Through air sampling, it was possible to evaluate microbial contamination in environments at high risk of infection, and to check the efficiency of ventilation system and the medical team's hygiene procedures. This study measured the concentration of particulate matter (PM) 2.5 or less microns and microbiological organisms in operating rooms (OR),
intensive care units (ICU) and emergency rooms (ER) in Ain Shams University Surgery Hospital, and to assess ventilation characteristics in operating rooms in the hospital. The passive air sampling was done from ICUs, ORs, and ERs in Ain Shams University Surgery Hospital. Also for each operating room, an observational checklist was done to record other factors that may affect air quality in the room. The evaluated air quality indices were: suspended (PM) 2.5 micrometer or less, culture media and microbial identification of bacteria and fungi, and temperature and relative humidity. The results showed that the highest mean found for bacterial (105.70±30.49) and fungi concentration (7.50±5.30) was in ER. The three settings did not differ statistically as regard levels of PM 2.5, temperature, and relative humidity. A positive correlation exits between bacteria and fungi concentration on one hand and relative humidity on the other. Diphteroid, CONS, MRSA, S. aureus, and Anthracoid were the most frequent isolated bacterial types, while Penicillium and Asperigillus fumigatus were the most frequent isolated fungi. In operating rooms, the percent of unmasked persons present and the temperature positively influence the bacterial count, while ventilation condition is negatively influencing fungi count, and the number of persons present in the operating room positively affects the PM level.