Indoor and Outdoor Air Microbial Contamination During Different Reconstruction Methods of Historic Buildings

The quality of indoor air is dependent on a number of factors, including the presence of microorganisms that colonize the building materials. The potential for health risks associated with microbial contamination is a significant concern during the renovation of buildings. The aim of this study was to assess the impact of two reconstruction methods for historic buildings on air quality. The two reconstruction procedures were facadism, which preserves only the façade, demolishing the rest of the building and constructing a new building, and complete reconstruction, which involves internal renovation with a less intensive demolition. A total of 70 + 70 air samples, as well as surface and dust samples, were collected throughout the course of the reconstruction of the two buildings. In the case of facadism, total colony counts were found to be 2-4 times higher indoors than outdoors, even at the initial stage of the works. High concentrations of <i>Aspergillus</i> and <i>Penicillium</i> spp. were detected. During the less intensive reconstruction, the total colony count in the indoor air samples was initially lower at almost every sampling point than at the outdoor levels. With regard to fungi, <i>Penicillium</i> species were initially present at lower conidia concentrations, followed by <i>Aspergillus</i> species over time. In both buildings, elevated concentrations of airborne fungi were detected during the main reconstruction period. The fungal genera found in the indoor air were also detected on surfaces and in dust samples. Outdoor air samples collected from the vicinity of the buildings revealed elevated fungal counts at multiple sampling points, particularly in the case of facadism. Disinfection with dry fogging was implemented twice throughout the entire interior of the buildings. Following the first disinfection process, there was no notable decrease in colony-forming unit (CFU) counts in either building. However, the second disinfection resulted in a reduction in microbial concentration in the air. Our study confirms that the renovation of historical buildings can result in an elevated prevalence of fungal bioaerosols, which can be harmful to occupants. While the impact of the reconstruction remained within the range of urban background variability at distant (>1 km) locations, it caused local microbial contamination, often exceeding the detection limit in near-site samples.