Photograph of  thick hairy fungal mycelia extending below a wooden subfloor in a home - how old is this mold and how long did it require to grow in this pattern? Age of Mold Contamination in Buildings

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How old is the mold growth:

This building mold contamination assessment article discusses how we can estimate the age and history of mold contamination in a building and how we can find evidence suggesting that a given mold contamination case is new, old, or includes both old and new fungal growth.

The appearance of mold genera/species varies widely as a function of the growth substrate (paper, wood, cloth) and moisture conditions.

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How Old is This Mold? Is it Possible to Determine the Age of Mold in buildings?

Photograph of moldy pantry shelf.For insurance adjusters, building investigators, and building owners, a common question that arises when reviewing building mold contamination is "when did this happen" or "did the recent building leak or problem cause all this mold?".

Here we explain how an experienced forensic investigator can find visual evidence that a mold problem in a building is of recent origin or if on the other hand the mold contamination in an area has been most likely present for a much longer time.

We will discuss: building conditions affecting the genera, species, and rate of fungal growth in buildings. Conditions permitting an assessment of probable age of mold growth in buildings. How to evaluate mold samples for indications of active mold growth. Evidence of Mold History in buildings, Evidence of Mold Age in Laboratory Samples. Insurance claims & water entry history & mold damage causation analysis.

A Single-leak event mold example

The mold photos above show the extent of fungal growth in a home which had no previous mold nor leak history but which experienced a burst-pipe flood followed by six months of inattention before an insurance claim was filed.

Article Contents

Thick fungal growth was present on some surfaces, and some fungal growth was present on most materials and surfaces with genera/species varying considerably depending on just what surface we examined.

For example, the edge of a door hosted a different genera/species than the face of the same door (different woods). (We conducted a survey to study just what fungal genera/species preferred just which building materials in this home.)

Photograph of moldy pantry shelf.

Was this mold due only to the burst pipe? The absence of other indications of recurrent water entry or rot supported the view that although the fungal growth was extensive, it was in response to a single event.

A multi-event long-standing leak condition mold example

Photograph of moldy pantry shelf. Photograph of moldy pantry shelf.

The mold photos above show thick dense fungal growth on rotted wood in a second building.

At the top of this page, a photo of similar-looking fungal growth as that shown in the "single-leak" case cited just above (by size, thickness, and extent below a wood subfloor) was was taken in a second building which had suffered decades of water entry.

It was much less likely that the mold growth we saw in the second building was due only to a single event flood.

Data like this can assist building owners and insurance claims adjusters in forming a reasonable opinion about the cause and age of mold contamination in a building.

In turn, that opinion can assist in setting priorities for building repair since if we do not correct the underlying causes for mold contamination in a building, mold growth is likely to recur and the investment in a given mold remediation project may be wasted.

A mycologist's view on setting an age on mold in buildings

Mycologists indicate and field experience by property inspectors confirms that fungal growth can occur in a building over a broad surface and quite rapidly, in as short a time as 2-3 days in some conditions and they add that it is not very reliable to guess at the "age" of a given mold colony.

Of course there are exceptions: some fungal growth such as "tree ears" and hard fungi produce slow-growing and durable structures over many years - the larger their size the older they are. But in general, mycologists are precisely correct: looking at a sample mold itself does not easily permit us to guess how long mold has been present on that surface.

A building scientist's view on setting an age on mold in buildings

But the good news for building forensic analysis is that other accompanying observations in a mold-contaminated building can provide compelling evidence regarding the age of mold infection there.

A building scientist, particularly if s/he has also some training in mycology, can in some cases sort out the probable history of mold contamination in a building by using evidence from a variety of supporting sources.

Most buildings more than just a few years old are likely to have experienced an occasional spill, leak, or other source of water entry which could, in turn, have been a gating factor in developing problematic mold growth in the structure.

Buildings more than 20 years old, particularly wood-frame structures and structures which use interior materials which are "mold friendly" such as drywall, fiberglass insulation, and carpeting, often have had more than one leak, wetting or flooding event and may have more than one location and source of mold growth.

In fact a careful inspection of most homes, even ones which have never reported a flood or mold problem, can quite often, perhaps usually, find some spot of fungal growth in the structure. (That does not necessarily mean that the building has a "mold problem".

In all but the most egregious and uncommon cases such as brand new construction which has been flooded, a hasty, superficial scan of a building or of visible mold in it should not alone be a basis for deciding whether or not a mold problem has been long-standing or has occurred only as the result of a single most-recent building leak or flood.

However, thoughtful observation and recording of certain building conditions along with careful, thorough site and laboratory work can provide insight into the probability that mold found indoors at a particular building and at a particular time is probably due to a specific building wetting event or, on the contrary, that mold in a building, or at least some of it, probably pre-dates the specific wetting or flooding event.

In our separate article on detecting evidence of building water entry see EVIDENCE oF AGE & HISATORY OF BUILDING WATER ENTRY or LEAKS

Conditions affecting the genera, species, and rate of fungal growth in buildings

Extremen mold contamination indoors (C) Daniel FriedmanThe appearance of mold genera/species varies widely as a function of the growth substrate (paper, wood, cloth) and moisture conditions.

Different species prefer different moisture levels and thus may appear on the same material such as drywall but at different heights from the floor if that section of drywall was wet from the floor level. (Stachybotrys chartarum prefers very wet conditions and appears low on the wall.

In the extremely mold-contaminated building shown at left, the author tested every surface of every different type of building material found inside the structure.

The results were interesting: different mold genera/species had strong preferences for different materials (no surprise).

Cladosporium and Ulocladium are often found somewhat higher on the same wall. Aspergillus sp. or Penicillium sp. often prefer still less wet conditions and may be found higher still or more uniformly spread over a drywall surface as they may grow more readily in conditions of high humidity even if the drywall was not actually wet to the touch.

What produces mold growth on a building surface is the combination of this presence of omnipresent fungal spores available in the general environment, the presence of a building material that a particular fungus will grow on (its food), and the presence of appropriate conditions of moisture and, to a lesser degree, temperature and perhaps light or darkness.

If these mold-conducive conditions have been present in a building for months or years, the probability that a fungal growth has appeared suddenly and as a sudden and brand-new mold colony is rather low.

Conditions permitting an assessment of probable age of mold growth in buildings

All of these mold-producing conditions are likely to pertain and should always considered in both field and laboratory examinations of moldy conditions in any building where mold contamination is present.

Of course for any specific case of mold contamination in a building, only some of these conditions will actually be present and determinant of mold growth in a particular building and case.

Water leak history sets a probable age of mold

Evidence of a history of recurrent water entry in a building will establish that mold-producing conditions have been present since the beginning of those water or wet conditions in and at the property. These include both:

Evidence of rot sets a probable age of mold

Photograph of moldy rotted floor trim.

Evidence of rotted wood components such as flooring, framing, floor joists, sill plates, or posts.

While water entry can occur suddenly and can be extreme (flooding, burst piping, sudden severe roof leaks), the conditions produced by a first-time and one-time event, if inspected days, weeks, or even several months after the event, will not include rotted components.

The floor trim in the photo above is not only moldy, a closer examination shows that it has rotted.

The rotted condition of the trim indicates long-term exposure to water and makes it less likely that the mold on the trim is due only to a single recent leak event.

Photograph of moldy floor trim wall interior.

The photo above examines the wall cavity behind this rotted floor trim, showing additional wood debris and mold growth on the cavity side of drywall in this building.

Evidence of exfoliating rust sets a probable age of mold

Photograph of light rust on a steel lally column. Photograph of exfoliating rust on a steel lally column.

Evidence of exfoliating rust on steel components such as steel Lally columns, teleposts, or steel heating furnaces or boilers, is evidence of recurrent or protracted wet conditions which are also mold-conducive.

"Exfoliating" or thick flaking rust, 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 at above left shows no building-related rust on a steel lally column in a dry crawl space. This area has not been subject to severe recurrent water entry. The second photo at above right 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.

Indirect evidence of moist conditions can suggest probable age of mold

Photograph: mold hidden behind basement wall panelingThe photo showing me 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 that a single event that soaks a building can produce a large mold reservoir that might be left un-attended for a long time.

Details about this case are at HIDDEN MOLD in PANELING.

Evidence of wood destroying insect activity is suggestive of moist or wet conditions as those invite insects into a building; other conditions such as wood-soil contact are also factors in the development of insect damage.

Evidence of age of mold in the forensic microscopy laboratory

Notes about these clues to the age of mold or mushrooms found in, on or around buildings (Identification of True Morel mushrooms)

Morel mushroom growing in a brick entry walk in Poughkeepsie (C) Daniel Friedman Photo guide to identification points on a true morel mushroom - edible (C) Daniel Friedman POISON WARNING: WHEN IN DOUBT THROW IT OUT DO NOT EAT IT

[Click to enlarge any image]

Watch out: people have been seriously poisoned, even died, from eating improperly-identified or improperly-selected mushrooms. When in doubt, throw it out, don't eat it.

Watch out also: even edible mushrooms may vary widely in taste and appeal, not just by genera/species but by the mushroom's growing conditions and the make-up of its soil. I would not eat a mushroom that has been found on chemically-treated ground (which includes soil under some apple trees) or close to a public roadway where salt or other treatments have entered the soil.

Research on Edible True Morel Mushrooms & Poisonous False Morel Mushrooms

Unambiguous, Ambiguous and Overlapping Mold Colonies in buildings

Especially in older buildings where there has been a recent sudden leak event associated with mold growth, it is often possible to identify pre-existing mold as well as mold-producing conditions.

In unambiguous cases, the "new" mold associated with the building leak event may, by luck, appear in a limited area of the building which maps the area wet by the recent leak, and separated by distance or building area from other moldy areas which in turn are associated with other building leaks or conditions.

The physical separation of wet areas and wet conditions may be sufficient to make a clear assignment of mold causation in such cases.

In ambiguous cases, there is fresh, active fungal growth, probably associated with a recent leak or flooding event in the building, which has grown entirely or partly overlapping pre-existing mold growth.

In this case the assignment of cause and age of mold in the building can be ambiguous. If an insurance claim is involved, insurance company policy details and internal claims adjustment guidelines will determine the extent to which insurance coverage will address building remediation and repair for these overlapped-occurrence mold conditions.

Determining Whether or Not a Mold Test Sample Shows Active Mold Growth

Moldy roof sheathing (C) Daniel FriedmanQuestion: how do we determine whether or not mold in a building is active vs. inactive?

Would you please let me know how an individual (or lab) would test for whether a mold found on attic sheathing is active vs. inactive?

Research on-line has told me the test for active vs inactive is whether it smears when you rub it. Is there a more technical test that can be done? Should a lab be able to tell me this when I supply a sample? - M.O.


The question of how we determine whether or not mold in a test sample is "active" is a bit misleading, although some surface test samples of mold do indeed give compelling evidence of recent active fungal growth.

Our site photograph of moldy roof sheathing (above) is an example. Is this mold growth "active" or "inactive", and does activity make much difference in risk to building occupants?

Here discuss visual clues that help determine the age of mold contamination in buildings or on building surfaces. There we explain what dried, desiccated, "old" mold growth may look like on a surface, in a test sample, and under the microscope. Among other factors, we distinguish between

Aspergillus niger (C) Daniel Friedman Aureobasidium pullulans (C) Daniel Friedman

Often we can confirm recent fungal growth in a tape sample by the presence of certain growth structures, hyphal buds, or even the state of a conidiophore.

Our photo of Epicoccum sp. fungal spores and hyphae (above left) collected from a building surface shows intact, fragile hydrated complete spores still connected to hyphae - this mold growth is recent and might indeed be considered "active mold growth" as would the intact, hydrated, and budding Aureobasidium pullulans spores shown in our second lab photo (above right).

This burst of Pleospora spores is clearly active. Similarly, for certain species that produce long fragile spore chains, the presence of long mold spore chains is certainly indicative of nearby active fungal growth, as these chains break up rapidly into individual spores when airborne.

Conversely, highly desiccated, fractured, or damaged fungal material that lacks budding hyphae or sporulating intact conidiophores are almost certainly "inactive" mold growth in the spot where sampled.

Sources of Mistaken Conclusions about Mold Risk in Buildings

Moldy books (C) Daniel Friedman

Watch out: "mold activity" or "mold inactivity" can be misleading conclusions about the risks associated with mold growth in buildings.

The moldy books in a college library (photo at left) were in the opinion of some people "an old inactive mold problem" but when workers began dehumidifying the area in preparation for a mold cleanup, visible clouds of Aspergillus sp. spores were released into the air by small air currents caused by simply walking down the aisle between stacks of books.

  1. Mold test sample size: Because a sample represents a small area of a building and of time, and because other materials may be present that the sampler did not see, detect, or test, a single sample is an indicator, not a conclusion about a building's condition.
  2. Multiple mold genera/species: Next, the conditions that produced fungal growth that was seen and tested mean that building conditions were ripe for mold growth, perhaps on other surfaces or in cavities or in less visible locations.

    For example we may see one species of mold on an attic roof underside, say Cladosporium cladosporioides, perhaps even desiccated by the heat of sun beating on the roof, while the conditions that produced that growth also produced a non-visible but more troubling Aspergillus sp. contaminating growth in building insulation in the attic floor.

    So at a given time, some mold on building surfaces may be "inactive" while nearby another genera/species may be growing like mad, or releasing spores like mad, i.e. "active".
  3. Dead toxic mold: Next, we suspect an underlying faulty premise that the salient question is whether or not a fungal growth is active or not in a building. In fact even currently inactive (no apparent recent fungal propagation) fungal presence on a surface can be a hazard since spores and fungal hyphal fragments may be toxic, pathogenic, or allergenic even in that state.

    That point remains true even if some misinformed "mold remediator" tries to "kill" the fungus with a fungicide or bleach.
  4. Smearing mold?: Further, he claim that smearing mold with a finger determines whether or not it is active or not is nonsense.

    A mold growth on a surface could be very desiccated and may not have actively propagated for a long time but still can be "smeared" with a finger wipe.

    But we agree that if you cannot smear suspected-mold or better, if you cannot obtain particles on an adhesive tape sample, then the surface may have been previously cleaned, and what remains may be no more than a cosmetic stain.
  5. Inoculation of mold: Finally, even if mold is "not actively growing" at the time that a sample is obtained from a surface (or in dust or air), the presence of a large amount of fungal material in a building can lead to rapid mold re growth and propagation when building conditions change to those more conducive to growth of the fungi already present.

    All mold is everywhere all the time in the form of spores ready to inoculate a material or surface. So when building conditions are ripe for mold growth, it is likely to occur.

    Predominantly, it is the building conditions that determine whether or not we will have a building mold problem, not the presence or absence of mold in general. But the presence of a large reservoir of pre-existing (inactive) mold can speed both the recurrence and the extent of a future mold contamination. "Inactive" does not mean "non-viable".

Watch out however: using a swab or culture test for "viable mold" in buildings can give very misleading results since what grows in the culture is what likes the culture, not necessarily what is present or dominant in the building.


These reasons explain why in addition to testing to confirm the presence of mold growth, and to confirm that it is not simply cosmetic, in cases of possibly costly mold cleanup or diagnosing a possible building contribution to indoor air quality complaints, is important to have an expert perform a competent inspection of the building.

Also see MOLD GROWTH on SURFACES for an index of what mold genera/species are frequently found on various building surfaces and materials.

To better understand the water entry or leak history of a building - gating factors in mold contamination, see EVIDENCE OF AGE & HISTORY of BUILDING WATER LEAKS


Continue reading at HIDDEN MOLD in PANELING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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