Filtering device for the nasal respiratory airways Description The present invention pertains to a device for filtering the air inhaled through the nasal openings and for retaining dust, extraneous materials, bacteria, pollen and like particles which can cause damages to the respiratory organs of the user. Many people suffer respiratory ailments and allergic reactions when are exposed to various pollens, dust or other particles entrained in the ambient air during certain periods of the year. There are also numerous workers, as for example persons working in construction, mining, medical professions, or cleaning services, who are continuously endangered by breathing large quantities of pollutants contained in the inhaled air. Various devices are known for protecting respiratory airways against smokes, pollu¬ tant gases, dust or other noxious contaminants in the air. Generally, a face mask, placed over the mouth and nasal respiratory openings in or- der to filter the air people breath, is the protecting device most widely used by the persons who are operating in contaminated work environments. The most commonly used masks are made of filtering material ( for example, of cotton or paper material ) with a rectangular flat-folded shaped form or with a semi¬ rigid cup-shaped form. Such masks are held over the user's mouth and nose by elastic cords which engage the back of the head or ears. The major disadvantage of this type of face masks is that it does not filter effecti¬ vely all of the air that is inhaled by the user because these masks do not conform ex¬ actly to the facial contour and do not form a good seal against the curves on the face of the wearer. For the purpose of overcoming this drawback, masks have been realized with pli¬ able bands of metal which can be shaped to fit to the nasal contour of the user in or¬ der to hold the masks in its correct filtering position. But, the usefulness of this expedient is frustrated by the continuous movements of the head and of the miming muscles that force the shape of the retaining band and loose its contact with the nose of the wearer. Further disadvantages of this type of mask are produced by the fact that it impedes a correct exhalation of the hot and wet air by the user. Consequently, the exhaled air by-passes the filter and can find a prefential outlet through the passageways formed around the nose and along the curves of the face. The hot and wet flow of exhaled air increases the temperature and the perspiration of the face and causes condensation on screen shields or eyeglasses of the wearer. It is obvious to understand that the sweat zones of the face collect dust and pollut¬ ants which cause discomfort to the user and could start chemical reactions irritating the skin of the face and the ocular mucosa. A further disadvantage of this type of masks is that it interferes with basic activities of the user, as talking, drinking, eating, and performing any other oral functions. Finally, these masks are uncomfortable to carry in a pocket or in a bag during stor¬ age and could cause embarrassment to the user because they attract the attention and the curiosity of the people during use. It is, therefore, the principal object of the present invention to provide a filtering de¬ vice for overcoming the above stated drawbacks while improving the quality of the inhaled air. These and other objects are attained according to the invention by a filtering device having the hereinafter claimed features. The objects and the features of the filtering device according to the invention will appear clear from the following description with reference to the accompanying schematic drawing, wherein: - the Figure 1 is a top perspective view of the filtering device according to the pre- sent invention; - the Figure 2 shows a side perspective view of the filtering device of figure 1; - the Figure 3 is a front view of the filtering device of figure 1; - the Figure 4 is a front/down view of the filtering device according to the present invention; - the Figure 5 is an embodiment of the filtering device according to the present in¬ vention; - the Figure 6 is an alternate embodiment of the filtering device according to the present invention; - the Figure 7 is a side and partially sectioned view of the filtering device according to the present invention in its working position; - the Figure 8 is a front view of the filtering device of figure 7. The drawings illustrate a preferred embodiment of the filtering device according to the present invention which is generally designated by the reference numeral 10. The filtering device substantially comprises two annular filter-supporting elements 11, filter elements 20 and elastic retaining means 30 which join together the said supporting elements 11 and retain the filtering device 10 in its correct working posi¬ tion. The filter-supporting element 11 has an elliptical shape that is particularly adapted to seat transversely within the nasal cavity C as illustrated in the Figure 7. In this position, the outer contour of the peripheral edge of the filter-supporting element 11 forms a seal with the surface of the nasal cavity C and forces all of the air inhaled by the user to pass through the central opening of the annular filter-supporting ele¬ ment 11 . The peripheral edge of the annular filter-supporting element 1 1 has a circular cross- section (Figures 2 and 3) in order to improve the sealing contact between the annu¬ lar element and the inner surface of the nasal cavity as well as to retain the filter- supporting element in its correct filtering position. Alternatively, the cross-section of the annular filter-supporting element 11 may be frusto-conical in shape. The outer surface of the peripheral edge in the annular filter-supporting element 11 may be provided with variously shaped protuberances to improve the accomodation of the filtering device 10 in the nasal cavity C. The annular filter-supporting element 11 is preferably made of materials which are sufficiently elastic to conform to the inner surfaces of the nasal cavities C, such as, for example, thermoplastic elastomeric materials and/or silicone elastomeric mate¬ rials. Each filter-supporting element 11 comprises at least a filtering element 20 extend¬ ing across its central opening in such a way that the flow of inhaled air is forced to pass through said filtering element 20. In the embodiment shown in Figure 5, the filtering element 20 has a single filtration layer but it will be understood that filtering elements having two or more spaced layers, as illustrated in Figure 6, may also be employed. Such filtering elements may have different porosity, in order to capture various sized polluting particles and they are, however, of such a thickness that they do not limit breathing to the point that the user is forced to breath through the mouth by- passing the filter elements in the nose. In the embodiment shown in Figures, the filtering element 20 present a flat shape but it will be understood that filtering elements 20 having a corrugated or pleated contour or an hemispherical shape, in order to increase the overall surface area of the filter, may also advantageously be employed. The filtering element 20 can be molded in one piece, together with the filter- supporting element 11, for example by means of known injection molding proc¬ esses, according to an embodiment in which the filtering device is preferably of dis¬ posable type. Alternatively, the filtering element 20 may be made removable from the filter- supporting element 11 according to embodiments which permit the sanitization and the reutilization of the same filtering device 10. For example, the filtering element 20 may be made as inserts which are joined to the filter-supporting element 11 by means of a mechanical connection and/or by the interposition of adhesive material. It is obvious that the mechanical connection and the interposition of adhesive material are such that they prevent the accidental de¬ tachment and inhalation of the filter element 20. Alternatively, the filtering element 20 may be made in the form of an elastic bag that can be removably wrapped around the annular filter-supporting element 11. The filtering element 20 may be made of different materials, for example cellulosic material or plastic material, with different porosity or different filter capacity de¬ pending upon the poilutans to be filtered. Preferably, the material forming the filtering element is chosen in such a manner that it allows mounting with the material forming the filter-supporting element by bonding, welding as well as mechanical connecting processes which are sufficiently known in the art as not to require further description. However, it will be understood that the features of the invention may also find an advantageous application in filtering devices 10 made of materials which are differ¬ ing from those described above. The filtering device according to the present invention comprises an elastic retain¬ ing element 30 formed into the shape of a "U". Such retaining spring 30 is adapted to engage the nasal septum, to retain the filtering device in a correct working posi¬ tion, to prevent accidental detachment, displacement or dislodgement of the filter¬ ing device to an unfiltering or trapped position in the nasal cavity and to facilitate its removal from the nose of the user. It is to be pointed out that the dimensions and the elasticity of the "U" shaped spring 30 are selected so that it may be adapted for use in a sufficient range of persons. The material used for the filtering device 10 may be made in skin tone colors of various human races but it may be realized in bright colourings and/or with various decorations. For example, the external surface of the elastic and "U" shaped retain¬ ing element 30 may be chromium-, gold- or silver-plated and/or enriched by mount¬ ing of ornamental stones in order to simulate the insertion of decorative rings or pins through skin perforations. In use, the filter-supporting elements 11 and their filtering elements 20 are inserted within the respective nasal cavities C of the user so that the elastic and "U" shaped retaining element 30 engages the central septum of the nose as illustrated in the Fig¬ ures 7 and 8. Obviously, the elastic and "U" shaped retaining element 30 may be provided with means for regulating the insertion depth of the filtering devices 10 within the nasal cavities C of the user. Once the insertion is completed, all of the air being inhaled by the user through the nasal respiratory airways is forced to pass through the filtering elements 20. Medical substances such as, for example, decongestant or antihistaminic substances, and/or aromatic ingredients such as, for example, lavender or mint based solutions, could be added to the filtering element 20 for benefiting the health of the user 12. After use or once the filtering elements 20 have been exhausted, the filtering device 10 can be removed from the nose and integrally eliminated in the case of disposable realization. In the case of repeated use, the used filtering elements 20 may be replaced with new filtering elements 20, then the used filtering elements may be sanitized and reused until their final replacement determined by deterioration or damage of the plastic material constituting them. It will be appreciated that a wide variety of filter-supporting elements 11 and filter¬ ing elements 20 may be prearranged with shapes, dimensions, filtering pores which are anatomically shaped and sized for use in different nasal morphologies and against different atmospheric pollutions. From what stated, it will be seen that the primary object of present invention is achieved by a filtering device 10 which overcomes the disadvantages associated with the known face masks of the prior art and improves the quality of inhaled air. It is well understood that modifications and variations may be made to the filtering device 10 forming the object of the present invention without departing however from the scope defined by the following claims. For instance, the filtering device 10 could also be formed by separate filter- supporting elements 11 each of them is provided with an elastic and "C" shaped re- taming element which is adapted to engage the outer edge of the nostril.