There are several factors that affect the hygiene in the surgical operating theatres. The HVACairside system plays an important role to maintain the adequate hygiene level in the operatingtheatres. The present paper displays comparisons between different HVAC airside designs toassess their impact on the surgery staff and patient health and comfort. The present workutilizes a numerical modelling technique to predict local flow field characteristics, heattransfer and air distribution profiles. To reduce the infection possibilities of the surgery staffand patients, it should reduce the backward flow from the polluted zones towards the cleanzones and the recirculated zones; also, the air infection by direct and indirect ways should beavoided. The present paper introduces suitable design of the HVAC airside system that isbased on application of the centralized partially ceiling supply flow with the two extractlevels to ensure the hygiene of the occupancy of the operating theatres.