4.2 Air quality and cleanliness

4.2.1 Room classes

In hospitals, for microbiological reasons, different requirements are imposed on the sterility of the supply air and the indoor air. The rooms in different areas of a hospital can be divided into the following two classes:

1) Room class Ⅰ: high or very high sterility is required. 2) Room class Ⅱ: ordinary sterility is required.

4.2.2 Air cleanliness

Multi-stage air filtration is required in the supply air process to remove all types of particulate contaminants, including microorganisms. Two-stage filtration is used for class Ⅰ rooms; in accordance with high or very high sterility requirements, three-stage filtration is used in class Ⅰ rooms.

The air filters used at each stage of filtration shall attain the following classes (see DIN24 184 and DIN24 185 Part 2):

1) First-stage filtration: EU4 class, or air filters of class EU4 or better 2) Second-stage filtration: EU7 class, or air filters of class EU7 or better 3) Third-stage filtration: class S; air filters of class R may also be used, but such filters must have a test certificate proving that they have met the oil mist method specified in DIN24 184, with a penetration rate of 0.1%¹).

To meet hygienic requirements, the air filters shall be installed as follows:

1) First-stage filtration is located immediately adjacent to the intake opening of the outdoor fresh air duct (to keep the air-handling components clean); if the outdoor fresh air intake is located outside the building, or the outdoor air passes through a tortuous duct, the first-stage filtration may be installed on the outside of the duct where it enters the building. 2) Second-stage filtration is located in the positive-pressure section at the very beginning of the supply air duct (to keep the ductwork clean). 3) Third-stage filtration is located in the positive-pressure section of the ductwork, as close as possible to the rooms or areas being supplied with air; in operating rooms it is best placed at the terminal position of the air duct.

1) The penetration rate must be limited to within 0.1% so as to meet the penetration requirements for the filter media in the installed state and to comply with the particle count.

4.2.3 Outdoor Air and Supply Air Volume

Hospital HVAC systems shall operate at the specified minimum outdoor air volume. However, in order to further reduce the content of airborne microorganisms and/or to maintain thermal balance, the supply air volume must be higher than the specified minimum outdoor air volume¹⁾; for the supply air volume of operating rooms, please refer to Section 5.6.2¹⁾.

If the use of recirculated air cannot compensate for the difference between the specified supply air volume and the minimum outdoor air volume, the outdoor air volume shall also be increased accordingly.

4.2.4 Recirculated Air

Recirculated air may be used under the following conditions:

(1) Only return air from one and the same room, or from rooms belonging to the same zone²⁾ (e.g., an operating room unit), may be recirculated.

(2) Recirculated air must, either alone or after being mixed with outdoor air, be filtered through filters equipped with the same number of filtration stages and the same filter classes as specified above for outdoor air filtration.

From a hygienic-toxicological point of view, when the return air contains harmful gases, return air must not be used in any form (see Section 5.6.3).

4.2.5 Inter-room Airflow

In accordance with the specified hygienic requirements, positive-pressure airflow is permitted only from areas with higher aseptic requirements to rooms with lower aseptic requirements. The rooms of an operating room unit must conform to the airflow pattern specified above. The airflow patterns between other areas of the hospital shall be specified by analogy.

The installed air handling system should be designed so that, by appropriately adjusting the air volumes between supply air and return air, the positive-pressure airflow is maintained in the specified direction. Consequently, relative to the surrounding environment, the protected area requires more air to be supplied mechanically and then discharged as exhaust air. The difference in air volume can flow into the surrounding areas through predetermined non-airtight passages (e.g., gaps around doors).

If, due to the functional requirements of the room, the tight closures provided (e.g., doors, gates, flap doors) permit only a small amount of non-airtight passage, the HVAC system may only be operated for a short time. In accordance with hospital management requirements, corresponding airlocks should be installed at places where doors and gates are frequently opened and closed.

Airlocks are usually installed at places where zones are separated:

(1) Between Class Ⅰ rooms and Class Ⅱ rooms.

(2) Between the air of Class Ⅰ rooms and the connected outdoor air.

(3) At the request of the hygienist, between rooms of a specified class and rooms of the same class, such as between operating rooms and ICU wards.

If appropriate measures are taken (e.g., installing automatically interlocked doors) to prevent the entrance and exit doors from being opened at the same time, these airlocks can fulfill their intended function; otherwise, it is impossible to achieve this purpose by means of the HVAC system. To minimize the amount of particles dispersed when doors are opened, it is recommended that sliding doors be installed preferably.

In consideration of the relationship between the air change rate and the standard pressure differential required by the hygienic requirements, the installation of effective isolation is particularly important when there is more than one passage between the protected area and other areas of the building, or when the protected interior area is equipped with shaft passages or openings (e.g., the doors of ventilation shafts, elevator doors). The safest way is to completely eliminate the adverse effects mentioned above right from the beginning of the building design.

1) When there are many persons in the room under examination and it is necessary to increase the outdoor air supply volume, the calculation shall be carried out in accordance with the requirements of DIN 1946 Part 2.

2) The hygienist decides which rooms belong to the same zone.

4.3 Noise Level

Structural measures shall be adopted as the primary means to reduce the noise generated by the air handling system, so that the noise transmitted—through the air or through the building structure—into rooms supplied with air by the air handling system is reduced to a level that does not exceed the specified maximum noise level indicators for that system.

5 Technical Hygiene Requirements

5.1 Outdoor Fresh Air Intakes and Exhaust Air Outlets

The fresh air source must meet strict cleanliness requirements. The outdoor fresh air intake shall not be located lower than 3m above ground level, because outdoor air close to the ground is easily contaminated by high concentrations of microorganisms and dust. At the same time, the required clearances must be taken into account to ensure that a sufficiently large distance is maintained between the intake and flat roofs or other horizontal surfaces. Owing to different meteorological conditions or different building structures, it is not possible to specify a general height for fresh air intakes.

When determining the location of the outdoor air intake, full consideration shall be given to existing or potentially future emissions from exhaust air, smoke, odorous gases, or other sources of interference. Where necessary, the opinions of relevant experts shall be sought.

The air intake shall not be handed over immediately to personnel without professional accreditation for management.

The vast majority of exhaust air is discharged into the atmosphere from the roof. The height, position, and structure of the exhaust outlet shall be determined such that, even when the exhaust air volume is reduced or natural wind affects the exhaust air, the exhaust will not become a burden or hazard to the surrounding environment or to the building itself. Where necessary, experts shall be consulted on such issues.

5.2 Ductwork System

5.2.1 General Requirements

The inner walls of ducts shall be even and smooth, for example by using galvanized steel sheet or materials of equivalent smoothness. Duct lengths shall be as short as possible. Flexible hoses shall only be used for connection to supply air outlets, and their length shall be permitted up to a maximum of 2m.

Ducts, shaped fittings, and joints shall be fabricated in accordance with aerodynamic principles; local negative pressure in the supply air ducts (in positive-pressure duct sections) caused by vortex flow—which would lead to deposition of dust particles inside the duct or infiltration of outdoor dust particles—shall be avoided as far as possible.

Downstream of the third filtration stage, the inner walls of all supply air ducts shall be easy to clean or to wipe with disinfectant solution; in the supply air ductwork downstream of the third filtration stage, flexible hoses, silencers, and dampers shall not be installed. Downstream of the third filtration stage, spiral-wound ducts are permitted; if lubricants are used in the manufacture of spiral-wound ducts, they must leave no volatile residues.

According to hygienic standards, for rooms within a building between which air migration is not permitted and which differ in class (e.g., rooms with different surgical functions), airtight dampers must be installed to separate the supply and exhaust air ductwork. On the supply air side with three-stage filtration, this separation must be located in the duct section immediately upstream of the third-stage filter. Components that do not belong to the air handling system shall not be installed in the ducts. For components whose installation within the air handling system is unavoidable (e.g., power supply cables for lighting inside ducts, heat-transfer medium pipes for air heating, or steam pipes for steam humidification), the installation must comply with the above requirements concerning the inner walls of ducts.

Components on the ducts, such as shut-off dampers and control dampers, must have inspection openings upstream of them to permit visual inspection, and their positions must be very clearly marked.

Building spaces such as installation shafts, cavities between double walls, or suspended ceilings shall not be used directly as supply air or return air plenums.

When assembling ductwork, care must be taken not to soil the inner walls; appropriate measures shall be taken to ensure that the inside of the ducts is kept clean. After duct assembly each day, the ducts shall be sealed to ensure that the required clean state is achieved inside the ducts; in this way, it is avoided that, before use, the second-stage filter (in the direction of flow), the rooms, and the ductwork between the second- and third-stage filters have to be cleaned again.

5.2.2 Outdoor Fresh Air Intake Ductwork

The fresh air intake ductwork must comply with Class I airtightness as specified in DIN24 194 Part 2, so as to avoid drawing in poor-quality air from within the building, thereby preventing cross-infection occurring within the hospital and the resulting risks. For this reason, where it is unavoidable that the intake is located far from the rooms being served, consideration must be given to whether to use a short intake duct with a long positive-pressure duct, or a long intake duct with a short positive-pressure duct; the former option should be given priority. The duct section between the intake and the central air handling room (cabinet) or unit should have access passages, or at least passages through which a person can crawl, or be provided with a sufficient number of cleaning openings, so that the inner walls can be mechanically cleaned and disinfected.

Even if the maximum wall roughness $R_{max}$ of the intake duct is 0.3mm, the intake duct must still meet the general requirement of a smooth inner wall.

The duct walls must be able to withstand mechanical loads and be wear-resistant.

Floor drains should generally be avoided; if unavoidable, the drain must not be directly connected to the public sewage system.

5.2.3 Supply Air Ductwork

In general, the ductwork should be designed to be as short as possible, especially within Class I room areas; the central air handling room or equipment should be located as close as possible to the rooms or areas requiring supply air.

In Class I rooms, the pressurized supply air ductwork must meet the Class III airtightness requirements specified in DIN V24 194 Part 2. Air leakage from the supply ductwork will not cause excessive pressure to build up in the building space. Experience has shown that it is relatively safe when the air extracted from the building space via the connected exhaust equipment is three times the duct leakage rate.

Upstream of each third-stage filter, an accessible test nozzle that a person can reach should be installed, so that test smoke can be introduced when necessary.

Where the third-stage filter is not installed at the final terminal, it is preferable to provide, outside the supplied room, a test pipe fitted with an 80mm diameter blanking cap, in order to monitor the degree of bacterial contamination of the air in the duct section between the third-stage filter and the room.

5.2.4 Air Outlet, Return Air and Exhaust Ductwork

To prevent air leakage from the ductwork inside the building, the positive-pressure duct sections should be as short as possible and meet the Class I airtightness requirements specified in DIN V24.

Exhaust air from the isotope treatment room must be discharged outdoors through an independently installed ductwork system and does not require filtration. Only under the German radiation protection regulations is filtration of the exhaust air required.

5.3 Airtight Dampers

Even when the air handling system is shut down, the duct network must not allow air movement within the ducts caused by wind or pressure differentials, which would degrade the air quality in the building. For this reason, it is necessary to install motorized airtight dampers in the ductwork, so that airtight closure is ensured not only when the system is shut down but also when the actuators of the standby power system fail.

If, at a given pressure differential of 100Pa and a damper flow cross-section of 1m², the air leakage does not exceed 10m³/h, this indicates that the airtightness requirements specified by the standard have been met.

To meet the above requirements, airtight dampers must be installed at least at the following locations in the supply air ductwork (this prevents supply air from bypassing the three-stage filtration and causing unhygienic air to flow back) and in the exhaust air ductwork:

  1. Where the system supplies air to areas of different classes, the dampers are located at the separation interfaces between areas of different classes.
  2. Where the system serves multiple floors, if isolation cannot be guaranteed due to prolonged system shutdown, the dampers are installed on each floor on the branch ducts above¹);

3）In areas of the same class, airtight dampers shall be installed at the boundaries of the areas where, in the view of hygiene experts, the air between rooms must be separated when the system is shut down²）.

4）In the supply air ducts and return air ductwork of systems serving areas with different hygiene requirements, airtight dampers shall be installed respectively at the grilles of the outdoor fresh air intakes, at the exhaust air grilles, and between the rooms connected to them.

Airtight dampers shall be installed upstream of the third-stage filtration or upstream of a group of third-stage filters connected in parallel, so that the filters can be maintained even while the system is in operation. When the airtight damper is located upstream of the final filter, it shall also be possible to carry out filter maintenance individually.

Fire dampers that have undergone type testing may be used as shutoff dampers after approval by the building supervisory authority.

Ordinary multi-blade dampers may be installed upstream of the first-stage filtration and downstream of the return air fan.

5.4　Smoke Exhaust Ducts and Fire Dampers

The design of the smoke exhaust ductwork shall ensure that it does not cause air movements inconsistent with hygiene standards. Fire dampers shall not be installed downstream of the third-stage filters. Fire dampers in the supply air ductwork of Class Ⅰ rooms shall be interlocked with the exhaust fan of the room, i.e., when the fire damper closes, the exhaust air is shut off at the same time, thereby preventing bacteria from intruding from adjacent areas.

5.5　Components of the Air Handling System

5.5.1　Installation of Components

Fan rooms, central control rooms, ventilation plants, filter rooms, and the places where components are installed must be convenient for maintenance and servicing, and the required materials must be able to be brought in very easily without entering Class Ⅰ rooms. As an exception, with the consent of the hygiene expert, special air conveyance systems with recirculating fan units equipped with cooling units are permitted to be installed. In this regard, particular attention must be paid in such cases to the required maintenance work, so that inspection is possible and the final third-stage filter can be replaced.

For central air handling units and ventilation plants, provision must be made for the air handling system and the rooms in which they are located to be inspected, serviced, and cleaned, so as to increase operational reliability and meet the stringent hygiene requirements.

5.5.2　General Requirements

It shall be ensured that the components, ventilation plants, and central air handling rooms (units) are designed to meet the requirements applicable to air ducts, and that they can be operated and serviced with the least possible effort, including the replacement of filters. Finally, the interior walls of the central control station and the ventilation plant shall be smooth so that they can be cleaned. Components of the air handling system can be serviced through the supply and exhaust ducts upstream and downstream, or it shall be possible to remove them conveniently and safely; this shall be taken into account when making the duct connections.

Central air handling units and ventilation plants must be open to inspection; at the very least, fans, filters, humidifiers, and dehumidifiers shall be fitted with inspection ports and interior lighting so that they can be observed.

The casing of the central air handling unit and the housing of the ventilation plant must comply with the requirements of DIN V24 194 Part 2 for tightness class Ⅱ, taking into account possible interference sources (duct connections, cable penetrations) etc. at the installation site. Indoor air handling units must meet the same drainage requirements as outdoor fresh air ducts.

5.5.3　Air Filters

The filtration efficiency required for the filter stages in Section 4.4.2 shall be maintained under all operating conditions. In particular, under the influence of high humidity, the tightness and operating condition of the filters shall also meet the above requirements. The filter media of the first- and second-stage filters shall not deliquesce or suffer substantial damage under the influence of high humidity, and their pressure drop shall not be affected in any substantial way.

1）During long winters, especially in cold weather, unwanted pressure differentials can cause air exchange in vertically interconnected shafts (such as stairwells). Therefore, when a system serves multiple floors, the installation of airtight dampers for the different floors is particularly important.

2）Areas belonging to the same room class which, from the hygienic point of view, require that air separation be ensured when the system is shut down, for example, surgical units with different functions within the operating room and the clean and non-clean areas of central sterilization. Only high-efficiency filters that have been tested by the type-testing method specified in DIN24 184 and that achieve the required filtration class may be installed. The filter medium used for third-stage filtration must be "hydrophobic"; to this end, the filter medium of an unused filter shall be tested according to the method specified in USA-MIL-STD282, and water must not penetrate at a test differential pressure of 2000Pa. For high-efficiency filter units installed in the filter frame, the seal between the filter and the frame shall be verifiable by means of a leak detector while maintaining a differential pressure above 2000Pa; the tightness shall be checked by measuring the leakage flow rate, which must not exceed 0.003% of the rated flow of that filter unit.

It must be avoided that the air temperature on the supply-air side of the high-efficiency filter falls below the apparatus dew point temperature, because around the dew point temperature conditions are most favorable for the growth of bacteria and fungi. The humidity of the airflow passing through the filter medium must also not exceed 95%; however, to avoid an unnecessary increase in filter resistance, the relative humidity should generally not be higher than 90% (see Section 5.5.5). To monitor the operating condition of the filters, a differential pressure gauge shall be provided for each filter bank; an excessively high differential pressure indicates that the filter has become clogged with dust. Where possible, it is most appropriate for the third filtration stage to install measuring instruments that can be shut off.

In order to ensure an accurate assessment of the filter condition, after completion of the system the following filter characteristic parameters shall be clearly marked on each filter: filter class, type of filter medium, rated flow rate, corresponding initial pressure drop and permissible final pressure drop. After each filter change, the operator shall record the date of the filter replacement in place.

5.5.4 Fans

The supply fan shall be installed between the first- and second-stage filters, and condensate must be prevented from entering the fan.

5.5.5 Air Humidifiers

The humidifier shall be installed upstream of the second-stage filter.

The humidifier shall be easy to inspect and observe, and shall have the length of humidification section specified for the respective humidification technology.

The humidifier shall be designed such that no water droplets are present in the supply air after humidification, and the air humidity at the end of the humidification section does not exceed 90%.

It must be ensured that during humidification no condensate occurs in the supply air system downstream of the humidifier, and that no condensate is produced even in the event of system shutdown, unexpected malfunction, or a significant reduction of the supply air flow.

The drain piping shall comply with the requirements of Section 5.5.6.

In the case of steam humidification, the steam shall not contain any substances harmful to health.

When spray humidification with recirculated water is used, the quality of the supply air must not be impaired by chemical substances in the spray water. In view of microbiological aspects, the water used for humidification must be of at least potable water quality. The water shall be treated, for example by ultraviolet disinfection, to prevent the growth of bacteria. If the water is chemically treated, it must be ensured that the supply air is indeed non-toxic (both acute and chronic). Hygienists shall be involved in the selection of the humidifier type.

The humidifier units shall have the performance required for their use, and shall be corrosion-resistant, cleanable and disinfectable.

5.5.6 Dehumidification Cooling Coils

The air dehumidification cooling coils shall be installed upstream of the second-stage filter.

The construction of the air cooling coils shall ensure that condensate is completely drained away.

Each dehumidification cooling coil shall be equipped with a condensate drain pan of sufficiently large size to allow the condensate to drain away smoothly. All wetted parts shall be cleanable and disinfectable.

Appropriate measures shall be taken to ensure that, both when the system is shut down and in all operating states, no solid, liquid or gaseous contaminants enter the supply airflow from the condensate; for example, a float valve shall be connected in series with the condensate drain pan, and the condensate drain and drain piping must not be directly connected to the public sewage system.

5.5.7 Droplet Eliminators

Droplet eliminators shall be installed upstream of the second-stage filter, and appropriate measures shall be taken to ensure that water droplets cannot be carried from the humidifier or the cooling coils into the downstream components of the system. They shall be corrosion-resistant, easy to clean and disinfectable.

5.5.8 Heat Recovery Units

(1) Overview.

From a hygienic point of view, there are two different approaches to heat recovery units in air handling systems.

1) Units in which particles and gases cannot leak from the exhaust air into the supply air (e.g., closed-circuit heat exchangers).

2) Units with or without partition walls, in which particulate matter and gases may leak from the exhaust air into the supply air due to air leakage.

Heat recovery units shall be installed between the first and second filtration stages as specified in Section 4.2.2.

(2) Units with no possibility of leakage.

Units which, due to their construction, do not allow particles (bacteria) or gases to leak from the exhaust air into the supply air may be installed directly without hygienic testing.

(3) Units with a possibility of leakage.

In units where particles (bacteria) and gases may leak from the exhaust air into the supply air, leakage may occur when the partition wall is damaged, since their heat exchange surfaces are continuously and alternately exposed to contact between contaminated exhaust air and intake outdoor fresh air.

For this type of unit, under the specified operating conditions, when outdoor air enters the system, the penetration of particles (bacteria) from the exhaust air into the outdoor fresh air shall not exceed $1:10^3$. The penetration concentration can be determined with the aid of a suitable gas (e.g., nitric oxide). The hygienic safety of such units shall be assessed in writing by two experts; one of the written assessments must be based on actual measurements, and the other expert must be a hygienist.

When the concentration in the affected area does not exceed the permissible value, recirculation of gases in the air is permitted.

Exhaust air from animal housing areas and from rooms with heavy odor loads must not be reused.

5.5.9 Silencers

The surface materials in silencers facing the airflow shall be wear-resistant, waterproof and corrosion-resistant, and shall be effectively protected against mechanical damage, e.g., by means of perforated plates or sheet metal and wire mesh.

Outdoor air silencers shall be installed downstream of the first filtration stage and upstream of the fan. Supply air silencers shall be installed upstream of the second filtration stage and, where necessary, may also be installed upstream of the third filtration stage.

5.5.10 Air Outlets

Supply air outlets must be cleanable and disinfectable; the parts directly facing the duct must be accessible and removable. The installation of supply air outlets should not, and must not, be changed arbitrarily.

The supply air outlets in operating rooms shall be designed and installed so as to prevent room air from flowing back into the supply air outlet.

Return air outlets in rooms where large quantities of lint are generated (e.g., operating rooms) shall be fitted with a fiber-retaining layer, which shall be easily removable without any tools. In operating rooms, 1200m³/h of the return air shall be extracted near the floor and the remainder near the ceiling; when exhaust air and return air are carried in separate ducts, at least the exhaust air shall be extracted from near the floor.

Exhaust air outlets must be easy to clean. The lower bottom edge of the exhaust air outlet shall be a few centimeters above the floor, and the bottom of the exhaust air outlet shall be sloped toward the operating room.

5.6 Air Handling Systems for the Operating Suite

5.6.1 Function of the Air Handling System

The air handling system in operating rooms must fulfill the following four mutually independent tasks:

1) To limit the content of airborne bacteria in the areas requiring special protection (the operating area and the instrument table, as well as the areas requiring protection mentioned below).

2) To ensure the required air flow rates between rooms (see Section 4.2.5). 3) Limit the concentration of anesthetic waste gases and other pollutant loads within the working area.

4) Maintain the required indoor conditions (eliminating the space heat load and other loads).

5.6.2 Supply Air Volume

For aseptic rooms with high requirements, when a mixing air supply system is used, the minimum supply air volume based on experience is 2400m³/h, which is defined as the "reference supply air value."

The average concentration reached by the pollutants (airborne bacteria) generated indoors after dilution (or in the entire return air) is the reference airborne bacteria concentration $k_{mathrm{r}}^{*}$, with no significant deviation anywhere in the room.

In air supply systems employing displacement airflow, the reference concentration of airborne bacteria in the protected zone can be calculated from the contamination degree $mu_{mathrm{s}}$ and the reduced supply air volume.

In order to determine a relative value that can be used to evaluate various air supply systems, an independent quantity is derived from a given pollutant load, namely the relative airborne bacteria concentration in the protected zone $E_{mathrm{s}}$, defined as follows:

$$E{mathrm{s}} = bar{k}{mathrm{s}}/bar{k}{mathrm{r}}^{*} = mu{mathrm{s}} cdot bar{k}{mathrm{r}}/bar{k}{mathrm{r}}^{} = mu{mathrm{s}} cdot dot{V}{mathrm{zu}}^{}/dot{V}_{mathrm{zu}} tag{1}$$

where

$$mu{mathrm{s}} = bar{k}{mathrm{s}}/bar{k}_{mathrm{r}}^{*} text{ denotes the contamination degree in the protected zone}$$

$bar{k}{mathrm{r}}$ — the average indoor airborne bacteria concentration at $dot{V}{mathrm{zu}}$;

$bar{k}{mathrm{r}}^{*}$ — the average indoor airborne bacteria concentration at $dot{V}{mathrm{zu}}^{*}$;

$bar{k}_{mathrm{s}}$ — the average indoor airborne bacteria concentration in the protected zone;

$dot{V}_{mathrm{zu}}^{*}$ — the reference supply air volume (2400m³/h);

$dot{V}_{mathrm{zu}}$ — the actual supply air volume.

With the "relative airborne bacteria concentration" serving as the evaluation index for the hygienic air quality of the protected zone, and with a permissible upper limit value $E{mathrm{szul}}$ specified, the minimum supply air volume $dot{V}{mathrm{zumin}}$ is:

$$dot{V}{mathrm{zumin}} = dot{V}{mathrm{zu}}^{*} cdot mu{mathrm{s}}/E{mathrm{szul}} = 2400 cdot mu{mathrm{s}}/E{mathrm{szul}} , (mathrm{m}^3/mathrm{h}) tag{2}$$

Under given operating conditions, the contamination degree depends not only on the air supply system, but also on a series of supply air parameters, especially the air volume itself. According to Equation (2), the minimum supply air volume can only be determined experimentally. For ideal mixed airflow, the corresponding contamination degree is $mu{mathrm{s}} = 1$, and all air supply systems are shown not to conform to this. The test method specified in DIN 4799 confirms $mu{mathrm{s}} < 1$.¹⁾

The upper limit values for the relative airborne bacteria concentration in the protected zone are specified as follows:

1) Operating rooms with particularly high aseptic requirements (Class A) (e.g., operating rooms used for organ transplantation, cardiac surgery, joint repair surgery, and alloarthroplasty).

2) Operating rooms with high aseptic requirements (Class B) $E_{mathrm{szul}} = 1$ (defining quantity).

It must be determined by the hygienist which operations require high aseptic requirements or particularly high aseptic requirements.

For operating rooms with particularly high aseptic requirements, the so-called minimum requirement value, i.e., $E{mathrm{szul}} = 2/3$, should not be achieved by raising the supply air volume above the aforementioned reference supply air value $dot{V}{mathrm{zu}}^{*} = 2400,mathrm{m}^3/mathrm{h}$, but rather by means of an air supply system with a good displacement effect ($mu_{mathrm{s}} leqslant 2/3$). Therefore, the following conditions are added for the air supply systems:

1) Class A operating rooms: air supply systems with a good displacement effect.

2) Class B operating rooms: air supply systems with a dilution or displacement effect.

These different aseptic requirements are determined by the different types of operations performed in the operating room. In addition, consideration must also be given to some

1) When the airborne microbial concentrations obtained during commercial surgical operations are used as product characteristics of the ventilation system, the concentrations shall be determined by test methods conforming to DIN4799. ...on the duration of individual operations, the daily operating schedule, the number of persons participating in the operations, the weakening of the patient's resistance, and other conditions.

In addition to a certain airborne bacterial concentration, the comfort of the surgical team and the benefit to the patient must also be considered when selecting the supply air system.^1)

Only when suitable streamlined lighting is used can the low-turbulence displacement flow ventilation ceiling fully develop its effect.

In addition, care must be taken not to place other obstacles affecting the airflow, such as monitors, ceiling pendants, or X-ray machines, in the airflow between the ceiling and the areas requiring special protection (operating field, instrument table).

5.6.3 Outdoor Fresh Air Volume

When the supply air passes through three-stage filtration, the airborne bacterial concentration can be neglected even if the supply air is mixed with recirculated air. Since anesthetic waste gases and disinfectant vapors cannot be removed by filtration, only outdoor air can effectively reduce the gas concentration in the working area. For this reason, the minimum outdoor fresh air flow $V_{aumin}$ must be 1200m³/h to ensure that, in conventional systems with mixed supply air, the anesthetic waste gas concentration in the working area—especially at the position where the anesthetist is located—remains below the toxicologically permissible threshold.^2)

Although anesthetic gas scavenging systems are generally installed to reduce the spread of anesthetic waste gases, such devices cannot completely eliminate them for technical or functional reasons, and it is often assumed that a certain amount of anesthetic waste gas is released into the operating room. When setting the value of the minimum outdoor fresh air volume, other harmful substances released in the operating room, such as disinfectants and cleaning agents, must also be taken into account.

The use of other types of supply air systems or different outdoor fresh air volumes is permitted, but care should be taken to ensure that excessively high concentrations cannot occur at the anesthesiologist's workplace or in the other working areas of the operating room. If for medical or technical reasons anesthetic waste gas scavenging may be required, the health safety of this area must be confirmed by a hygienist.^3)

5.7 Operation of the Air Handling System under Special Conditions

5.7.1 Operation during Periods of Non-Normal Use

In the operating department, only appropriate measures can ensure operation during periods of non-normal use.

1) The airflow between the individual rooms must still be maintained from higher cleanliness to lower cleanliness.

2) When the third-stage filter is not installed at the terminal, the minimum average air velocity in the supply air duct downstream of this filter shall be 2m/s, the purpose being to prevent contamination of the operating department. Where terminal filters are installed, only an excess of supply air or exhaust air is required to maintain the necessary air flow between adjacent rooms. The supply air and return air ducts need not be kept in operation during periods of non-normal use, but it must be ensured that no internal airflow can enter the rooms through the duct system. Moreover, in order to fulfill all the intended functions of the rooms, the necessary openings of the rooms (doors, pass-through windows, valves, etc.) should be closed during periods of non-use.

Cooling and humidification should also be provided outside the periods of normal use.

The time spent on cleaning and maintenance of the operating department should be counted as working time. The air handling system that maintains the airflow pattern required within the operating department may only be switched off for urgent maintenance or repair work. The shutdown period should be kept as short as possible, and the number of hours of continuous operation should be monitored by means of the installed operating-hour counter.

For rooms with a supply air surplus, the exhaust fan should shut off automatically when the supply fan fails, so as to prevent a reversal of the airflow direction required relative to the adjacent rooms. For rooms with an exhaust air surplus, the exhaust fan should be shut off first to prevent backflow.

After the disinfectant vapors have been completely discharged, the operating mode must first be switched before the aforementioned partial operation can be realized.

1) Stricter requirements apply in special environments.

2) According to present knowledge, even when anesthetic gas scavenging systems are used, some spread still occurs; taking Halothane as an example, it is approximately 500mL/h. Calculated with an outdoor fresh air flow of 1200m³/h and assuming uniform dilution in the room, the volumetric concentration of Halothane reaches 0.4ppm. Investigations to date have shown that the maximum concentration near the diffusion source (the position where the anesthesiologist is located) is about 10 times the average concentration. The maximum permissible concentration of Halothane is 5ppm.

3) The examination shall be carried out by a hygiene expert, with corresponding consideration of the displacement effect of the supply air system and the actual conditions.

5.7.2 Operation in the Event of Power Supply Interruption

The operation of air handling systems for Class I rooms, with the exception of cooling and humidification, must be guaranteed even in the event of an interruption of the normal power supply system. For this purpose, the backup power supply is not necessarily required to be an uninterruptible power supply.

8 Cleaning and Disinfection of Air Handling Systems

Particular attention must be paid to regular cleaning of air handling systems and, where necessary, disinfection of the air humidifiers (including water collection pans, air cooling coils, and droplet separators). When regular cleaning and sterilization of the ductwork between the second filtration stage and the rooms, or between the second and third filtration stages, cannot be carried out, or when disinfection is unnecessary for hygienic reasons, it must be determined in advance that the requirements for maintaining cleanliness during duct assembly must be observed. Cleaning and disinfection work is limited to the final section of ductwork between the third-stage air filter and the supply air outlet.

The cleaning and sterilization of the duct sections downstream of the third-stage air filter (Class I rooms) should be carried out immediately before acceptance of the hospital, or immediately after a short-term interruption of these systems. This work is performed by the hospital's designated responsible personnel or by persons commissioned by the hospital administration. In any case, after all installation work on the third-stage air filters has been completed, the ductwork downstream of the third-stage filter and the sealed rooms must be disinfected.

9 Acceptance

9.1 General

The owner must carry out technical and hygienic acceptance testing of the air handling in accordance with all relevant standards and regulations.

To achieve this purpose, technical acceptance according to Section 9.2 and hygienic acceptance according to Section 9.3 should be carried out simultaneously.

9.2 Technical Acceptance

9.2.1 General

Technical acceptance testing is carried out by a professional engineer at the corresponding construction stage. Routine acceptance should comply with the provisions of DIN18 379. In addition, hospital-related acceptance must be carried out according to the requirements described in Sections 9.2.2 and 9.2.3 and in Appendix A (omitted).

All required acceptance tests must be documented.

9.2.2 Testing HEPA Filters for Leakage and Frame Airtightness

The main test is for leakage of the HEPA filters (non-destructive testing). Leak detection is performed using the following methods:

  1. Testing the filter medium for leakage before filter installation, using the oil mist method mentioned in DIN24 184.
  2. Testing the airtightness of the frame, testing the filter mounting frame (channel).
  3. Testing the filter medium for leakage and frame airtightness in the installed state using the particle counting method [see Appendix B.2 (omitted)].

To ensure the accuracy of the measurements, the influence of interfering airflows (induced airflows) must be avoided.

9.2.3 Testing Airflow Direction

Testing of the airflow direction specified in Section 4.2.5 is carried out with the doors closed, using smoke. For rooms within the surgical department, in addition to measuring the nominal flow rate, the reduced supply air volume permitted according to Section 5.7.1 (during periods of non-normal use) must also be measured.

9.3 Hygienic Acceptance

Hygienic acceptance must be completed by a hygienist. The inspections and tests include:

  1. Inspection of the air handling system and the rooms supplied with air by this system (see Appendix B.1); this inspection is carried out by a professional engineer.
  2. Hygienic inspection of the air conditioning system in all areas with hygienic requirements, especially Class I rooms; the insp The inspection must be carried out after technical acceptance and thorough cleaning and disinfection, and before the room is put into use.

This inspection includes: particle counting; determination of the airborne bacteria concentration; checks of the airflow direction; and examination of the humidifier water for microorganisms and for additives that inhibit microbial proliferation. The inspection of the humidifier water must cover all humidifiers in the system, including the humidifiers used in Class Ⅱ rooms.

In addition, under specific circumstances, the leakage rate of heat recovery units in which leakage may be present is to be checked as required by Section 5.5.8.

All required acceptance measurements must be documented.

10 Maintenance and Routine Inspection of the Air Handling System after Handover

A fault-free air handling system is one of the most important elements in the safe operation of a hospital. For example, the user must carry out regular maintenance and check that it is in good condition; such inspections are best performed in accordance with the German Mechanical Engineers' Association Standard 21.184 or the "Maintenance 85" recommended by the administration of the national mechanical and electrical engineering sector. 1)

In addition to cleaning the components of the air handling system, the central stations and the ventilation installations, the maintenance of filters is of particular importance. Regular inspection of the filters is just as indispensable as the replacement of filters once they have reached their final resistance. The inspection intervals should be short so that faults which occur or become apparent can be rectified in good time; the required intervals are therefore determined according to local conditions. The dust loading of the air filtration is expressed by the differential pressure.

In addition, the user should arrange for:

  1. At least after the high-efficiency filters have been replaced, the leak test and airtightness check to be repeated in accordance with Section 9.2.2.
  2. The airflow direction to be re-checked at regular intervals as described in Section 9.2.3, paying particular attention to the effect on the air volume flow caused by changes in filter resistance.

10.2 Hygienic Inspection

The user should arrange for:

  1. A hygienic inspection to be carried out once a year.
  2. Dust particle counting and airborne bacteria measurement to be carried out immediately after the third-stage filters have been replaced.
  3. If repairs are carried out that may affect hygienic requirements, the responsible hygienist to be consulted. The hygienic inspection also includes a microbiological examination, in particular a requirement to inspect all humidifiers.