Interpreting lab test results for indoor mold is currently not standardized. Therefore, it would not be surprising if you give 10 different people same copy of lab test results for interpretation and you get 10 different opinions. However, if you tell the 10 people what you wanted to find out from those results and also give them visual assessment results and the history of the building, you are likely to get most of the people arriving at the same conclusion. In other words, lab test results are meant to answer a question or prove/disprove a hypothesis that was formulated prior to collection of samples. This is only possible if you have a well defined objective for collecting mold samples for lab testing. The 3 common objectives for collecting mold samples are:
- To determine if building materials or contents are colonized by mold. Mold may or may not be readily visible with unaided eyes depending on the stage of growth and the color of the mold against that of the material it’s growing on.
- To determine whether the indoor environment has “normal and typical” types and amounts of airborne mold spores. Mold spores are found everywhere including indoors. Mold becomes a problem when it grows indoors. A number of molds that grow indoors, for example, some species of Stachybotrys and Aspergillus are known not only to cause allergy but also produce highly toxic byproducts (mycotoxins). Such molds when present in high numbers pose a health risk to the occupants.
- To determine if mold removal was effective. This is sometimes referred to as “Clearance testing”. Mold removal or remediation is meant to bring the mold levels to normal condition.
Once the sampling objective has been defined, the next thing is to decide on the most appropriate types of samples to collect. The types of samples you need to collect is determined by the type of data that would answer a question or prove/disprove your hypothesis.
Types of Mold Samples
Air Samples.
Air samples can be collected in 2 different ways. By impacting air on some growth media (culture air samples) or impacting air on inert media (non-culture air samples). The choice of either method depends on the objective of the investigation. If you are interested in determining whether specific species of mold are present in the air then you should collect culture air samples. If you want to determine how contaminated the air is, then non-culture air samples are the most appropriate. You use both sampling methods if you are interested in determining the species present and also the level of contamination.
Lab test results for air samples are usually qualitative and quantitative. Without standards and guidelines, interpretation of results of mold air samples relies on comparisons of indoor vs. outdoor results. There are three primary sources of mold spores found in indoor environment. These are outdoor air carried in through doorways and windows; spores carried in on people, pets, or items brought into the building; and mold that grow and produce spores indoors as a result of excess moisture.
Surface Samples
Surface samples include bulk (pieces of building material and dust), swab and tape-lift samples. The primary purpose for collecting surface samples is to determine whether building materials and content are colonized and by what type of mold. With the exception of dust samples we do not recommend quantifying mold on surfaces of building materials. Quantification data obtained from these type of samples is usually not a good representative of the actual contamination level in the building and in most cases this data is not accurate and can therefore be very misleading. And even if the data was accurate, it cannot be used to determine risk of mold exposure and therefore has no practical use in determining health risk.
So, What Do Your Mold Lab Test Results Mean?
Mold lab test results should never be interpreted on their own. Your own visual assessment during the inspection and the building history should be given more weight. Some labs offer help with results interpretation but for them to provide meaningful interpretation you should provide them with building assessment data and what you were trying to determine through lab testing.
Determining if building materials or contents are colonized by mold
Tape-lift samples are the most appropriate for determining if building materials or contents are colonized by mold. In cases where scotch tape cannot be used to collect a sample, e.g., on wet surface, dry swabs can be used to collect the samples. Bulk samples may also be used. Presence of both mycelia and spores is an indication of mold growth.
Determining whether the indoor environment has “normal and typical” types and amounts of airborne mold spores
Air samples are the most appropriate to determine whether the indoor environment has “normal and typical” types and amounts of airborne mold spores. “Normal and typical” types and amounts of airborne mold spores would be defined as the average amount and types of mold spores found in buildings with no history of water damage or ventilation problems. A building with normal and typical amounts of mold spores may have several spore types at levels that are lower than those found outdoors. Even in buildings with no history of water damage, spores of Penicillium/Aspergillus may be prevalent exceeding the absolute levels and relative percentages of these spores outdoors. Moisture-indicator molds such as Chaetomium, Stachybotrys, and Ulocladium species should be absent.
When the relative airborne spore concentration is greater indoors than outdoors, it indicates that the source of spores is not outdoor air alone, but also an indoor source, such as mold growth associated with a leaky roof, leaky foundation, plumbing leak, or any significant moisture source that is sustained over a long period.
Determining if mold removal was effective
To determine if mold removal was effective, visual inspection of the surfaces that have been remediated is key in assessing the effectiveness of the remediation effort. The remediated surfaces should show no evidence of present or past mold growth. The visual assessment is backed up with laboratory testing of surface and air samples. Presence of a few airborne indicator mold spores after mold removal does not mean remediation was not effective.
Some professionals believe air sample lab test results are not reliable and “clearance testing” performed after mold removal should just be visual. However, air testing is an important additional tool to ensure the building occupants are not exposed to elevated levels of airborne fungal spores and spores were not spread everywhere during the remediation process.
