The Invisible World

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Contributed by Jeffrey C. May, May Indoor Air Investigations, Tyngsborough, MA

As an IAQ professional I have to be keenly observant about visual clues that point to potential sources of contamination. But a lot of the evidence I gather isn’t visible to the naked eye.

Unlike many IAQ professionals who send samples to outside labs for analysis, I use microscopy to examine most of the samples I take. I have a camera mounted on my microscope and take photomicrographs of what I see, so that I have a visual record of the contaminants that could negatively impact IAQ (I include micrographs in my reports).

Sometimes the microscopic evidence confirms the  conclusions that I’ve drawn from visible evidence, but sometimes what I’m seeing under the microscope points me in a different direction altogether.

Here are some examples:

Case Study #1. My client who had mold allergies asked me to look at a house she had purchased and was having renovated prior to moving in. She was concerned about possible mold in the basement, which had experienced some water intrusion over the years.

Efflorescence on a foundation wall due to moisture intrusion

Efflorescence pointed the way to minor leakage at a capillary crack between two foundation pours, visible after she had all the drywall and insulation in the old finished basement removed. Like many people do, she had mistaken the feathery look of efflorescence for mold growth.

I had one client who was told that she needed a $3,000 mold remediation in her basement and that staying in the house until the work was done would threaten her health. She was living in a hotel when she called me. The “mold” on her foundation wall was efflorescence.

That didn’t mean, however, that the client’s basement was free of mold growth. Some of the original framing from the old basement family-room walls was still in place, and except where decayed due to moisture intrusion the framing appeared to be intact. I didn’t see any visible mold on visible framing surfaces, but I was still suspicious because I’ve often found mold growth on construction materials behind basement finished walls.

Mold growth requires moisture. Below-grade, such moisture can be supplied by water intrusion. Some types of mold including Aspergillus and Penicillium species can grow when the relative humidity is over 80%. Since below-grade spaces are naturally cool and damp, even in the absence of water intrusion or leakage, they are prone to developing mold-growth problems.

I took a few tape samples of dust on stud surfaces facing the foundation wall. These samples confirmed that mold growth was present. I also found evidence of mite activity in the sample, which didn’t surprise me since mites often forage on mold growth.

Cladosporium mold spores, mite, and mite egg

I was also concerned that there might be mold growth on the foundation wall, leading to a need for basement cleaning or remediation prior to construction of finished basement space, so I took another tape sample. It contained evidence of mold growth, but it also contained something else: actinomycetes. These bacteria resemble fungi because they grow and reproduce with hyphae and spores.

Actinomycetes wrapped around a fiberglass fiber

Long-term exposure to one type of actinomycetes (thermophilic) is associated with a serious and potentially life-threatening respiratory illness called hypersensitivity pneumonitis or HP: one of the few respiratory illnesses that physicians recognize as being caused by environmental exposures to bioaerosol.

Case study #2: An elderly couple had sold their large Victorian and moved into a condo. They called me because they were constantly cleaning dust from surfaces, and they were both experiencing a lot of allergy symptoms. The tape sample that I took of the house dust consisted of almost 50% starch granules. I asked the woman if she or her husband used cornstarch body powder or tossed pizza dough into the air in her kitchen. She replied that they hardly cooked anything and neither of them used body powder.

I took a sample of the duct dust, and it, too, contained starch granules. The extensive presence of starch in the air could certainly account for the mold growth I found in the stinky furnace-humidifier tray. The husband and wife were being exposed to spores from this mold growth.

Furnace Humidifier Tray Full of Mold

How did all of that starch get into their condo in the first place? The couple found out that their condo was the last unit in the building to sell, and it had been used meanwhile to mix wallpaper paste for other units. The paste is made from cornstarch powder mixed with water.

Case study #3: I was contacted by a mold remediator who was quite upset. The post-remediation verification (PRV) testing of a basement remediation job he had completed confirmed that Aspergillus mold spores were still present in the air and in the foundation dust. The remediator went back to the property and did additional remediation work and cleaning – only to be disappointed when the property again failed the PRV test. Frustrated, he requested my assistance to help determine why the property was still contaminated. I expected to find obvious signs of mold growth in the dust, but the only particles that were present were spray-paint spheres.

Microscopic Spray Paint Spheres With Pigment Inclusions

Whenever paint is sprayed, a great deal of paint aerosol is created. The larger liquid particles adhere to the surface being sprayed, but some of the smaller particles in the air dry out and remain airborne or settle onto other surfaces. The sizes and shapes of these particles resemble those of Aspergillus mold spores; the lab that the mold assessor used was, shall we say, less than competent.

This case study leads me to one of my pet peeves: incorrect lab analyses. For example, even if mold spores are correctly identified, most labs do not report the presence of fungal conidiophores or of spores in chains or clusters. In consequence, the results can be misleading.

Individual mold spores observed in a dust sample are not necessarily evidence of mold growth, since the spores could have come from another source and been carried on airflows to that location. If indoor samples contain fungal conidiophores or Penicillium and/or Aspergillus spores in chains or clusters, indoor mold growth is present, regardless of any comparison made between spores in the outdoor air versus spores in the indoor air. 

Here is some advice that flows from these case studies:

  1. If you send samples to an outside lab, try to find a lab that will indicate the presence of chains or clusters of Penicillium and/or Aspergillus mold spores or of fungal conidiophores, rather than just report on spore concentrations.
  2. The history of the use of a property is important, including whether the relative humidity in a below-grade space has been adequately controlled since construction.
  3. Exterior grading and roof-water control are of particular importance for a property with a finished basement.

About the Author: Jeffrey C. May is founder and Principal Scientist of May Indoor Air Investigations LLC in Tyngsborough, MA (978-649-1055). A nationally recognized speaker on IAQ topics, he is the author of five books on IAQ, including My House is Killing Me! A Complete Guide to a Healthier Indoor Environment. A member of MIAQC and IAQA (which inducted him into its Hall of Fame), Jeff is a Council-Certified Microbial Consultant (ACAC), a Certified Indoor Air Quality Professional (AEE), a former adjunct faculty member in the Department of Work Environment at U MA Lowell, and a Board member of the Massachusetts Association of the Chemically Injured and the Chemical Sensitivity Foundation. He holds a B.A. from Columbia College (chemistry) and an M.A. from Harvard (organic chemistry). More information about his professional experience and publications is available on


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