A system configured to reduce exposure of an individual to microorganisms, and toilet room comprising the system. The present invention relates to a system configured to reduce exposure of an individual visiting a room exposed to or containing microorganisms, which exposure subjects the individual to potential infection with microorganisms. The article “The Size of SARS-CoV-2 and its Implications” by Benedette Cuffari, M.Sc. (News-Medical.net - An AZoNetwork Site) reports that the diameter of the SARS-CoV-2 virus has been found to range between 50 nm to 140 nm. The virus is reported embedded in respiratory droplets that typically are 5 - 10 µm in length. Because of this size an individual who ingests, inhales, or is otherwise exposed to SARS-CoV-2 positive respiratory droplets, can be exposed to hundreds or thousands of virus particles which immediately increases the probability of infection with the virus. Respiratory droplets can be transmitted through coughing, sneezing, contact with contaminated surfaces, or through inhaled aerosols, and many attempts have been made and means proposed to reduce the risk of being infected with virus, such as SARS-CoV-2. Such attempts and means include e.g. wearing a mask, using alcohol disinfection, and practicing safe social distance measures. Within the context of the present invention the term “room” is used for a space or enclosure that can be occupied or where something can be done. The room is a part or division of a building enclosed by walls, floor, and ceilings, a separate booth within a building or outside a building. Rooms include detached enclosures and cubicles. Rooms within the scope of the present invention includes, but is not limited to: toilet rooms, meeting rooms, e.g. a MuteBox from the Danish design company MuteBox, Torvegade 15, 7100 Vejle, Denmark, telephone boxes, hospital wards, smoking rooms, compartments in trains and other places, isolation rooms, peadiatric cubicles, hotel rooms, clean rooms, operating rooms safety spaces, and waiting rooms. Rooms that are alternatives to toilet rooms are referred to below as “similar confined spaces”. Preferably the rooms are closed or closeable rooms. Although the invention is described below with reference to a “toilet room” emphasize is made that the below description applies equally to other “similar confined spaces”. The inventor of the present invention advances the hypothesis that the concentration of expired virus is heavily increased in a room, such as a toilet room, even if the room is subjected to ventilation, e.g. conventional ventilation as prescribed by legal guidelines. The hypothesis suggests that many people get infected with virus and bacteria when visiting a room. One of the reasons are that some particles have a certain mass whereby they are held in a stationary zone in which gravitational pull and the vertical upwards directed component of the ventilation force are counterbalanced. Such particles are thus not evacuated and remains in this stationary zone to be highly injurious to health of people visiting the room, such as toilet rooms. Within the context of the present invention the term “toilet” is used in the sense of a fixture comprising a bowl used for defecation and/or urination. Thus urinals are also toilets in the sense of the present invention. A “flush toilet” uses water or cacuum for the removal of faeces and urine. The term “vacuum toilet” means as flush toilet that use suction for the removal of faeces and urine resulting in a minimal requirement of water (0.5 to 1.5 liters). The term “toilet room” means, in the context of the present patent application, the compartment or room with the “toilet”. It should be emphasized that ventilation is traditionally established for comfort, and for comfort only, thereby reducing presence of smells, feeling of stuffiness, humidity, CO VOCs (volatile organic compounds) and other gases, etc, none of which are fatal to visitors of the toilet room, as e.g. bacteria and vira are, such as e.g. SARS-CoV-2 is. Conventional, assisted ventilation of a toilet room is conducted in view of energy consumption and economy considerations as a minimum requirement, whereby air in a toilet room shared in turns is exchanged 5-6 times per hour, or a bit more in shared bathrooms. So conventionally a toilet room has a steady, rather slow ventilation, via the top of the toilet room, e.g. via the ceiling, both of which is acceptable in view of the intended purpose. Other kinds of rooms may have similar ventilation. Some small rooms have no ventilation options at all when its door is closed. In Denmark “The Executive Order No. 1615 of 13 Dec. 2017” (Executive Order on building regulations 2012 (BR18)) prescribes that evacuation from bathrooms and toilet rooms in residential units should be adjustable to minimum 15 l/s. In toilet rooms without bath/shower and in sculleries, evacuation should be adjustable to minimum 10 l/s. The evacuation of air within these kinds of small rooms are typically conducted via an exhaust outlet close to the ceiling of the toilet room, as in particular defaecation gases, such as amongst others methane, carbon dioxide, nitrogen and sulphurous gases, tend to ascend to the ceiling rather than descend towards the floor. Air expired by an infected individual holds wet respiratory particles that can contain virus. The expired wet particles are heavier than the defecation gases and will not immediately ascend towards the ceiling in response to conventional evacuation. Thus infectious particles expired by an individual during his/her stay in a toilet room or similar confined space remains an increased infection risk to any other individual subsequently entering and temporarily staying in the same toilet room or similar confined space in which people can be caught in between equivalent forces acting in both downward and upward direction, creating a tug-of-war scenario, such as in public toilet rooms or other toilet rooms used by different people. Such toilet rooms include but are not limited to toilet rooms at hospitals, kindergartens, airplanes, railway stations, service stations, schools, offices and factories. As examples can be mentioned that staff toilet rooms in busy health units, and in schools during breaks, often are occupied 50% of the time with up to 15 different users. Hence one user can alone during a work period visit the toilet room 2-3 times and potentially cross infect 30-45 other visitors alone. Such an event comes in groups. Accordingly there is strong reason to believe it is exactly this behavior that may contribute in spreading of SARS-CoV-2 to a large number of people despite hand hygiene, face masks, and preventive wearables etc. – in particular if the toilet visitor is classified as a so-called “super spreader”. The outbreak of SARS-CoV-2, its contagiousness, severity, SARS- CoV-2 viral mutations, and a significant increase in antimicrobial resistance (AMR), require new improved systems and ways for ventilation of shared toilet room. It is a main aspect of the present invention to provide a system of the kind mentioned in the opening paragraph for reducing the risk involved in airborne transmission of respiratory microorganisms, in particular transmission of respiratory vira and bacteria. It is a further aspect of the present invention to provide a system of the kind mentioned in the opening paragraph having reduced energy consumption. It is yet a further aspect of the present invention to provide a system of the kind mentioned in the opening paragraph which has improved utilization of the energy involved with ventilation. It is yet a further aspect of the present invention to provide a system of the kind mentioned in the opening paragraph which subjects the visitor of a toilet room to reduced draft. The novel and unique features whereby these and other aspects are achieved according to the present invention consist in that the system comprises - at least one air supply unit having a first air intake port and a first discharge port, - an air cleaning unit having a second air intake port and a second discharge port, which air cleaning unit is configured for cleaning air, which is evacuated by the exhaust unit and is received from the exhaust outlet of the exhaust unit, of microorganisms, - a recirculation unit provided at a top of the toilet room or similar confined space and having a third air intake port and at least one third air discharge port, wherein the third air intake port is connected to the second discharge port, and wherein - the recirculation unit is configured for forcing at least a part of the cleaned air into the toilet room or similar confined space via the at least one third air discharge port. For simplicity the “toilet room, or similar confined space,” is referred to as “toilet room” in the following description. It is however emphasized that this should not be construed as limiting the potential applications of the system of the present invention, as said system can be applied to any kinds of similar small rooms and closable confined spaces. The term “replacement air” or “replaced air” means air supplied to the toilet room from the at least one air supply unit, recirculation unit, and/or from said units in combination. A toilet room is a small closable and confined room in which an infected individual will move around within the toilet room, thereby posing a risk of spreading expired infectious microorganisms inside the toilet room, e.g. if he/she sneezes or coughs during the stay at the toilet room. Droplets are larger than 100 µm and can travel less than 1 meter. So droplets tend to fall to the ground within 5 seconds and are less prone to be inhaled, e.g. as an aerosol. Nevertheless droplets constitute an infection risk. Aerosols have particles smaller than 100 µm. As an example a virus-laden aerosol, that is expired from an infected individual, will typically be present within, optionally beyond, 1 m from the infected individual and float in the air in a toilet room for hours. Furthermore, respiratory aerosols contain non-volatile components including proteins, electrolytes, and other biological species, causing the evaporation rate to be slower than that of pure water (“Evaporation and dispersion of respiratory droplets from coughing”, L. Liu, J. Wei, Y. Li, A. Ooi,; Indoor Air 27, 179– 190 (2017). doi: 10.1111/ina.12297; pmid: 26945674). Thus the next individual entering the toilet room is exposed to inhale the infectious aerosol expired to the toilet room, even if the virus-infected individual is no longer present in the toilet room or has recently left the toilet room. Thus another individual can enter the same toilet room shortly after the first sick individual without any knowledge of the infection risk he/she faces when inhaling the air in the toilet room, because expired airborne microorganisms remain in the toilet room some time after the sick individual has left the toilet room. Accordingly the expired air can maintain suspended in the air inside the toilet room as aerosols or droplets together with e.g. defecation gasses. The gasses and expired air may stay inside the toilet room for a period sufficient for the next individual that enters the toilet room to be infected if the expired air contains microorganisms. The exhaust unit may serve to evacuate air from within the toilet room. The at least one air supply unit may replace only a part or all of the air evacuated by the exhaust unit. If only a part of the air inside the toilet room is evacuated the remaining air to replace evacuated air is recirculated air that has been cleaned by the cleaning unit after having been evacuated by the exhaust unit. The evacuated air is treated in the air cleaning unit and is set free of microorganisms by the cleaning unit, and is then forced inside the toilet room again by the recirculation unit from the top of the toilet room. In an embodiment according to the present invention recirculated air may constitute all of the replacement air or at least the majority of replacement air. Optionally the amount of air evacuated by the exhaust unit corresponds substantially to the sum of the amount of air delivered by the at least one air supply unit and the amount of air from the recirculation unit to avoid a pressure difference across the toilet room. The air cleaning unit may comprise one or more of an electrostatic filter, an ionization apparatus, a HEPA filter and a UV-C disinfection unit. In a simple embodiment of the system according to the present invention the at least one air supply unit is the environment around the toilet room, and the first air intake port, and optionally the first discharge port, can simply be the door to the toilet room. Every time the door is opened air will flow in and out of the toilet room, and some air inside the toilet room will inherently be replaced. Often the door to the toilet room is left open or partly open so that natural exchange of air takes place. The “at least one air supply unit” may also be a combination of a mechanical unit, such as a fan or pump, and natural ventilation via the door to the toilet room. Generally the system of the present invention removes contaminated air and replaces it with fresh or cleaned replacement air in a controlled fast manner during a cleaning cycle, thereby reducing, or even eliminating, infection risk after the cleaning cycle is completed. In a preferred embodiment the majority of the replacement air to the toilet room is recirculated air that has passed the air cleaning unit and thus is free of microorganisms. Conventional toilet rooms used by plural different users, such as public toilet rooms or toilet rooms at schools and hospitals, have constant, typically mechanical, evacuation of air from the toilet room into the surrounding environment, and as a result the evacuation has inherent high energy consumption. However for the present invention mechanical or natural evacuation need not run constant, as recirculated air is supplied to the toilet room as clean air and the potentially contaminated air in the toilet room is thus more or less replaced. In an embodiment of the present invention the recirculation unit may comprise an air distribution unit to thereby speed up air distribution within the toilet room, and thus reduce time for replacing the air inside the toilet room subsequent to a toilet visit, and in response to evacuation by the exhaust unit after the visiting individual has left the toilet room. Advantageously the air distribution unit may comprise a perforated diffuser plate suspended to, arranged close to, or constituting a ceiling of the toilet room, or of the similar kind of confined space, and being arranged below the at least one third air discharge port. Preferably the perforated diffuser plate substantially covers the ceiling or constitutes the ceiling. By means of the perforated diffuser plate clean recirculated air can be provided to the toilet room as a lamina due to passing through the perforations or openings of said perforated diffuser plate. The openings or perforations of the perforated diffuser plate divides the air evenly over the entire surface of said perforated diffuser plate. In this way it can be ensured that an even air flow without swirls is injected into the toilet room at same volumetric velocity at any cross- sectional point, section or area of the toilet room, and thereby press a lamina of cleaned recirculated air towards the floor of the toilet room for the potentially contaminated air present inside the toilet room to be evacuated by the exhaust unit. The recirculated air may be warm to further promote air lamination. By arranging the recirculation unit at or in the vicinity of the ceiling of the toilet room the expired air and gasses can expediently be forced towards the exhaust unit and immediately be less accessible for inhalation by the next user of the toilet room in case the next user by mistake enters the toilet room before the system is ready for it, thus after a predetermined period that has been established as a safe period in view of replacing the air in the toilet room with a sufficient amount of cleaned air, which is free of microorganisms, and optionally free of smelly gasses, and optionally with air from the at least one air supply unit, which latter air also may have passed the perforated diffuser plate if the first discharge port is arranged above said perforated diffuser plate. The recirculation unit may further comprise a manifold arranged above the perforated diffuser plate, which manifold may have a manifold inlet, which is or communicates with the third air intake port, and a plurality of manifold discharge ports in form of at least one third air discharge port. The manifold may serve to improve spreading of the recirculated air across the area of the perforated diffuser plate before the recirculated air is pressed through said perforated diffuser plate. The first discharge port can be in communication with an inlet opening to a chamber above the perforated diffuser plate, which chamber houses at least the manifold. The recirculation unit may further comprise an air transfer layer provided at an upper face of the perforated diffuser plate. The air transfer layer may be configured to adjust, optimally homogenize, the transfer velocity and mass transfer of cleaned air forced across the air transfer layer and the adjacent perforated diffuser plate, and then further into the toilet room. The air transfer layer may be a felt layer and advantageously be configured to reduce the transfer velocity of the cleaned air forced across the perforated diffuser plate, which cleaned air is pressed into the toilet room, thereby avoiding draft, swirl formation, and turbulence inside the toilet room. Optionally the air transfer layer can be composed of one or more filter layers or filters, e.g. include a HEPA filter and/or an electrostatic filter, to remove small particles from the recirculated cleaned air, thereby serving as an additional cleaning component. The air transfer layer advantageously distributes cleaned air over the perforated diffuser plate on the upmost inlet face of said perforated diffuser plate so that a substantially homogeneous outlet flow is created over the entire perforated diffuser plate. Optionally one or both of the air cleaning unit and the recirculation unit may also comprise a temperature adjusting unit to e.g. increase the temperature of the air evacuated by the exhaust unit, or of the recirculated clean air provided to the toilet room. The first discharge port of the at least one air supply unit can in some embodiments be arranged in communication with an inlet opening to the toilet room or similar confined space, or be an air inlet opening to the toilet room or similar confined space. The first discharge port can be an existing ventilation opening to an existing toilet room. The first discharge port can be arranged at any convenient location, however it may be preferred that the first discharge port is close to the ceiling of the toilet room, for air to be supplied close to or together with the recirculated clean air. Emphasize is made that the at least one air supply unit may be comprised of several air supply subunits, some of which being natural air supply units, such as the door, a window or a vent channel, and other air supply units being mechanical air supply units, such as fans. The recirculated cleaned air is forced into the toilet room and assumes a lamina that presses the existing air within the toilet room vertically downwards towards the floor of the toilet room. To expedite removal of potentially contaminated air within the toilet room the exhaust inlet of the exhaust unit may be located close to the floor to assist in evacuation, when the lamina of recirculated clean air directs the potentially contaminated air downwards. In a preferred embodiment the exhaust inlet may be located in the vicinity of a toilet of the toilet room, such as e.g. below the toilet, to thereby instantaneously evacuate gasses, droplets and airosols that are pressed towards the floor and cannot rise to the ceiling due to the pressure of the lamina of air coming from above. According to the system of the present invention gasses, droplets and aerosols do not stay for a substantial time in the toilet room as in a conventionally ventilated toilet room. In an alternative embodiment the exhaust inlet can be located in the toilet, e.g. be part of a vacuum toilet. So the at least one air supply unit may have an first discharge port that is arranged at or above the ceiling of the toilet room to assist in replacing contaminated air in the toilet room, optionally assist in forcing the air inside the toilet room, and thus expired air, towards the exhaust unit by means of an air carpet or lamina of cleaned recirculated air and optionally air from the surroundings and delivered by the at least one air supply unit. Any contaminants present in the toilet room are this way forcingly discharged and exterminated, and gases, water vapors, and microorganisms that may exist in different lamina and areas inside the toilet room will be forced towards the exhaust inlet and out of the exhaust outlet thereby being evacuated to the air cleaning unit and the recirculation unit. Optionally a part of the cleaned air is discharged to the environment to preserve air mass balance at equilibrium. Wet microorganisms may be heavier than gases, such as methane and hydrogen sulphide, and by themselves be affected by gravity and move downwards. Gravity however is slow compared to the pressure subjected by the lamina including or consisting of recirculated air, which increases the volumetric velocity of replacement air towards the floor of the toilet room, and thus makes the toilet room ready for revisiting very fast. The system of the present invention may be operative when an individual is inside the toilet room, but most preferred a cleaning cycle is initiated only after an individual has left the toilet room, and most preferred with the door to the toilet being closed. So to get the optimum effect of the system of the present invention it is preferred that the door is closed when a user leaves the toilet room. Otherwise the capacity of the system will inherently be weakened, although not completely inefficient. Automatic door closing means may therefore advantageously be implemented at the door, such as e.g. a door spring. Other kinds of door closers, simple or motorized, are within the scope of the present invention. The duration of the cleaning cycle is adjusted in accordance with parameters of the toilet room and of the system, such as the size of the toilet room, the position of the intakes, discharges, inlets and outlet, etc. The toilet room may be closed the whole duration of the cleaning cycle. Once the cleaning cycle period is over the toilet room can be accessed again. Optionally one or more sensors that monitor and output parameters of microorganism content in the toilet room before, during, and/or after a cleaning cycle may be included in the system, and such parameters obtained by the one or more sensors be used to define the length of the cleaning cycle, optionally co control volumetric speed of replacement air. The system may include a display unit provided outside the toilet room and configured to display the progress of the cleaning cycle. Advantageously the location of the first air intake port can be provided in accordance with the above-mentioned Executive Order on building regulations 2012 (BR18). A first volumetric velocity of air recycled to the toilet room by the exhaust unit may be between 72 m/h – 720 m/h, preferably around 360 m/h. Preferably a second volumetric velocity of air supplied to the toilet room by the at least one air supply unit alone and/or by the recirculation unit alone, or by said units in combination, may correspond substantially to the first volumetric velocity to avoid air pressure differences across the toilet room that e.g. could create a positive pressure that could force the door to the toilet room open, or create a negative pressure that could make it difficult to open the door to the toilet room. The air mass balance preferably should be maintained at equilibrium. The system may comprise a control unit configured to control one or more of the exhaust unit, the at least one air supply unit, the air cleaning unit, the recirculation unit, the display unit, the first volumetric velocity, the second volumetric velocity, and communicate with optional sensors. Optionally the control unit may also comprise a timer associated with the display unit and being configured to, in dependence of at least one predetermined set value based on parameters known to the control unit, control duration of the cleaning cycle. The display unit displays information about the existing status of the cleaning cycle, and thus of the air conditions within the toilet room at any time. The display unit expediently serves as a nudging unit that informs users of the toilet room when and/or of how much of the air in the toilet room that has been replaced when entering the toilet room. The timer may start the cleaning cycle via the control unit, optional in response to receiving a signal from a moving sensor arranged at or in the vicinity of the toilet room or the anteroom. The sensor can be arranged at the door of the toilet room, or register behavior of the toilet user, such as the user leaving the toilet room, the user entering the toilet room, or be arranged at or in the anteroom to register the user entering or leaving the anteroom of the toilet room. Embodiments wherein the timer is started manually to actuate the control unit to initiate the cleaning cycle are also foreseen within the scope of the present invention. The timer may be configured to inform users of the toilet room when consecutive accesses to the toilet room are safe based upon knowledge and experimentation of the duration of the period the toilet room needs to be closed down to be sure that any contaminations are removed from the toilet room, during which period the cleaning cycle is conducted. In the alternative or additionally measurements of the sensor of microorganism content can be relied on. Once the contaminated air inside the toilet room has been replaced with cleaned air by the recirculation unit and/or with fresh air from the air supply unit, a new individual can safely enter the toilet room at minimum risk of been infected by any viral or bacterial infection the previous visitor potentially may suffer from. The control unit may also control and adjust the amount of recirculated air in relation to air supplied by the at least one air supply unit. The ventilation of a conventional public accessible toilet room that complies with the above-mentioned Executive Order on building regulations 2012 (BR18) typically runs at no stops between frequent visits. In the system of the present invention the air inside the toilet room can be replaced within minutes after the user has left the toilet room. When the next user enters the toilet room the potentially contaminated air inside the toilet room has been substantially or completely replaced by clean non-contaminated air, and none of the units of the system need to be in operation after a cleaning cycle, as the air in the toilet room is now replacement air. When the toilet room has been visited again the system starts a new cleaning cycle. The total energy consumption required for operation of a toilet room equipped with the system according to the present invention for cleaning on demand is expected to be substantially reduced to about less than 1/3 of the energy consumption required for conventional continuous ventilation of a frequently used toilet room. Further the combination of the units of the system according to the present invention decreases downtime of e.g. a toilet room between safe visits. Some toilet rooms have ventilation that starts when light is turned on or when the door is opened to the toilet room, which is very uncomfortable to the user during the stay at the toilet room. As the system according to the present invention preferably is operated between visits this further disadvantage is also eliminated. In one embodiment of the system according to the present invention the microorganism that the system removes from the toilet room is an airborne microorganism, including but not limited to an airborne virus or an airborne bacteria. Examples an expired airborne virus include SARS-CoV, SARS-CoV- 2, MERS-CoV, influenza virus, including avian influenza virus, piglet influenza virus, zika influenza virus, rhino virus, adenovirus, measles virus or respiratory syncytial virus. The system according to the present invention may be implemented in a main system in which several toilet rooms are arranged next to each other and share the same at least one air supply unit and the same control unit, and optionally same air cleaning unit. As soon as a cleaning cycle of one toilet room has been completed the system can proceed to clean another used toilet room and the cleaned toilet room can be accessed. In some embodiments the system may be dimensioned with capacity to clean more that one toilet room at the time. The air supply unit and control unit, and optionally the air cleaning unit that operates several adjacent toilet rooms in accordance with the system of the present invention, may share a common anteroom. If the toilet room is a toilet room of a plurality of adjacent toilet rooms, these toilet rooms may then be sharing a common anteroom, and optionally be operated one by one. The invention further relates to a toilet room comprising the system of the present invention. The system may be implemented in an existing toilet room or be an integral part of a pre- fabricated toilet cabin. The invention will now be described with reference to the drawing in which Fig. 1 is a schematic view of the principles of ventilation of a conventional toilet room illustrating a tug-of-war scenario of potential infectious particles, Fig. 2 is a schematic view of an exemplary embodiment of a system according to the present invention, Fig. 3 is a schematic front view of a display of a nudging device, Fig. 4 is a schematic top view of a toilet room according to the invention with a view to the manifold, Fig. 5 is an enlarged scale view of the air cleaning unit seen in top left corner of fig. 2, and Fig. 6 is a schematic top view of three adjacent toilet rooms sharing a common anteroom and being provided with controlled ventilation according to the system of the present invention. The invention is described below with reference to principle sketches of exemplary embodiments. It should be noted that combination of feature of different embodiments are within the scope of the present invention. Although the below detailed description refers to a toilet room emphasize is made that the system of the present invention is suitable for installation in similar spaces, in particular similar confined spaces. Up-scaling are possible for larger spaces. The potentially infectious particles are shown grossly exaggerated for illustrative purposes and visualization in the figures. Fig. 1 shows a toilet room 1 having a conventional ventilation unit 2 arranged at the ceiling 3. A toilet 4 is provided suspended to a wall 13 above the floor 5. The toilet room 1 is separated from an anteroom 6 by a door 7, and the anteroom 6 has a hand washing equipment 8. The potentially infectious particles P can have a mass causing them not to be evacuated by the ventilation force F , nor to drop to the floor due to the gravitational force F . In lamina zones where F is substantially equal to F some potentially infectious particles P will not be evacuated by conventional evacuation via the ceiling exhaust outlet 9, and the air in the room will still contain infectious particles P when the next user enters the toilet room 1. Fig. 2 shows a toilet room 10 provided with an embodiment of a system according to the present invention. An exhaust unit 11 comprises an exhaust pipe 45 extending along a wall 13 of the toilet room 10, in the present exemplary embodiment the vertical wall 13 behind the suspended toilet 4. The exhaust pipe 45 of the exhaust unit 11 has an exhaust inlet 12a for evacuation of air from the toilet room 10, as indicated by arrow F . The exhaust inlet 12a is arranged below the toilet 4, and an exhaust outlet 14 is arranged above the ceiling 3 of the toilet room 10. The system comprises an air supply unit 15 schematically illustrated as a flow pipe 16 extending above the ceiling 17 of the anteroom 6. The air supply unit 15 has a first air intake port 18 and a first discharge port 19 that supplies air from the surroundings to a ventilation chamber 20 above the ceiling 3 of the toilet room 10, wherein the ceiling 3 is provided as a perforated diffuser plate 21 on top of which is provided an air transfer layer 22. The door 7 to the toilet room 10 may serve as a first air intake port and/or first discharge port when the door 7 is left more or less open or is opened. An air cleaning unit 23 has a second air intake port 24 by means of which said air cleaning unit 23 receives exhaust air evacuated from within the toilet room 10 via the exhaust outlet 14 of the exhaust unit 11. The air cleaning unit 23 comprises an electrostatic filter 25 in series with a downstream ionization apparatus 26, and a further downstream flow adjusting unit 27, which is in fluid communication with a manifold 28 provided in the ventilation chamber 20 above the perforated diffuser plate 21 on top of the toilet room 10. The flow adjusting unit 27 may comprise a suction means, such as a fan 43 (schematically shown in fig. 5), for sucking air into the exhaust unit 11 from within the toilet room 10 through the air cleaning unit 23, and for blowing at least a first part of the cleaned air, which may be the major part, into the recirculation unit 29 for said cleaned air to be supplied to the toilet room 10 from the ceiling 3. A second part of the cleaned air may be vented to the surroundings via regulatory vent 33. The regulatory vent 33 may e.g. discharge an amount of air corresponding to the amount of air supplied by the at least one air supply unit 7;15 and/or by the recirculation unit 29, where the door 7 also allows air from within the toilet room 10 to be replaced when said door 7 is open/opened, as mentioned above, to maintain an equilibrium between total mass of incoming air and total mass of outgoing air from the toilet room 10. The manifold 28, the perforated diffuser plate 21 and the air transfer layer 22 is part of the recirculation unit 29 that from a second discharge port 30 of the air cleaning unit 23 receives the air cleaned by said air cleaning unit 23. The manifold 28 of the recirculation unit 29 has a third air intake port 31 and at least one third air discharge port 32, wherein the third air intake port 31 is connected to the second discharge port 30. Further features of the manifold 28 will be discussed below in relation to fig. 4. The flow adjusting unit 27 is under the control of a control unit 34 that is schematically shown as a box 34 at the hand washing equipment 8. The control unit 34 can however be placed at any convenient location where it is accessible for various operations by an operator. The control unit 34 may even be coupled to a remote main control that controls and monitors several toilet rooms from a remote location. A remote main control can e.g. lock a toilet room from the remote location, e.g. if the predetermined high degree of cleaning has not been reached within a set duration of cleaning cycle. Optionally one or more further cleaning cycles can be initiated by the remote main control to reduce concentration of infectious or other undesired matter inside the toilet room. A single control unit may have similar properties and functions as the main control. The control unit and/or the main control may be configured to change operation conditions, such as volumetric velocities of various air flows and duration of air cleaning cycle. As indicated by dotted lines 35a,35b the control unit 34 is operatively connected to the air cleaning unit 23, the recirculation unit 29, and to a nudging device 36 arranged visually accessible for the user of the toilet room, e.g. at the door 7. The nudging device 36 presents information to users of the toilet room 10 who can follow the air replacement process on a display 37. A printed circuit board (PCB), or other kind of computer means, and suitable software may be part of the control unit 34 to control at least the operative units 11,23,27,29, as well as other components and functions of the system. An example of a display 37 of a nudging device 36 is illustrated in fig. 3. The display 37 shows the current status of the cleaning process, which in the example is that 89% of the contaminated air has been replaced with air from the air supply unit 15 and the recirculation unit 29 in common. The flow adjusting unit 27 may simply be, or include, a ventilator dimensioned to recycle and/or vent between 300 – 900 m/h of potentially contaminated exhaust air from the toilet room back to the toilet room again as cleaned air, thus replacement air, depending on process parameters, especially the volume of the toilet room and/or vent a part to the surroundings. Other kinds of suction/blowing means than ventilators are within the scope of the present invention, e.g. air compressors and air pumps. The manifold 28 is seen best in the schematic top view of fig. 4. The manifold 28 is an air flow dividing manifold having the main target of distributing the cleaned air received from the air cleaning unit 23 across the air transfer layer 22, and thus across the diffuser plate 21. The manifold 28 has a main manifold pipe 38 and two open-ended blower branch pipes 39 that extend crosswise the manifold pipe 38 whereby opposite third air discharge ports 32 of the blower branch pipes 39 discharge recycled, cleaned replacement air into the air chamber 20 opposite each other on opposite sides of the main manifold pipe 38, thereby optimizing high distribution of the cleaned air. The two blower branch pipes 39 are arranged spaced at a distance from each other, whereby the manifold 28 has a substantially H-shape when viewed in the orientation of fig. 4. More than two blower branch pipes 39 can be arranged, as convenient for a given system. The third air discharge ports 32 each have a blow bag 40 to further distribute the recycled clean replacement air inside the air chamber 20 and across the diffuser plate 21 and across the air transfer layer 22, so that said recycled clean replacement air can be distributed evenly into the toilet room via the diffuser plate, as indicated by arrows F in fig. 2, as a lamina that presses the existing air inside the toilet room 10 towards the floor 5 to be evacuated into exhaust inlet 12. As shown in the enlarged scale view of fig. 5 of the air cleaning unit 23, the flow adjusting unit 27 can have a valve means 41, such as a pressure valve, damper, gate or flap valve, to regulate the ratio between air flow to the recirculation unit 29, as indicated by arrow F , and air out of the regulatory vent 33, as indicated by arrow F , respectively. The valve means 41 can be controlled by the control unit 34 in response to measurements by e.g. an air flow meter 42 and a sensor means 44 to allow exhaust of between 1% - 50% of the air cleaned by the air cleaning unit 23. The sensor means 44 may be configured to measure content of e.g. CO and volatile organic compounds (VOCs), including methane and optionally hydrogen sulphide, in the air cleaned by the air cleaning unit 23. Fig. 6 is a schematic top view of a multi-toilet facility 46 with three adjacent toilet rooms 10a,10b,10c sharing a common anteroom 6 and being provided with ventilation in accordance with the system of the present invention. The adjacent toilet rooms 10a,10b,10c have respective manifolds 28a,28b,28c arranged in respective air chambers 20a,20b,20c above respective perforated diffuser plates (not shown) and air transfer layers (not shown). Each toilet room 10a,10b,10c has a respective separate door 7a,7b,7c and separate nudging device 36a,36b,36c. A common air supply unit 15 and a common air cleaning unit 23 in turns operate the three adjacent toilet rooms 10a,10b,10c of the multi-toilet facility 46. To that aspect the exhaust inlet 12a,12b,12c of each respective toilet room 10a,10b,10c is in fluid connection to a common exhaust pipe 45 via respective first valves 47a,47b,47c. Exhaust air is in turns evacuated from within a respective toilet room 10a,10b,10c. The exhaust air is drawn through the common air cleaning unit 23, and is via common air supply pipe 48 in turns delivered to the respective manifolds 28a,28b,28c via respective second valves 49a,49b,49c inserted in the common air supply pipe 48. The capacity of the system of the present invention operated on the multi-toilet facility 46 is, as also mentioned above, utilized to in turns replace air within one of the toilet rooms of the adjacent toilet rooms 10a,10b,10c at the time. If e.g. the central toilet room 10b is operated, the adjacent two toilet rooms 10a,10c are assumed clean and accessible for use due to having been cleaned previously. During cleaning of central toilet room 10b, its corresponding first valve 47b is open to allow evacuation of potentially infectious air within said toilet room 10b, for passing said air on to the air cleaning unit, and then via its corresponding recirculation unit 29a,29b,29c add at least some of the cleaned air to the toilet room 10b again from the top of said toilet room 10b via its open respective second valve 49b. During cleaning of the central toilet room 10b the first valves 47a,47c associated with the adjacent toilet rooms 10a,10c are closed, so that the cleaning unit 23 only cleans one toilet room at the time, and thereby utilizing the capacity of the cleaning unit to a maximum. If one of the toilet rooms 10a,10c, e.g. toilet room 10a, is not at cleaned status during cleaning of the central toilet room 10b, the control unit 34 may initiate locking of toilet room 10a until the cleaning of toilet room 10b is completed. No evacuation takes place from the respective adjacent toilet rooms 10a,10c while the central toilet room 10b is operated. The control unit may be responsible for the sequential cleaning of adjacent toilet rooms in any arbitrary order on demand using a system having capacity to clean just one toilet room at the time. A toilet room equipped with the system according to the present invention can be manufactured as a unitary separate integral toilet cabin unit to be installed at any appropriate location, e.g. exterior to a building lacking toilet facilities. Also several toilet rooms of a plurality of toilet rooms sharing a common anteroom can be manufactured as an integral unit to be installed at any desired location in need of e.g. additional or new toilet facilities. The system of the present invention can also be retrofitted on existing toilet rooms. Several unitary separate integral toilet cabin units can also be stacked on top of each other if little area is available for installation. As a result of cleaner air within the toilet room means cleaner surfaces within the toilet room, so the system of the present invention will not only reduce microbiology in the air inside the particular toilet room or similar confined space, but also weaken transmission via/from the surface and vital touch points within said toilet room or similar confined space, including surfaces such as water faucet, sink (if available), flush button, door handle, toilet brush, toilet seat, bin edges, etc. Several systems according to the present invention can be installed in large rooms.