Visits have been made to thirty operating suites in the British Isles. Each suite was visited three times at approximately 4-month intervals and observations made on an average of five half-day working sessions on each occasion. Measurements were made of air temperature, humidity, air movement and radiation temperature and many details of the suites and working conditions recorded. At suitable intervals the operating room staff were questioned as to their feelings of thermal comfort using Bedford's 7-point scale. Over 2500 sets of replies were obtained. Visible sweating was also noted.

The effect on comfort and the extent of visible sweating of the many items recorded was then explored by means of a sequential multiple regression analysis. Although air temperature was by far the most important factor affecting thermal comfort, all the variables named above exerted a significant effect. In addition, a number of other conditions including age, sex and race produced minor differences.

Surgeons and anaesthetists were found to differ from other staff in their thermal preferences, the surgeons liking a cooler and the anaesthetists a warmer environment. Although most surgeons were comfortable at temperatures around 66·5°F. (19°C.), at 50% relative humidity and 25 ft./min. air movement with the average amount of thermal radiation from the operating room lamp, it would be necessary to keep the temperature down to 64·5°F. (18°C.) if no more than one surgeon in twenty was to be uncomfortably hot. At this temperature nearly half the anaesthetists, who mostly preferred temperatures around 71°F. (21·5°C.), would feel too cold. Variation in the clothing worn by different staff members seems to be the only way of resolving this difficulty.

The average temperature in the operating rooms visited was over 72°F. (22°C.), and 75°F. (24°C.) was exceeded on about 25% of occasions.