# Comparison of Methods to Identify and Monitor Mold Damages in Buildings
## 1 Introduction  
Increased humidity in building materials leads to fungal growth in both visible and hidden structures. In addition to the deterioration of the colonized materials, mold growth has been associated with adverse effects on human health [1, 2, 3]. Previous studies have correlated certain fungal taxa to different building materials [4, 5, 6, 7, 8, 9], as well as to specific building structures and moisture problems (e.g., condensation in thermal bridges and water leakage) [10, 11, 12]. This knowledge is crucial to select suitable moisture-damage indicator species that are able to grow on building materials. They should be discriminated from occasional species, unable to grow in the indoor environment, as is the case for plant and/or animal parasites, or mycorrhizal fungi [12, 13]. Using indicator species during the monitoring of buildings allows the efficient detection of moisture problems, relevant for both buildings and occupants' health. In addition, such important information is also necessary to investigate and promote the use of materials less prone to microbial colonization. Various methods have been used in the microbiology of the built environment, i.e., microscopy, particle counting, culturing, and DNA and chemical analyses [6; 14; 15; 16]. The sampling of residential buildings is carried out either from the air, dust, building materials and surfaces, or directly from microbial growth when evident [17]. The selection of methods, including the sampling strategy and analytical techniques, implies inevitable biases for all studies. The main limitation of culture-based methods is that the cultivable fraction of the microbiota is minor. In particular, [PERSON] et al. [18] estimated that cultivable fungi are only about 11% (ratio uncultivable/cultivable = 8.8) of the total fungi present in a given environmental sample. In addition, few researchers are familiar with the taxonomy and ecology of the fungi occurring in buildings. Thus, a gradual shift toward DNA-based methods has taken place in the last decades. Among them, the real-time quantitative PCR (qPCR) has been widely used in research to detect and quantify many fungal species, genera, or higher taxa in indoor dust [19]. DNA metabarcoding, based on the high-throughput sequencing of an amplified DNA marker, has become a key tool for surveying environmental microbial communities in the last decades. In several studies, the mycobiota of the built environment have been surveyed through DNA metabarcoding analyses of the internal transcribed spacer (ITS) of the nuclear rRNA operon [20; 21; 22; 23]. However, DNA analyses are not able to distinguish between live and dead fungi or identify specifically those that are capable of growing on building materials. In this regard, the direct microscopy of mold growth, obtained by tape lifts, may be a simple and quick alternative to identify biologically active indicator fungi [12]. Unfortunately, microscopy examinations have a relatively low sensitivity, limited to the small surface area sampled, and require the presence of identifiable structures and highly skilled fungal taxonomists. Thus, certain groups, such as yeasts and sterile mycelia, are generally underestimated by microscopy. Despite the high potential of DNA techniques and their increased application in research, their practical use by building practitioners/ consultants is still limited. In addition to the required skills in DNA analyses, their scarce use may be in part due to the lack of taxonomic and ecological knowledge about the vast amount of operational taxonomic units (OTUs) that can be detected in the built environment. In the last years, some building consultant companies have expanded their services, using qPCR assays on dust samples as a monitoring tool-kit for the early detection of moisture/mold problems.