“Fungal-Friendly” Is Not Always Warm and Humid: A Rapid Diagnostic Aerobiological Assessment with the Fungal Susceptibility Score for Airborne Fungi at Castello D’Albertis Museum of World Cultures (Genoa, Italy)

Airborne fungi can threaten both cultural heritage materials and indoor air quality, even under temperature and relative humidity conditions not commonly associated with fungal proliferation. This study evaluated culturable airborne fungi in two museum rooms, a semi-confined Lodge, and the surrounding Garden at Castello D’Albertis Museum of World Cultures in Genoa, Italy, during one museum closing day and one high-attendance opening day. Air samples were collected by active impaction on Malt Extract Agar and Rose Bengal Agar. Fungal isolates were identified using morphological and molecular methods, and the Fungal Susceptibility Score was applied as a comparative/priorization tool to estimate potential risks to paper, wood, plaster, and people. Cladosporium was the dominant genus under both sampling conditions, detected in all eight sampling occasions (100%), and was also consistently recovered from visibly affected books. The indoor/outdoor fungal concentration ratio increased from approximately 0.41 during the closed day to 1.06 during the opening day. Isolates provisionally identified as C. pseudocladosporioides and Cladosporium sp. showed ligninolytic and cellulolytic activity, with C. pseudocladosporioides displaying a significantly higher gallic-acid halo-to-colony ratio (4.53 ± 0.30 vs. 1.76 ± 0.04, n = 9; p < 0.0001). Fungal Susceptibility Score values were generally higher for material substrates than for people, although patterns varied among sampling sites and opening conditions. These findings indicate that moderate indoor relative humidity does not exclude fungal biodeterioration risk and support integrating fungal concentration, taxonomic composition, and substrate specific indices in museum monitoring and conservation planning.