1.3.1 Protecting Key Areas and Local Purification

China's national standard, the Construction Standard for Hospital Clean Operating Departments, has been in effect since October 1, 2000. This Standard classifies clean rooms into 4 grades. Among them, clean operating rooms are graded according to the bacterial concentration and air cleanliness of the operating table area and the surrounding area, which was the first time this had been done internationally. Article 29 of the Standard stipulates: "Grade Ⅰ, Ⅱ and Ⅲ clean operating rooms shall adopt a local centralized air supply mode; the area of the centrally arranged air supply outlets, i.e., the size of the operating zone, shall be commensurate with the grade of the operating room—not less than $6.2mathrm{m}^2$ for Grade Ⅰ (of which the portion dedicated to the head shall be not less than $1.4mathrm{m}^2$), not less than $4.6mathrm{m}^2$ for Grade Ⅱ, and not less than $3.6mathrm{m}^2$ for Grade Ⅲ."

Such a stipulation is completely different from the fairly popular industrial cleanroom practice (e.g., in Japan): full-room unidirectional flow or turbulent flow mode.

The guiding idea behind this stipulation is to apply the mainstream zone theory to protect key areas, so as to achieve the goal of both improving the cleanliness performance and being more economical.

Protecting key locations and local purification were borrowed from German practice. What Germany promoted in hospital operating rooms was not full-room purification with uniform air supply; rather, it focused on keeping the airborne bacterial concentration in the key area (operating table and instrument table) within a defined range, and on providing a qualified background environment for the working area.

This technical route of German hospital clean operating rooms is inseparable from the two concepts of pollution degree $Umathrm{s}$ and relative pollution degree $Emathrm{x}$ proposed in 1977 by Professor Esdorn of the Technical University of Berlin [1] (for definitions, see Reference [2]).

The German standard DIN1946 Part 4 "Ventilation and Air Conditioning in Hospitals" adopted the above concepts and stipulated that $Umathrm{s}=1$ is permitted for the so-called general aseptic operating room, and $Umathrm{s}=2/3$ for the so-called highly aseptic operating room.

The introduction of the pollution degree concept gave an impetus to the development of what Germany calls aseptic operating rooms—clean operating rooms—along a mode different from the general practice of industrial cleanrooms at that time. Its main advantages are:

(1) Using bacterial concentration as the basis of judgment suits the habits of medical staff.

(2) It gave the basic condition for meeting the so-called general aseptic requirement—an air change rate of $20mathrm{h}^{-1}$ of clean air—considering that such an air change volume would satisfy the hygienic requirements of the so-called general aseptic operating room; that is to say, a clean operating room must be supplied with clean air, and the air change volume must be at least equivalent to $20mathrm{h}^{-1}$.

(3) It established the concept that the working area is the key area, and put forward the proposition of delivering the $20mathrm{h}^{-1}$ air change volume in a concentrated manner above the working area, so that the bacterial concentration achieved in the operating zone is only half of the indoor average concentration, i.e., the pollution degree drops to 0.5. This is precisely the difference between centrally arranged filters and the conventional dispersed arrangement of filters.

(4) It did not stop at the concept, but combined the concept with the device for its implementation, developed the operating room supply air ceiling unit, and transferred this development to the Weiss company; this is also the basic model of the supply air ceiling units for clean operating rooms that we see today in many European clean operating rooms (including those of Trox Technik and Nickel of Germany and Luwa of Switzerland).

However, the deficiencies exhibited by the pollution degree concept are:

(1) No absolute standard of bacterial concentration was set for clean operating rooms, and this is not reflected in DIN1946 Part 4 either. Although the basic air volume of $2,400mathrm{m}^3/mathrm{h}$ was derived through measurements and the like, no bacterial concentration standard attainable with this air volume was given, nor were the relevant conditions (such as area and number of people) specified. As a result, comparisons between such relative quantities are of little significance and of limited operability, and the adoption of $2,400mathrm{m}^3/mathrm{h}$ (or $20mathrm{h}^{-1}$ air changes) appears arbitrary.

(2) Esdorn's pollution degree concept is, in essence, an approach pursuing a minimum air volume; his scheme is applicable only to the so-called general aseptic operating rooms, which is obviously of great limitation, and the supply air volumes in the standards of other European countries, such as the United Kingdom and Switzerland, are much larger than that of Germany. This point was also made by Professor Schmidt, who had worked at the Weiss company; he held that the above minimum-air-volume idea was not correct, since the added investment is very small compared with the benefits it produces in avoiding infections and ensuring surgical success, as well as in reducing losses from possible legal proceedings. He instead proposed that the air volume for the so-called highly aseptic operating room must be greater than $7500mathrm{m}^3/mathrm{h}$.

(3) Neither the result of $U_mathrm{s}=0.5$ obtained by Professor Esdorn, nor the results on the supply air area and the protected zone area summarized by Professor Schmidt from a large number of measurements (see Table 1 of Reference [2]), reached the level predicted by theory; no basically feasible theoretical calculation method was provided, which also reduced the operability in practice. In fact, several of the ideas reflected in the concept of pollution degree and in DIN 1946 Part 4 are not unfamiliar to us.

This pollution degree approach, put forward by Germany in 1977, holds that a clean operating room need not undergo whole-room purification; it is only necessary to reduce the bacterial concentration locally and to effectively protect the critical area, and to concentrate the clean air corresponding to an air change rate of $20,mathrm{h}^{-1}$ and deliver it above the operating table. The essence of this approach is precisely the combination of the concepts of "local purification" and the "mainstream zone".

China's Technical Measures for Air Cleanliness^[3], which was adopted by the national appraisal and review meeting in July 1977 and officially published in 1979, was the first to put forward the concepts of "overall purification" and "local purification":

"Whenever, through air cleaning and other comprehensive treatment measures, the entire working area within a room is made into a clean air environment, this practice is called overall purification." "Whenever only a local working area of the room—a specific local space—is made into a clean air environment, this practice is called local purification." "Where the process requirements are satisfied, local purification should be adopted as far as possible."

Of course, such local purification also brings a purifying effect to the whole area.

1.3.2 The Mainstream Zone Concept and Calculation of Non-uniformly Distributed Concentration

The mainstream zone is a concept first proposed in China in 1978^[4]: in a turbulent-flow cleanroom, the region within a certain range below the supply air outlet is the mainstream zone, which has the highest cleanliness; near the return air outlet there is a very small return air outlet zone, whose cleanliness can be either higher or lower than the room average concentration, depending on whether the non-uniform distribution coefficient $Psi$ is greater or less than 1, but it equals the room average concentration under theoretical uniform distribution ($Psi_{mathrm{c}}=1$); in addition, the other regions on both sides of the supply outlet are turbulent regions containing eddies, customarily abbreviated as the eddy zone (which does not mean that the entire region consists of eddies), and their cleanliness is markedly lower than that of the central mainstream zone. The non-uniform distribution of the three zones is shown in Figure 1.

Figure 1　Non-uniform distribution of the three zones

According to the mainstream zone concept, for a clean space with a HEPA filter at its terminal, since the supply air concentration is extremely small, we have: the room average concentration under non-uniform distribution

$$overline{N}{mathrm{v}} = N{mathrm{s}} + Psi,frac{60G times 10^{-3}}{n} approx Psioverline{N}$$

Mainstream zone concentration　　　$N{mathrm{a}} = Psi{mathrm{a}}overline{N}$

Eddy zone concentration　　　$N{mathrm{b}} = Psi{mathrm{b}}overline{N}$

Return air outlet zone concentration　　$N{mathrm{c}} = Psi{mathrm{c}}overline{N}$.

where: $Psi$——non-uniform distribution coefficient, $Psi = left(dfrac{1}{varphi} - dfrac{beta}{varphi} + dfrac{beta}{1+varphi}right)left(varphi + dfrac{V_{mathrm{b}}}{V}right)$;

$Psi{mathrm{a}}$——mainstream zone non-uniform distribution coefficient, $Psi{mathrm{a}} = 1 - dfrac{beta}{1+varphi}$;

$Psi{mathrm{b}}$——eddy zone non-uniform distribution coefficient, $Psi{mathrm{b}} = dfrac{1+varphi-beta}{varphi}$;

$Psi{mathrm{c}}$——return air outlet zone non-uniform distribution coefficient, $Psi{mathrm{c}} = 1$;

$n$——air change rate;

$overline{N}$——room average concentration under uniform distribution, $N = N_{mathrm{s}} + dfrac{60G times 10^{-3}}{n}$;

$N_{mathrm{s}}$——supply air concentration;

$G$——particle generation rate per unit volume;

$beta$——ratio of the particle generation rate in the mainstream zone to that in the whole room;

$varphi$——entrainment coefficient of the mainstream zone;

$dfrac{V_{mathrm{b}}}{V}$——ratio of the eddy zone volume to the whole room volume.

I.3.3 Mainstream Zone Pollution Degree

$Psi_{mathrm{c}}=1$ indicates that the concentration in the return air outlet zone is the overall average concentration under uniform distribution. Past theoretical research results have already pointed out that the working zone should be placed under the mainstream zone. In order to assess the degree of superiority of centrally arranging the air supply outlets and placing the working zone under the mainstream zone compared with the conventional dispersed arrangement, the following considerations exist.

(1) This degree is expressed by the ratio of the mainstream zone concentration to the room average concentration, i.e., $Psi_{mathrm{a}}/Psi$. Since the room average concentration includes the mainstream zone concentration, the superiority of the mainstream zone may not be reflected in this ratio. Meanwhile, when the mainstream zone scheme with centrally arranged air supply outlets is adopted, agreement on the room average concentration is prone to divergence, because how many measurement points should be located in the mainstream zone is related to their respective weighting coefficients and is difficult to determine, so the operability is not strong.

(2) This degree is expressed by the ratio of the mainstream zone concentration to the return air outlet concentration or the room average concentration under uniform distribution, i.e., $Psi{mathrm{a}}/Psi{mathrm{c}}$, that is, expressed by $Psi_{mathrm{a}}$. Although it is operable, i.e., the concentration in the return air outlet zone (which is equivalent to the return air concentration) can be measured, since working points cannot be placed in the return air outlet zone, their ratio is not the most concerning. The aforementioned German pollution degree concept is just such a ratio; it cannot answer a question of concern after the air supply outlets are centrally arranged: how about the peripheral zone without air supply outlets? In addition, although the return air outlet concentration can be measured, obviously the concentrations of return air outlets at different locations differ considerably, which requires measuring all of them so as not to cause too much distortion.

(3) This degree is expressed by the ratio of the mainstream zone concentration to the eddy zone, i.e., the peripheral zone concentration, i.e., $Psi{mathrm{a}}/Psi{mathrm{b}}$, which can explain the problem relatively clearly. As mentioned earlier, the eddy zone refers to the turbulent zone containing eddies on both sides of the mainstream zone; its concentration represents the concentration of other main areas in the room apart from the mainstream zone and the return air outlets, that is, the concentration of the areas surrounding the mainstream zone. Therefore, $Psi{mathrm{a}}/Psi{mathrm{b}}$ can reflect the concentration difference between the working zone and the non-working zone. Meanwhile, the concentrations of these two zones can be measured separately by arranging sampling points according to zones.

In summary, the pollution degree of the mainstream zone relative to the peripheral zone is called the mainstream zone pollution degree, expressed as $Psi{mathrm{a}}/Psi{mathrm{b}}$, which is clearly distinguished from the German pollution degree, i.e., $Psi{mathrm{a}}/Psi{mathrm{c}}$.

I.3.4 Grading Calculation

(1) Plan of the operating room. The interior plan of the operating room can be divided into several zones by function, drawing on the division in the relevant military standard[5], as shown in Figure 2. In Figure 2, zone "1" is considered based on an operating table length of 1.8m, which can be opened at both ends to 2.1m for exceptionally tall patients. The spacing between zones is basically 0.6m.

Figure 2 Functional zone division (turbulent flow and vertical unidirectional flow)

(2) Number of surgical personnel. The number of surgical personnel is related to the personnel allocation scheme in the operating room. During surgery, medical staff gather densely around the operating table; the surgeons stay close to the operating table, and they are the chief surgeon, the first assistant, the second assistant, and the head nurse (or scrub nurse), with an anesthesiologist near the head. Meanwhile, the more modern the medical equipment, the fewer personnel are needed, because in principle one instrument requires one person to attend to it; when instruments become modern and multifunctional, the number of units decreases, and the auxiliary personnel also decrease. Auxiliary personnel generally number between 8～12; after using modern equipment, generally no more than 10. The planar layout of surgical instruments is shown in Figures 3～6[6].

(3) Air supply area. Since the air supply is concentrated on the working zone, the size of the air supply area is very important. If the air supply area is too large, the required air volume will be too large, or the air velocity will be too low under a given air volume, and the cost is high; if it is too small, the protection of the working zone will be affected.

Now three cases are distinguished, namely the minimum, intermediate, and maximum standards.

The minimum standard is to enclose the working zone containing only the operating table within the mainstream zone; the intermediate standard includes the area 0.6m around the operating table; the maximum standard may include the area 0.9m beyond the sides of the operating table (the actual figure in the provisions of the "Standard" is slightly smaller, therefore the calcu the calculation results can be more reliably guaranteed).

Figure 3　Plan layout example 1 for cardiac surgery instruments

  1. Heart-lung machine　2. Instrument table　3. Medicine cabinet　4. Electrosurgical unit　5. Defibrillator　6. Electric sternum saw　7. Work table　8. Suction and oxygen supply　9. Anesthesia table　10. Anesthesia machine　11. ECG machine　12. Heat exchanger　13. Basins and buckets

Figure 4　Plan layout example 2 for cardiac surgery instruments

  1. Pendant tower for drawing off 5 types of gases　2. Extracorporeal blood circulation machine　3. Infusion hanging rail　4. Indicator　5. Monitor　6. Instrument table　7. Basins and buckets

Figure 5　Plan layout example 3 for cardiac surgery instruments

  1. Anesthesia machine　2. ECG machine　3. Anesthesia table　4. Instrument table　5. Instrument tray　6. Microscope　7. Basins and buckets　8. Infusion stand　9. Electric drill　10. Suction and oxygen supply

Figure 6　Plan layout example 4 for joint surgery instruments

  1. Anesthesia machine　2. Anesthesia table　3. Instrument table　4. Instrument tray　5. Electrocautery device　6. Pneumatic drill　7. Gas and infusion hanging rail　8. Basins and buckets

Here, the mixing and weakening phenomenon at the boundary of the weak jet airflow should be considered (see Figure 14-3 in Reference [7]); the boundary may shrink by an angle of 10.7° (the region including the 10.7° angle still belongs to the main flow zone).

According to the type of shadowless lamp, the net indoor height is 2.9～3m, the operating table is 0.8m high, the incision height on the human body is 0.3m, and the working plane is 1.1m high; the shrinkage distance on each side is then [(2.9～3)−1.1]tan10.7°=0.34～0.36m.

(4) Bacterial generation rate. The bacterial generation rate is not calculated from personnel density; it must be calculated directly. There are no direct data yet on the bacterial generation of indoor surfaces, so the surface dust generation rate can be used as a reference, i.e., it is assumed that the bacterial generation of the surfaces represented by 8m² of floor is equivalent to that of one person at rest. The maximum bacterial generation of a person at rest can be taken as 300 bacteria/(person·min); taking the measuring personnel as 2 persons, then

$$text{Total bacterial generation}=left(frac{text{room area}}{8text{m}^2}+2right)timesfrac{300}{text{min}}text{bacteria}$$

(5) Coefficients. $varphi$ is taken, according to the full-coverage ratio of the supply air face, as 0.4, 1.3, 0.26, 0.2 in turn; $beta$ is taken, according to the full-coverage ratio of the supply air face, as 0.7, 0.65, 0.8, 0.85 in turn; $dfrac{V_text{b}}{V}$ is taken, according to the full-coverage ratio of the supply air face, as 0.45, 0.65, 0.32, 0.24 in turn.

The calculation results are listed in Table 1.

If the full-coverage ratio of the supply air outlet is very large, with $beta$ reaching 0.95, and return air outlets fully arranged on both sides as standard, $varphi=0.05$, then $varphitext{a}=0.095$, and it can be calculated that $Psitext{a}/Psitext{b}approx0.048$. Therefore $Psitext{b}=1.98$, from which it can be seen that the concentration in the peripheral zone is about 20 times that in the main flow zone; if the main flow zone falls within the Class 100 category, then the peripheral zone is within the Class 1 000 range.

I.3.5　Conclusion

Since the operating area is the location where dust generation is concentrated in the operating room, according to the main flow zone theory, if this area is placed within the main flow zone, then This is greatly beneficial to improving the cleanliness of the main flow zone and the entire room. Calculation results show that if the uniformly distributed air supply outlets in an industrial cleanroom (per the Class 1 000 air change rate standard) are changed to a concentrated arrangement, the operating area can achieve Class 100 results (with not only high cleanliness but also a good airflow pattern), while the peripheral zone remains at Class 1 000. By analogy, the class of all key areas can be raised by one level[8]. It should be noted that, in order to shorten the operation time, the concept of self-purification air change rate was adopted[9]; the air change rates of the above-mentioned Class 1 000, 10 000 and 100 000 operating rooms are slightly higher than the conventional values, while for non-operating-room rooms they are lower than the conventional values.

Table 1　Calculation results related to the main flow zone

Operating room scale Supply air area/m² Medical staff Air change rate/h⁻¹ Air volume/(m³/h) Supply air velocity/(m/s) Room non-uniformity coefficient Working zone non-uniformity coefficient $N_a$/(count/m³) $overline{N}$/(count/m³) $N_a$/(particles/L) $overline{N}$/(particles/L) German pollution degree $Psi_a/Psi_c$ Main flow zone pollution degree $Psi_a/Psi_b$
General clean operating room (30.8m², h=3m) 1.3×2.15=2.8 As-built measurement (2 persons) 10 persons at rest 20 1 848 0.18 1.08 0.48 28.0 63.1 13.9 31.3 0.76 0.57
General clean operating room (30.8m², h=3m) 1.3×2.15=2.8 As-built measurement (2 persons) 10 persons at rest 20 1 848 0.18 1.08 0.48 46.7 105.2 32.6 73.9 0.76 0.57
General clean operating room (18.9m², h=3m) 1×1=1 As-built measurement (2 persons) 6 persons at rest 15 851 0.24 1.1 0.76 72.0 104.2 36.5 52.8 0.48 0.29
General clean operating room (18.9m², h=3m) 1×1=1 As-built measurement (2 persons) 6 persons at rest 15 851 0.24 1.1 0.76 96.5 139.7 54.7 79.2 0.48 0.29
Clean operating room (30.8m², h=3m) 1.8×3=5.4 As-built measurement (2 persons) 11 persons at rest 30 2 772 0.14 0.84 0.37 14.4 32.7 7.0 16.0 0.37 0.21
Clean operating room (30.8m², h=3m) 1.8×3=5.4 As-built measurement (2 persons) 11 persons at rest 30 2 772 0.14 0.84 0.37 26.4 60.0 14.9 33.8 0.37 0.21
Special clean operating room (37.8m², h=3m) 2.4×3=7.2 As-built measurement (2 persons) 12 persons at rest 110 12 441 0.48 0.61 0.27 2.6 6.0 1.3 3.0 0.27 0.17
Special clean operating room (37.8m², h=3m) 2.4×3=7.2 As-built measurement (2 persons) 12 persons at rest 110 12 441 0.48 0.61 0.27 4.6 10.6 3.4 7.8 0.27 0.17

Note: $N_a$——average bacterial concentration in the working zone; $overline{N}$——average bacterial concentration in the room; $N_a$——average dust concentration in the working zone; $overline{N}$——average dust concentration in the room.

I.3.6　References

[1] Esdorn H, Nouri Z. Vergleichsuntersuchungen über Luft füh rungssysteme mit Mischströmung in Operationsräumen. In Operationsraum HLH28(1977)Nr 4.

[2] Shen Jinming. German hospital standards and operating room design. HV&AC, 2000,30(2):33.

[3] Air Cleanliness Technical Measures Compilation Group. Air Cleanliness Technical Measures. Beijing: China Architecture & Building Press, 1979.

[4] Xu Zhonglin. Calculation method for non-uniform distribution in cleanrooms. Proceedings of the Founding Conference of the HVAC Professional Committee of the Architectural Society. 1978.

[5] YFB 001-1995　Architectural technical code for clean operating departments of military hospitals.

[6] Mei Zili (ed.). Air Conditioning Design for Medical Buildings. Beijing: China Architecture & Building Press, 1991.

[7] Xu Zhonglin. Principles of Air Cleanliness Technology. Shanghai: Tongji University Press, 1998.

[8] Xu Zhonglin, Zhang Yizhao, Zhang Yanguo, et al. Study on the mechanism and characteristics of new air distribution modes in clean spaces. HV&AC, 2000,30(3):1-7.

[9] Xu Zhonglin (ed.). Cleanroom Design. Beijing: Seismological Press, 1994.

Ⅱ International