Aspergillus/Penicillium and Unidentified Spores: What Should You Know?

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Penicillium spores are very similar to Aspergillus spores

Penicillium spores

Indoor air sampling for mould spores may be conducted to help in evaluating the air quality after occupants’ complaints of ill health, to determine the effectiveness of remediation procedures, to assess health hazards or to proactively monitor indoor air quality.

Mould spores enter a building from outdoors through:

  • air intakes for the heating, ventilation, and/or air conditioning system (HVAC),
  • doors and windows
  • contaminated building materials and contents.

Interpreting Air Sample Results For Mould Spores

Since under normal conditions indoor mould spores originate from outdoors, the common practice in mould investigation is to compare indoor airborne fungal populations and their concentration to those of outdoor samples.

While indoor/outdoor comparison can work fairly well for culturable air samples or non-viable samples with easy to identify fungal spores, it may not work all the time for non-viable air samples when the dominant spore types both indoors and outdoors are Aspergillus/Penicillium, unidentified spores, basidiospores, or ascospores.

Comparing Indoor/Outdoor Concentrations of Aspergillus/Penicillium and Unidentified Spores

Trichoderma spores are similar to penicillium or Apergillus spores

Trichoderma spores

Spores of Aspergillus and Penicillium are rounded or slightly elongated, and smooth or ornamented depending on the species. Since these spores have no other distinguishing characteristics, laboratories usually lump them together as Aspergillus/Penicillium when they analyze air by direct microscopy (total spore counts).

Aspergillus spores are very similar to those of Penicillium

Aspergillus spores

Unfortunately, there are many other genera and species which produce spores that are very similar and difficult to differentiate from Aspergillus and Penicillium.

For example, spores of some species belonging to the following groups may easily be misidentified as Aspergillus/Penicillium: Trichoderma, Absidia, Acremonium, Aphanocladium, Beauveria, Chromelosporium, Cladosporium, Phialophora, Gliocladium, Metarrhizium, Monocillium, Mortierella, Mucor, Paecilomyces, Syncephalastrum, Verticillium and many others.

Fortunately most of these are not as common indoors as the Aspergillus and Penicillium species.

Unidentified spores including ascospores and basidiospores also present problems when it comes to indoor/outdoor comparison if these spores are in high concentrations both indoors and outdoors. Trying to compare indoor/outdoor samples for these categories of spores could be erroneous since one could be comparing spores belonging to totally different genera.

The Solution

Syncephalastrum spores may be similar to Penicillium or Aspergillus spores

Syncephalastrum spores

Presence of significant numbers of Aspergillus/Penicillium and unidentified spores (including ascospores and basidiospores) in the indoor environment is indicative of poor air quality.

However, it’s difficult to tell from these results whether the indoor spores originated from outdoors and whether the spores belong to Aspergillus/Penicillium or other moulds with similar spores. The solution to this problem would be to collect culturable air samples.

Culturable air samples could help in characterizing most of these moulds if they grow and produce spores in culture.


  1. Brian G. Shelton, Kimberly H. Kirkland, W. Dana Flanders, and George K. Morris (2002). Profiles of Airborne Fungi in Buildings and Outdoor Environments in the United States. Appl Environ Microbiology; 68(4): 1743–1753
  2. Smith, E. Grant (2000). Sampling and Identifying Allergenic Pollens and Molds, Blewstone Press, San Antonio, Texas.
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Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at
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