Indoor surfaces in the built environment seldom are free of mould propagules from the extant environment. Vegetative and dormant forms of fungi including viable conidia, dormant-resistant chlamydoconidia, and various sexual stages can bind and persist differentially in dust. Select fungi with appropriate dampness can colonize metabolically susceptible surfaces, particularly cellulosic coated gypsum wallboards and ceiling tiles.¹,²,³,⁴,⁵ Construction activities, external and internal, can increase airborne fungal densities to objectionable and unhealthy levels.⁶,⁷,⁸,⁹ Green et al¹⁰ reported the use of a halogen immunoassay (HIA) to document the allergenicity of airborne fungal hyphal fragments and broken spores. The immunoglobulin E (IgE)-based assay documented allergen release from intact germinating conidia as well as the terminal ends of growing hyphal fragments. The data suggested that the total submicron fungal particles could magnify the overall aeroallergen load in the environment beyond that indicated by identifiable spore counts.

Air filters and air duct insulations may support hidden select, reproducing fungal populations contributing to various indoor syndromes (Figure 69.1).¹¹,¹²,¹³,¹⁴ Water reservoirs and rains in particular may harbor persistent biofilms of aspergilli and fusaria.¹⁵,¹⁶,¹⁷,¹⁸ These inherent colonizations, sometimes cryptic within materials as well as on surfaces, can result in indoor air densities significantly distinct in time and composition from those recoverable in the extant airborne mycota.¹⁹,²⁰ Chronic water leaks from plumbing or exterior wall fissures, catastrophic water incursions following floods and storms, and persistent high relative humidity can facilitate fungal growth and metabolism that render the built environment unsuitable for its intended use.²¹ Porous structural- or indoor-finishing materials in human habitats inundated with water for over 48 hours, particularly following catastrophic flooding when drying of materials is impractical, generally are recommended for replacement.⁴,²²

Occasionally new, but mostly aged, structures become colonized with fungi without recognized water or construction events. Typically, such colonization can be attributed to the miasma stresses of the structure use and aging (ie, building foundation or envelope integrity issues). In some instances, mostly with new materials, contaminating fungi are not identifiable within the extant geography. The fungus may be from the environment of material processing, sometimes inherent on or within the materials themselves, or from material exposures during shipping and storage.²³,²⁴

Crystalline gypsum (CaSO₄•2H₂O) bonded between two layers of paper is used worldwide for interior fire-retardant acoustic walls. The product is variable in composition both by design on intended use and the inclusion of inhibitors or nutritive formulations, for example, starch or recycled content.⁵,²⁵,²⁶,²⁷ Products from the same manufacturer may be inhibitory or stimulatory for growth of select fungi varying with the source of raw materials and climatic conditions.⁵,²⁸ The paper layers impacted with sufficient moisture support growth and metabolism of fungi, and the core itself may be impacted (eg, after flooding).²⁵

The spectrum of fungi capable of growth and metabolism on gypsum wallboards is broad, typically reflecting the adjacent extant environment, and identifiable often with the genera Aspergillus, Alternaria, Chaetomium, Cladosporium, Penicillium, Stachybotrys, and Ulocladium.⁵,²⁹

Krause et al³⁰ demonstrated that commercially available gypsum wallboards, some treated with protective coatings, developed visually detectable mold growth during exposure to bottled water. Untreated wallboard inundated by mold during the exposure appeared devoid of viable
fungi following cleaning with household chlorine compounds (see chapter 15). The tests were conducted under ambient conditions in a room setting. One end of various uninoculated gypsum board panels was suspended in bottled water to a depth of 1 inch. Various genera such as Penicillium, Cladosporium, then Aspergillus, and later Ulocladium were detected sequentially in time at 1 to 6 weeks. Fungi were not observed in ostensively dry areas about 6 inches above the ascending water stain. Stachybotrys was not observed, or was mold observed, on dry control panels maintained in the same room. Several protective coatings delayed fungal development. The extent of growth and splotchy appearance varied among the panels. The source of the fungi in these experiments is problematic. These genera were probably sourced mostly from the extant atmosphere and handling in the test room, but some may have been inherent to the wallboard. Delgado et al³¹ reported postprocessing survival and recalcitrance to disinfection chemicals by heat-resistant fungi on paperboard packaging materials. Additional such findings have been mentioned by Rico-Munoz.³² Menetrez et al³³,³⁴ demonstrated that steam-sterilized coupons of gypsum wallboard, plain and treated with different coatings, supported varied levels of visually detectable growth after inoculation with Stachybotrys chartarum. Cleaning and disinfection regimen applied with commercially available products indicated some potential for remediation. The presence of inherent mould among the gypsum boards and the varied coatings may be questioned; it was not part of the study design. Andersen et al²⁷ examined 13 unused gypsum boards acquired at four different building supply stores that represented three types and two brands. Twelve discs were cut from each panel, submerged with rubbing in 96% ethanol, air-dried, wetted, and stored in sealed Petri dishes at 22°C to 23°C. Aspergillus hiratsukae (Neosartorya), Chaetomium globosum, and S chartarum were the most prevalent species observed that developed on all 13, 11, and 7 panels, respectively. The fungi presumably were em-bedded in the kraft paper surrounding the gypsum core.²⁷ This was the first report verifying the potential of A hiratsukae for colonization of gypsum wallboards and for survival following disinfection procedures. This species at room temperatures (20°C-35°C), in addition to developing conidiophores and conidia similar to those of Aspergillus fumigatus, produces cleistothecia encasing asci containing heat-resistant ascospores.²⁷,³⁵ The maximal temperature for growth on laboratory media such as malt extract is between 45°C and 50°C, but the dormant thermoduric ascospores may survive temperatures above 90°C.³⁵,³⁶,³⁷

The A hiratsukae is an emerging pathogen in the fumigati complex, and, as such, its potential colonization of a built environment may be of greater concern for immunocompromised inhabitants in comparison to noxious fungi of lesser or unknown virulence in such as Avicularia versicolor, C globosum, or S chartarum.³⁸,³⁹,⁴⁰

Lewinska et al⁴¹ used scanning electron microscopy (SEM) and microcomputed tomography to visualize structural changes in gypsum board and plywood samples when exposed to water, C globosum, and S chartarum over an 8-week period. The S chartarum seemed restricted to surface growth on the inoculated paper-layered gypsum wallboard, but inoculated plywood was penetrated into the core. The growth of Chaetomium on both substrates was somewhat atypical. Details of the type of gypsum and plywood or presence of potential inhibitory components were not available. Fungal growth was not reported on the uninoculated controls.

Mesophilic fungal isolates identified as C globosum, common colonizers in damp buildings, have demonstrated restricted growth, if any, at 37°C to 42°C, although their ascospores may survive exposures at temperatures above 50°C. Such isolates are of possible rare risk for involvement in onychomycosis, otitis, peritonitis, or keratitis, but any associations with invasive mycoses seem unlikely, particularly in noncompromised hosts.⁴²,⁴³ Some thermotolerant chaetomia capable of growth or survival near 50°C have been recognized as emerging human pathogens in rare infections of competent- and immunocompromised individuals.⁴²,⁴³,⁴⁴ Thermotolerance among these fungi, some of which are yet to demonstrate ascomata, may reside with clusters of chlamydoconidia-like cells.⁴⁵,⁴⁶

Isolates of the S chartarum complex⁴⁷ (notorious for toxicity after ingestion) have maximal growth temperatures below 40°C and probably higher survival temperatures if encased in paper, but their capacities for infections remains questioned.²⁰,²⁶,⁴⁷,⁴⁸ The proclivity of this species complex to colonize and metabolize paper coatings of gypsum boards, particularly the inner wall, can result in a hidden source of noxious odors following catastrophic or undetected moisture events. Colonization by the S chartarum complex on inner and outer surfaces of interior walls may not be reflected in the detectable airborne mycota because of sluggish conidial release and size versus Aspergillus and Penicillium.⁴⁹ Price and Ahearn⁵⁰ earlier noted the development of S chartarum on uninoculated
gypsum board controls. Visually detectable fungal colonization on gypsum wallboard is often restricted to untreated surfaces (Figure 69.2).⁵¹