INTRODUCTION

There is a saying that relates to sample collection: “If you don’t know how to interpret the results, then don’t take the sample”. In general, this is good advice. Therefore, I am going to explain how I interpret wall cavity samples.However, the first statement that I am going to make is that professional judgment should always be a primary factor in evaluating industrial hygiene data. The numerical guidelines presented in this article are secondary methods for evaluating the data.

Why not just use destructive testing?

First or all, destructive testing, conducted without proper engineering controls, can further contaminate the test area. But wall cavity sampling has a second advantage compared to destructive testing alone.The statement that “fungal spores and hyphae are invisible to the unaided eye” is readily accepted by most mold investigators. But, the corollary “therefore visual inspection of surfaces for fungal contaminants, by itself, is inadequate to detect the presence of fungal contaminants” seems to be highly controversial. Figure 1 contains a photograph of the interior surfaces of five pieces of drywall. The five surfaces were judged to be “clean” by three separate individuals at the time the samples were collected.As indicated in Table 1, two of the pieces of drywall had a surface growth of culturable Aspergillus versicolor and/or Penicillium, and came from wall cavities with significant concentrations of culturable Aspergillus and Penicillium. In this particular instance, visual inspection was not an acceptable method for detecting the presence of significant concentrations of fungal contaminants that were present inside the wall cavity.

Table 1. Wall Cavity Concentrations Associated with Visibly “Clean” Drywall Samples.

I sometimes hear about wall cavities that were sampled and found to contain mold, but were “perfectly clean” when the wall was opened during destructive testing. This is often used as an argument that wall cavity sampling is prone to “false positives” (laboratory reporting spores when they really weren’t there). Personally, I find it difficult to grasp the logic of such an explanation (a spore that wasn’t there grows and produces a fungal colony in a lab culture?). It is more probable that spores and hyphae were present, but that they were not detected by visual inspection when the wall was opened.

What about the sample volume?

Let’s calculate the volume of air contained in typical stud bays (space between studs) in houses, because that affects how much air can be withdrawn during sampling. A stud bay 8 feet high and 16 inches wide contains an air volume of about 79 liters, while a stud bay located under a window set at 30 inches contains about 27 liters of air. However, the area under the window is exactly where we want to collect many of our samples, so let’s assume there are 27 liters of air in the “sample container”.If we are interpreting the laboratory results qualitatively (just what is there, not how much is there), then the volume of air we withdraw from the stud bay does not matter. However, if we decide to interpret the laboratory results quantitatively (not only what is there, but how much is there), then the volume of air withdrawn from the stud bay (the sample container) becomes important. The reason is that as air is withdrawn from the wall cavity, a like volume of fresh air is drawn into the wall cavity, diluting the sample. As a practical limit, in order to maintain the integrity of the sample, no more than about 10 % of the available air should be withdrawn during sample collection. If the sample is being drawn from a single stud bay located under the typical window, and if numerical guidelines are to be applied to the sample results, then the maximum sample volume should be limited to about 3 liters.

Where Should Samples Be Collected?

Mold follows moisture. A wall cavity subjected to a floor-level water spill will have most of the wetness localized near the baseboard. Therefore, I generally collect the sample near the baseboard, not in the middle of the wall. If the water intrusion occurred at the ceiling, gravity might still cause most of the wetness in a wall cavity to be localized near the baseboard. However, in that case, I will sample both near the baseboard and near the ceiling.

What are we looking for in wall cavities?

In a previously published study describing 150 wall cavity samples [Spurgeon, J.; AIHA Journal; 64: 40-47; 2003], only Aspergillus and/or Penicillium species were detected in 69 % of the wall cavities in which culturable fungi were detected. This result, which was supported by other studies referenced in the article, suggests that culturable Aspergillus and/or Penicillium, or Aspergillus/Penicillium type spores, are frequently the primary “indicators” of a contaminated wall cavity. Therefore, basing numerical guidelines on “total spore counts” (which may include Myxomycetes, rusts, smuts, etc.) as an indication of contamination may tend to bias the conclusion. The data in Table 2, which are similar to two actual samples I was asked to compare, illustrates this concept. Table 2. Example Sample Results for Two Wall Cavities (spores/m3)

The total spore counts in samples W-1 and W-2 are similar. However, the total spore count in sample W-1 is alm

ost entirely due to common plant pathogens, with very few Aspergillus/ Penicillium type spores detected. In sample W-2, over 50 % of the total spores were due to Aspergillus/Penicillium type spores. When assessing the condition of a wall cavity, I generally focus on the total concentration of “indicator genera”, which commonly include Aspergillus, Chaetomium, Stachybotrys, Ulocladium, etc. Therefore, I would conclude from these data that location W-1 had not been subjected to water intrusion, while location W-2 had been subjected to water intrusion.Second, I report my sample results as “standardized” data, whereas a number of reports I have reviewed did not. For example, reporting a result as “100 spores” is not standardized, and does not provide a basis for comparing that result with any other result. Reporting a result as “100 spores per cubic meter of air” is standardized based on the sample volume, and that result can be compared to other similarly standardized sample results. Third, anyone who collects samples should be aware that “if you don’t know how you are going to interpret the data, then don’t collect the sample”. So, can wall cavity data be interpreted? In my opinion, the answer is yes; and in at least four ways.

Dominant Contaminants

The first method is by determining the dominant types of mold spores detected in the wall cavity. For example, Stachybotrys chartarum requires wet conditions for growth, while Aspergillus versicolor prefers near-wet conditions. As conditions in the wall cavity move down the moisture scale, then Chaetomium or Ulocladium may become dominant. Finally, in wall cavities that have dried out, Penicillium and Cladosporium tend to be dominant.

Numerical Guidelines: Wall Check Sampler

The second method is to use numerical guidelines to interpret the sample results. However, even though I am about to discuss numerical guidelines, professional judgment should always be included in the decision making process. Let’s discuss the WallChek (Aerotech Laboratories) sampling device first. I was asked to analyze a significant number of wall cavity samples collected using the WallChek. Without going into a lot of detail, I found that wall cavities containing less than about 7,000 spores/m³ of Aspergillus/Penicillium type spores appeared to be in a different group than wall cavities containing more than this concentration. Therefore, for wall cavity samples collected using the WallChek:
1. Data interpretation should be based on Aspergillus/Penicillium type spores and other “indicator genera” rather than total spores, and
2 . Wall cavities with Aspergillus/Penicillium type spore concentrations less than 7,000 spores/m³ were probably not subjected to water intrusion and may be classified as Uncontaminated.
3. Wall cavities with Aspergillus/Penicillium type spore concentrations of 7,000 spores/m³ or higher were probably subjected to water intrusion and may be classified as Contaminated.

Numerical Guidelines: Bi-Air CassetteThe Bi-Air cassette collects both fungal spores and culturable fungi. In addition, it has a higher collection efficiency compared to the WallChek device, so the reported concentrations will be higher than those obtained with the WallChek. Table 3 contains the numerical decision criteria I generally use when interpreting wall cavity data collected using the Bi-Air cassette.

Table 3.a. Numerical Guidelines for Culturable Aspergillus/Penicillium (cfu/m³) Collected using the Bi-Air Cassette.

Table 3.b. Numerical Guidelines for Aspergillus/Penicillium Type Spores (spores/m³) Collected using the Bi-Air Cassette.

Culturable-to-Total RatioThe third method for interpreting wall cavity data is to evaluate the concentration of culturable fungi detected relative to the concentration of total fungal spores detected. The ratio of culturable spores to total spores may provide some indication as to age of the incident that caused the fungal contamination. However, this method can only be used with the Bi-Air cassette.This method is based on the assumption that as a wall cavity begins to dry, the fungal spores will begin to die. Therefore, if the water intrusion incident that affected the subject wall cavity occurred recently, a significant portion of the fungal spores should still be viable (culturable), and the ratio of culturable spores to total spores should be high.Conversely, if the water intrusion incident that affected the subject wall cavity occurred in the past, a significant portion of the fungal spores should be nonviable (not culturable), and the ratio of culturable spores to total spores should be low. The data in Table 4 illustrate this concept.

Table 4. Example Wall Cavity Data (fungi/m³).

If the percent of culturable fungi are used to estimate the percent of viable fungi in the wall cavity, then we see that a much higher percentage of spores were still alive in Wall # 1 as compared to Wall # 2. Although this is not a very precise timing method, it may provide some insight into which walls were damaged by the current water intrusion (Wall # 1) as opposed to some previous water intrusion (Wall # 2).

Correlation with Airborne Concentrations

The fourth method of interpreting wall cavity samples is attempting to determine if the hidden fungal reservoirs are affecting the indoor air. In my experience, it is not infrequent that even low concentrations of airborne Aspergillus versicolor, when detected persistently in air samples, indicates that hidden mold reservoirs are present in wall cavities. However, in order to detect such as association, both the air samples and the wall cavity samples must be cultured, and the fungi identified to the species level. This is the second advantage of using the Bi-Air cassette, which can collect both fungal spores and culturable fungi as part of the same sample.

SUMMARYThe objective of any sampling method is to provide data that can be interpreted; the method has to have utility. The data obtained with wall cavity samples can be interpreted, by assessing the types of fungal spores that were detected, using numerical guidelines, and by calculating the ratio of culturable fungi to total spores. Collecting both culturable fungi and fungal spores provides at least some information as to the relative age of the incident causing the mold. But, more important, collecting culturable fungi provides the opportunity to associate airborne exposures with hidden mold reservoirs.