1. Pathogenic bacteria are removed from air by relatively simple filters having an efficiency of 99·9% for 5-μ particles.

2. To prevent ingress of contaminated air from other parts of the hospital an operating-room should be pressurized by a flow of filtered air. An input flow of 1200 cu. ft. (34 cu. m.) per min. will ensure this if mechanical exhaust fans are replaced by pressure-relief valves.

3. Other rooms of the suite should be ventilated in this way with flow rates depending on door area, and with pressure gradients adjusted by relief-valve loadings to cause air flow from dirtier to cleaner zones.

4. All openings between the rooms of an operating-suite should be fitted with doors to prevent airborne cross-contamination and high-level loss of ventilating air.

5. The relative merits of turbulent and downward-displacement ventilation have been studied and are discussed. The choice between them depends on the sites from which pathogenic bacteria are mainly disseminated during operations; these are not yet fully understood but the available evidence favours the displacement system. Methods for producing turbulent and displacement ventilation are described.

6. Discomfort from overheating of operating-rooms is largely due to faulty design and siting of sterilizing equipment. In Britain, if these faults are avoided, ventilation without cooling equipment can give comfortable conditions except on very rare occasions.