INTRODUCTION: Surgical site infections are among the most frequent Health Care Associated Infections with severe impact on mortality and high economic costs; the role of air microbiological contamination in surgical site infections was amply discussed in the scientific literature, highlighting differences in air contamination rates between different ventilation systems, number of people present and door opening rates. MATERIAL AND METHODS: The aim of our study was to monitor the presence of bacterial air contamination in operating theaters and its relationship with number of people and type of airflow over a period of nine years (January 2010-November 2018) at Messina's University Hospital. The Rho of Spearman test was used to evaluate differences in microbial contamination between empty and working theaters. The impact of the number of people on colony-forming unit values was assessed by performing a stepwise multiple regression analysis. The differences between the results recorded over the nine-year study period were evaluated using the variance analysis. Software R was used for the statistical assessment. RESULTS: Air samples were taken in each operating theater over nine years. A total of 1,425 samples were collected with a positivity rate of 37.3%. The median bacterial contamination rate was 30 cfu/m3 in empty theaters, while this rate was significantly higher (P < 0.001) in working theaters, where it reached 85 cfu/m3. A statistically significant difference was identified between laminar and turbulent airflow systems, with higher bacterial contamination rates with the latter (p<0.001); in these cases, the value of bacterial contamination (microbial count) was higher in the presence of a higher number of individuals in the operating theater (p<0.001). CONCLUSION: Our study demonstrates the complexity of the Operating Theatres environment, in which a good ventilation system is, indeed, only one prerequisite for clean air, but other factors, such as the behavior of healthcare professionals and environmental conditions, can influenced the bacterial count.