The ventilation system is not the most important source to cause surgical site infections via the air. More important is the skin of both staff and patients. The literature did not reveal any reduction of the risk of surgical site infections resulting from the employment of ultra-clean air-systems during surgical procedures, the one exception being high risk operations such as orthopaedic implant surgery. Both ultra-clean air and antimicrobial prophylaxis can reduce the incidence of surgical site infections. If HEPA filters are used, they are only necessary directly in the operating rooms. Other rooms such as the washroom, the anaesthesia preparation room or corridors which are connected to the OR do not have to be treated with HEPA filters. If a laminar air-flow system is installed, there are some factors which have to be considered. The number of operating lamps and the heads of the operating team affect the function of the air ceiling as they form thermic and air-flow resistance and create turbulences. Also, forced air-warming systems, which are used to maintain normal body temperatures for patients during surgery, disturb the ultra-clean field through the air emitted from the blankets used. Moreover, any medical equipment which is cooled by integrated cooling blowers can influence an ultra-clean air system. Existing ventilation systems are not able to create good room conditions for all persons inside the OR. Therefore new ways have to be found to create a room climate taking into account the level of activity.