Basics of Mold Testing

Once you have had your home tested by a professional, how do you determine if the results are accurate? Hopefully, you have hired an ethical professional who has the proper training and expertise to perform the testing; just in case, as a homeowner, you should have a little understanding on how to interpret the sample results.

While there are several different types of samples used in the industry – and just as many sampling devices – two kinds of samples are the most common. These are the non-viable air sample and the surface sample.

Non-Viable Air Samples
The non-viable air sample is collected by pulling a known amount of air over a sticky slide. Contents of the air are deposited on the slide. This includes mold spores, insect parts, fibers, and anything else in the air. Some labs report on the other contents of the slide, while some only report on the mold. Several things can be inferred from the additional contents, but to keep this simple, we will concentrate on the mold spores.

The spore counts will be listed in the number of spores of a category, spores per cubic meter of a category, total spores, and total spores per cubic meter. The number of spores is the actual number of spores on the slide, while the spores per cubic meter is derived from a formula based on the amount of air pulled over the slide.

Both findings are important along with the location of the samples. An outdoor control sample should always be taken, and an air sample should never be taken in an unoccupied zone such as an attic or crawlspace. The sample taken from the investigated area is to be compared to the control sample.

There is a lot to interpreting non-viable spore samples, and a homeowner cannot be expected to understand all the aspects of the testing. A lot comes from experience and dealing with a large number of samples. I always recommend that you remember the following and don’t be afraid to ask questions or even another opinion:

  1. In most situations, spore counts in the home should be similar to or below the outdoor control;
  2. The type of mold spores in each sample should be similar;
  3. Variations in the samples can occur, but this doesn’t necessarily mean there is an abnormality in the home – you must look at the amount and type of spore in the variation;
  4. Just because spore levels in the home are higher than outside does not mean there is a problem – levels must be considered significantly higher to cause alarm;
  5. Actual counts of spores should also be considered – one spore in the sample can read as 13 spores per cubic meter or more, depending on the amount of air passed over the slide when the sample was taken. One spore may not indicate a problem!

Surface Samples
I have found surface samples to be one of the most misused samples out there. A surface sample can be used to determine the type of mold present, or if there are a large number of spores on a surface due to cross contamination. However, many people have been using surface samples to indicate abnormal fungal growth. This should not be the case if mold is visible.

A surface sample can be taken using a swab, tape lift, or stick slide. All are proper methods of sampling; the only difference is how you record the area sampled. The area sampled is only in question when quantitative results are desired; in most cases, the samples are qualitative and only used to determine what type of mold is present.

How results are reported vary by lab, but most use a scale, such as “occasional, few, moderate, numerous.” Wording and categories may change between labs, but in this case, “occasional” and “few” would be viewed as normal, while “moderate” and “numerous” would be viewed as abnormal. The scale is typically listed at the bottom of the results.

Here are some items to remember when reviewing surface samples:

  1. Surface samples do not indicate how much growth is present on a surface, just in the sampled area;
  2. For the most part, surface samples are only for mold identification;
  3. All surfaces may contain some amount of spores, even if everything is normal;
  4. Generally, surface samples should not be used to determine abnormal conditions or remediation protocol.

The final item of this list is the most important, and should be further explained. The best example that can be given is as follows:

There are two walls in the home, Wall A and Wall B. Wall A has one square inch of mold on it that is very dense, and Wall B is fully covered (80 square feet for an 8x 10 wall), but it is not very dense. Two samples are taken from each wall. The results from Wall A come back with “Numerous” spore levels for one genius of mold, where Wall B comes back with “Moderate” and “Few” spore levels for several genera of mold. According to the samples, Wall A has a problem, while Wall B does not. Someone could determine based on the samples that Wall A needs remediation more than Wall B. In actuality, Wall A has had all the mold removed by the sample, while Wall B has a significant problem. The variation comes from the density and area sampled.

This just gives a rudimentary example of why surface samples cannot be used to determine the need for remediation. I see this happening the most in crawlspaces, with recommendations being made over the density of the growth – not the actual coverage area.

With surface samples, only take them for what they can actually tell you. Again, as a homeowner, don’t be afraid to ask questions or get a second opinion!


Install weather stripping.

This may seem like a simple item, but many people forget about it. Check around your windows and doors and see if they are leaking, or if you can see daylight. If so, install weather stripping to seal this up! There are various types, and each one has it use. Ask someone at your local hardware store which one works for your situation or you can research your options online. Also, while door socks can help, many people will remove them in the summer. Just because you are not uncomfortable in the summer with the draft under the door, it’s still there and costing you money! If you do this, also remember to pick up a door sweep while you are at the store getting your other weather stripping supplies.

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