3-Steps in Determining the Age of Mold

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3 steps in determining the age of mold

Can you tell how long the mold has been there? We get this question a lot. Unfortunately, you probably won’t be able to do more than just guess on your own. An indoor environmental professional (IEP) will be needed to help “guesstimate” the relative age of molds. They are usually described as New Mold (within the past few days to months), Old Mold (within the past few months to a year), or Older Fungi (longer than a year ago).

The question comes up mostly with regard to landlord/tenant disputes, insurance claims, and those struggling with mold illness – how old is that mold? So, a simple guess is usually not defensible. You will need supporting conditions to defend your age guestimate; otherwise, you might lose that landlord dispute, your insurance company may deny your claim “as pre-existing”, or you may not correctly identify that exposure event that triggered your mold illness.

Type of Environment Impacts Mold Growth Rate

Wild mycelial networks can survive for thousands of years. As free and wild creatures, fungi only perform the work that benefits them and their immediate environment.  Wild mycelial networks are much more dynamic than cultivated ones, where we can view the home environment as a type of cultivated environment.

When a mycelial network is fed the same substrate for an extended period of time, it may stop producing the enzymes required for digesting another substance and thus undergo senescence or “slow in its growth” and go dormant. Upon contact with a novel substrate, fungi often go into a period of stasis in which their growth halts as it decide how best to consume the new food source (McCoy, 2016).

Determining the Approximate Age of Molds

Forensic techniques that are more involved than simply looking at the moldy area and guessing are used to determine the age of the mold. The IEP relies on sampling and identifying supporting conditions like wood rot, metal corrosion, wetting events, and food sources to help determine when the molds started growing.

The experts agree that establishing the approximate age of mold growth based solely on visual identification is neither reliable nor defensible. But by 1) identifying the mold (genus and species), 2) accounting for the conditions conducive to mold growth, and 3) identifying other mold growth conditions, one can narrow down the time when it started to:

  1. Newer Mold Growth – Within the past few days to months
  2. Older Mold Growth – Within the past 6 months to a year
  3. Historic Mold Growth – Older than a year.

What complicates matters is that when mold growth isn’t noticed right away or is ignored, thinking it’s not that bad or is hidden, layers of mold activity will develop, which will be a mix of Newer, Older, and Historic mold growth.

I. Mold Sampling for Mold Type and Moisture Needs

Mold growth is not an exact science. Molds grow at rates dependent on the available resources and the overall ambient conditions of the area (like temperature, relative humidity, etc.). However, the experts agree that molds are ubiquitous in all environments. Lab analysis for Genus and Species-level identification can help sort out the players in your moldy environment.

Mold colors age progression gwe
Figure 1 – the more colorful molds are usually the “primary colonizer” molds, where seen here are the hydrophilic “older age” molds on the right side that had started spreading down the wall and transitioned into the “colorful” mesophilic molds • photo by victor j. Coppola, greenworks environmental, llc.

1. Establish Normal Fungal Ecology (NFE)

Molds are around us all of the time. As they are microscopic, you shouldn’t be able to see them. Lab analysis can help establish what normal mold concentrations are. Add to that seasonal variability and a baseline can be established. 

The labs can also help reveal if the molds are actively growing by identifying hyphal fragments. Elevated levels of hyphal fragments help to establish mold activity.

2. Identify Mold Type (Genus and Species)

By first identifying the molds, one can get a good idea of its initial approximate age by identifying its water needs.

  • Slow Colonizers (Xerophiles) – These low-water-loving fungi grow at low water activity or equilibrium relative humidity (ERH) and are an important part of the indoor fungal community, of which Aspergillus penicilloides is a xerophilic tolerant species. Your heating, ventilation, and air conditioning (HVAC) system, along with libraries and museums, are their favorite haunts in your home. Grain and food production are also favorite environments. But they usually take a long time to grow to levels considered to be of concern.
  • Rapid Colonizers (Mesophilic) – There are a vast number of molds referred to as “Rapid Colonizers” often associated with the genus Aspergillus, Penicillium, and Cladosporium. These molds can grow over large surface areas in just a few days. They are moderate water lovers (Mesophilic) relying on relative humidity in the 50-60% range. This is a large group of fungi where over a thousand species have been identified (so far). Many of which colonize right alongside “water lovers.”

    Note – GWE has found from field sampling several hundred homes spread out over a decade that if you pay close attention to the variety of organisms present in your sampling, you may see a mesophilic or hydrophilic pattern emerge. 
  • Water Lovers (Hydrophilic) – These organisms are known as water damaged building (WDB) indicator organisms and require a lot of moisture over an extended period of time in order to germinate, grow, and reproduce. Some common water lovers are Stachybotrys chartarum, Chaetonium globosum, Aspergillus versicolor and A. niger.

    For Example: Stachybotrys chartarum is a tertiary colonizer that produces slimy spores whose dispersal via aerosolization is not as effective as dry spores. Because of its biology, at least 9 months of chronic wetness is needed for Stacht to establish itself (Chin S. Yang, 2007).

II. Assorted Conditions Contributing to Building Mold Age

All of the following mold-producing conditions are candidates for mold production. They should be considered in both field and laboratory studies where mold is anticipated. Albeit only some of those conditions listed may be present, they all help contribute to Guesstimating the age of your mold:

Back of cabinet mold
Figure 2 – slow-growing / water-loving stachybotrys are hidden inside the wall behind the cabinet. How long does it take for mold to “break through” a wall and contaminate this cabinet? Gwe has found that many molds of differing water preferences can occur in the same area. Relying on just one to establish the age of the mold does not give the big picture • photo by victor j. Coppola, greenworks environmental, llc.

1. History of Water Leaks

Evidence of recurrent bulk water entry in a building is indicative of mold-producing conditions being present since the start of those wet conditions that include:

  • Water-Entry Conditions – Missing gutters, leaders, turnouts at the foundation, in-slope grade, improper ventilation, plumbing leak history, and flood lines, etc.
  • Effects of Water-Entry – Water Stains, efflorescence, visible mold growth, rot/insect damage, elevated moisture content / saturated conditions. (Janet Macher, 1999)
Bleeding tooth mold
Figure 3 – bleeding tooth mold in a newer renovation of an older home. Bet this mold would freak you out! • photo courtesy of tim lehman, owner of timely home inspections.

2. Mushroom Fruiting Structures

Mushrooms are indicative of long protracted periods of most conditions that, in nature, can take months to develop into a mushroom. Their growth cycle goes through several stages of growth, from spore inoculation and mycelial network development to the emergence of fruiting bodies (the mushroom). For example, “Most 4” to 8” inch diameter logs in the wild (or cultivation) fruit in one year from the time of (spore) inoculation, with some producing (fruiting bodies) in only 6 months while others take up to two years.” (Stamets, 2011)

Mushrooms on beam
Figure 4 – desiccated (dry) mushrooms growing on the bottom of biodeteriorated structural beams. This is the “fruiting stage” for molds. Did you know that molds will fruit when they have exhausted the resources they are feeding on? • photo by victor j. Coppola, greenworks environmental, llc

3. Evidence of Wood Decay or Rot

Wood decay (the most common being Brown, White, Soft, and Dry Rot) is any species of fungus that digests moist wood, causing it to rot. Mold spores require a moisture content higher than the fiber saturation point (FSP) of the wood species they are located on, typically between 27% and 30%. Once favorable conditions are available, mold spores germinate and develop by extending a hyphal tube into the soft wood substrate. Some attack dead wood (such as Brown Rot), while others attack living trees. They are all good indications that molds have been present for some time (Anagnost, 2007).

Visible presence of wood decay in investigated areas confirms long-term moisture exposure. 

  • Evidence of Rotted Components – Flooring, trim, Framing, Floor Joists, sill plates, posts, window sash. While water intrusion can occur suddenly, the conditions of a first-time event will not include rotted components. Rotted conditions are a sign of long-term moisture and mold exposure.
Rotten beam
Figure 6 – structural beam biodeterioration aka “wood rot” in 1960’s home • photo by victor j. Coppola, greenworks environmental, llc.

4. Evidence of Exfoliating Rust

Thick flaking rust (aka “Exfoliating”) or even rust penetration of components is to be distinguished from light, superficial rust that appears readily on unprotected metal surfaces after a single wet event.

The photo below shows exfoliating rust on a steel lally column, clear evidence that this space has been subject to recurrent and/or prolonged (many months) water entry.

  • Rust on Steel Components – Like Lally columns, teleports, or steel heating furnaces or boilers are evidence of recurrent wet conditions, which are also mold conducive.
H2o lally cu
Figure 7 – lally column oxidation “rusting” • photo by victor j. Coppola, greenworks environmental, llc.

5. Indirect Evidence of Moist Conditions

Pulling away perfectly-nice-looking wall paneling to disclose a decades-old mold contamination issue from a single-event basement flood makes the point that a single event that soaks a building can produce a large mold reservoir that might be left unattended for a long time.

  • Wood Destroying Insect Activity – Their presence suggests prolonged moist or wet conditions, such as ants, termites, powderpost beetles, wood-boring beetles, etc.
  • Other Biologics – that are attracted to mold, mildew and elevated indoor humidity/water-damaged indoor environments like booklouse (psocids), mites, springtails, fungus-feeding beetles (foreign grain beetle), fungus gnats, insect-relatives (spiders, millipedes, centipedes, sow bugs, mites), rodents, etc. (Merchant, 2008)
Book lice
Figure 8 – closeup photo of psocids (booklice) • ℅ texas a&m.

Note – Fungal-associated insect activity can indicate:

  • Recent fungal growth (like in new homes) on new wall boards and framing from the lumber yard.
  • Older fungal growth (like in established homes) where it takes months of moldy/moist conditions to become established (Merchant, 2008).
  • Mold Appearance – Condition of the molds themselves, are they:
    • Dry/desiccated – signifying older molds or intermittent moisture.
    • Hydrated and fresh – indicative of active growth

“Hyphae of a number of leaf surface fungi (Alternaria, Cladosporium…) are remarkably resistant to desiccation. After extended periods of drying, desiccated hyphal tips were found to resume growth within 1 hour after returned to moist conditions” (Burge, 1995)

III. Other Factors in Determining the Age of Mold

Every site has its own set of unique factors that an experienced investigator might only see once or twice in a career. This is why an experienced investigator is essential in helping wrap everything up into one sound conclusion as to this is why we think the molds are this age.

06-damprid-30 year old one
Figure 9 – 30-year-old “damp check” moisture absorber container in 1980’s home under kitchen leak • photo by victor j. Coppola, greenworks environmental, llc.

There are numerous other factors beyond those listed above that can play a role in helping establish the age of molds. Be mindful of unique factors specific to your environment.

  • Length of time in the home
  • Assorted non-conventional  “DIY” fixes to moisture management
  • PH of food sources, water activity & soil type
  • Temperature and ventilation
  • wood soil contact and cross contaminants
  • Intermittent / wind driven / decade drip

Making the Presumption of Mold Age

While the technology is still advancing, someday we might have a test  or some neat gadget to definitively establish the age of molds. But for now, while any one or two of the above items may appear to be insignificant, string many seemingly insignificant contributing factors together, and a pattern begins to emerge. As an experienced investigator, you can then identify a tipping point when molds started to grow beyond normal ecology.


Microorganisms associated with damp indoor environments (molds, bacteria, fungi) are known to be one of the main causes of degradation of indoor air quality (IAQ) through their production of airborne particles and biotoxins. These nano-particles composed of mold spores can pose serious health hazards to occupants. Hence the reason why so many folks are interested in how long the mold has been around. GreenWorks is a Building Biology and indoor air quality (IAQ) firm that specializes in issues that degrade IAQ. We would love to hear how you determined the age of your mold problem by contacting us.


  • Anagnost, S. (2007). Wood in the Built Environment—Conditions for Mold and Decay. In C. S. Yang, Sampling and Analysis of Indoor Microorganisms (pp.155-178). Hoboken: John Wiley & Sons, Inc.
  • Burge, H. A. (1995). Future Research Needs (Section VIII). In H. A. Burge, Bioaerosols: Indoor Air Research Series (p. 113). New York: CRC Press, LLC.
  • Centers for Disease Control & Prevention. (2022, November 11). Facts about Stachybotrys chartarum. Retrieved from: https://www.cdc.gov/mold/stachy.htm
  • Chin S. Yang, P. A. (2007). A Retrospective and Forensic Approach to Assessment of Fungal Growth in the Indoor Environment. In P. A. Chin S. Yang, Sampling And Analysis of Indoor Microorganisms (pp. 215 – 227). Hoboken, NJ: John S. Wiley & Sons.
  • Inspectapedia. (2023). How Old is This Mold? Retrieved from Age of Mold Contamination in Buildings
  • Janet Macher. (1999). Bioaerosols: Assessment and Control. Cincinnati, OH: American Conference of Governmental Industrial Hygienists.
  • Leong, S.(2011, Jan 31). NIH National Library of Medicine. The Extreme xerophilic mould.
  • Maurice, S. (2011). Modeling the effect of temperature, water activity, and pH on the growth of Serpula Lacrymans. Journal of Applied Microbiology, 1436-1446.
  • McCoy, P. (2016). Radical Mycology: A Treatise On Seeing and Working with Fungi. Chthaeus Press.
  • Merchant, M. (2008, October 1). Insects in the City. Retrieved from Texas A7M Agrilife Extension.
  • Pettersson, O. V., & Leong, S.-l. L. (2011). Fungal Xerophiles (Osmophiles). Retrieved 12 27, 2023.
  • Stamets, P.(2011). Mycelial Running: How Mushrooms Can Help Save the World. Ten Speed Press.
  • Thomas Verdier (2014, October). Building & Environment, Vol 80, P. 136-149. From Science Direct.

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Victor Coppola

Victor Coppola is a Building Biologist with decades of diverse environmental experience spanning natural resource management and contaminated sites to Indoor Air Quality (IAQ) and building hygiene matters. He focuses on the three most prevalent issues degrading today's built environment: Moisture Odor & Mold (M.O.M.). Mr. Coppola and his Team are sought out by individuals and professionals concerned about IAQ to those actually suffering from Biotoxin Illness. His hybrid blend of environmental interests and services enable him to give a unique perspective into today's changing environment. Think your home is killing you, better call Victor and his Team at GreenWorks.

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