The mysterious mould outbreak - A comprehensive fungal colonisation in a climate-controlled museum repository challenges the environmental guidelines for heritage collections

Within the last decade, fungal colonisations have increased in Danish museum repositories. The growth is unexpected, as many Danish museums strive to comply with the environmental guidelines for heritage collections. When fungal growth develops in heritage collections, it threatens the heritage preservation and occupational health of museum staff. Therefore, it is crucial to prevent. This study characterised a fungal colonisation in a 1450m² museum repository, striving to meet the guidelines for heritage collections with relative humidity below 60%. After fungal discovery, the repository was examined twice by an environmental laboratory with morphological identification of fungi and quantification of fungal biomass based on fungal enzyme activity. However, the reports were not sufficient to qualify a recovery process. A research study with a broader approach was conducted to further elucidate the problem. The study included 1) building examination, 2) fungal surface sampling and morphological ID, 3) ID of fungal isolates with DNA sequencing, and 4) activated fungal air sampling and morphological ID. Although the relative humidity was measured to meet the guidelines for heritage collections with no evidence of moisture or microclimate, hyaline and white fungal colonies were distributed on heritage artefacts throughout the repository. There was no growth on interior and building structures. Cultivation of air samples on DG18-agar and V8®agar showed the presence of common indoor fungi, while artefact samples cultivated on the same media showed no growth. In contrast, cultivation of air samples and artefacts samples on the low water activity agar MY50G followed by DNA-sequencing showed high concentrations of the xerophilic fungi A. halophilicus, A. domesticus, A. magnivesiculatus and A. vitricola, belonging to Aspergillus section Restricti. These fungi are characterised by growing at low water activity corresponding to low relative humidity. The museum repository seemed to provide this environment with relative humidity below 60%. The study emphasised that examining the same fungi using different approaches may obtain very different results. Furthermore, the study questioned if the environmental guidelines for heritage collections adequately prevent the risk of xerophilic fungal growth. Xerophilic fungi are not adequately included in the risk assessment underlying the preventive conservation framework. Consequently, the risk is not included in the revised environmental guidelines accepting RH between 40-60% to support more sustainable heritage storage. It has not been studied if the revision would increase the risk of xerophilic fungal growth before it was accepted and implemented. This study indicates that it could be the case. Close collaboration between mycologists and museum professionals may develop more standardised and targeted detection and prevention practices for heritage repositories. The risks of xerophilic fungal growth should be included in the preventive conservation framework ensuring heritage preservation and occupational health of museum staff.