Mold Testing – Air Quality Lab Interpretation

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Mold Testing Petri Dish Impactor

Mold Testing Air Quality Lab Interpretation

People with asthma and mold allergies are the first to ask about mold testing. Since their symptoms are primarily respiratory, it is only logical they ask for “air quality testing.” Other individuals looking for peace of mind or suffering much more severe symptoms sometimes also ask for mold testing air quality services. This blog is about air quality testing and some of the variables and limitations that should be considered in every mold testing job.

Part of our job is to help develop a mold testing air quality sampling plan specific to your project, and to help interpret the lab results in light of discoveries made during the visual inspection. I’ve heard mold testing described as “an art and a science,” and I agree with that sentiment. Since it’s not a federally regulated contaminant it has been a little bit of the wild west in terms of who tests, what they test for, and how the results are interpreted. For a customer – that should scare you! – because it has meant that anyone could call themselves mold inspectors without having adequate training or professional credentials to demonstrate their competence.

Mold Testing – When to Test Air Quality?

Most experts, including the EPA and California Department of Public Health, suggest only testing under very specific conditions. In general, a moisture and mold inspection by a qualified professional is far more useful in determining if a mold problem exists. However, there are some circumstances that warrant additional air quality testing. The most commonly referred to guidance for how to perform mold inspections and when/how to test for mold is the AIHA “Green Book,” Recognition, Evaluation and Control of Indoor Mold.

Mold Testing Air Quality

Mold Testing AIHA Green Book

Here are three general reasons that justify mold testing:

1) Determining the scope of contamination.

2) Determine the type of contamination.

3) Clearance testing.

Mold Testing – What Type of Air Quality Testing Method?

If mold testing is pursued, these are the three most common types of samples collected:

  • Air sampling: the most common form of sampling to assess the level of mold. Sampling of the inside and outdoor air is conducted and the results to the level of mold spores inside the premises and outside are compared. Often, air sampling will provide positive identification of the existence of non-visible mold.
  • Surface samples: sampling the amount of mold spores deposited on indoor surfaces (tape, and dust samples). Dr. Ritchie Shoemaker, MD is a strong believer in dust sampling using the Environmental Relative Moldiness Index (ERMI).
  • Bulk samples: the removal of materials from the contaminated area to identify and determine the concentration of mold in the sample.

In this blog, we focus on air quality testing. There are four ways of air quality testing for mold:

  1. Spore trap non-viable (non-culturable) sampling. While controversial, this is the most common type of “mold testing” we see performed in the industry. A calibrated pump draws a known volume of air over a greased slide. This is an “impaction” method of collecting spores. Based on a direct microscopic examination of a portion of the slide, the lab extrapolates how many spores are present per cubic meter, and can identify many common genera of mold. One weakness of direct visual examination is that some common genera, such as Penicillium and Aspergillus, cannot be differentiated and reported cumulatively. Another obvious weakness is that heavier spores, such as Stachybotrys, may be present but not aerosolized and therefore underreported if only air sampling is performed.
    Mold Testing Air Quality

    Mold Testing Handheld Sporetrap    Mold Testing Air Quality Testing – Alex Stadtner using pump and Sporetrap

  2. Pitri Dish-style, viable (culturable) sampling. The second most common type of air sampling for mold is also an impaction-style collector. A petri dish with a growing media is placed beneath a pin-holed cap and air is drawn over the petri-dish. The petri-dish is then incubated and visually inspected so the number of colony forming units (CFUs) of different types of molds are identified and counted. Common types of mold growth media are Malt Extract Agar (MEA), Potato Dextrose Agar (PDA), and Hay Infusion Agar. A strength of this method is that molds can be especiated. So in addition to differentiating between genera like Penicillium and Aspergillus, using viable (culturable) analysis you can identify if the mold is Aspergillus niger, or Penicillium marneffei. This can be valuable information since there are hundreds of species within each genera, and some are more toxigenic than others. Some weakness of this sampling method include the fact that different molds prefer different growth media, temperatures and moisture levels, but the lab will just use a “standard setting” unless instructed otherwise. So to get a more complete picture of the viable airborne spores in one space you may need to collect numerous petri-dish samples using different agars and to be incubated under different growing conditions. Some molds grow well under lab conditions, and others don’t. Some molds produce powerful chemical weapons used against neighboring colonies, which can influence the number of CFUs counted for both species.
    Mold Testing Air Quality

    Mold Testing Petri Dish Impactor

  3. Microbial Volatile Organic Compound (MVOC) sampling. When molds are wet and have food they grow. When mold is “eating” it produces metabolic byproduct gases known as MVOCs. Air samples can either be collected in a summa canister or sorbent tube, and the lab analyzes the air sample for a limited set of microbial VOCs. MVOC indicator gases include but are not limited to the following: Furan, 1- and 2-Pentanol, 2-Hexanone, 2-Heptanone, 1- and 3-Octanol, and Geosmin. Most MVOC scans are limited in that they do not analyze for every possible MVOC, and most MVOCs have not been directly tied to a single species of mold so extrapolating from just MVOC data is rather limiting. If you smell “musty” odors you are probably inhaling MVOCs. It’s a sign of active microbial growth and can be tested via air sampling. 
    Mold Testing Air Quality

    MVOC Testing for Mold

    Mold Testing Air Quality

    MVOC Mold Testing with Sorbent Tube or Summa Canister

  4. Mycotoxin Testing. Mycotoxins are secondary metabolites produced by fungi. These are basically self-produced chemical weapons wich can harm other fungus, and can also cause disease and death in humans. Although the primary concern for humans is for ingestion, inhalation is also of concern. Few indoor environmental consultants are currently testing for mycotoxins. The three most commonly analyzed mycotoxins include aflatoxin, trichothecenes, and ochratoxin. The lab utilizes polymerase chain reaction (PRC) analysis to identify and quantify mycotoxins in air samples. Samples are collected in 37mm cassettes, preloaded with 0.45 um pore-sized filters, and sampling times are  longer than typical sporetrap grab samples. Whereas the above referenced types of lab analysis rely on a lab technician’s ability to visually identify and count… PCR has over a 95% confidence rate and is more automated.

Mold Testing – How to Interpret Lab Results and Air Quality Data?

So you’ve had an inspection and received the lab results… what does it all mean? Companies that just send convoluted lab reports without further explanation should be put out of business. It is the role of the Certified Indoor Environmental Professional to help interpret the lab results in combination with what was gleaned from the visual inspection.

Sporetrap lab results should include, at the VERY minimum, one outdoor sample and a handful of indoor samples. The outdoor sample is used as a control to compare to the indoor samples. Because there can be such variation in sporetrap samples the statistical significance from collecting so few samples is very limited. Most labs simply share a chart showing how many of which type of genera were present.

Mold Testing Air Quality

Sporetrap Chart – Mold Testing Non-Viable, Non-Culturable

Some labs offer their own “limited interpretation,” such as the MoldScore from EMLab P&K. “MoldSCORE™ is a specialized method for examining air sampling data. It is a score between 100 and 300, with 100 indicating a greater likelihood that the airborne indoor spores originated from the outside, and 300 indicating a greater likelihood that they originated from an inside source…” Below you can see some obvious “spikes” that EMLab identified as statistically significant higher counts of Cladosporium and Pen/Asp indoors compared to outdoors.

Mold Testing Air Quality

Mold Testing Lab Interpretation

Some Building Biologists refer back to the Healthy Home Standard for guidance on interpreting sporetrap results:

Mold Testing Air Quality

Building Biology Sporetrap Interpretation Method

Viable (Culturable) results are usually presented in CFU’s, and different labs use different set points to determine low, medium or high probability of significant mold growth. Building Biologists often refer to the Healthy Home Standard suggested interpretation guidelines:

Mold Testing Air Quality

Building Biology Viable Culturable Lab Interpretation

There is much less standardization for interpreting these last two sample types. MVOC results may come with lab interpretation such as this: “Levels below 8 ng/L are typical for most homes and should not cause great concern for healthy individuals. Levels between 8 and 30 ng/L indicate a low level of mold which, generally, affects people who are sensitive to molds. Levels above 150 ng/L indicate that a high level of active mold growth is present and it is likely that nearly all occupants of the home will be affected.”

Mold Testing Air Quality

Microbial VOC (MVOC) Lab Analysis

Mycotoxin results may be paired with an individual’s blood test results, if they are working with an environmental doctor. We are often called in as a result of an environmental medicine practitioner finding that an individual’s body is reacting to mycotoxins… and then we have to find where that exposure is occurring – usually at home or work.

1) Decide if environmental sampling would add value to the case, 2) define a sampling plan that would answer a question or prove/disprove a hypothesis, 3) perform testing in accordance with industry guidelines, 4) only use qualified laboratories with good track records and the appropriate certifications, 5) interpret data in combination with information collected during the inspection, 6) provide the interpretation in a user-friendly manner written to the “right audience.”

Until mold testing becomes more regulated it will remain “an art and a science.”


Healthy Building Science is an environmental consulting firm which provides mold testing and mold inspection services for commercial, residential, multi-family, buildings, offices, industrial and manufacturing workplaces, hospitals and medical facilities, and single-family homes in the greater San Francisco Bay Area and all of Northern California including the cities of San Francisco, San Jose, Oakland, and Sacramento.