The minimum fresh air volume for each operating room shall be determined according to the following requirements, and the maximum value shall be taken.

(1) The fresh air volume calculated according to the fresh air change rates in Table 4.1: calculated per person per hour, the Swiss standard is 80m³, the German standard is 60m³, and the Japanese standard is 3～5 times/h. This table adopts 60m³ and 3～6 times/h.

Due to the characteristics of the layout and functions of operating rooms, such as the use of automatic sliding doors for entering and exiting and the presence of an exhaust system indoors, the leakage air volume is generally larger than that of industrial cleanrooms, and the positive pressure value is difficult to achieve; the values adopted in the design should be selected correctly.

(2) The fresh air volume required to compensate for the indoor exhaust (not less than 200m³/h for each operating room) and to maintain the indoor positive pressure value.

(3) The fresh air volume required for personnel respiration.

For the above, Class Ⅰ operating rooms should not be less than 1 000m³/h per room (among which ophthalmology-specific operating rooms with smaller areas should not be less than 800m³/h per room); for Class Ⅱ and Ⅲ clean operating rooms, each room should not be less than 800m³/h; for Class Ⅳ clean operating rooms, each room should not be less than 600m³/h.

[Reference] The Hospital Design and Management Guidelines published in Japan in 1998 stipulate a minimum fresh air volume of 5 times/h; the Applications volume of the 1999 edition of the Handbook of the American Society of Heating, Refrigerating and Air-Conditioning Engineers also stipulates a minimum fresh air volume of 5 times/h; Part 4 of the Federal German standard DIN1946 gives 70m³ per person per hour for wards, and although no value is given for operating rooms, it is obviously to be higher than this figure, but it does give a total fresh air volume of 1 200m³/h per operating room; the Swiss standard adopts 80m³/h per person. Taking into account the installation of the exhaust system, the assumed number of people (extra-large: 12 persons, large: 10 persons, medium: 8 persons, small: 6 persons) and the stipulation of a minimum of 60m³/h of fresh air per person, these standards are all rather high, with the German fresh air volume being the largest; its consideration is that the Halothane usage in an operating room is 500mL/h, and if the fresh air reaches 1 200m³/h, the halothane concentration can be maintained at $dfrac{500,text{cm}^3}{1,200,text{m}^3} approx 0.4times10^{-6}$, while near the anesthesiologist it will be 10 times higher than this concentration, i.e., $4times10^{-6}$, which is just below the maximum allowable concentration of this gas, $5times10^{-6}$.

According to the explanation in the "Code":

(1) The German consideration may be used as a reference, but for Class Ⅳ operating rooms where minor ordinary operations are performed, the anesthetic dosage may all be relatively small, and the release of anesthetic gas should not be continuously at high concentration, whereas the standard stipulates that the exhaust is continuous; therefore, it may be considered to reduce the fresh air volume to half of it, approximately 600m³/h.

(2) This is also the most important point, namely, to ensure that when the door is open, the indoor airflow can flow outward at a certain velocity to resist the intrusion of outside air. Assuming that a Class Ⅰ operating room maintains an airflow velocity of 0.1m/s, and the area after the door is opened is 1.4×1.9=2.66m², then 956m³/h of fresh air is required; for Class Ⅱ and Ⅲ operating rooms maintaining a flow velocity of 0.08m/s, 766m³/h is required. In addition, it is fairly commonly reported that operating rooms feel rather stuffy; therefore, this clause appropriately increases the fresh air. In addition to stipulating the fresh air change rate and the fresh air volume per person later, the minimum fresh air volumes for Class Ⅰ, Class Ⅱ-Ⅲ, and Class Ⅳ are set at 1 000m³/h (ophthalmology-specific operating rooms generally have very few surgical personnel and the rooms are also small, so 800m³/h may be adopted), 800m³/h, and 600m³/h, respectively.