Seasonal concentrations and size distributions of culturable bacterial aerosols inside and outside university classrooms - A case study in Beijing

The risks of cross-infection among occupants will be greatly increased by the high occupant densities and mobility inside and outside university classrooms. In order to know the distribution characteristics of culturable bacterial aerosols in such public environments, during normal didactic activities, indoor and outdoor bacterial aerosols were sampled from December 2018 to December 2019 by using Andersen six-stage impactor in 11 university classrooms. The yearly average concentration of outdoor bacterial aerosols was 314 +/- 229 CFU/m(3), and the size mainly distributed within 2.1-4.7 mu m. For unoccupied condition, indoor culturable bacterial aerosols were dominated by outdoors (R-2 = 0.78), the yearly average concentration was 254 +/- 130 CFU/m(3), and the size mainly distributed within 1.1-3.3 mu m. For occupied condition, the yearly average concentration of indoor bacterial aerosols was 494 +/- 263 CFU/m(3), and the size mainly distributed in the range of 1.1-4.7 mu m. With human presence, occupants instead of outdoor air becoming the primary source of indoor culturable bacterial aerosols. The seasonal emission rates of bacterial aerosols resulted from occupants were in the order of summer > autumn > spring > winter, corresponding to 1538 +/- 646, 850 +/- 292, 592 +/- 254, and 388 +/- 219 CFU/person/h. Air temperature, relative humidity, and PM10 mass concentrations were main environmental factors (p < 0.01) to positively influence the concentrations distribution of indoor culturable bacterial aerosols. The results can provide basic data for evaluating the risks of occupants expose to bacterial aerosols and improving indoor microbial air quality in university classrooms.