Background and Objectives: Transmission of pathogenic micro-organisms through the air is very dangerous for the society health. It is one of the most important issues that currently has faced the majority of hospitals with increasing nosocomial infections. Bio-aerosols are linked with a wide range of health effects including communicable diseases, acute toxic effects, and allergies and nosocomial infections, which can threaten health of personals, patients, and their companions admitted in hospitals. . Given that hospital infection rates has a direct relationship with the density and type of bioaerosols,, therefore, the distribution and abundance of micro-organisms, which wasthe main objective of the study becomes important. Materials and Methodes: This was a descriptive-analytic study in which seven wards of Valiasr hospital were selected randomly. The wards selected were Internal, laboratory, infants and children, CCU, Men surgery, women and labors. Operating rooms and outside the hospital environment were also studied. Air samples were taken according to the NIOSH standard instructions and Anderson procedure with a flow rate of 3.28 L per 2 minutes on mannitol salt agar, nutrient agar, EMB agar, blood agar, and sabarose dextrose agar media. Out of 240 samples taken, 200 samples showed growth. Then, the samples were transported to laboratory immediately and were incubated for 48 h at 37˚C and the experiment temperature was 22-27˚C, and afterwards each sample was counted andtested. At the end, the microorganisms density was determined in term of CFU/M3. Results: The highest average concentration of pollution occurred in Infectious Ward (238.51 CFU/ M3 in spring and 167.02 CFU/M3 in autumn) and the lowest one was related to the CCU, where showed no fungi growth during both seasons. Despite the environment sterilization, the highest percentage of fungi (Aspergillus Niger) and yeast observed in the hospital air was 42.45 percent in spring and 44.26 percent in autumn respectively. Moreover, Staphillus Epidermithis (25.93 percent)and gram-positive bacillus were the highest percentage of bacteria identified in air samples. Conclusion: From the findings of this study, it can be concluded that the concentration of bio-aerosols in different hospital wards expect in CCU was more than recommended and similar studies and in terms of species was similar to other studies. Therefore, the hospital authority is recommended to reduce the amount of the pathogenic and environmental bio-aerosols through controlling individual traffic, changing the disinfectants and their applying procedure on the wards surface, establishing standard and suitable ventilation systems.
