Portuguese primary schools: dust filter samples, a simple tool for assessing bacterial contamination indoors?

This study aimed to assess seasonal variations in bacterial contamination across school settings (canteen, classroom, gymnasium, library, and toilet) with dust filter samples, to identify high-risk areas and inform targeted hygiene practices and public health strategies in educational environments. Dust filter samples were collected in 10 schools within the Metropolitan Lisbon Area. Sterilized coffee filters were placed inside the disinfected vacuum tube for sampling. Samples were collected in the warm (N=33) and cold seasons (N=34), from canteens (N=14), classrooms (N=33), gymnasium (N=9), library (N=9), and toilets (N=2). Settled dust was collected from shelves, plinths, and floors around students' desks and near the door in all sampled rooms. After extraction with NaCl+Tween 80 solution, samples were inoculated onto Tryptic Soy Agar (TSA), Violet Red Bile Agar (VRBA), and MacConkey Agar (MAC). Results from the cold season indicate that gymnasiums (TSA = 1.8×10⁴ CFU/m²) and libraries (VRBA = 1.4×10⁴ CFU/m²) were contamination hotspots, although libraries showed lower contamination on TSA (1.8×10³ CFU/m²). Canteens (2.3×10³ CFU/m²) exhibited elevated Gram-negative bacteria, while classrooms had the lowest contamination on VRBA (5.0×10² CFU/m²). For the warm season, canteens showed higher contamination on TSA (6.5×10³ CFU/m²), while classrooms showed higher contamination on VRBA (4.5×10³ CFU/m²). For MAC results, gymnasiums showed higher contamination (2.0×10³ CFU/m²). Seasonal and spatial differences significantly influence microbial air contamination in schools. Gymnasiums and classrooms are potentially critical hotspots, particularly for MAC and TSA contamination results. TSA contamination results showed high variability (e.g., SD = 23,645 in canteens), suggesting fluctuating total bacterial contamination, likely tied to occupancy or cleaning schedules. These findings emphasize the need for seasonally adaptive hygiene protocols and focused monitoring in high-traffic areas to mitigate exposure risks.