Mold Growth in Structures: An Overview - ACS Publications

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Mold Growth in Structures: An Overview Stephen L. Quarles Division of Agriculture and Natural Resources, University of California Cooperative Extension, Richmond, C A 94804

The advent of litigation related to indoor air quality in structures has made non-decay fungal growth on wood and wood-based materials more than an aesthetic issue for the consumer. Although mold growth on non-wood based materials occurs, growth on wood-based materials has been a critical component of indoor air quality litigation in commercial and residential structures. The predominance of wood-based materials in construction, and the use of both initially wet and dry wood-based products in structures, has resulted in some specific problems regarding fungal growth on these products, and fungal growth that can be "expected" in wood-framed structures. The common use of wood-based products has also lead to some misconceptions regarding the growth requirements of the non-decay fungi, particularly with regard to minimum moisture requirements and wood constituents that can be utilized as a food source. The objectives of this chapter are to present field-based observations regarding fungi that are commonly found on wood-based materials, and review research-based information on growth requirements.

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© 2008 American Chemical Society In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


171 Durability issues for wood and wood-based construction materials used in buildings have traditionally focused on structural factors. Regarding fungal infestations, damage from decay fiingi was of primary importance because of the impact these organisms have on reductions in the strength of wood-based construction materials. In a literature survey that reviewed research on the strength loss associated with decayed solid wood, Wilcox (7) reported reductions of 50% or more, in brown rotted wood and depending on the particular mechanical property, with mass losses of 5% or less. In evaluating the impact of decay fiingi on structural performance the extent of decay must also be considered. Since decay progresses from one location to another within a wood sample, the process isn't uniform and therefore advanced decay can be observed locally without structural failure occurring in the member. In these situations the loading distribution on the member is also critical. Mold and sapstain fungi were previously considered an aesthetic issue, almost always with a negative connotation. However, in the case of certain sapstain (blue stain) fiingi in selected products, the stain could be considered a desireable feature. With the recent inclusion of personal injury components associated with mold growth in building construction and materials litigation though, the perception and importance of mold has changed dramatically. The objectives of this paper are to review the growth requirements of mold and decay fiingi and discuss related implications that have been observed in field work and interaction with experts in mold-related litigation. Fungal growth on 'green' (non-kiln dried) and rewetted wood products will also be discussed.

Growth Requirements The general growth requirements for wood-inhabiting fiingi are well known and include a favorable temperature, sufficient oxygen, a food source (materials with accessible carbon containing compounds) and sufficient moisture (2). Two other requirements, favorable p H and the availability of certain essential elements, can also be included in this list. Control of fungal growth in service is usually accomplished by controlling moisture, or where that isn't feasible, by adding biocides to the product. Although there is general agreement that decay fiingi require liquid water for growth to initiate and propagate, there has been some disagreement regarding the minimum amount of moisture required for mold growth. Similarly, whereas there is no question regarding the strength reducing capacity of decay fiingi, there has been some disagreement as to whether mold fiingi can structurally degrade wood. This is particularly true where a mold fungus has been reported to produce cellulase, such as some Trichoderma species. Some cellulases are actually a multienzyme complex that can decompose accessible cellulose (2); however, some organisms produce only one type of cellulase (endo or exo) and

In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

172 thus they may have limited capacity to completely degrade cellulose even i f it is accessible. These two factors will be discussed in more detail in the following sections.


Moisture Zabel and Morrell (2) state that 'free water on the surfaces of the cell lumina' is required for fungal growth. Free water on the surface impies that at least locally, the moisture is above the fiber saturation point, typically taken to be approximately 28% (oven dry basis). This condition also indicates that the relative humidity, again at least locally, is 100% (i.e. at saturation), with an associated water activity, a , of 1. Water activity is measured specifically by the water vapor pressure of a material comparted to the water vapor pressure of pure water at the same conditions (5), which leads to confusion when it is used in a manner which equates it to a relative humidity index. Other documents clearly state that mold growth on the surface of wood can occur at humidity conditions far less than saturation. For example, Flannigàn and Miller reported minimum water activity levels (a ) of 0.8 and lower for some mold fungi (4). These results are in agreement with those of Viitanen, and other research cited by Viitanen (5). TenWolde (6) reported the ability of mold fungi to grow at moisture contents less than the fiber saturation point of wood, implying again that free water is not required. Based on these contradictory results, it is perhaps understandable that there is confusion in the the non-refereed technical journals and trade publications regarding minimum moisture content. As an example, Wemhoff (7) reported that 'mold begins to form at about 17%, and rotting begins at about 27%'. At 70°F (21°C), the 17% moisture content would occur at a relative humidity between 80 and 85%, in line with the published 0.8 minimum a requirement (4 - 6). The 27% value is the approximate fiber saturation point. w

w

w

In an informal survey of forest products pathologists, conducted by this author in 2001, there was general agreement among the respondents that liquid water was required at the location where the fungus was active (S). Most of the respondents indicated that one potential reason for the confusion regarding minimum moisture content and relative humidity requirements lies in the difficulty in accurately measuring local moisture content and relative humidity at the micro scale, particularly in wood cell lumens. The moisture content usually reported in studies is a bulk or average value (5, 6), measured at some distance from the specimen. Depending on the dimensions of the test specimens, there can be moisture gradients present that would mask the actual moisture content at the surface. Another factor commonly discussed by the respondents was the difficulty in accurately controlling ambient relative humidity at elevated levels, and the sensitivity of the moisture content of wood at these conditions. For example, at 70°F (21°C), the nominal equilibrium moisture content is 20.6% at a



173 relative humidity (RH) of 89%, but it is at the fiber saturation point at 95% (RH) (P). Although the fiber saturation point is technically reached at a relative humidity of 100%, this information is indicative of the difficulty in accurately measuring moisture content in wood at elevated humidities. Finally, some of the respondents thought that the presence of the gelatinous hyphal sheath could explain some of the apparent contradictions in the literature regarding moisture content requirements. In this case, the hyphae would be in intimate contact with the food source. The argument for the liquid water requirement is based on the external digestion process that occurs with fiingi. Water serves as a diffusion medium for enzymes and other metabolites produced by the fungus to reach the carbon-based food source, and for break down products to return to the hyphae. The argument for elevated moisture contents, but at levels lower than the fiber saturation point, lies in the concept of 'available water' (70). In this case, the more loosely held bound water in the secondary sorption layers at given sorption sites in wood would have thermodynamic properties closer to that of liquid water, thereby allowing the necessary diffusion process to occur. Some of the sorption theories (e.g., B E T ) and models that have been developed assume thermodynamic properties of some secondary sortion layers equal to or similar to liquid water, particularly at the moisture contents achieved at elevated relative humidity levels Another factor that could have led to confusion regarding minimum relative humidity levels is related to the substrates used in research where these levels are determined. For example, Clarke, et al. (72) reported that hightly xeroplilic mold fiingi can survive at humidity levels as low as 75%. In this study, samples were collected in a mold infested house by pressing contact agar plates on the affected material, then incubating the plates in the laboratory. The agar plates were evaluated over the course of the study, and where growth occurred, the mold fungus was identified. Given the use of agar plates as the growth substrate, with its accompanying high water content and the associated accessibility of the agar media, the determined minimum relative humidity absolutely has to be lower that that determined with mold growth on wood and wood-based products where the food wouldn't be as accessible. Experiments conducted on agar could be evaluating, for example, the sensitivity to dehydration of surface hyphae and the fruiting portions of the fungus. Other studies have also used growth on agar media to evaluate minimum relative humidity conditions required for growth (75), but as indicated, studies have also been conducted on wood where minimum humidity levels of approximately 80% have been reported (5). However, even given the results reported by Clarke, et al. regarding minimum relative humidity (75), they still report that the most important factor affecting fungal growth on building materials is the local relative humidity and temperature because of their combined impact on the availability of free water for the fungus.


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Accessibility of the Food Source As previously stated, multienzyme complexes are known to be associated with the degradation process of cellulose and hemicellulose in wood (2) but nonenzymatic systems are known to be required in association with these enzymes for decay to occur (see Goodell et al. chapter, in this book). Some confusion has occurred, particularly among industrial hygienists, in interpreting potential structural damage to wood-based products from mold fungi that produce cellulases. The presumption is that when cellulase is produced, structural degradation of wood will occur. However, Cowling and Brown (14) reported that the enzymes associated with the decomposition of wood were too large to penetrate its gross cellular structure, implying that cellulase enzymes alone were not sufficient to decompose wood. In addition, mold fungi do not control the pH of their micro environments, nor do they produce appropriate non-enzymatic agents which are required for cellulose decomposition. A summary of the findings of Cowling and Brown (14, 15) is shown in Table 1. This work clearly shows that non-enzymatic reactions must accompany, or precede the enzymatic reactions that lead to the depolymerization of cellulose and hemicellulose. As stated by Goodell (16), T h e absence or presence of a key component... may be all that separates degradative ability or mechanism for lignocellulose degradation.' The data provided in Table 1, combined with the increased understanding of the underlying mechanisms associated with decomposition of wood by decay fungi, help explain the possible sources of food when mold fungi are found on wood-based construction materials. These results clearly show that when mold growth is observed on solid wood products, that this growth is occurring either: 1) on sapwood, where sugars and other low molecular weight carbohydrates are found in parenchyma (storage) cells, or 2) on other organic debris that has settled or been applied to the surface (in the case of some coatings) and which is being used as the food source. With the increased use of faster grown trees, and with the associated relative increase in sapwood content, mold growth can more readily be observed on plywood and oriented strandboard (OSB) products when these are wetted. For aspen-based OSB, this would be particularly true because of the aspen's large sapwood zone. Industrial hygienists involved in indoor air quality litigation have speculated that the adhesive used in OSB has resulted in the observed mold growth, but this, and other more directed research, has indicated that the adhesives commonly used OSB don't contribute to mold growth (77). These results also help explain the occurrence of mold growth on gypsum wall board (Figure 1). Discussions in the field have centered on mold growth on the paper faces commonly used on gypsum products. A factor that some don't consider is the contribution of the starch-based adhesive used to bond the paper to the gypsum substrate. Starch can be readily metabolized by most mold fungi and does not require even the production of cellulase enzymes. The larger pore


175 Table 1. Size of gross cellular structure of cell wall capillaries in three materials. Material Median


Water-swollen Wood Cotton Wood Pulp

Size of opening (Â) Maximum

10 5 25

35 75 150

NOTE: Units are Angstrom, A. The smallest cellulases are approximately 125°A in diameter by 140°A. SOURCE: Data obtained from Reference 13.

size of the pulp fibers may allow more water to be held in capillaries and the combination of greater access to the fiber and the inclusion of the starch make this an better growth medium for mold fungi.

Common Mold Fungi In Buildings California and other regions in the western United States have traditionally used non-kiln dried ('green') lumber in construction. Wet lumber is solid piled at the lumber mill and shipped either to the lumber yard or the construction site. In either of these cases, only the lumber on the edge of the unit can even partially dry until the unit is unbundled and the lumber installed in the structure. The moisture content of the sapwood of 'green' lumber is usually in excess of 100% (oven dry basis), and therefore the available moisture is more than adequate to support fungal growth (75). With the transition in recent years to second growth trees for most of our construction lumber, a greater proportion of studs and joists contain sapwood and therefore they have more starch and sugars in the sapwood parenchyma cells. These components are a ready food source for the mold fungi. The time lumber spends in a solid unit can be up to four to six months, which is sufficient time for fungal growth to occur. As a result, it is common for new construction to have fungal growth 'built in' (Figure 2). This author conducted a survey of fungal growth on 'green' lumber at selected construction sites and lumber yards in northern and southern Califorinia in 2001 - 2003. The survey consisted of taking either bulk (short end cuts from the lumber) or tape lift samples from the surface of the lumber. For tape lift samples, transparent tape was pressed against the fungal growth on the lumber surface, and then attached to the inside of a plastic zip-lock bag. These samples were sent to an American Industrial Hygiene Association (AIHA) Accredited Laboratory for identification of mold growth. The most commonly identified fungal species on the 'green' lumber were sapstain fungi in the Ceratocystis/ Ophiostoma group. Occasionally Gonatobotrium sp. was also identified.



Figure J. Stachybotrys atra growing on gypsum wall board. The source of water was a leak at the deck to wall interface located above this ceiling space.


In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. 1

Figure 2. The use of non-kiln dried ('green ') lumber in construction in California and other portions of the western United States has has resultedfungal growth being "built in in many buildings. Note the abrupt transition in fungal growth on this stud, coinciding with the transition from sapwood (fungal growth) to heartwood (no fungal growth).



178 Gonatobotrium is a mold fungus that lives parasitically on Ceratocystis/ Ophiostoma. Many sapstain fungi have evolved to be dissimulated via bark beetles, and therefore they have developed sticky spores, probably as an adaptation to insect dispersal (18). For this reason, evidence of sapstain fungi (pigmented hyphae throughout the sapwood portion of the cross section) is often observed in the log yard, and in fallen trees in the forest. Pigmented hyphae can also be seen in the sapwood portion of processed lumber, as well as dark colored spores on the lumber surface, also limited to the sapwood. Even after the unit is unbundled and the lumber surface dries, these spores are not considered to be as readily airborne as the spores of other mold fungi whose spores are transmitted predominantely via air currents. However, because the spores of fiingi in the Ceratocystis/Ophiostoma are indistinct, and therefore not readily identifiable alone (i.e., without the fruiting body), they could often be missed or combined in a generic category. Because the spores of these sapstain fiingi are held in a sticky mass once released from the asci, many industrial hygienists consider the health implications negligible regarding the common occurrence of these fungi in residential construction. In order to avoid the occurrence of sapstain fungal growth on 'green' lumber, some lumber mills have started applying anti-sapstain chemical treatments, either in a spray system or by dipping lumber and timbers in a vat. The antisapstain chemicals are intented to limit sapstain fungal activity until the bundled lumber can get to the lumber yard or construction site. Kiln drying is also an option, however, in the west the drying capacity of available kilns doesn't meet the production needs. 'Green' Douglas-fir is still desired by some contractors, reportedly because it is easier to work with (e.g., easier to drive a nail through it). 'Green' lumber is still readily available. Dry kilns are sometimes used to perform a heat treatment procedure on lumber intended for use in packaging (pallets, etc). Kiln temperatures reached during Heat Treatment (HT), a sterilzation process applied to lumber products used in packaging applications for international trade, are adequate to kill any fungal spore on, or in, the lumber during the drying process. A n ' H T ' grade stamp indicates that the lumber core reached a temperature of 56°C (about 135°F) for at least 30 minutes, but does not imply that the moisture content of the lumber has been reduced to any specified level. Surface temperatures reached on material with either H T or Kiln Dried (KD) stamp assure that sapstain fungi was killed while in the kiln. If this material is rewetted, newly deposited mold fungi whose spores are dissimulated by air currents will grow. In order to demonstrate this occurrence, this author has conducted fungal growth surveys, again using tape-lift samples, on lumber exhibiting a K D or H T stamp, and also showing evidence of fungal growth. Whereas fiingi in the Ceratocystis/Ophiostoma group were consistently found on 'green' lumber, they were not observed on K D / H T stamped lumber. A number of mold fiingi were identified on these samples For example, a few different kinds of mold were identified on the the stud shown in Figure 3, including Trichoderma sp.,


179 Pénicillium/Aspergillus sp., and Cladosporium sp. This finding points out the importance of storing H T or K D under cover in the lumber yard and on construction sites. Whereas it is generally thought that the health risk presented by Ceratocystis/Ophiostoma sp. is very minor, the same cannot be said regarding exposure to other mold fungi (see Chapter by Jellison in this book).


Summary Information and data from scientific and non-peer reviewed technical journals was reviewed. O f particular interest was information regarding the minimum moisture requirements necessary to support growth of mold and decay fiingi, and enzymatic and non-enzymatic processes that would affect the ability of mold and decay fungi to utilize two of the basic chemical components found in wood (cellulose and hemicellulose) as a food source. Some of the experimental factors that could have led to the differences in published minimum moisture requirements were discussed. These included the difficulty in measuring moisture content and relative humity at a very local level, and the variability of data obtained using agar media to simulate conditions for mold growth on wood and wood-based products. Results of fiingal growth surveys on lumber in lumber yards and on construction sites was also discussed. Sapstain fiingi in the Ceratocystis/ Ophiostoma group were found almost exclusively on 'green' lumber. Occasionally the mold fungus Gonatobotrium sp. was also found. Fungal growth on kiln-dried (KD) or heat-treated (HT) lumber that was rewetted was always from a mold species whose spores were disseminated by air currents.

Acknowledgements This author gratefully acknowledges the useful suggenstions and contributions of Professors Barry Goodell (University of Maine) and Susan E. Anagnost (Syracuse University of New York) resulting from their review of this manuscript.

References 1. 2. 3.

Wilcox, W. W. 1978. Review of Literature on the Effects of Early Stages of Decay on Wood Strength. Wood and Fiber 9(4):252-257. Zabel, R . A . and J.J. Morrell. 1992. Wood Microbiology: Decay and Its Prevention. Academic Press, Inc. New York. 476 pp. Shaughnessy, R.J., P.R. Morey, and E.C. Cole. 1999. Prevention and Control of Microbial Contamination. In: Bioaerosols: Assessment and Control. Ed: J. Macher. American Conference of Governmental Industrial Hygienists, Cincinnati, O H . Chapter 10, In Development of Commercial Wood Preservatives; Schultz, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


Figure 3. The mold fungus Trichoderma sp. was identified on the surface of this 2x4 that was in an outside storage area in a lumberyard and rewetted by rain. Note the "KD HT" stamp on the lumber.


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Flannigan, Β. and J.D. Miller. 1993. Indoor Humidity and the Building Envelope, In: Bugs, Mold & Rot II. W . B . Rose and A . TenWolde, Eds. National Institute of Building Sciences, Washington, D C . pp. 43-50. Viitanen, H . A . 1997. Modelling the Time Factor in the Development of Mould Fungi - the Effect of Critical Humidity and Temperature Conditionson Pine and Spruce Sapwood. Holzforchung. 51(1) 6-14. TenWolde, A . 1993. Indoor Humidity and the Building Envelope, In: Bugs, Mold & Rot II. W . B . Rose and A . TenWolde, Eds. National Institute of Building Sciences, Washington, DC. pp. 37-41. Wemhoff, P. 2001. Mold, A Poltergeist. Home Energy. 18(l):19-23. Personal Communication. 2001. Professor Barry Goodell, University of Maine, Orono, M E . Dr. Peter Laks, Michigan Technological University, Houghton, M I . Dr. Paul Morris, Forintek Canada Corporation, Vancouver, Bristish Columbia, Canada, Professor Elmer Schmidt, University of Minnesota, St. Paul, M N . Professor Wayne Wilcox, University of California Berkeley, Richmond, C A . Dry Kiln Operator's Manual. 1991. W.T. Simpson, Ed. United States Department of Agriculture Forest Service Forest Products Laboratory. Madison, WI. 274 pp. Personal Communication. Professor John Straube, University of Waterloo, Waterloo, Ontario, Canada. Skaar, C. 1988. Water in Wood. Springer-Verlag, New York. 263 pp. Clarke, J.Α., C . M . Johnstone, N.J. Kelly, R.C. McLean, J.A. Andeson, N . J . Rowan, and J.E. Smith. 1999. Building and Environment. 34:515-521. Nielsen, K . F . 2002. Mould Growth on Building Materials. Secondary Metabolites, Mycotoxins, and Biomarkers. Ph.D. Thesis. Danish Building and Urban Research. Hørsholm, Denmark. Cowling, E . and W. Brown. 1969. Structural Features of Cellulosic Materials in Relation to Enzymatic Hydrolysis. Advances in Chemistry Series 95. American Chemical Society, Washington, D C . pp. 152-187. Green III, F. and T.L. Highley. 1997. Mechanism of Brown-Rot Decay: Paradigm or Paradox. International Biodeterioration & Biodegratation. 39(2-3): 113-124. Goodell, B . 2003. Brown-Rot Fungal Degradation of Wood: Our Evolving View. In: Wood Deterioration and Preservatation: Advances in Our Changing World, B . Goodell, D.D. Nicholas, and T.P Schultz, Eds. A C S Series 845. American Chemical Society, Washington D.C. pp 97-118. ΑΡΑ - The Engineered Wood Association. 2005. Laboratory Test of Mold Growth on Wood Adhesives. Form No. TT-103. Tacoma, W A . 3 pp. Wood Handbook: Wood as an Engineering Material. 1999. Reprinted from Forest Products Laboratory General Technical Report FPL-GTR-113. Forest Products Society, Madison, WI. FPS Catalogue No. 7269. Webster, J. 1970. Introduction to Fungi. Cambridge University Press. Cambridge, Great Britain. 424 pp.


Mold Growth in Structures: An Overview - ACS Publications

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