Investigations and evaluation of mold related problems rely on sampling techniques to determine the presence or absence of mold growth. In the indoor environment presence of mold is an indirect indicator of the occupants’ potential exposure to those molds. There are several mold sampling methods and understanding the advantages and disadvantages of each method is important. Each sampling method has its advantages and disadvantages, meaning that no single method is capable of fully answering all the investigation questions. Therefore, a complex mold investigation may require more than one sampling method. Before choosing a sampling method, it’s important to understand what kind of data each method can provide and how to interpret that data. You may want to learn more about our mold inspection and control course.
Methods for Sampling of Indoor Mold
Air sampling methods (culture plates, spore traps)
When people talk of spore traps they refer to air sampling using filter membranes or adhesive-coated slides such as Air-O-cell, Allergencos or Micro 5. However, sampling air using culture plates is also a form of spore trapping. Results for spore traps are reported as number of spores per cubic meter of air (spores/m3). One big disadvantage for spore traps is that the mold types can only be identified to genus level and a significant number of spores may have to be reported as “unidentified spores”.
Results of samples collected using culture plates are reported as number of colony forming units (cfu) per cubic meter of air (cfu/m3). The advantage of culture plate sampling is that the types of mold present can be identified to species. A few molds may fail to produce spores that aid in identification. Such molds are reported as “non-sporulating isolates (NSI).
When mold releases spores into the air the spores end up settling on surfaces such as those of the carpet. When the carpet is disturbed these spores together with dust are re-suspended in the air and are inhalable into the lungs and can induce adverse health effects. Sampling of carpet dust will give us an idea of the amount and types of spores present on the carpet. Spores can also be brought into the building through soiled articles such as shoes. Results for dust samples are reported as number of colony forming units (cfu) per gram of dust (cfu/g). The recovered molds can either be identified to genus or species.
Surface Swab and Wipe Sampling
Surfaces can be sampled for mold contamination using either a swab or wipe. This method is particularly useful when there is no visible mold growth. The results are reported as colony forming units (cfu) per square inch (cfu/inch2) or per square centimeter (cfu/cm2). The recovered molds can be identified to genus or species.
Tape Lift and Bulk Sampling
Tape lift sampling is the recommended method for sampling visible mold growth. If the surface being sampled is wet, then bulk or swab/wipe sampling should be used. The purpose of tape lift and bulk sampling is to determine the type of molds present. Results are provided as a list of the molds identified in rank order. The ranking is rather subjective and depends on which areas of the slide the analyst scanned. If sampling large areas of mold contamination, it’s important to collect more than one sample because a single sample may not be a good representative of the diversity of molds present in the building.
While some labs try to quantify the amount of mold on tape lift and bulk samples, such data is often not only inaccurate and misleading but it is also of no practical use.