Tehran University of Medical Sciences Journal of Air Pollution and Health 2476-3071 6 1 2021 10 25 14 29 Maryam Dashti Ardabil University of Medical Science Armin Sorooshian University of Arizona Mehdi Vosoughi Ardabil University of Medical Science Seyed Ahmad Mokhtari Ardabil University of Medical Science Hadi Sadeghi Ardabil University of Medical Science Abbas Norouzian Baghani Department of Environmental Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran 2021 09 15 2021 10 09 Introduction: Hospitals are sensitive places owing to the contagious nature of diseases transferred by patients to others such as health care workers and staff. Materials and methods: The aim of the present work is to evaluate the type and concentration of bacterial and fungal bio-aerosols in the indoor air of four operating rooms (ORs) and four wards in Khalkhal, Iran during 2019. A total of 192 bacterial and fungal samples were collected. Results: Mean total concentrations of airborne bacteria for ORs and wards were between 11±1.2 to 48±3.1 CFU/m3, respectively, while for airborne fungi values ranged from 95±5.6 to 51±1.2 CFU/m3, respectively. The predominant genera of airborne bacterial isolated (ORs vs. wards) were Staphylococcus epidermidis (72% vs. 58%), Group D Streptococcus (4% vs. 17%), Group A Streptococcus (13% vs. 3%), and Staphylococcus saprophyticus (6% vs. 4%). In addition, the main fungal species identified (ORs vs. wards) were Cladosporium sp. (37% vs. 38%), Penicillium sp. (28% vs. 22%), and Aspergillus Niger (21% vs. 12%). A statistically significant correlation was observed between the mean concentration of bio-aerosols and population density (p<0.05). Conclusion: Furthermore, a statistically significant difference was observed between the mean concentrations of bio-aerosols and the values recommended by WHO (p<0.05), linked presumably to inadequate disinfection, improper design and operation of standard central ventilation (SCV), and the high density of visitors and patients. Addressing such issues can help reduce airborne fungi and bacteria in hospital. https://japh.tums.ac.ir/index.php/japh/article/view/295 https://japh.tums.ac.ir/index.php/japh/article/download/295/268