FIELD

This invention relates to an air filtration device and a method for the filtration of air. More particularly, this invention relates to an air filtration device and method for removing pathogens from the air. The invention may be adapted for use in any substantially enclosed environment, including, but not limited to, residential buildings, commercial buildings, hotels, hospitality venues and vehicles.

BACKGROUND

Substantially enclosed environments, for example residential buildings, commercial buildings, hotels, hospitality venues and vehicles, may suffer from poor air quality. Poor air quality may be adverse to the health of any persons in the relevant enclosed environment. Poor air quality may take several forms. For example, poor air quality may include the presence of particulate matter in the air, the presence of pathogens (viruses, bacteria and moulds) and the presence of volatile organic compounds (VOCs) - which may be the cause of unpleasant odours. Poor air quality may also include high CO2 levels which are associated with headaches, sleepiness, poor concentration, loss of attention, increased heart rate and slight nausea and stagnant, stale, stuffy air.

Air filtration systems which remove one of particulate matter, pathogens or VOCs from the air are well known. However, systems which remove all three are less common.

One known method of destroying pathogens is the use of ultraviolet germicidal irradiation (UVGI). UVGI is a disinfection method that uses short-wavelength ultraviolet (ultraviolet C or UV-C) light to kill or inactivate microorganisms.

UV light is electromagnetic radiation with wavelengths shorter than visible light but longer than X-rays. UV is categorised into several wavelength ranges, with short- wavelength UV (UV-C) considered "germicidal UV". Wavelengths between about 200 nm and 300 nm are strongly absorbed by nucleic acids in living organisms. The absorbed energy can result in the death or inactivation of the organism. Microorganisms including pathogens such as viruses, bacteria and moulds have less protection against UV and cannot survive prolonged exposure to it. It follows that a UVGI system is designed to expose (or irradiate) an environment to be sterilised of pathogens with a sufficient intensity of UV radiation.

However, as well as mircoorganisms, UV-C light is hazardous to most living things, including humans and animals. Skin exposure to germicidal wavelengths of UV light can produce rapid sunburn and skin cancer. Exposure of the eyes to this UV radiation can produce extremely painful inflammation of the cornea and temporary or permanent vision impairment.

Another potential risk associated with UVGI is the UV production of ozone. Oxygen molecules present in the air, when exposed to certain wavelengths of UV radiation, can form ozone, which can be harmful to humans and animals when inhaled. Ozone can trigger a variety of responses, such as chest pain, coughing, throat irritation, and airway inflammation. It can also reduce lung function and harm lung tissue. Ozone can worsen bronchitis, emphysema, and asthma, leading to increased medical care.

A further challenge for existing air filtration systems it that the volume of air that they can filter in a given timeframe is relatively small.

It is desirable to provide an air filtration system which can remove pathogens from the air, which utilises UV irradiation, but which mitigates or obviates the risks discussed above. It is further desirable to provide an air filtration system which effectively simultaneously removes particulate matter, pathogens VOCs, CO ₂ and Ozone from the air. In addition, it is desirable to provide an air filtration system which can filter a greater volume of air in a given timeframe than existing systems. Finally, it is desirable to provide an alternative air filtration system which may obviate or mitigate one or more disadvantages of existing air filtration systems, whether discussed above or otherwise.

SUMMARY

According to an aspect of the present disclosure, there is provided an air filtration device comprising: an inlet and an outlet; an airflow conduit between the inlet and outlet, the airflow conduit defined by: first and second filter arrangements; an irradiance chamber; and a fan; wherein: the fan is configured to move air along the airflow conduit in a first flow direction from the inlet to the outlet via the first and second filter arrangements and the irradiance chamber; the irradiance chamber includes a UV light source configured to emit UV light which irradiates air moved through the irradiance chamber by the fan; the first filter arrangement is located upstream in the first flow direction of the irradiance chamber and the first filter arrangement is opaque to the UV light emitted by the UV light source so as to substantially prevent UV light emitted by the irradiance chamber from passing out of the air filtration device via the first filter arrangement; and the second filter arrangement is located downstream in the first flow direction of the irradiance chamber and the second filter arrangement is opaque to the UV light emitted by the UV light source so as to substantially prevent UV light emitted by the irradiance chamber from passing out of the air filtration device via the second filter arrangement.

By being opaque to the UV light emitted by the UV light source, the first and second filter arrangements prevent UV light produced by the UV light source of the irradiance chamber from exiting the air filtration device where it could be harmful to humans or animals near the device. In addition, the second filter arrangement, by removing any ozone, produced by the UV light, in the air filtered by the air filtration device, ensures that no ozone exits the air filtration device where it may be harmful to humans or animals near the device.

If a UV light source which does not result in ozone production is used (e.g. a UV light source which produces UV light with a wavelength that does not cause ozone production is used), it may not be desired to provide the second filter arrangement with a filter medium which removes ozone from air passing through the second filter arrangement.

The second filter arrangement may comprise a filter medium which removes ozone from air passing through the second filter arrangement.

If a UV light source which results in ozone production is used (e.g. a UV light source which produces UV light with a wavelength that causes ozone production is used), it may be desirable to provide the second filter arrangement with a filter medium which removes ozone from air passing through the second filter arrangement. The first filter arrangement may comprise a filter medium which removes particulate matter from the air passing through it.

By removing particulate matter upstream of the irradiance chamber this ensures that the particulate matter does not pass to the irradiance chamber and, as such, the UV light in the irradiance chamber isn’t obstructed or absorbed by the particulate matter and can therefore be more readily absorbed by pathogens, thereby increasing the effectiveness of the irradiance chamber of killing or deactivating pathogens.

Furthermore, if the first filter arrangement is upstream of the fan, by removing particulate matter from the air, the first filter arrangement prevents particulate matter from coming into contact with the fan, thereby reducing the risk that particulate matter causes a reduction in the operating efficiency of the fan, or, in extreme cases, causes the fan to malfunction.

The second filter arrangement may comprise a filter medium which removes particulate matter from the air passing through it.

This may further remove particulate matter from air filtered by the device. In some examples the filter medium of the first and second filter arrangements may be such that the filter medium of the first filter arrangement is configured to remove particulate matter which is greater in diameter than a first diameter and the filter medium of the second filter arrangement is configured to remove particulate matter which is greater in diameter than a second diameter, and the first diameter may be greater than the second diameter.

The filter medium of the second filter arrangement may be absorbent and/or catalytic.

The filter medium of the second filter arrangement may comprise granulated active carbon.

The filter medium of the first filter arrangement may be absorbent and/or catalytic.

The filter medium of the first filter arrangement may comprise granulated active carbon. The fan may be located downstream in the first flow direction of the irradiance chamber, and upstream in the first flow direction of the second filter arrangement. In this way the fan will suck air through the irradiance chamber. It is thought that sucking air through the irradiance chamber (as opposed to blowing it through the irradiance chamber) will reduce the occurrence of eddy currents and/or turbulence in the air flowing through the irradiance chamber. This will ensure that the air flowing through the irradiance chamber will be exposed to the UV light in a relatively uniform manner. In addition, reduction of turbulence will help to maximise airflow through the device, meaning that the device can filter more air in a given timeframe.

A first direction of airflow of air entering the air filtration device at the inlet may be substantially perpendicular to a second direction of airflow of air passing through the fan and/or irradiance chamber.

This may help to maximise airflow through the device, meaning that the device can filter more air in a given timeframe.

A third direction of airflow of air exiting the air filtration device at the outlet may be substantially perpendicular to a second direction of airflow of air passing through the fan and/or irradiance chamber.

This may help to maximise airflow through the device, meaning that the device can filter more air in a given timeframe.

The air filtration device may further comprise an air quality sensor and a controller, the air quality sensor being configured to provide an air quality signal to the controller which is indicative of a quality of the air passing through the air filtration device.

This allows the device to monitor a quality of the air passing through the device, which may be used to determine the effectiveness of the device during its operation.

The controller may be configured to provide a fan speed control signal to the fan based on the air quality signal. This enables the controller to control the speed of the fan (and hence the rate of air passing through the device) as a function of a quality of the air. For example, if a quality of the air is reduced, the controller may decrease the fan speed to increase the effect of the irradiance chamber on the air passing through the device.

The controller may be configured to provide a UV light source intensity control signal to the UV light source based on the air quality signal.

This enables the controller to control the UV light source intensity (and hence the amount of UV light in the irradiance chamber) as a function of a quality of the air. For example, if a quality of the air is reduced, the controller may increase the UV light source intensity to increase the effect of the irradiance chamber on the air passing through the device.

The air filtration device may comprise an exterior air quality sensor and a controller, the exterior air quality sensor being configured to provide an exterior air quality signal to the controller which is indicative of a quality of the air in the exterior environment of the air filtration device.

This allows the device to monitor a quality of the air in the environment (e.g. room) in which the device is located, which may be used to determine the level of filtration required in the environment in which the device is located.

The controller may be configured to provide a fan speed control signal to the fan based on the exterior air quality signal.

This enables the controller to control the speed of the fan (and hence the rate of air passing through the device) as a function of a quality of the exterior air. For example, if a quality of the exterior air is reduced, the controller may increase the fan speed to increase the amount of air being filtered by the device in order to provide more air filtered by the device into the environment within which the device is located, to thereby attempt to improve the quality of the exterior air.

The irradiance chamber may include a reflector configured to contain and redirect UV photons, produced by the UV light source, within the irradiance chamber. In this way, the UV photons produced by the UV light source can be contained and concentrated in the irradiance chamber. This ensures that the UV photons produced by the UV light source have the maximum possible opportunity to interact with the air passing through the irradiance chamber. In this way, the effectiveness of the UV light within the irradiance chamber in interacting with pathogens and sterilising the air passing through the irradiance chamber of said pathogens is maximised.

According to a second aspect of the present invention there is provided a method of filtering pathogens from air using an air filtration device; the air filtration device comprising: an inlet, an outlet, an airflow conduit between the inlet and outlet, the airflow conduit defined by: first and second filter arrangements, an irradiance chamber comprising a UV light source, and a fan; the method comprising: the fan moving air along the airflow conduit in a first flow direction from the inlet to the outlet via the first and second filter arrangements and the irradiance chamber; the UV light source emitting UV light which irradiates air moved through the irradiance chamber by the fan; and the first filter arrangement, located upstream in the first flow direction of the irradiance chamber, being opaque to the UV light emitted by the UV light source and thereby substantially preventing UV light emitted by the irradiance chamber from passing out of the air filtration device via the first filter arrangement; and the second filter arrangement, located downstream in the first flow direction of the irradiance chamber, being opaque to the UV light emitted by the UV light source and thereby substantially preventing UV light emitted by the irradiance chamber from passing out of the air filtration device via the second filter arrangement.

A filter medium of the second filter arrangement may remove ozone from air passing through the second filter arrangement.

The benefits of the first aspect of the invention apply mutatis mutandis to the second aspect of the invention.

It should be recognised that features discussed above in relation to one aspect of the invention may, where appropriate, be applied in relation to any other aspect of the invention. BRIEF DESCRIPTION OF DRAWINGS

These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is schematic view of an air filtration device according to an embodiment of the present invention; and

Figures 2 and 3 show an example of a filter arrangement which may form part of an air refining filtration device according to the present invention.

DETAILED DESCRIPTION OF DRAWINGS

Figure 1 shows a schematic view of an air filtration device 10 according to an embodiment of the present invention. The air filtration device 10 comprises an inlet 12 and an outlet 14. An airflow conduit is defined between the inlet 12 and the outlet 14. The airflow conduit is defined within the air filtration device 10 by first and second filter arrangements 16, 18, an irradiance chamber 20 and a fan 22. The airflow conduit may only be a part of the route taken by the air between the inlet 12 and the outlet 14.

The fan is configured to move air along the airflow conduit in a first flow direction A from the inlet 12 to the outlet 14 via the first and second filter arrangements 16, 18 and the irradiance chamber 20. In the present embodiment the fan 22 takes the form of axial-flow fan which includes a rotor (not shown) rotated by a motor (again, not shown) about a rotation axis. The rotor includes blades that force air to move parallel to the rotation axis about which the blades rotate. In the present example, the rotation axis is the same as an airflow axis B. It will be appreciated that in other embodiments any appropriate type of fan can be used to move air along the airflow conduit. For example, the fan may be of a centrifugal or cross flow type. Alternatively, the fan may utilise bellows, the Coanda effect or convective or electrostatic propulsion. Although the fan in the present embodiment is driven by an electric motor, in other embodiments the fan may be driven by any appropriate power source.

In the present embodiment the first and second filter arrangements 16, 18, irradiance chamber 20 and fan 22 are arranged such that they all lie along the airflow axis B. The airflow conduit of the air filtration device 10 (which may be said to include the irradiance chamber, the fan, and a part of each of the first and second filter arrangements) is generally linear and lies along the airflow axis B. The generally linear nature of the airflow conduit maximises the ease with which air can pass through the system as it does not have to change direction as is passes between the filter arrangements, irradiance chamber and fan. It also maximises efficiency of airflow by minimising turbulence. In other embodiments the airflow conduit may be non-linear - for example it may be meandering or include one or more right angles.

Figure 1 includes a virtual ‘cut out’ (indicated in dashed lines and with reference numeral 24) in an outer wall of the irradiance chamber 20 so that the inside of the irradiance chamber is visible. For the avoidance of doubt, the cut out is included for the purposes of clearly showing the invention: it is not present in the air filtration device in reality. The irradiance chamber 20 includes a UV light source 26 configured to emit UV light which irradiates air moved through the irradiance chamber 20 by the fan. In the present example the UV light source is a Philips ® Master PL-L 55W/840 (marketed by Signify Netherlands B.V., High Tech Campus 48, 5656AE Eindhoven, The Netherlands). In other embodiments the UV light source may be any light source capable of producing UV light, and, in particular, UV-C light having a wavelength between about 100 and about 280 nm, which is capable of killing or inactivating microorganisms. Examples of such microorganisms include pathogens such as viruses (including, among others, COVID-19), bacteria and moulds.

The UV light source 26 is supported within the irradiance chamber 20 by a UV light source support (not shown). Any appropriate UV light source support may be used.

The irradiance chamber 20 includes a casing which is formed from a material which is opaque to the UV light produced by the UV light source. In one example, the material is steel, but it may be any appropriate material - for example other metals or UVC opaque plastic materials. A casing 20a of this nature prevents light produced within the irradiance chamber 20 by the UV light source from passing out of the air filtration device. There is an air inlet and an air outlet of the irradiance chamber 20 from which UV light could potentially escape, but this is prevented as discussed in more detail below. The first filter arrangement 16 is located upstream in the first flow direction A of the irradiance chamber 20. The first filter arrangement 16 is opaque to the UV light emitted by the UV light source 26 so as to substantially prevent UV light emitted within the irradiance chamber 20 from passing out of the air filtration device 10 via the first filter arrangement 16. That is to say, the first filter arrangement 16 ensures that any UV light produced by the UV light source 26 does not pass from an air inlet to the irradiance chamber 20 to the exterior of the air filtration device 10.

The second filter arrangement 18 is located downstream in the first flow direction A of the irradiance chamber 20. The second filter arrangement 18 is opaque to the UV light emitted by the UV light source so as to substantially prevent UV light emitted within the irradiance chamber 20 from passing out of the air filtration device via the second filter arrangement. That is to say, the second filter arrangement 18 ensures that any UV light produced by the UV light source 26 does not pass from an air outlet of the irradiance chamber 20 to the exterior of the air filtration device 10.

By being opaque to the UV light emitted by the UV light source 26, the first and second filter arrangements 16, 18 prevent UV light produced by the UV light source 26 of the irradiance chamber 20 from exiting the air filtration device, where it could be harmful to humans or animals near the device.

The second filter arrangement 18 also comprises a filter medium (not shown) which removes ozone from air passing through the second filter arrangement 18. The presence of a filter medium within the second filter arrangement 18 which removes ozone provides reassurance that if any ozone is produced by the UV light source, it will be filtered from the air so that no ozone exits the air filtration device where it may be harmful to humans or animals near the device.

The filter medium of the second filter arrangement may be absorbent (so that it absorbs ozone) and/or catalytic (such that it promotes reaction of the ozone into a less harmful species). The filter medium may also absorb or pacify VOCs and CO2. For example, the filter medium may promote the conversion of CO2 to O2. The filter medium of the second filter arrangement 18 in the present example comprises (compressed) granulated activated carbon, although any suitable filter medium may be utilised. In a particular embodiment the granulated activated carbon may be RC412 carbon with a pore size of 0.02mm.

In other embodiments, if a UV light source which produces UV light with a wavelength that does not cause ozone production is used, it may not be necessary to provide the second filter arrangement with a filter medium which removes ozone from air passing through the second filter arrangement.

The first filter arrangement 16 comprises a filter medium which removes particulate matter from the air passing through it. By removing particulate matter upstream of the irradiance chamber 20 this ensures that the particulate matter does not pass to the irradiance chamber 20 and, as such, the UV light in the irradiance chamber 20 is not obstructed or absorbed by the particulate matter and can therefore be more readily absorbed by pathogens, thereby increasing the effectiveness of the irradiance chamber of killing or deactivating pathogens. In other embodiments, the first filter arrangement need not include a filter medium which removes particulate matter from the air passing through it.

Furthermore, given that the first filter arrangement 16 is upstream of the fan 22, by removing particulate matter from the air, the first filter arrangement 16 prevents the removed particulate matter from coming into contact with the fan 22, thereby reducing the risk that particulate matter causes a reduction in the operating efficiency of the fan 22, or, in extreme cases, causes the fan 22 to malfunction.

The filter medium of the first filter arrangement 16 may be absorbent and/or catalytic. The filter medium may absorb or pacify VOCs and CO2. For example, the filter medium may promote the conversion of CO2 to O2. The filter medium of the first filter arrangement 16 in the present example comprises (compressed) granulated activated carbon. However, in other examples the first filter arrangement 16 may include any appropriate filter medium. In a particular embodiment the granulated activated carbon may be RC412 carbon with a pore size of 0.02mm.

The second filter arrangement 18 comprises a filter medium which removes particulate matter from the air passing through it. This may further remove particulate matter from air filtered by the device. In other embodiments, the second filter arrangement 18 need not include such a filter medium.

In the present example the filter medium of each of the first and second filter arrangements is such that the filter medium of the first filter arrangement is configured to remove particulate matter which is greater in diameter than a first diameter and the filter medium of the second filter arrangement is configured to remove particulate matter which is greater in diameter than a second diameter, the first diameter being greater than the second diameter.

There are a number of ways that this functionality may be achieved. In one example the filter medium of the first filter arrangement may have a structure such that its pore size is greater than that of the filter medium of the second filter arrangement, which is determined by the structure of the filter medium of the second filter arrangement. In another example, the filter medium of the first and second filter arrangements may be the same but the speed of the air passing through the first filter arrangement may be greater than the speed of air passing through the second filter arrangement (this may be achieved, for example, by the flow area of the inlet being smaller than the flow area of the outlet). In this way, the air passing through the first filter arrangement will interact with the filter medium of the first filter arrangement for less time than the air passing through the second filter arrangement will interact with the filter medium of the second filter arrangement. This will result in the filter medium of the second filter arrangement being able to remove particles from the air passing through the second filter arrangement which are smaller in size than those removed from the air passing through the first filter arrangement by the filter medium of the first filter arrangement.

In the present example, the fan 22 is located downstream in the first flow direction A of the irradiance chamber 20, and upstream in the first flow direction A of the second filter arrangement 18. In this way, the fan 22 will suck air through the irradiance chamber 20. It is thought that sucking air through the irradiance chamber 20 (as opposed to blowing it through the irradiance chamber 20) will reduce the occurrence of eddy currents and/or turbulence in the air flowing through the irradiance chamber 20. This will ensure that the air flowing through the irradiance chamber 20 will be exposed to the UV light in a relatively uniform manner. In addition, reduction of turbulence will help to maximise airflow through the device 10, meaning that the device can filter more air in a given timeframe. In other examples, the fan may be located in any appropriate position in the airflow path. For example, the fan can be located: i) downstream of the inlet and upstream of the first filter arrangement; ii) downstream of the first filter arrangement and upstream of the irradiance chamber; or downstream of the second filter arrangement and upstream of the outlet.

Figures 2 and 3 show side and end views respectively of an example of a filter arrangement 28 which may form the first or second filter arrangement of the air filtration device discussed above.

The filter arrangement 28 is generally cylindrical. It has a cylindrical (or circumferential) surface 30, a closed end 32 and an open end 34 for connection to the remaining portion of the air filtration device. The cylindrical surface 30 comprises a mesh defining a large number of openings. The filter medium of the filter arrangement is located in an internal cavity of the filter arrangement defined in part by the cylindrical surface 30.

In the case where the filter arrangement 28 is the first filter arrangement of the air filtration device, the openings defined by the mesh of the cylindrical surface 30 constitute the inlet 16 of the air filtration device. A first direction C of airflow of air entering the air filtration device at the inlet may be substantially perpendicular to i) the direction of airflow A of air passing through the fan and/or irradiance chamber and/or ii) a second direction D of airflow exiting the first filter arrangement 16.

This may help to maximise airflow through the device, meaning that the device can filter more air in a given timeframe. In particular, using the circumferential surface of the filter arrangement to provide the inlet (as compared to the end surface of the filter arrangement) means that the inlet has a greater surface area, meaning that it possible to suck a greater amount of air into the device.

In the case where the filter arrangement 28 is the second filter arrangement of the air filtration device, the openings defined by the mesh of the cylindrical surface 30 constitute the outlet 14 of the air filtration device. A third direction E of airflow of air exiting the air filtration device at the outlet may be substantially perpendicular to i) the direction of airflow A of air passing through the fan and/or irradiance chamber and/or ii) a fourth direction F of airflow entering the second filter arrangement 18.

This may help to maximise airflow through the device, meaning that the device can filter more air in a given timeframe. In particular, using the circumferential surface of the filter arrangement to provide the outlet (as compared to the end surface of the filter arrangement) means that the outlet has a greater surface area, meaning that it possible to vent a greater amount of air out of the device.

The air filtration device 10 may further comprise an air quality sensor and a controller, the air quality sensor being configured to provide an air quality signal to the controller which is indicative of a quality of the air passing through the air filtration device 10. In this case, the air quality sensor may be located within the device, at a location downstream of the irradiance chamber, such that it can provide a signal which is indicative of a quality of the air within the device downstream of the irradiance chamber.

This allows the device to monitor a quality of the air passing through the device 10, which may be used to determine the effectiveness of the device 10 during its operation.

The quality of the air measured by the air quality sensor may be the concentration of a certain chemical species within the air, for example O2, a particular VOC, CO2 or ozone.

The controller may be configured to provide a fan speed control signal to the fan 22 based on the air quality signal.

This enables the controller to control the speed of the fan 22 (and hence the rate of air passing through the device) as a function of a quality of the air. For example, if a quality of the air is reduced, the controller may reduce the fan speed to increase the effect of the irradiance chamber 20 on the air passing through the device 10. For example, if the controller receives an air quality signal which indicates a reduction in air quality, the controller may provide a fan speed control signal to the fan in the form of a reduced drive voltage, which results in a reduced operation speed of the fan. The controller may be configured to provide a UV light source intensity control signal to the UV light source 26 based on the air quality signal.

This enables the controller to control the UV light source intensity (and hence the amount of UV light in the irradiance chamber 20) as a function of a quality of the air. For example, if a quality of the air is reduced, the controller may increase the UV light source intensity to increase the effect of the irradiance chamber 20 on the air passing through the device 10.

For example, the UV light source intensity control signal, may, if the UV light source is capable of varying the intensity of its output as a function of power provided to it, control the intensity of the UV light source directly. Alternatively, the UV light source may be formed of a plurality of light source elements which are each capable of emitting UV light, and the UV light source intensity control signal may control the number of the light source elements that are energised, thereby controlling the total intensity of the UV light source.

The air filtration device may comprise an exterior air quality sensor and a controller, the exterior air quality sensor being configured to provide an exterior air quality signal to the controller which is indicative of a quality of the air in the exterior environment of the air filtration device (for example, in a room in which the air filtration device is located).

In this case, the exterior air quality sensor may be located at the exterior of the device (or interior to the device at a location upstream of the first filter arrangement), such that it can provide a signal which is indicative of a quality of the air to the exterior of the device (or just downstream of the inlet).

This allows the device to monitor a quality of the air in the environment (e.g. room) in which the device is located, which may be used to determine the level of filtered air required in the environment in which the device is located.

The quality of the air measured by the exterior air quality sensor may be the concentration of a certain chemical species within the air, for example O2, a particular VOC, CO2 or ozone. In addition or alternatively, the quality of the air measured by the exterior air quality sensor may be the concentration of particulate matter within the air having a diameter above a desired threshold.

The air quality sensor and/or exterior air quality sensor may be an infra-red chemical concentration sensor or an infra-red particulate matter sensor.

In one example, the air quality sensor and/or exterior air quality sensor may be an infrared CO2 sensor. The effective measuring range is from 0 to 5000ppm. The sensor may utilise non-dispersive infrared (NDIR) technology and include temperature compensation.

In one example, the air quality sensor and/or exterior air quality sensor may be a particulate matter sensor. Such a sensor may operate by measuring and sorting (also known as ‘binning’) particulates in the range of about 1 microns to about 10 microns.

The particulate matter may measure the number of PM 10 particles which interact with the sensor in a given period of time. PM 10 particles are particles with a diameter of 10 microns and smaller. PM 10 particles irritate exposed mucous membranes such as the eyes and throat. The particulate matter may measure the number of PM2.5 particles which interact with the sensor in a given period of time. PM2.5 particles are particles with a diameter of 2.5 microns and smaller. PM2.5 particles can pass into the alveoli and may exacerbate or cause heart and lung conditions.

The controller may be configured to provide a fan speed control signal to the fan 22 based on the exterior air quality signal.

This enables the controller to control the speed of the fan (and hence the rate of air filtered by the device being output into the environment) as a function of a quality of the exterior air. For example, if a quality of the exterior air is reduced, the controller may increase the fan speed to increase the amount of air being filtered by the device (in a given time) in order to provide more air filtered by the device into the environment within which the device is located, to thereby attempt to improve the quality of the exterior air. By way of example, if the controller receives an exterior air quality signal which indicates a reduction in exterior air quality, the controller may provide a fan speed control signal to the fan in the form of a increased drive voltage, which results in an increased operation speed of the fan.

Returning to Figure 1 , the irradiance chamber 20 of the present embodiment includes a reflector 36. The reflector 36 in the present embodiment takes the form of the reflective inner surface of casing 20a. In other embodiments, the reflector 36 may take any appropriate form and may be supported within the irradiance chamber 20 in any appropriate manner.

The reflector is 36 is configured to contain and redirect UV photons, produced by the UV light source 26, within the irradiance chamber 20.

In this way, the UV photons produced by the UV light source 26 can be contained and concentrated in the irradiance chamber 20. This ensures that the UV photons produced by the UV light source 26 have the maximum possible opportunity to interact with the air passing through the irradiance chamber 20. In this way, the effectiveness of the UV light within the irradiance chamber 20 in interacting with pathogens and sterilising the air passing through the irradiance chamber of said pathogens is maximised.

Other embodiments of the invention may not include a reflector.

To deliver the required level of exposure of UV light to pathogens (i.e. to kill or inactivate the pathogens) within the air passing through the irradiance chamber 20, the following calculations can be used to confirm the dosage of the UV light.

In one example, the internal volume of the irradiance chamber 20 is 0.0428 cubic meters (1.511 cubic feet). The maximum distance between the light source and a portion of irradiance chamber through which the air to be irradiated passes is 10cm. This may be referred to as the maximum irradiance distance. In an example in which the light source is distributed around the circumferential edge of a cylindrical irradiance chamber, this will be the distance between the light source and the centre of the chamber. It will be appreciated that, in other embodiments, rather than a cylinder, the irradiance chamber may have any appropriate cross-sectional (perpendicular to airflow direction) shape - for example, triangular, square or hexagonal.

The velocity of the air through the irradiance chamber 20 is defined by the fan velocity - 0.15 cubic meters per second (317.8 cubic feet per minute).

As previously discussed, the UV light source in the present example is a Philips PL-L 55W, has an irradiance level of 156 pW / cm ² at a distance of 1m. The minimum irradiance level for the irradiance chamber is determined as the irradiance level at the maximum irradiance distance. It follows that the minimum irradiance level for the irradiance chamber (i.e. at a distance of 10cm) is 15600 pW / cm ² . This is calculated using the ‘inverse square law’.

The minimum fluence (at the centre of the irradiance chamber in the example given above) of the irradiance chamber is calculated as follows.

The exposure time of the air within the irradiance chamber 20 is given by the internal volume of the irradiance chamber divided by the fan velocity 0.0428 cubic meters divided by 0.15 cubic meters per second = 0.28 seconds.

The minimum fluence is given by the product of i) the number of UV light sources in the chamber (in the present case, 4); ii) the minimum irradiance level; and iii) the exposure time of the air within the irradiance chamber. This is:

4 x 15600 pW / cm ² x 0.28 s = 17472 pWs / cm ²

It is known that the fluence necessary to inactivate or kill most pathogens is 80 J / m ² (8000 pWs / cm ² ). Provided that the minimum fluence of the irradiance chamber is equal to or exceeds about 80 J / m ² (8000 pWs / cm ² ), then the irradiance chamber will effectively inactivate or kill most pathogens in the air passing through the irradiance chamber. As such, the system upon which the calculations above are based will operate to effectively inactivate or kill most pathogens in the air passing through the irradiance chamber as the fluence provided by the system exceeds 80 J / m ² (that is, the minimum fluence 17472 pWs / cm ² exceeds 8000 pWs / cm ² ). Air travelling through any part of the irradiance chamber will receive in excess of 80 J / m ² of UVC radiation, thereby inactivating or killing most pathogens in the air.

It will be appreciated that the equation above can be rearranged to determine operating characteristics for other systems to be effective in inactivating or killing pathogens.

For example, it is possible to control the irradiance level (in power per unit area) of the light source of the irradiance chamber, and the time that the air is in the irradiance chamber (by controlling the fan speed) to ensure that the minimum fluence (in energy per unit area) provided to the air as it passes through the irradiance chamber is equal to or exceeds the level of 80 J / m ² required to inactivate or kill most pathogens.

This also means that, because reducing fan speed results in an increase in minimum fluence, the irradiance level of the light source can be reduced when the fan speed is reduced to maintain the same minimum fluence - as such, a low power mode of the air filtration device is possible in which fan speed and irradiance level of the light source are both reduced whilst maintaining a minimum fluence level sufficient to inactivate or kill most pathogens. However, it will be appreciated that in such a low power mode the amount of air which is filtered by the air filtration device in a given time is reduced.

In the above examples it is noted that the fluence necessary to inactivate or kill most pathogens is 80 J / m ² (8000 pWs / cm ² ). However, the fluence required within the irradiance chamber may be determined by the specific pathogen(s) that the irradiance chamber is required to inactive or kill.

The list below indicates the fluence (or dosage) in J/m ² required to achieve 99% inactivation of a particular species of pathogen.

Bacteria (J/m ² )

Bacillus anthracis 45.2

B. megatherium sp. (spores) 27.3

B. megatherium sp. (veg.) 13.0

B. parathyphosus 32.0

B.suptilis 71.0

B. suptilis spores 120.0 Campylobacter jejuni 11.0

Clostridium tetani 120.0

Corynebacterium diphteriae 33.7

Dysentery bacilli 22.0

Eberthella typhosa 21.4

Escherichia coli 30.0

Klebsiella terrifani 26.0

Legionella pneumophila 9.0

Micrococcus candidus 60.5

Micrococcus sphaeroides 100.0

Mycobacterium tuberculosis 60.0

Neisseria catarrhalis 44.0

Phytomonas tumefaciens 44.0

Pseudomonas aeruginosa 55.0

Pseudomonas fluorescens 35.0

Proteus vulgaris 26.4

Salmonella enteritidis 40.0

Salmonella paratyphi 32.0

Salmonella typhimurium 80.0

Sarcina lutea 197.0

Seratia marcescens 24.2

Shigella paradysenteriae 16.3

Shigella sonnei 30.0

Spirillum rubrum 44.0

Staphylococcus albus 18.4

Staphylococcus aureus 26.0

Streptococcus faecalis 44.0

Streptococcus hemoluticus 21.6

Streptococcus lactus 61.5

Streptococcus viridans 20.0

Sentertidis 40.0

Vibrio chlolerae (V.comma) 35.0

Yersinia enterocolitica 11.0

Yeast Bakers’ yeast 39

Brewers’ yeast 33

Common yeast cake 60

Saccharomyces cerevisiae 60

Saccharomyces ellipsoideus 60

Saccharomyces sp. 80

Mould Spores

Aspergillus flavus 600

Aspergillus glaucus 440

Aspergillus niger 1320

Mucor racemosus A 170

Mucor racemosus B 170

Oospora lactis 50

Penicillium digitatum 440

Penicillium expansum 130

Penicillium roqueforti 130

Rhizopus nigricans 1110

Viruses

Hepatitis A 73

Influenza virus 36

MS-2 Coliphase 186

Polio virus 58

Corona Virus 20

Novel Virus 16

Rotavirus 81

Adeno Virus 20

Coxsackie 18

Based on the above information the fluence level of the irradiation chamber can be selected so that it exceeds the fluence value required to inactivate the desired species of pathogen. It should be understood that the examples provided herein are merely exemplary of the present disclosure and that various modifications may be made thereto without departing from the scope defined by the claims.