Common Building Molds and Their Hazard Classes

0 Flares 0 Flares ×

Building molds are common on damp materials with a water activity value equal to or greater than 0.90.

Building molds and yeasts such as strains of Aspergillus fumigatus, Trichoderma spp., Exophiala spp., Stachybotrys spp., Phialophora spp., Fusarium spp., Ulocladium spp., and yeasts such as Rhodotorula spp. grow well on very wet building materials.

Materials with a water activity value ranging from 0.90 – 0.85 are colonized by Aspergillus versicolor while those with water activity values of 0.85 or slightly less are colonized by Aspergillus versicolor, Eurotium spp., Wallemia spp., and Penicillium spp., such as Penicillium chrysogenum and Penicillium aurantiogriseum.

Hazard Classes of Building Molds

Building molds on ceiling wood dominated by Penicillium

The mold Penicillium growing on ceiling wood

In some countries building molds have been grouped into 3 hazard classes based on associated health risk. These classes are similar to risk groups assigned to microorganisms handled in laboratory environments.

  • Hazard Class A: includes fungi or their metabolic products that are highly hazardous to health. These fungi or metabolites should not be present in occupied dwellings. Presence of these fungi in occupied building requires immediate attention.
  • Hazard class B: includes those fungi which may cause allergic reactions to occupants if present indoors over a long period.
  • Hazard Class C: includes fungi not known to be a hazard to health. Growth of these fungi indoors, however, may cause economic damage and therefore should not be allowed.

See which hazard class each of the common building molds belong to at Common Indoor Molds.

Is Sampling and Testing for Building Molds Necessary?

The mould Dicyma olivacea from insulation material

The mould Dicyma olivacea from insulation material

Yes. Building occupants need assurances that they were not exposed to building molds that may cause health problems. Some of the objectives for laboratory testing are:

  • To determine which moulds are growing in the building and hence the level of protection required for both the occupants and remediation staff.
  • To determine the presence/absence of airborne spores, their composition and concentrations in situations where occupants complain of mould related ill health but with no obvious visible mould growth.
  • To determine if spores from visible growth sources had become airborne.
  • To detect and quantify certain mould species.
  • To determine the effectiveness of remediation work.


Gravesen S, Nielsen PA, Iversen R, Nielsen KF. (1999). Microfungal Contamination of Damp Buildings–Examples of Risk Constructions and Risk Materials. Environmental Health Perspectives, Supplements Volume 107, Suppl 3:505-8.

Sedlbauer, K., (2002): Prediction of mould fungus formation on the surface of and inside building components. Doctoral Dissertation, Fraunhofer Institute for Building Physics.

0 Flares Twitter 0 Facebook 0 LinkedIn 0 0 Flares ×
The following two tabs change content below.
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

Join Us to Get Free News and Tips

Mold and bacteria testing tips, news and insights that we only share with our private newsletter subscribers.




No thanks, I don't need today's most important news.
905 290 9101
0 Flares Twitter 0 Facebook 0 LinkedIn 0 0 Flares ×

Free Report Reveals Simple Ways to Improve Your Lab Results

This free 16-page report was written to provide you with the information we've learned after decades of analysis and interpretation.

  • 3 simple ways to improve laboratory reports
  • Understanding terminology used in laboratory reports
  • How to interpret laboratory results for airborne fungal samples
  • And much more...
Close this popup

Enter your email address below to get instant access