Q & A: Standards for Interpreting Mold Samples Mold testing indoor air quality - understanding types of mold growth and mold spores from mold test kits reports.

Q. My company conducts IAQ assessments primarily in residential buildings. Is there a direct correlation between total airborne mold spore counts conducted on microscope slide samples and counts based on collection and growth on agar plates? There are some industry- accepted standards for culture plate sampling (CFUs/m3) but none for spore counts. With regards to assessment and clearance, what constitutes a problem? What criteria should be used to accurately state whether a mold problem does or does not exist? How do I interpret high spore counts with low counts using culture plate techniques?
- Ron, Florida

A. Statistically total spore counts are always significantly correlated with counts conducted on agar plate samples. Though correlation coefficients are significant they are relatively modest indicating that only about 30% of the variability associated with the data can be ascribed to the direct relationship between total mold and culturable-viable sampling results. The variability that cannot be accounted for has to be due to other factors.

One of the most important of these is the variation in apparent viability from one set of results to another. On average total mold spore to culturable mold ratios are in the range of 10:1 that is that there are 10X more dead mold spores airborne than there are live ones. Sometimes the ratio is as low as 2-3:1. I have seen ratios as high as 2000:1.

Mold spore viability depends on a variety of environmental factors. These include (1) how active mold infestation is and (2) mold spore history relative to the time it was released, exposure to ultraviolet light and drying conditions. Most mold spores that are airborne are no longer alive. As such they cannot grow on culture media. Spore viability appears to vary from genus to genus with highest apparent viabilities reported for Cladosporium and Penicillin. Mold types with low apparent viabilities appear less frequently on culture plates than they do on sample slides. This is the case for Epicoccum and Alternaria.

It is good practice to conduct both culturable-viable and total mold spore counts in house mold assessments. The ratio can be used to ascertain how active the infestation is. A low ratio (on the order of two to five) indicates a relatively active infestation and a need to re-mediate both the infestation and its cause. High ratios (typically greater than 10) indicate that the infestation is less active with remediation focused on infestation sites and less on the cause of the initial infestation. A high spore count and low culture count indicates a relatively inactive infestation. Since allergenicity is independent of viability, the high spore count is significant. It indicates that a significant exposure risk exists.

I use 10,000 spores per cubic meter as my guideline value in residences. This is based on some research work in Australia in which we were able to show a correlation between symptoms and total airborne mold levels. Levels above 15,000 s/m3 are of particular concern. These are results that are based on Burkard samplers or their equivalent (laboratory calibrated Allergenco samplers) and counting at 1000X. Culturable-viable results should never be higher than total mold spores sampling results.

What is a "safe level"? That is a difficult question. In residences concentrations based on the Burkard total mold spore method (or equivalent) should not exceed 10,000 S/m3 with 5,000 S/m3 as a target level. Concentrations dominated by one genus such as Penicillium or Aspergillus even at 10,000 S/m3 are unacceptable. In such cases levels should not exceed 5,000 S/m3. Repeated testing is recommended when as few as one Stachybotrys spore is found on a slide or culture plate.
One cannot state in regards to a mold problem whether a house is safe or not. More realistically a house can be described as relatively safe when total airborne mold concentrations are less than 5,000 S/m3 and no evidence of mold infestation is present.