Airborne pollutants in heritage spaces: A study case of the Sephardic Museum’s indoor air quality in Toledo (Spain)

• IAQ was assessed in a 14th-century Mudéjar synagogue turned museum. • VOCs and PM levels were higher during public visiting hours. • Microbial diversity varied by location and visitor presence. • cel5B and canA genes linked to biodeterioration were detected. • Showcases offered partial protection against pollutants and microbes. Indoor Air Quality (IAQ) is a critical factor in the preventive conservation of cultural heritage, particularly in historic buildings that house valuable artworks. This study presents a multidisciplinary assessment of IAQ in the Synagogue of El Tránsito, home to the Sephardic Museum (Toledo, Spain). Air samples were collected from five locations—three showcases, the Great Prayer Hall, and an outdoor courtyard—during both open and closed museum hours, to evaluate the impact of visitor presence. The chemical pollutants (CO, NO 2 , O 3 , VOCs, CH 2 O, PM 10 , PM 2.5 ), physical parameters (temperature, relative humidity (RH), CO 2 ), and microbiological contaminants (bacteria, fungi, and biodegradative genes), using culture-based methods, qPCR, and amplicon sequencing, were analysed. Results revealed seasonal humidity fluctuations and elevated O 3 and CH 2 O levels, particularly in showcases. VOCs were originated from indoor sources, such as paints and cleaning agents. Particulate matter concentrations were significantly higher in the Great Prayer Hall during hours with public, indicating human activity as a major contributor. The bacterial and fungal counts were higher in the Great Prayer Hall than at the showcases, meaning that these offer partial protection despite not being airtight. Genes related to cellulose ( cel5B) and stone degradation ( canA ) were detected. Microbial community composition was influenced by both spatial variation and human presence. Specific taxa, including Cladosporium, Malassezia, Thermosinus, and Enhydrobacter , were significantly correlated with environmental parameters such as temperature, RH and CO 2 levels. This study supports the development of conservation protocols and advocates for international standards to protect cultural assets and public health in museums.