How To Interpret Laboratory Results for Airborne Fungal (Mould) Samples

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Laboratory results for airborne mould and bacteria concentrations can be difficult to interpret for two major reasons.

  • Maximum exposure limits have not been set
    Currently, there are no set maximum exposure limits (MELs) or threshold level values for airborne indoor mould and bacteria concentrations. Setting MELs would be difficult for reasons which include limitations in air sampling techniques (Kung’u, 2004), variability in sensitivity to microbial exposure among the human population, occurrence of a large number of different types of biological and chemical pollutants in indoor environment (Morey, 2001) and limited data on exposure-response relationship.
  • Failing to clearly define air sampling objectives
    In some cases investigators fail to clearly define the objectives for taking air samples for microbial analysis. A well defined sampling objective helps the investigator to design an appropriate sampling strategy which includes the data required and hence the type of samples to take (i.e., viable or non-viable), the minimum number of samples to take, when to take them and how to interpret the resulting data. An initial walkthrough of the building under investigation is recommended prior to designing the sampling strategy.

Principle guidelines in data interpretation

In absence of set MELs, concentrations and composition of indoor airborne mould and bacteria are compared with those of outdoor (Miller, 2001, Health Canada, 2004). The investigator, therefore, should have some background information on:

  • typical airborne fungal or bacteria concentrations including seasonal, diurnal and temporal variation in concentrations and composition for outdoor and indoor environments,
  • factors affecting airborne microbial concentrations,
  • mycological or bacteriological terms used in the reports.

Understanding Terms Used in Laboratory Reports

Terms used in non-viable analysis report

Mould spores sampled from air

Amerospores: single celled spores (width to length ratio <15:1). Since spores from many groups of fungi can be referred to as amerospores, this term is not useful in reports.
Basidiospores: spores produced by a group of fungi called basidiomycetes, for example mushrooms, some wood-rotting fungi.
Ascospores: spores produced by a group of fungi called ascomycetes
Smut: refer to spores produced by smut fungi.
Rust: refer to spores produced by rust fungi.
Myxomycetes: slime moulds. Not true moulds.
Hyphal fragments: refer to fragments of the filamentous structures (hyphae) that make up the body of moulds by branching extensively to form a complex network called mycelium.

Terms used in Viable Analysis Report
Colony Forming Units (CFU): for moulds, a colony refers to a group hyphae. The term “Units” refers to the mould component(s) from which a colony developed. The unit could be a single spore or a single hypha, a group of spores or hyphae or a mixture of both spores and hyphae.
Non-sporulating (non-sporing) isolates: refer to moulds that fail to produce spores either because they have lost this ability, conditions were not suitable or requires very long periods to produce spores. Fungi that are strictly non-sporulating are called mycelia sterilia (sterile mycelia).
Sp (plural written as spp).: short form for species, e.g., Penicillium sp or Penicillium spp referring to a single species or more than one different species of Penicillium respectively.

How to interpret laboratory reports

Air samples on RCS agar strips

Air samples on RCS agar strips

As indicated above interpretation of airborne concentration of indoor moulds and bacteria is primarily based on experience and professional judgement. Basic knowledge in ecology of moulds is helpful. For example if lab results show that dominating spores for an indoor sample were mainly smut and rust spores while the outdoor sample showed dominant moulds as Aspergillus/Penicillium, then the investigator would suspect the samples were swapped and may want to resample again. To be able to interpret the results there are key steps to follow:

  1. Review the air sampling objective Let us say the primary objective of the investigation was to determine if there were elevated mould spore concentrations and/or indoor amplification sources in complainant room.
  2. Bearing in mind the sampling objective, compare total airborne spore concentrations from complainant room with those from outdoors and non-complaint rooms. This would answer the question whether levels of mould spore concentration were elevated in complainant room compared to outdoor and non-complaint room.
  3. Compare the dominant spore types (and their concentrations) from complainant room with those from control samples. Our objective was also to know whether there were amplification sources in the complainant room. The data we need to answer this question are:
    • Dominant moulds present in complainant room but not in the control samples. For examples if Cladosporium is dominating in complainant room but is insignificant in outdoor and control sample, we can conclude the source is in complainant room, even though Cladosporium can originate from outdoor.
    • Presence of indicator moulds. These are moulds frequently found in water damaged buildings. These include Aspergillus and Penicillium species, Acremonium spp., Sporobolomyces spp., Stachybotrys chartarum, Memnoniella echinata, Tritirachium oryzae, Ulocladium botrytis, U. chartarum, Cladosporium spp., and Chaetomium spp.
Air Sample taken with an Andersen Sampler

Air Sample taken with an Andersen Sampler

Some of these moulds such as Aspergillus fumigatus, A. niger, Penicillium oxalicum, P. thomii, and Cladosporium species, may also originate from outdoor environment. Building history i.e., whether there has been previous water problem would give further evidence as to whether there was mould growth in the complainant room. Register for the course How To Interpret Mold Results to earn more.

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Kung’u, J. N.(2004). Limitations and Considerations in Air Sampling, Sample Analysis and Result Interpretation for Airborne Mould Spores. Inoculum, 55(5):1-4.

Miller, J.D. (2001). Mycological Investigations of Indoor Environments. In: Microorganisms in Home and Indoor Work Environments. Diversity, Health Impact, Investigation and Control. Edited by Brian Flannigan, Robert A. Samson and J. David Miller. Taylor & Francis, London and New York 2001, hard back ISBN 0-415-26800-1. Pages 231-246.

Morey, P. R. (2001). Microbiological Investigations of Indoor Environments: Interpreting Sampling Data- Selected Case Studies. In: Microorganisms in Home and Indoor Work Environments. Diversity, Health Impact, Investigation and Control. Edited by Brian Flannigan, Robert A. Samson and J. David Miller. Taylor & Francis, London and New York 2001, hard back ISBN 0-415-26800-1. Pages 275-284.

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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
Dr Jackson Kung'u

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