Most surgical wound infections are acquired in the operating room from the patient's own microbial flora. The remainder are acquired mainly from the staff in the operating room during surgery. The inanimate environment (e.g., walls, floors, and surgical instruments) has little relevance to the spread of infection. Because the air is an important route of spread in joint prosthesis operations, the routine use of an ultraclean air system and exhaust-ventilated clothing is frequently recommended. The value of such a system in other types of clean surgery is doubtful, but other measures, such as the following, may provide similar results at less cost: reduction of the number of persons in the operating room; a policy of not opening doors during operations; the use of comfortable, washable, bacteria-impermeable clothing by the operating-room staff; and concentration of the airflow over the operation site rather than over the whole operating room.