mold spores were unlikely to spread.  Second, because the mold growth had occurred inside the wall cavities (behind the sheetrock), the spores were relatively well sealed from the indoor air.

Mold test results from house with mold in the walls.

Now let’s look at the data for the basement with the wet rug.  340,000 spores per cubic meter for Penicillium/Aspergillus.  This is an extremely high spore count, close to 1,000 times greater than a typical outside sample.

Results of basement mold testing

This tells us a couple of important pieces of information.  First, the quantity of mold growth in a home is less important than the location of the mold growth.  Even though the second home we examined had mold covering hundreds of square feet, those mold spores were trapped within the walls and had no one of affecting the indoor air quality.  In the other example, even though the mold growth was limited to a small area rug, those spores were easily disturbed and therefore, readily distributed into the air.



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