Abstract:
An air purifier has an air flow path with a dielectric body interposed across the path. The dielectric body may be made of, for example, quartz or alumina fibers or silica granules or sponge so that it is porous to air and transmissive to ultraviolet (“UV”) light. A source of ultraviolet light emits UV 1  and UV 2  light into the airflow path upstream of the dielectric body and at least UV 2  light into the dielectric body itself. The UV light forms ozone. The ozone, as well as water vapor in the air, naturally attaches to the dielectric body which concentrates these materials in the dielectric body. The UV light irradiating the ozone and water in the dielectric body causes the formation of highly reactive hydroxyl radicals which assist in sterilizing the incoming air.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to an air purifier and to a method of air purification.  
           [0002]    It is known that ultraviolet (“UV”) light sterilizes DNA so that biological material (such as viruses, bacteria, molds, yeasts, and pollens) exposed to UV light either dies or cannot reproduce. This property of UV light has been utilized to sterilize air in a building by simply placing UV lamps in the building&#39;s air ducts. One drawback with this approach is that biological material may not be exposed to UV light for a sufficient time to be sterilized. To address this drawback, it is known to utilize a porous air filter and mount a UV light for reciprocating movement across a face of the filter. In operation, a fan draws air through the filter resulting in biological material becoming trapped in the filter. The irradiation of the filter with the reciprocating UV light acts to kill this trapped biological material. However some biological material, namely viruses, readily pass through porous filters and would not, therefore, be sterilized with the combination of a porous filter in conjunction with a UV lamp. Furthermore, UV light degrades a porous filter requiring frequent replacement of same.  
           [0003]    In our U.S. Pat. No. 5,656,242 issued Aug. 12, 1997, we describe several air purifiers which sterilise air with UV radiation. In one embodiment air is drawn through a filter and a perforated metal plate into a primary radiation cavity containing a UV light. The filter traps biological material which is exposed to a low UV dose via the perforations in the metal plate. In another embodiment, air is drawn along a U-shaped path defined by a filter transmissive to UV2 and blocking UV1. UV1 and UV2 radiation generated by a lamp in the first leg of the U-shaped path forms sterilising ozone (O 3 ) in this leg; the UV2 which passes through the filter into the second leg of the U-shaped path breaks down this ozone. Water misters in this second leg result in the disassociated ozone forming hydroxyl radicals (OH) which further sterilise the air. Thus, the air is sterilised directly by the UV radiation and also indirectly by the UV radiation creating ozone and hydroxyl radicals. While this embodiment results in an effective purifier, water misters may not be readily available and increase maintenance needs of a system.  
           [0004]    Therefore, there remains a need for an effective air purifier.  
         SUMMARY OF INVENTION  
         [0005]    An air purifier has an air flow path with a dielectric body interposed across the path. The dielectric body is fabricated so as to be porous to air and transmissive to ultraviolet (“UV”) light. A source of UV light emits UV light into the dielectric body and, optionally, also into the air flow path upstream of the dielectric body. The UV light may form ozone. Ozone, as well as water vapour in the air, naturally attaches to the dielectric body which concentrates these materials in the dielectric body. The UV light irradiating the ozone and water in the dielectric body causes the formation of highly reactive hydroxyl radicals which assist in sterilising the incoming air.  
           [0006]    Accordingly, in one aspect, there is provided an air purifier comprising: an air flow path; a dielectric body which is porous to air and transmissive to ultraviolet light interposed across said air flow path; and a source of ultraviolet light for emitting ultraviolet light such that ultraviolet light is present in said dielectric body.  
           [0007]    According to another aspect of the invention, there is provided an air purifier comprising: an air flow path; a dielectric body interposed across said air flow path, said dielectric body being porous to air and fabricated of at least one of silica, silicon dioxide, aluminum oxide, magnesium fluoride, calcium fluoride, barium fluoride, strontium fluoride, lithium fluoride, quartz and sapphire; and a source of ultraviolet light for emitting ultraviolet (“UV”) light such that ultraviolet light is present in said dielectric body According to a further aspect of the present invention, there is provided a method of air purification comprising: passing contaminated air through a dielectric body which is porous to air and transmissive of ultraviolet (“UV”) radiation; and UV irradiating said dielectric body.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    In the figures which illustrate example embodiments of the invention,  
         [0009]    [0009]FIG. 1 is a schematic side view of an air purifier made in accordance with an embodiment of this invention,  
         [0010]    [0010]FIG. 2 is a schematic top view of the purifier of FIG. 1,  
         [0011]    [0011]FIG. 3 is a schematic cross-sectional view along the lines  3 - 3  of FIG. 2,  
         [0012]    [0012]FIG. 3 a  is a graph of UV intensity versus radial distance, and  
         [0013]    [0013]FIG. 4 is a schematic side view of an air purifier made in accordance with another embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    Referencing FIGS.  1  to  3 , an air purifier  10  has a housing  12  with an air intake  14  and an air exhaust  16 . Within housing  12 , an intake plenum  18  extends from the air intake  14 , through a dust filter  19 , to the suction inlet of a blower  20 . An outlet plenum  22  extends between the outlet of the blower and an annular wall  24  inwardly depending from housing  12 . Annular wall  24  has a concentric aperture covered with an ultra-violet (“UV”) reflecting screen mesh  26  which allows the flow of air but which reflects UV. An annular dielectric body  30  extends between annular wall  24  and a second annular wall  32  inwardly depending from the housing to define a central cavity  34  and a peripheral annular cavity  36 . Dielectric body  30  is enveloped by a screen mesh sleeve  38 , a particulate filter  40 , and a chemically absorbent filter  42 . Sleeve  38  may, optionally, be provided with a UV coating on its inside surface such that it allows the transmission of air but reflects UV. The second annular wall  32  has a central opening  37  and a peripheral annular, UV reflecting, screen mesh section  44 . The gas containing tube  46  of an ultraviolet lamp  50  extends through the opening  37  of wall  32  into cavity  34 . The ballast  52  of lamp  50  is secured to wall  32 .  
         [0015]    Walls  24  and  32  along with the wall of the housing  12 , define a UV chamber  60 . The walls of this UV chamber many have a UV reflective coating. The outer cavity  36  opens into an exhaust plenum  62 .  
         [0016]    An inner member, shown as annular inner wire mesh  66 , lines the inside wall of the dielectric body  30  and an outer member, shown as annular outer wire mesh  68  is embedded within the dielectric body  30 . A voltage source  70  (FIG. 3) is connected (through a switch-not shown) between the inner mesh  66  and mesh sleeve  38 , on the one hand, and outer wire mesh  68 , on the other. Each mesh might be in the form of thin metal (Al with gold, rhodium or nickel coatings) radial blades which would reflect UV by grazing incidence but intercept significant amounts of light.  
         [0017]    The intake and exhaust plenums  18  and  62  may be coated with a UV absorbing paint which, optionally, may be impregnated with a UV activated biocide such as TiO 2 .  
         [0018]    The UV lamp  50  may emit UV1, UV2 and UV3 radiation. UV1 radiation is defined as UV radiation below approximately 185 NM in wavelength, UV2 is defined as radiation between 185 and 300 NM in wavelength and UV3 is defined as UV radiation above 300 NM in wavelength.  
         [0019]    UV1 radiation photo dissociates O 2  into ground state atomic oxygen (O) and water vapor into hydroxyl free radicals (OH) and hydrogen (H). UV2 radiation photo dissociates O 3  into O 2  and excited atomic oxygen (O*). These dissociation processes create powerful oxidants which can oxidize both bio-aerosols and volatile organic compounds rendering them either harmless, or converting them into species which are readily absorbed by filters. UV3 radiation does not photo dissociate any gaseous species but can excite photo catalysts, such as surfaces of TiO 2  and similar semiconductor catalysts.  
         [0020]    All of these species will attach to surfaces in the annular dielectric  30  resulting in a concentration of the processes of oxidation. In this regard, when voltage source  70  is switched in, photo-electrically generated electrons from the inner wire mesh  66  and mesh sleeve  38  flow towards the oppositely charged outer mesh  68 . These electrons attach to particulate and to the outer wire mesh  68 . Such charge attachments retard the flow of the particulate enhancing the UV exposure by increasing the exposure time. In addition, electrostatic attachment of the particulate to the outer filters is enhanced increasing the efficiency of the filtration of the particulate.  
         [0021]    The photoelectric effect is enhanced at shorter wavelengths for many materials. Thus using the inner wire mesh  66  as the cathode, which is near the lamp, would allow UV1 to be used to eject photo-electrons. An alternate method to using a mesh would be to coat a thin metal transparent conductive film directly on the lamp. Such cathodes (usually called semitransparent) are commonly used in optical sensing devices. This cathode should absorb only a tiny amount of UV1 exiting the lamp but could be highly photo-emissive by virtue of the enhanced quantum efficiency at shorter wavelengths. A very thin layer of gold, nickel, rhodium or other metal might be used. Cesium iodide or cesium telluride (in small quantity or low concentration) might also be used.  
         [0022]    The inner mesh cathode  66  of the dielectric body may be coated with a UV reflective coating or may be constructed with a UV reflective material such as aluminum or aluminum coated with rhodium. This would concentrate the UV1 and UV2 in the central cavity  34  increasing the kill of bio-aerosols and photo-dissociative effects in the air. In addition, UV enhancement in the central chamber will not be at the expense of UV reaching the dielectric body if the reflective coating has a low absorbance. This occurs since the intensity of light inside the central cavity  34  will increase proportionately to the reflectance of the inner mesh cathode  66 . Even though the cathode will transmit a smaller percentage of the light striking it, a larger amount of light will be available at its surface. Thus, a higher intensity of UV can be gained inside the central cavity  34  while preserving the flux into the annular dielectric body  30 .  
         [0023]    The UV reflective coating of the UV chamber  60 , the inner mesh cathode  66 , and the screen mesh sleeve  38  may be comprised of rhodium coated aluminum which can exhibit both high reflectance and a photoelectric effect. It may also be pure aluminum with a very thin protective film to protect it from oxidation but which will allow photoelectrons to escape. Such a film might be comprised of pure aluminum oxide, magnesium fluoride, or other fluoride material. The coating might also be comprised of an alkali metal with high UV reflectance in pure form and high photoelectric effect with a thin oxidation protective film such as a fluoride. The mesh size of the screen mesh sleeve  38  is chosen so that the preponderance of UV light reaching the sleeve is reflected. One way of achieving this is to keep the mesh size less than one tenth the size of the smallest wavelength to be reflected in a conductive mesh. In this fashion the mesh could serve as a particulate filter as well as a light reflector.  
         [0024]    Unlike UV2 and UV3, UV1 radiation forms ozone which is a toxic gas. Consequently, it is desirable that most, or all, of the UV1 radiation be absorbed within central cavity  34  so as to reduce the prospect of ozone leaking from purifier  10 .  
         [0025]    The radial extent of the central cavity  34  of UV chamber  60  may therefore be dependent on the largest wavelength of UV1 produced by lamp  50 . More particularly, in some embodiments of the invention, it may be desirable to have the most of the UV1 at no greater than 170 NM. In such instance, even with the radial extent of the inner cavity being on the order of a few mm, most of the UV1 radiation will be absorbed by the air of the inner cavity  34 . On the other hand, if the lamp produces UV1 at up to 185 NM, the radial extent of the inner cavity would need to be on the order of at least 10 cm for most of this radiation to be absorbed while traversing the inner cavity.  
         [0026]    The dielectric body  30  is formed so as to be porous to air. Consequently, an air flow path is defined from purifier air intake  14 , through the blower  20 , into the central cavity  34  of the UV chamber  60 , then through the annular dielectric body  30 , the screen mesh sleeve  38 , outer particulate filter  40 , outer chemically absorbent filter  42 , into the outer cavity  36  and out the air exhaust  16 .  
         [0027]    The dielectric body  30  is fabricated of a porous dielectric material which transmits UV radiation. Suitable materials could include:  
         [0028]    1. Silicon dioxide or pure silica in the form of fiber, sponge or frit  
         [0029]    2. Silicon dioxide or pure silica in the form of an aerogel or xerogel  
         [0030]    3. Silica gel granules  
         [0031]    4. Silica gel granules coated onto silicon dioxide fibers or frit  
         [0032]    5. Aluminum oxide (high purity) fibers, frit or granules  
         [0033]    6. Aluminum oxide (high purity) coated aluminum fiber  
         [0034]    7. Magnesium fluoride, calcium fluoride, barium fluoride, strontium fluoride or lithium fluoride powers, fibers, frits or coatings on transmitting or reflecting substrates.  
         [0035]    8. Quartz fiber, quartz fiber with silica gel coating.  
         [0036]    9. Sapphire fiber.  
         [0037]    Other dielectric matrices with air passageways may also be used. Two properties are, however, needed: that the dielectric body transmit UV (UV1, UV2 and UV3) and that ozone and water vapour attach to the dielectric body. The latter property increases the availability of these species for photo-catalytic reactions which convert UV light into hydroxyl free radicals.  
         [0038]    Water vapour and ozone will attach (i.e., bond) to all dielectrics to at least some extent. However, in some dielectric materials this property is particularly pronounced. For example it is well known that silica gel can absorb up to 30% of its mass of water vapor and ozone. For any dielectric material, the ability to attach to ozone and water vapour will increase if the material is provided with a large surface area. This suggests that the porous dielectric body should have relatively small pores to increase surface area (limited only in that the pores should not be so small as to inhibit the admission of the molecules of water and ozone).  
         [0039]    Optionally, the dielectric body is fabricated of a material which more strongly absorbs UV1 radiation than it does UV2 radiation. This may be desirable where the radial extent of the inner cavity is such that an appreciable portion of the UV1 radiation is not absorbed in the air of the central cavity  34 . One suitable dielectric material with this property is quartz which, depending on the grade, will absorb more strongly at wavelengths below 185 nm than for wavelengths above 185 nm. Another material which may be suitable is aluminum oxide, provided it has sufficiently high purity to transmit UV.  
         [0040]    The outer particulate filter  40  may be a pleated fabric filter or a fiber filter, which will trap biological contaminants such as virus, bacteria and moulds. UV light that transmits through the screen mesh sleeve  38  will sterilize the biological material on the filter. The outer chemically absorbent filter  42  may be a charcoal or zeolite filter, both of which will trap gaseous chemical contaminants as well as biological material. The life of outer filter  42  will be enhanced if it is placed after (in the air flow sense) the particulate filter. This will insure that its absorbent material pores do not clog with micro particles. Filter  42  will serve to remove any residual organic breakdown fragments from the photochemical reactions that oxidize volatile organic compounds in the dielectric body  30  insuring the safety of the device.  
         [0041]    Since UV2 and UV3 are expected to penetrate into the two outer filters  40 ,  42 , photocatalytic materials such as TiO 2  may be added to either or both of these filters. This will produce a continuous cleaning effect, which may serve to cleanse the filters of organic particulate material, enhancing their lifetimes. The dielectric body  30  will also produce hydroxyl free radicals in the gas phase which will be entrained in the gas flow and which will also serve to continuously clean the filters of particulate. The dielectric body may also be coated with a photocatalytic material such as TiO 2  to enhance the destruction of volatile organic compounds.  
         [0042]    Since UV3 will be readily transmitted through many materials, it is expected to make its way through the outer filter  42  and into the outer cavity  36 . The outer wall of housing  12 , which can also be coated with photo catalytic material, can then absorb UV3. Since the appropriate concentration of this material will act as a strong UV absorber, the outer wall will both absorb residual UV and add to the overall volatile organic compound removal by the device.  
         [0043]    In operation, both the blower  20  and UV lamp  50  are activated and the switch to voltage source  70  is closed. The voltage then polarizes inner mesh  66 , mesh sleeve  38 , and outer mesh  68  establishing an electric field between the inner mesh and the mesh outer mesh and between the mesh sleeve and the outer mesh. Further, the UV radiation from lamp  50  results in photo-emission of electrons from the inner mesh  66  such that this mesh acts as a cathode. These electrons are attracted toward the outer mesh (which therefore acts as an anode) but attach themselves to the dielectric body  30  along the way. The body retains the static charge owing to its high electric impedance. (Note that a dielectric body  30  fabricated of quartz fibres is particularly advantageous in this regard due the high electrical resistance of quartz). This effect enhances the electric field established in the body  30 . Blower  20  draws contaminated air from intake  14 , though intake dust filter  19 , and expels it into the central cavity  34  of UV chamber  60 . The pressurized air in the inner cavity  34  moves downstream from the inner cavity  34  through dielectric body  30  to the outer filters  40  and  42 . In doing so, much of the biological material (such as bacteria and viruses) in the air becomes trapped in the electric fields set up between meshes  38 ,  66 , and  68 . Further, water vapor and ozone in the air is absorbed by the dielectric body  30 . These materials are converted to OH both in the gas phase and on the dielectric fill. As the air passes through outer filters  40  and  42 , residual biological material and chemicals are removed from the air. Both are destroyed by the wash of residual UV2 and by OH that is entrained in the air.  
         [0044]    With lamp  50  activated, FIG. 3 a  graphically illustrates the intensity of UV1, UV2 and UV3 radiation as a function of radial distance in the lamp cross-section illustrated in FIG. 3. Turning to these figures, it will be seen inner cavity  34  of UV chamber  60  is flooded with UV1, UV2 and UV3 light (section  100 ) and the dielectric body  30  is flooded with (predominately) UV2 and UV3 light (section  102 ). A small amount of UV2 light passes through screen mesh sleeve  38  and into filters  40  and  42  (section  104 ). The UV reflective coatings of walls  24  and  32  as well as of housing  12  and inner mesh  66  enhance the intensity of UV radiation in the central cavity  34 . The UV reflective coating on walls  24  and  32  and on the screen mesh sleeve  11  enhance the intensity of UV radiation in the dielectric body  30 . The UV absorbing coatings of intake plenum  18  and exhaust plenum  62  help ensure that any UV light reaching these extremities of purifier are absorbed and do not leave the purifier (section  106 ).  
         [0045]    The UV radiation produced by lamp  50  produces the following chemical reactions.  
                 O   2     +   UV1     →     O   +   O             (   1   )                   O   2     +   O     →     O   3             (   2   )                   O   3     +     UV1                 or                 UV2       →       O   *     +     O      2               (   3   )                   O   *     +       H   2        O       →     20      H             (   4   )                   O   *     +     O   2       →     O   3             (   5   )                     H   2        O     +   UV1     →       O                 H     +   H             (   6   )                 O   +     O   2       →     O   3             (   7   )                               
 
         [0046]    As will be appreciated by those skilled in the art, these reactions have been simplified. In fact, other free radicals (such as H and HO 2 ) and compounds (such as H 2 O 2 ) will play roles.  
         [0047]    UV1 radiation produced by lamp  50  photo-dissociates oxygen (O 2 ) in the air resulting in the formation of ground state atomic oxygen (reaction (1)). This atomic oxygen is highly chemically reactive. A large portion of this atomic oxygen reacts with O 2  to form ozone (O 3 : reaction (2)). Ozone may be further photo-dissociated by UV1 or UV2 to form excited atomic oxygen (O*: reaction (3)). As will be appreciated by those skilled in the art, the optimum UV wavelength for dissociating ozone is about 250 nm. This excited atomic oxygen is even more chemically reactive than the oxygen formed in reaction (1) and rapidly attacks any water vapor present to form OH by reaction (4). The excited atomic oxygen can also be deactivated by oxygen (O 2 ) and nitrogen (N 2 ) in the air to form ground state atomic oxygen which then reacts with O 2  to reform ozone (reactions (5), (6) and (7)).  
         [0048]    Atomic oxygen, ozone (O 3 ) and hydroxyl radicals (OH) will react with organic compounds and break them into oxidized fragments. However, OH removes most organic compounds at rates up to ten orders of magnitude faster than ozone. Further, ozone is a toxic gas. OH, on the other hand, is not a hazard because it is so chemically reactive that is cannot survive more than a few second in normal air. Thus, unlike ozone, it cannot accumulate.  
         [0049]    In view of the forgoing it is desirable to create as much OH and a little ozone as possible. This means enhancing reactions (3) and (4) relative to reactions (5) to (7). This is achieved by dielectric body  30  which traps ozone, thereby increasing the rate of its photo-dissociation by reaction (3), and which traps water vapor and ozone for use in reaction (4).  
         [0050]    A highly porous dielectric body can absorb water or ozone to up to about 30% of its weight. The high absorbency and higher density of the dielectric body  30  relative to air results in an enhancement of the volume density of water and ozone of about three orders of magnitude. The dielectric body will absorb water vapor even when relative humidity is low making it unnecessary to add water vapor to the system.  
         [0051]    Because UV1 is primarily or entirely contained within inner cavity  34  of UV chamber  60 , it will be apparent that atomic oxygen is primarily formed in the inner cavity (reaction 1). Ozone will therefore be formed (by reaction 2) in the inner cavity and in the dielectric body. Because the body  30  is primarily radiated with UV2, little ground level atomic oxygen (O)—which generates ozone—will be formed in the body. Instead, the UV2 irradiating the body will primarily photo-dissociate the ozone trapped by the body resulting in excited atomic oxygen (reaction (3)). Given the high concentration of water vapor in the body  30  and the presence of excited atomic oxygen there, OH (by reaction (4)) is formed primarily in the dielectric body.  
         [0052]    If a suitable dielectric material is added to the inner cavity  34 , or if a porous UV1, UV2 and UV3 transmitting dielectric is coated onto the lamp walls, the production of OH by reaction (6) will increase relative to reactions (1) and (2). This enhancement results from the high absorption of H 2 O relative to O 2  onto the surfaces of many dielectrics (e.g. silica gel or aluminum oxide). This effect can be useful in embodiments in which it is desirable to further minimize ozone production.  
         [0053]    For example, by applying a pure silica gel coating  90  (FIG. 1) which is a few millimeters thick to the light emitting tube  46  of the lamp  50  of FIG. 1 or  4 , the H 2 O present in the coating will absorb all the UV1, converting the H 2 O directly to OH. This will reduce the ozone production but will not block UV2 and UV3 radiation from the lamp.  
         [0054]    As noted, the OH and atomic oxygen will fragment (oxidize) organic compounds thus destroying bacteria and viruses in the air. This will also result in fragmentation of other volatile organic compounds and organic pollutants which may be in the air, thereby reducing their concentration.  
         [0055]    Organic compounds may stick to the dielectric body  30 . However, OH will rapidly attack these surface contaminants thereby fragmenting these materials. If the fragmented materials continue to stick, they continue to be fragmented until, in many cases, water vapor and carbon dioxide results. Carbon dioxide (CO 2 ) is not absorbed by zeolite or charcoal. Thus, where the outer chemically absorbent filter  42  is fabricated of such materials, CO 2  will float away and out of the purifier. Since the concentrations of volatile organic compounds are small (less than a part per million) compared to the ambient concentration of CO 2  (about 300 parts per million), any increase in CO 2  caused by the oxidation of volatile organic compounds by the purifier is negligible compared to other sources and will pose no health risk.  
         [0056]    The UV light itself will also act to sterilize biological materials in the intake air. This is particularly so in respect of material trapped by the electric field in the body  30  or trapped in outer filter  42  in view of the increased time during which such biological materials is exposed to the UV light.  
         [0057]    Ozone reaching the outer filter  42  is readily absorbed. While it is absorbed on the filter it will be broken down by the (small) amount of UV (UV2) radiation reaching outer filter  42  and will form OH. This reaction can be facilitated by adding a catalytic mesh (with a material such as TiO 2 ) to these filters.  
         [0058]    Screen mesh  38  could be replaced with a porous wall formed of fused UV reflecting grains having a diameter approximating that of the UV2 radiation. These UV reflecting grains could, for example, be spheres of aluminum, high purity silica, or grains of barium sulfate. It might also be fabricated out of aerogel matrices with the desired average pore sizes.  
         [0059]    While lamp  50  is described as emitting UV1, UV2 and UV3 radiation, air will still be purified by the purifier  10  (albeit not as efficiently or completely) if the lamp emitted solely UV1 or UV2 radiation. Further, two or three lamps could be provided, one which emits UV1 radiation into the airflow path upstream of the dielectric body, a second one which emits UV2 light into the dielectric body itself and a third one which emits UV3 radiation for use in the outer filters and outer wall.  
         [0060]    [0060]FIG. 4 illustrates an air purifier  100  in accordance with another embodiment of this invention. Turning to FIG. 4, wherein like parts have like reference numerals, housing  112  of purifier  100  is tubular. Air inlets  114  in one end of the housing feed to blower  120 . An outlet plenum  22  extends between the exhaust of the blower and the central cavity formed by the annular dielectric body  30 . An annular plate wall  132  abuts the end of the dielectric body  30  remote from plenum  22 . Baffles  180  extend between housing  112  and an end of annular particulate filter  142 . A chemically absorbent outer filter  144  extends between particulate filter  142  and air exhaust  162 . Lamp  50  through the annulus formed by the particulate filter  140  and the annulus formed by dielectric body  30 . As well as the inner and outer annular wire mesh  66 ,  68  associated with the dielectric body, there is an inner and outer wire mesh  166 ,  168  associated with the filters  140 ,  142 . Like meshes  66 ,  68 , meshes  166 ,  168  are polarised with a voltage source (not shown). With purifier  100 , when blower  120  is activated, air flows out from the blower into dielectric body  30 , then out from the body to between body  30  and the wall of housing  112 . Air then passes into particulate filter  40 , then through outer filter  42  and out exhaust  162 . Unlike purifier  10  (FIG. 1), there are no filters surrounding dielectric body  30 . Instead, filters  140 ,  142 , while concentric with lamp  50 , are separate from the body  30 . With this arrangement, UV light falls directly on the particulate and chemical filters. Appropriate screen meshes could be added to enhance UV2 in the cavity  134  inside the two filters  140 ,  142 . In addition, photoelectric effect mesh electrodes  166 ,  168 , if added to filters  40  and  42 , enhance their effectiveness. Instead of a mesh electrode, one method of producing a cathode might entail a coating of cesium iodide or similar material on an inner face of one of the filters. This coating would absorb wavelengths shorter than 185 NM and produce photo-electrons at such wavelengths. It would also be transparent at wavelengths longer than 200 NM. Thus, the cathode would inhibit the emission of UV1 past filters  140 ,  142  by blocking the ozone producing UV but still allow UV2 and UV3 to be emitted which would sterilize the filters  140 ,  142  and aid photochemical processes.  
         [0061]    A basic purifier in accordance with this invention would comprise a source of UV which irradiates a suitable dielectric body interposed in the airflow path of the purifier. The effectiveness of the purifier is enhanced by the addition of a cathode and anode to attract and trap charged particles for UV irradiation. Further improvement in efficiency is obtained with the addition of the each of the other features described, such as the described filters and coatings.  
         [0062]    Other modifications will be apparent to those skilled in the art and, therefore, the ambit of the invention is set out in the claims herefollowing.