Insights

Understanding Indoor Mold Growth: Estimating Time of Mold Growth

J.S. Held Acquires Stapleton Group & Launches Strategic Advisory Practice

Read More close Created with Sketch.
Home·Insights·Articles

The material in this paper was researched, compiled, and written by J.S. Held. It was originally published by The Microscope Journal.

Introduction

Mold growth on surfaces indicates that moisture is, or has been, present in an indoor environment. Direct microscopic examination (by a mycologist) is the primary method to determine whether mold or other fungi have grown on a particular surface. This analysis, while generally straightforward, provides the important answer to the question of whether mold growth is present. Mold growth can be described as the presence of vegetative (hyphae or mycelium) or sexual structures (fruiting bodies) with or without associated spores. 

An important question that mycologists who work on indoor fungi often hear from industrial hygienists and other professionals within the indoor air quality industry regards whether it is possible to establish the time of mold growth in an indoor environment. Specific knowledge about the growth rate of mold genera or species can make it possible to estimate the amount of time which it took for growth to take place with reasonable scientific certainty.

This paper discusses the rate of fungal growth indoors and the possibility of estimating the time of growth from a water loss. The following information may be of particular interest to industrial hygienists, forensic engineers, building consultants, and insurance adjusters.

Conditions for Indoor Mold Growth

The conditions for mold growth outdoors differ significantly from indoors. Fungal growth outdoors is subject to far harsher environmental conditions such as extreme fluctuations in temperature and moisture, humidity, or pH variations. Fungi/molds growing indoors have a more favorable environment to complete their life cycle. For mold to grow indoors, four structural elements are required: fungal spores, food sources, moisture, and time. Without sufficient time, fungal colonies are not visible, even if the other three conditions are met. Therefore, after a moisture event, it is crucial to dry moist surfaces quickly to prevent mold growth. Mold growth occurs when spores contact free water and germinate. The moment spores germinate is technically considered growth. This process may take hours depending on a particular mold species. However, at this stage, growth on surfaces is not yet visible to the naked eye. This process occurs at the microscopic level. It is not until days or even weeks later that we see the results of mold colonization on surfaces (Figure 1).

Figure 1 - Various mold colonization on drywall board.

Mold Growth Rates

All living organisms have rates of growth, i.e., growth over time, and fungi are no exception. The growth rate can vary between major groups and even within species in the fungal kingdom. When describing the growth of fungi in culture media, mycologists usually provide the growth rate over a certain time, and this growth is measured for a particular species in its most optimum cultural environment. Fewer optimum conditions indoors (e.g., moisture, temperature, competition with other fungi) slow the growth of fungi. The orange mold, Chrysonilia sitophila, a highly allergenic species with an infrequent occurrence indoors, can cover a culture plate or moist lumber within 24 to 72 hours. Another common mold, Trichoderma harzianum, and other species within the genus can grow rapidly, about 22.0 mm per day, and cover the entire culture plate within a brief time. Very few studies have looked at the rate of indoor mold growth on natural substrates such as drywall or wood.  For example, Elansky et al., 2003 looked at growth of Stachybotrys chartarum on wallboard and found the growth rate to be approximately 2.5 mm per day, slightly lesser growth rate than in culture media (i.e., about 3 mm) under optimum growth condition. We can use published available rate of growth from culture studies (their optimum controlled growth environment) described in mycological literatures then apply the measured growth rate to estimate the amount of growth colonizing surfaces indoors (less optimum and slower growth rate).

The common mold Stachybotrys chartarum, a hydrophilic (water-loving) species, has a much slower rate of growth compared to faster-growing molds, about 2.5 mm per day on wallboard or about 3.0 mm per day in culture. Other common molds like species of Aspergillus, such as A. versicolor and A. niger, have rates of growth of 9.0 mm and 12.0 mm per day, respectively. Common indoor Penicillium species such as P. chrysogenum and P. brevicompactum have growth rates of 6.5 mm and 4.0 mm per day, respectively. Other molds may require additional time, e.g., 7 to 14 days to sporulate. An example of such mold is Chaetomium globosum (Figure 2) as well as other species within this genus. This species grows about 70.0 mm in 7 days vegetatively but requires an additional 7 days or so for sporulation (fruiting body production). And, of course, we cannot ignore the growth rate of the most ubiquitous mold, Cladosporium. Common indoor species such as C. cladosporioides, C. sphaerospermum, C. halotolerans, C. allicinum, or C. herbarum have an average growth rate of about 28.0 mm in 7 days. In addition, many species within Cladosporium have a wide range of temperature tolerance, growing in cold refrigerator temperatures as well as in hot attics during summer months, making it the most widely encountered mold within the built environment.

Figure 2 - Measurement of growth of Chaetomium globosum colony on wallboard.

Other Fungi and Long-Term Growth

Other fungi that are not considered “mold” can have much more complicated rates of growth, taking months or even years. For example, wood-decay basidiomycetes are not only capable of superficial growth on wood surfaces (like most molds) but can penetrate deep inside the wood tissue and cause decay in the wood material and compromise the structural integrity of the wood. Common evidence of such decay is often the appearance of the wood, depending on the species found, which can typically appear as “cubical” (Figure 3), causing the wood to become so soft that one can easily penetrate it using a knife. The process of decay itself can take much longer— months, or even years. Other macro-fungi such as indoor saprophytic mushrooms like Coprinus species also result from months of growth. It is not unusual to find the growth of these mushrooms indoors where there has been prolonged water exposure. Another often-encountered macro-fungus, Peziza domiciliana, has been recorded on indoor carpets and even on drywall (Figures 4, 5). From the time the minute fruiting bodies are formed to about 14.0 mm in diameter can take approximately 30 to 35 days. This conservative estimation is purely based on the visible formation of fruiting bodies and does not even take into consideration the amount of time it takes for the development of subtending vegetative growth prior to sexual fruiting body formation.

Figure 3 - Cubical appearance of wood-decay caused by a brown-rot fungus.

 

Figure 4 - Growth of Peziza domiciliana fruiting bodies at various stages of maturity.

 

Figure 5 - Peziza domiciliana fruiting bodies at a late stage of maturity on drywall.

 

Estimating Mold Growth Rates Indoors

By knowing the detailed growth rate, we can estimate the amount of time it took for mold colonies to reach a certain size. The age of the mold or macro-fungi is not being determined in this type of fungal analysis. This is no different than if a dried-up corn plant used for Halloween decoration were to be aged. While we can tell how long it would take for a corn plant to reach maturity (growth rate) from the time of planting the seed to full maturity, once dried up, we cannot determine if its age is one or 10 years old. Similarly, we cannot determine the age (at least based on current scientific techniques) of a piece of dried wallpaper with colonies of molds growing on it (Figure 6). However, we can measure the diameter of the colonies and estimate how long it would have taken for that mold colony to reach that diameter. Careful identification and colony measurements of all fungal colonies growing on either the bulk samples or on surfaces indoors must be determined. For example, the growth rate of Stachybotrys chartarum is approximately 3.0 mm per day. If the colony measurement on the drywall board in a laundry room is 76.0 mm (about 3 inches) in diameter, it would have taken about 25 days for S. chartarum to reach such size. Contrary to some beliefs that such growth could have occurred within a brief time, like over a long weekend or within a week after the discovery of such loss, it would have taken a much longer time aligned with its biological rate of growth. The same concept can also be applied to other growth rates, provided that detailed growth descriptions are available in the literature. The determination of the growth rate of fungi indoors is an estimation of the time of growth, made with reasonable scientific certainty.

Figure 6 - Measurement of growth of Chaetomium elatum colony on dry wallboard.

Conclusion

It is possible, based on fungal growth rate, to estimate how long it would take for a fungal species to reach a certain colony size based on the radial growth on surfaces. This information can be used to calculate and estimate the time of growth from a date of a loss and may aid interested parties in filing/handling related claims. If you or your organization have concerns regarding fungal growth following a related loss, be sure to consult with experts with relevant mycological experience and insights.

Acknowledgments

We would like to thank our colleague Dr. Payam Fallah for providing insights and expertise that greatly assisted this research.

Dr. Payam Fallah is a Senior Technical Fellow (Mycology) in J.S. Held’s Environmental, Health and Safety practice. He has over 25 years of experience in mycology and indoor air quality as a consultant and expert in investigations throughout North America and the Caribbean. He has evaluated mold cases involving commercial and residential buildings regarding mold growth, aerosolization and exposure, as well as fungal wood decay compromising the structural integrity of building materials. Dr. Fallah holds a PhD from the University of Illinois and is an adjunct faculty member at McCrone Research Institute, where he teaches Introductory and Advanced Indoor Air Quality classes. He is also the director of the microanalytical laboratory at J.S. Held and a consultant and expert in mold litigations regarding indoor air quality issues, working closely with insurance company adjusters regarding mold claims.

Dr. Fallah can be reached at [email protected] or +1 425 292 1565.

References

[1] Domsch, K. et al. 2007. Compendium of soil Fungi. 2nd edition.

[2] Elansky, S., et al. 2003. Growth of Stachybotrys chartarum strains on natural and artificial substrates. Botanica Lithuanica 9:171-177.

[3] Francuz B. et al. 2010. Occupational Asthma Induced by Chrysonilia sitophila in worker Exposed to coffee Grounds. Clinical and Vaccine Immunology. 17: 1645-1646.

[4] Krause M. et al., 2006. Controlled Study of Mold Growth and Cleaning Procedure on Treated and Untreated Wet Gypsum Wallboard in an Indoor Environment. Journal of occupational and Environmental Hygiene 3: 435-441.

[5] Macher J. Bioaerosols: Assessment and Control.1999.

[6] Pady, S. 1939. Observation on the rate of growth of ascocarps of Peziza domiciliana. Mycologia. 31:53-55.

[7] Samson, R. et al. 2019. Food and Indoor Fungi. 2nd edition.

Find your expert.

This publication is for educational and general information purposes only. It may contain errors and is provided as is. It is not intended as specific advice, legal, or otherwise. Opinions and views are not necessarily those of J.S. Held or its affiliates and it should not be presumed that J.S. Held subscribes to any particular method, interpretation, or analysis merely because it appears in this publication. We disclaim any representation and/or warranty regarding the accuracy, timeliness, quality, or applicability of any of the contents. You should not act, or fail to act, in reliance on this publication and we disclaim all liability in respect to such actions or failure to act. We assume no responsibility for information contained in this publication and disclaim all liability and damages in respect to such information. This publication is not a substitute for competent legal advice. The content herein may be updated or otherwise modified without notice.

You May Also Be Interested In
White Papers & Research Reports

Indoor Air Quality: Health Effects of Airborne Mold & How Mold Is Measured

Mold is a common concern following remediation of water intrusion and/or claims regarding potentially adverse health effects of indoor environments. Adequate assessments require an expert understanding of molds, potential health effects, and appropriate investigative and...

Perspectives

Beyond Mold, Asbestos & Lead: The Role of Industrial Hygienists in the Construction Industry

Mold, asbestos, and lead are familiar hazards to most, but they are not the only hazards found on modern construction sites. In fact, industrial hygienists are called to help reduce health risks to workers posed...

Perspectives

Water Damage Restoration: A Guide to Advanced Structural Drying

Advanced Structural Drying (ASD) is the effective, efficient drying of water-damaged buildings and dwellings, using comprehensive knowledge and tools. The process includes the collection of appropriate data for utilization and application of scientific and technical...

 
INDUSTRY INSIGHTS
Keep up with the latest research and announcements from our team.
Our Experts