When samples are sent to the lab for analysis, the recovered data is presented to you in a report that provides concentrations or presence/absence information for the organisms of interest. Figuring out what all of the columns mean can be a challenge. However, some of this data can be very helpful in interpreting the data.

Most spore trap reports include columns for the amount of air sampled, the number of spores counted, an extrapolation of this count if only part of the slide was counted, and a calculation of spores/m³ of air for each type of spore. In some cases only the extrapolation count is presented. Let’s assume that the entire sample was counted and the actual number of spores seen is recorded on the report.

The amount of air sampled is usually listed in liters. The number of liters sampled is derived from the flow rate of the sampling device (often between 10 and 15 liters/min) and the length of time the sampler was operated. So if you collect a 5 minute sample at 15 liters/minute you have collected 75 liters of air.

Concentrations are listed on the report in terms of spores/m³ of air. Therefore, the number of spores counted is converted to spores/m³ by dividing the spore count by the amount of air sampled in cubic meters.

So how does this information help you? Let’s say the report lists a concentration of 13 Stachybotrysspores/m³ of air. How many actual spores were counted to reach this number? We look at the spore count for Stachybotrys and see that only a single spore was seen. With only a single spore captured on a sample there is a significant chance that that one spore was the only one present in the entire room. It could also represent the calculated concentration, or it could be a gross underestimate of the actual number. These situations have similar probabilities. Note that if you had collected a 10 minute sample at 15 liters/minute you would have collected 150 liters or 0.150m³, and if only one spore were seen, the calculated concentration would be about 7 spores/m³.

Statistically, the probability of a calculated number representing the actual concentration becomes significant at an actual count of about 10 spores. This represents a concentration for a 5 minute sample at 75 liters/min of 133 and for a 10 minute sample 67. This explains, in part, why we tend to consider spore counts less than 200 of little quantitative importance, regardless of the taxon.

Of course, concentrations are not the only aspect of the data that are important. The consistent presence of even low levels of some taxa is likely to indicate growth somewhere in the space. For example,Stachybotrys tends to be found in very low levels and infrequently in outdoor air. If it is consistently present in indoor air then an indoor source is probably present. Remember, however, that as far as exposure is concerned, low concentrations of these fungi are not of concern.

Now let’s consider a dust sample analyzed quantitatively for culturable fungi or bacteria. On the dust sample report will be columns that indicate how much dust was actually used in the assay, how many colonies were recovered, and the calculated colony forming units/gram of dust. So you send in a sample that weighs 1 gram.

In the laboratory the dust is sieved, then a small portion is separated for analysis. This small portion we will say for simplicity’s sake is 100 milligrams. The sample is suspended in liquid (let’s say 1ml) and 0.1 ml of this suspension is placed on a culture plate. This is a simplification of the process but should suffice for this discussion.

As an example, let’s assume you recover 10 fungal colonies on the plate. What does this mean in terms of cfu(colony forming units)/gram of dust? In order to discover this concentration we have to account for the small amount of dust actually used in the analysis and for the small amount of the suspended sample we used for plating. If we used 100mg of dust (0.1g, and 0.1 ml of the suspension then we have to multiply the 10 colonies by 100 (or divide by 0.1×0.1=0.01. Thus, our result is 1,000 cfu/gram of dust. Actually, we would report cfu/gram of dust for individual taxa.

So if we had 1 Stachybotrys spore on the culture plate, the final concentration of Stachybotrys colony forming units would be 100 per gram of dust. This sounds like a significant number, but note that it is based on a single Stachybotrys colony.

In actual practice, most labs dilute the original suspension by factors of 10 and incubate all of the plates. Then the plate with the most appropriate number of colonies for each taxon is chosen for calculating cfus. If the most concentrated dilution (which is essentially 1:10) is overloaded, then the 1:100 plate (or a higher dilution) is chosen. This procedure sometimes leads to low counts for individual taxa, especially if there is an abundance of one fast growing fungus. In these cases, as discussed above, it is important to consider the low colony counts if quantitative assessments are being made.

The bottom line: it is important to note the number of spores or colony forming units on which the reported concentrations for any type of quantitative sampling are based. If the number of spores or colonies (by taxon) is less than 10, then extreme caution should be used in interpreting the data quantitatively.

This article originally appeared in the January 2010 issue ofIndoor Environment Connections. Reprinted by permission.

About Dr. Harriet Burge Dr. Harriet Burge is EMlab P&K’s Director of Aerobiology and Chair of EMLab P&K’s Scientific Advisory Board. Widely considered the leading expert in indoor air quality (IAQ), Dr. Burge pioneered the field more than 30 years ago. She has served as a member of three National Academy of Sciences committees for IAQ, including as Vice-Chair of the Committee on the Health Effects of Indoor Allergens.View Dr. Burge’s Curriculum Vitae.