7.3.1 Centralized Air Supply

(1) In Class Ⅰ–Ⅲ clean operating rooms, the air supply outlets shall be centrally arranged above the operating table, so that the operating table and its surrounding zone are located in the main stream zone formed by the clean airflow. The Class 100 zone of Class Ⅰ clean auxiliary rooms shall also have air supply outlets centrally arranged above it. The area of the air supply outlets shall be selected according to the values listed in Table 7.15, and shall not exceed 1.2 times those values. For Class Ⅳ clean operating rooms and auxiliary rooms of all classes, the air supply outlets may adopt the conventional dispersed arrangement.

Table 7.15 Area of centrally arranged air supply outlets in clean operating rooms

Operating room class Air supply outlet area/m²
Class Ⅰ <2.4×2.6=6.24m²: operating table (width 1.8m, 0.6m from the table edge), with <0.4m, <0.4m, <0.9m, <0.9m reserved on the four sides; head-dedicated operating room within Class Ⅰ: <1.44m² (<1.2m, <1.2m)
Class Ⅱ <1.8×2.6=4.68m²: operating table (width 1.8m, 0.6m from the table edge), with <0.4m, <0.4m, <0.6m, <0.6m reserved on the four sides
Class Ⅲ <1.4×2.6=3.64m²: operating table (width 1.8m, 0.6m from the table edge), with <0.4m, <0.4m, <0.4m, <0.4m reserved on the four sides

[Reference] ① Although there are many forms of air supply terminals, arrangement methods, and air supply areas for Class Ⅰ–Ⅲ operating rooms, such as side supply, oblique supply, and top supply, nowadays, out of consideration for protecting the critical area, clean operating rooms neither require the airflow to be in a unidirectional flow state throughout the room nor desire the air outlets to be dispersed arbitrarily. In most cases, the outlets are centrally arranged above the operating area, and this top-supply, bottom-return arrangement is considered to provide the best protection for the critical area. It embodies a typical approach different from the previous laminar flow or turbulent flow: it does not emphasize the typical extrusion effect or dilution effect of the airflow, but instead fills the operating area with clean, sterile airflow, thereby reducing the bacterial concentration in the critical area.

② According to the main stream zone theory, after the air supply outlets are centrally arranged, under the original air volume of the original air cleanliness class, the class of the operating area can be raised one level higher, while the other areas in the room remain at the original level, and the bacterial concentration in the operating zone is also reduced by half. Since Class Ⅳ operating rooms have lower requirements, this provision is not made.

A schematic diagram of the main flow zone is shown in Figure 7.6.

Figure 7.6　Main flow zone when the three zones are non-uniform

  1. Air supply terminal　2. Main flow zone　3. Eddy zone　4. Return air inlet zone　5. Dust source

③ Since surgical incisions differ, healing requirements differ, and protection grades vary, the required characteristics of the supply airflow also differ. Class Ⅰ special clean operating rooms adopt a unidirectional airflow mode, which works on the principle of extrusion and displacement. Class Ⅱ and Class Ⅲ clean operating rooms, owing to their lower discharge air velocity, cannot obtain sufficient momentum to maintain unidirectional flow; they adopt a low-turbulence displacement airflow, which works on the filling principle and has quasi-unidirectional flow properties. Class Ⅳ quasi-clean operating rooms use mixed supply airflow, which works on the dilution principle. Only the top air supply mode is required, and no special requirements or provisions are imposed on the arrangement of the supply air outlets.

(2) In a Class Ⅰ clean operating room, the airflow in the Class 100 operating zone (room) must be unidirectional flow.

① The operating zone of a Class Ⅰ operating room must maintain a unidirectional flow pattern, and there are also strict requirements for the cross-sectional velocity and its uniformity at the working surface; in general, the supply air outlets are required to be provided with a certain flow-equalizing layer. Unidirectional flow patterns can be divided into vertical unidirectional flow and horizontal unidirectional flow. Through long-term technical and economic comparisons and long-term operational practice, opinions abroad have tended to converge: clean operating rooms should adopt vertical unidirectional flow. At present, horizontal unidirectional flow is no longer used in newly built high-grade operating rooms in China, and local unidirectional flow has widely replaced whole-room unidirectional flow.

② For a Class Ⅰ clean operating room that must maintain a unidirectional flow pattern, the supply airflow rate can be calculated from the required cross-sectional velocity and the supply air area. However, the velocity of 0.25～0.3m/s required by the Code is the average cross-sectional velocity at the working surface height of the operating zone, and it is also the lower-limit velocity that must be maintained during operation; it cannot be directly adopted in design calculations. This is because the supply airflow of local laminar flow will undergo momentum exchange with the airflow of the surrounding zone, so the supply airflow velocity will attenuate gradually, and its attenuation is greater than that of full unidirectional flow. Therefore, the discharge velocity at the supply air outlet should be greater than the cross-sectional velocity of the operating zone. Experimental research results show that the discharge velocity should be within 0.40～0.42m/s, and in design the discharge velocity may be taken as approximately 0.44～0.48m/s.

③ The verification of a Class 100 clean zone (room) is not merely a measurement result; its airflow pattern should also be unidirectional flow. Because During static testing, results of less than 3.5 particles/L can also be obtained under high air change rates, but this is not Class 100 in the true sense, because its airflow is non-unidirectional, its anti-interference performance is very poor, and its self-cleaning time is also long, which does not meet the requirements of a clean operating room. To achieve clean, uniform, single-directional and parallel unidirectional airflow, the coverage ratio of HEPA filters, the average cross-sectional air velocity, and the velocity uniformity are all important conditions.

[Reference] Relevant standards requiring unidirectional flow for Class 100.

The following standards all clearly stipulate that a unidirectional airflow pattern shall be adopted to ensure the Class 100 air cleanliness class.

① National Standard of the People's Republic of China, "Code for Design of Clean Room" (GB50073), Clause 6.3.3.

② International Standard "Cleanrooms and Associated Controlled Environments" (ISO-14644-1), Part 4 "Design, Construction and Operation of Cleanroom Facilities", Clause A2.

7.3.2 Requirements for the Arrangement of Final-Stage HEPA Filters

(1) When the final-stage HEPA filters in a Class 100 clean area are arranged in a concentrated manner, they shall meet the following requirements:

1) When arranged on the air supply surface at the lower part of the plenum box, or on the top surface of the plenum box parallel to the decorative layer or the flow-uniformizing layer, the coverage ratio shall be not less than 0.75.

$$text{Coverage ratio} = frac{text{Net cross-sectional area of HEPA filters}}{text{Total area of the section where HEPA filters are arranged}}$$

2) When arranged on the side of the plenum box, they shall be arranged on one side or on opposite sides; the coverage ratio on the side shall also be not less than 0.75, and measures for thorough mixing of the airflow inside the plenum box are required.

3) When restricted by the storey height and by the rule that maintenance inside the room is not allowed, an air supply surface with a leak-prevention function may be adopted and the filters arranged outside the plenum box, but as close to the plenum box as possible; the airflow inside the plenum box must be provided with thorough mixing measures, and the ratio of the area through which clean airflow passes on the air outlet surface to the total area of the outlet surface shall be not less than 0.85.

4) The final-stage HEPA filters of the Class 100 clean operating zone shall be arranged in a concentrated manner; it is not emphasized that they must be arranged at the end of the plenum box. For the two arrangement forms 1) and 3), as long as the coverage ratio meets the requirement, their effectiveness has been verified in practice. Form 2) has relatively poor unidirectional flow characteristics.

[Reference] The usual practice for a Class 100 clean operating zone is to arrange the HEPA filters at the end of the plenum box, and its effectiveness is realized by having the terminal filters act as the final barrier. However, this traditional terminal device also has its shortcomings, namely that it is very difficult to install so many HEPA filters over such a large installation area and to produce clean (completely filtered without leakage), uniform (well-developed airflow distribution), unidirectional and parallel (perpendicular to the filter face) airflow. This is equivalent to requiring every HEPA filter on the entire air supply surface not only to perform the filtration function itself, but also to serve as an orifice-plate-like flow-uniformizing and airflow-distributing device, as well as to be leak-free like a blind plate; the "coupling" of these three functions makes the requirements for the full-coverage HEPA filter approach exceptionally high — whether it is the plenum box body or the filters and their joint surfaces, the slightest leakage will cause the entire device to fail. Therefore, the traditional terminal device is not only difficult to fabricate, complicated to construct and troublesome in leak testing, but also expensive.

[Reference] If the third approach is adopted, that is, a filter box composed of HEPA filters and a casing is installed outside the terminal device, and the terminal device contains an airflow mixing device and a leak-prevention layer, the original localized "leak" becomes an overall "non-leak", thereby playing the role of blocking leakage. Since the filtered clean airflow enters the plenum box, the requirements for the plenum box body and its joint surfaces are greatly reduced, and the fabrication, construction and leak-testing processes are also simplified.

[Reference] Today, the operating room air supply ceiling with a leak-prevention layer has been developed into a product; its specifications are shown in Table 7.16. Compared with the traditional terminal device, its advantages are shown in Table 7.17: Table 7.16 Specifications of the Leakage-Proof Clean Air Supply Ceiling

Ceiling model / Parameter JTH100-Ⅰ JTH50-Ⅱ JTH30-Ⅲ
Application Class Ⅰ special clean operating room Class Ⅱ standard clean operating room Class Ⅲ general clean operating room
Suitable area Extra-large upper limit 45m², expandable by 1.2 times Extra-large upper limit 45m², expandable by 1.2 times Large upper limit 35m², expandable by 1.2 times

Table 7.17 Comparison between Traditional and New Dedicated Terminal Air Distribution Devices for Operating Rooms

Item Traditional dedicated terminal air distribution device for operating rooms New dedicated terminal air distribution device for operating rooms
Principle Traditional filter air supply outlet concept Adopts the advanced leakage-proof layer concept
Characteristics Three functions of filtration, flow equalization and leak sealing integrated into one Three functions of filtration, flow equalization and leak sealing "decoupled"
Airflow characteristics Poor airflow uniformity, airflow easily diffuses, poor disturbance resistance Unidirectional flow, good cross-sectional velocity uniformity, dense and parallel airflow, good disturbance resistance
Filtration characteristics Filter installed inside the terminal device, traditional mechanical sealing, filter needs to be leak-tested in the operating room Terminal filter installed in a separate dedicated box, zero-pressure sealing, filter does not need leak testing
Leakage Filter and mounting frame prone to leakage No risk of leakage
Terminal device Sheet-metal box, handmade, large dimensional fabrication errors Aluminum alloy profile box, divided into four independent boxes, can be factory-fabricated, small dimensional fabrication errors
Transportation and installation Inconvenient to transport, fabricated on site, hoisted as a whole unit Easy to transport, can be assembled on site, very convenient
Maintenance Maintenance and filter replacement must be performed in the operating room, terminal device needs to be disassembled and reassembled No need to replace filters in the operating room, device is semi-permanent, no disassembly or maintenance required
Appearance Highly arbitrary, perforated plate is unattractive, flow equalizing mesh becomes loose over time Unified pattern and specifications, no perforated plate, flow equalizing mesh will not become loose

[Reference] The performance of the leakage-proof clean air supply ceiling is shown in Tables 7.18 and 7.19.

Table 7.18 Performance of the Class Ⅰ Clean Ceiling

Air supply ceiling / Class Ⅰ operating room Leakage-proof clean air supply ceiling Standard Leakage-proof clean air supply ceiling Laboratory measurement Leakage-proof clean air supply ceiling Project measurement example 1 Leakage-proof clean air supply ceiling Project measurement example 2 Project measurement with conventional air supply mode
Basic performance Dust concentration in operating area/(particles/L) ≥0.5μm Maximum value ≤3.5 0.05 0.05 0.05 0.35~2
Basic performance Dust concentration in operating area/(particles/L) ≥0.5μm Statistical value ≤3.5 0.05 0.05 0.05 0.35~2
Basic performance Dust concentration in operating area/(particles/L) ≥5μm Maximum value 0 0 0 0 0
Basic performance Dust concentration in operating area/(particles/L) ≥5μm Statistical value 0 0 0 0 0
Basic performance Dust concentration in surrounding area/(particles/L) ≥0.5μm Maximum value ≤35 0.8 0.8 0.2 3.5~35
Basic performance Dust concentration in surrounding area/(particles/L) ≥0.5μm Statistical value ≤35 0.5 0.5 0.2 3.5~35
Basic performance Dust concentration in surrounding area/(particles/L) ≥5μm Maximum value ≤0.3 0 0 0 0~1
Basic performance Dust concentration in surrounding area/(particles/L) ≥5μm Statistical value ≤0.3 0 0 0 0~1
Basic performance Settling bacteria concentration in operating area/[CFU(φ90Ⅲ·30min)] ≤0.2 / 0 0.2 0~1
Basic performance Settling bacteria concentration in surrounding area/[CFU(φ90Ⅲ·30min)] ≤0.4 / 0.2 0.3 0~2

Table 7.19 Performance of Class II and Class III Clean Ceilings

Class II and Class III Operating Room ＼ Supply Air Ceiling Leakage-Preventing Clean Supply Air Ceiling
Standard Measured in Class II Project Standard Measured in Class III Project
Basic Performance Dust Concentration in Operating Zone/(particles/L) ≥0.5μm Maximum ≤35 0.1～0.5 ≤350 2.5～4.9
Statistical Value ≤35 0 ≤350 2.9～4.8
>5μm Maximum ≤0.3 0 ≤3 0
Statistical Value ≤0.3 0 ≤3 0
Dust Concentration in Surrounding Zone/(particles/L) ≥0.5μm Maximum ≤350 0.9～1.6 ≤3 500 4.8～7.9
Statistical Value ≤350 0.7～1.3 ≤3 500 4.8～7.8
>5μm Maximum ≤3 0 ≤30 0～0.5
Statistical Value ≤3 0 ≤30 0～1.2
Settling Bacteria Concentration in Operating Zone/[CFU/(φ90 dish·30min)] ≤0.75 0.2～0.3 ≤2 1～0
Settling Bacteria Concentration in Surrounding Zone/[CFU/(φ90 dish·30min)] ≤1.5 1.0 ≤4 1～2

(2) For clean zones below Class 100, the arrangement of the final HEPA filters shall meet the following requirements:

1) In clean zones below Class 100, when the air supply outlets are arranged in a concentrated manner, the final HEPA filters within the outlets may be arranged either in a concentrated manner or in a dispersed manner;

2) In order to make the air discharge from the supply air surface relatively uniform, a flow-equalizing layer (including perforated plate) must be provided on the air discharge surface.

7.3.3 Requirements for the Arrangement of Return Air Outlets

(1) All clean rooms in the clean operating department shall adopt return air at the lower part on both sides; when the distance between the two sides does not exceed 3m, return air at the lower part on one of the two sides may be used. Clean rooms with concentrated air supply outlets and clean auxiliary rooms with local Class 100 areas shall not adopt return air at four corners or four sides. The four corners cannot reduce the eddy zone; as can be seen from Figure 7-6, the eddy zones along the four sides of the air outlet always exist.

Adopting return air at the lower part on both sides is to ensure, as far as possible, the two-dimensional movement of the supply airflow, and this is even more important for Class 100 zones; according to experiments, with return air on four sides, the room-average turbulence intensity is more than 13％ higher than with return air on two sides. Therefore, return air at the lower part on both sides should be adopted for all clean rooms, and return air at four corners or four sides should not be adopted. In addition, the outlet area for four-corner return air is too small; in rooms with local Class 100 areas, the return air velocity cannot be controlled below 1.6m/s, so the height of the return air outlet has to be raised. In some projects, the upper edge of the return air outlet is as high as about 1.2m, which is extremely unreasonable. The four corners themselves also reduce the effective space and area of the operating room.

Rooms wider than 3m generally require return air on two sides; if return air outlets are provided on only one side, the pollution generated while working on the other side will flow across the working zone on this side, resulting in cross-contamination.

(2) The upper edge of the return air outlet opening shall not be more than 0.5m above the floor, and the lower edge of the opening shall be no less than 0.1m above the floor. The air velocity at indoor return air outlets shall not exceed 1.5m/s, and the return air outlets of Class I clean operating rooms should preferably be arranged continuously. The air velocity at corridor return air outlets shall not exceed 3.5m/s.

This measure is mainly to enable the clean, sterile supply airflow to cover the operating table and to avoid airflow short-circuiting. According to domestic research achievements, return air at the lower part of the side walls on both sides of the operating room gives a more ideal airflow than return air at four corners or four sides, with better results, and no eddy zone will appear in the middle of the supply airflow.

When the width of the operating department does not exceed 3m, single-side lower return air may be used; clean operating rooms wider than 3m should adopt return air at the lower part on both sides. The strip-shaped return air outlets on both sides of a Class I clean operating room should preferably be arranged continuously, which can make the sterile effect in the operating zone even better.

[Reference] The height of the return air outlet must be such that the curved airflow is below the working plane (0.7～0.8m); meanwhile, the return air outlets of a unidirectional-flow cleanroom must be arranged continuously in order to reduce the turbulent zone; furthermore, in order to reduce the noise caused by the fluttering of the outlet louvers, the return air velocity must be limited, and this value has been proven to be usable by extensive field measurements in projects. So as not to affect the hygienic corner and the skirting, and taking into account the flange edge width of the return air outlet, Therefore, the lower edge of the return air opening should not be too low; for this reason, it is required that the upper edge of the return air opening be no higher than 0.5m above the floor, and the lower edge be no less than 0.1m from the floor.

(3) All clean operating rooms should adopt self-circulating return air within the room itself. This is to conform with the requirement that each operating room be equipped with an independent unit as far as possible, so as to facilitate control; in addition, no excess pressure valve is installed to return air to the corridor, so as to reduce cross-contamination between operating rooms through the corridor. In short, the return air of each room is required to be handled through the room's own circulation.

[Reference] Although the excess pressure valve is simple to install and has relatively high control sensitivity, when it is fully closed and the indoor positive pressure is still below the preset value, it can no longer be controlled; after long-term use, some excess pressure valves fail to close tightly, resulting in serious leakage. Excess pressure valves are fairly widely used in industrial cleanrooms, but are hardly ever used in biological cleanrooms. To prevent cross-contamination, biological cleanrooms do not adopt corridor return air, and also avoid excessive air volume infiltrating into the corridor, which can easily cause pressure fluctuations in the corridor. Biological cleanrooms often contain odors, so organized exhaust is preferred over unorganized exhaust. Moreover, the excess pressure valve destroys the flat, smooth interior surface of the room, adding one more dust-accumulating surface; for an operating room where cross-contamination is strictly controlled, it virtually adds another passage to the outside. Therefore, no excess pressure valve is installed to exhaust air to the corridor. If an independent exhaust outlet fitted with an excess pressure valve is used, discharging the gas independently to the outdoors when the indoor positive pressure is high, this is permitted.

7.3.4 Requirements for the Layout of Indoor Exhaust Outlets

Clean operating rooms must be provided with an upper exhaust outlet, whose position should preferably be at the top on the patient's head side. The intake air velocity at the exhaust outlet should not be greater than 2m/s.

Since anesthesia equipment is often installed on the right side of the patient's head in the operating room, this provision can effectively remove harmful gases from the room. Personnel are concentrated in the operating room, and combined with the heat from high-intensity lighting and medical equipment, harmful gases accumulate at the top under the influence of heat; therefore, upper exhaust is the most effective. Although the exhaust velocity field dissipates fairly quickly, if the exhaust velocity at the outlet is further controlled, the interference of the exhaust with the supply air can be completely eliminated.

7.3.5 No Additional Air Purification Units Should Be Installed in Clean Operating Rooms

It has been emphasized that no additional air purification units should be installed in clean operating rooms, as also explained earlier. However, adding one into the system as a self-circulating unit is permissible. Generally, non-medical equipment should not be installed in clean operating rooms, because it also affects the airflow performance. Installing an ordinary air conditioner together with a self-purifier in the operating room is not allowed. Such an approach is purely aimed at achieving the cleanliness target rather than establishing a safeguard system, because the hidden danger of contamination on the coil surface and in the drain pan inside the air conditioner has not been eliminated. At present, there are many types of self-purifiers on the market, including air filtration self-purifiers, shielded ultraviolet self-purifiers, and so on. Although these self-purifying devices all have a certain bacteria-removal effect, they have obvious limitations and cannot replace the clean air-conditioning system.

7.3.6 Electrostatic Purification Devices Are Not Permitted

The Code stipulates that electrostatic purification devices must not be used as the terminal supply-air purification facility of the clean air-conditioning system, nor be installed directly in clean operating rooms and clean auxiliary rooms as independent units.

Electrostatic purification devices (or electrostatic precipitators) can be used in ordinary environments, but must never be used in cleanrooms as terminal devices, because their purification efficiency is only below 0.99, far inferior to that of HEPA filters; moreover, both power failure and electrical discharge can cause secondary dust generation, and their rated air volume is relatively small. As mentioned above, clean operating rooms are intended to establish a safeguard system; to ensure that nothing goes wrong, it is required that a HEPA filter or sub-HEPA filter be installed after the electrostatic precipitator.

[Reference] The performance of an electrostatic purification device often lies not in the initial test but in long-term operation. As long as an electrostatic precipitator has high-voltage corona point discharge, the generation of ozone and nitrogen oxides is inevitable. During operation, the amounts of dust, ozone, and nitrogen oxides generated are often much higher than in the initial test; especially when back-mixing occurs, dust and bacteria can enter the sterile area without any obstruction, which is absolutely unacceptable from the viewpoint of a safeguard system. Even if the electrostatic precipitator has a sterilizing effect, its sterilization efficiency is not 100%, and the bacterial corpses themselves are pyrogens. Even if some electrostatic precipitators are equipped with negative ion generators, the lifespan of negative ions is very short: they exist for at most 4～5min in clean air and less than 1min in polluted air, while the ... generated at the same time Ozone has a much longer lifespan, and long-term use will increase the indoor ozone concentration. It is evident that using an electrostatic precipitator in an operating room does not reduce the risk of infection, but rather adds a hidden hazard.

[Reference] Electrostatic purification devices were once used in operating rooms in the 1960s and 1970s. Owing to advances in filtration technology, especially the emergence of antibacterial filters, together with the various drawbacks of electrostatic purification devices exposed during use, they are no longer used.

If an electrostatic purification device is indeed still to be used today, a HEPA filter or sub-high-efficiency filter must also be installed at its air outlet as the supply air terminal, as a safeguard measure. At present, it is often used abroad as a purification facility for outdoor (fresh) air, which is more reasonable.