FIELD OF THE INVENTION

The present invention relates to a face mask, having an oral and nasal air filter able to remove particles and aerosols from inhaled air, while providing excellent breathability and comfort for extended periods of time. The invention also relates to a filter material suitable for use in such a device.

BACKGROUND TO THE INVENTION

The spread of Influenza through the global populous is recurrent in history, the most well known being the "Spanish flu" pandemic of 1918, the avian Influenza A H5N1 virus ("Bird flu") epidemic of 2004, and the presently predicted H1 N1 viral strain ("Swine flu") pandemic of 2009. The three strains which cause most pandemics belong to group A of the influenza virus and, unlike the other two groups (B and C), this group infects a vast variety of animals (poultry, swine, horses, humans and other mammals). Influenza A virus continues to cause global problems, both economically and medically.

Much has been done to control and prevent another pandemic from occurring with many anti-influenza products (vaccines and treatments) currently on the market. Presently, Amantadine is the principal antiviral compound against Influenza infections, but its activity is restricted to Influenza A virus. Anti-neuraminidase inhibitors, such as Zanamivir (Relenza) and Oseltamivir (Tamiflu), are a new class of antiviral agents licensed for use in the treatment of both Influenza A and B infections. The role of vaccines and antivirals in a pandemic may be limited due to the time and cost involved in production and the current limited supply. Moreover, their safety is questionable. With the recent news of a probable H1 N1 pandemic the need to prevent any opportunities of transmission of the virus in populous environments has risen.

The inhalation of air contaminated by harmful virus and/or other micro-organisms is a common route for infection of human beings, particularly health and others caused to work with infected humans or animals. Air exhaled by infected patients is a source of contamination. At the present time the risk of infection by the so called "Swine flu" H1 N1 virus is of particular concern. Masks incorporating a suitable filter material would be ideal for use as a barrier to prevent species-to-species transmission of the virus. Air filters believed to remove such virus and/or other micro-organisms are known. One type of such a filter comprises a fibrous or particulate substrate on which is deposited, upon the surface and/or into the bulk of such fibres or particles, a substance which captures and/or neutralises virus and/or other micro-organisms of concern.

Examples of disclosures of such filters are listed in the following. US 3,871 ,950 and US 4,181 ,694 disclose hollow fibres of acrylonitrile polymers for ultrafilter use, primarily for filtering aqueous media. US 4,856,509 discloses a face mask wherein select portions of the mask contain a viral destroying agent such as citric acid. US 5,767,167 discloses aerogel foams suited for filtering media for capture of micro organisms such as virus etc. US 5,783,502 discloses a fabric substrate with anti viral molecules, particularly cationic groups such as quaternary ammonium cationic hydrocarbon groups bonded to the fabric. US 5,851 ,395 discloses a virus filter comprising a filter material onto which is deposited a virus-capturing material based on sialic acid (9-carbon monosaccharides having a carboxylic acid substituent on the ring). US 6,182,659 discloses a virus-removing filter based on a Streptococcus agalactiae culture product. US 6,190,437 discloses an air filter for removing virus from the air comprising a carrier substrate impregnated with "iodine resins". US 6,379,794 discloses filters based on glass and other high modulus fibres impregnated with an acrylic latex. US 6,551 ,608 discloses a porous thermoplastic material substrate and an antiviral substance made by sintering at least one antiviral agent with the thermoplastic substance. US 7,029,516 discloses a filter system for removing particles from a fluid comprising a non- woven polypropylene base upon which is deposited an acidic polymer such as polyacrylic acid. US 2004/0250683 discloses a filter material comprising a network of fibres with an acidic substance deposited thereon, which may be an acrylic polymer. US 2005/0247608 discloses a filter block which may be treated with various anti viral polymers, principally cationic polymers.

WO 2001/07090 discloses a filter for removing micro-organisms comprising a substrate having a reactive surface and a polymer on its surface which includes cationic groups for attracting micro organisms. WO 2002/058812 discloses an air filter with microencapsulated biocides. WO 2003/039713 discloses a filter material said to have an anti pathogenic effect, including an effect against virus, based on a fibrous substrate partly coated with a polymer network containing pendant functional groups which may be acidic groups. WO 2005/070242 discloses an inhalation filter made of fibres treated to impart an electrical charge to catch particles such as virus. GB 2035133 discloses a membrane filter with a water-insoluble polymer, preferably a PVA, on its surface. Use of such a filter material in gas mask cartridges is suggested. JP 2001/1621 16 discloses an antibacterial filtration medium in which a self- cross-linking acrylic resin is used to bind a silver-organic idine antibacterial agent to a fibrous substrate. JP 2005/198676 discloses the use of a water-hardenable resin emulsion to bind citric acid to an antiviral face mask.

There is an ongoing need to improve such filters, particularly in view of the risks of transmission of "swine flu" and other pathogens such as Bird flu, Influenza, TB, SARS, and protection from gaseous and particulate pollutants. There is also a need to provide masks that are comfortable to the wearer and can be worn for long periods of time without the build up of moisture in which bacteria and fungi readily incubate, or inducing an allergic reaction. The present inventors have identified filter materials which facilitate removal of harmful virus and/or other micro-organisms (e.g. bacteria, viruses, fungal spores) and particles (e.g. dust) from inhaled air, while at the same time removing malodours, enabling the use of such materials in an improved air pollution face mask and filter.

SUMMARY OF SOME EMBODIMENTS OF THE INVENTION

One embodiment of the invention is filtration material (51 ) for an air pollution breathing mask comprising:

- a core layer (52) that is a non-woven material impregnated with sodium chloride,

- an atmosphere-facing outer layer (58) formed from a silver nano-particle-coated non- woven material,

- a skin-facing outer layer (60) formed from a silver nano-particle-coated non-woven material,

- a non-woven filtration layer (54) disposed between one side of the core layer (52) and the atmosphere-facing outer layer (58), and

- a non-woven activated carbon layer (56) disposed between the other side of the core layer (52) and the skin-facing outer layer (60). Another embodiment of the invention is filtration material (51 ) as described above, wherein the non-woven material in any of the core layer (52), atmosphere-facing outer layer (58), skin-facing outer layer (60) comprises a polypropylene, terylene, polyethylene, polyester, nylon, PET or PLA non-woven material. Another embodiment of the invention is filtration material (51 ) as described above, wherein the non-woven filtration layer (54) comprises melt-blown polypropylene non- woven material. Another embodiment of the invention is filtration material (51 ) as described above, wherein the non-woven activated carbon layer (56) comprises a polypropylene, polyethylene, polyester, nylon, PET or PLA non-woven material.

Another embodiment of the invention is filtration material (51 ) as described above, wherein the atmosphere-facing outer layer (58) and skin-facing outer layer (60) each comprise a polypropylene, polyethylene, polyester, nylon, PET or PLA non-woven material.

Another embodiment of the invention is an air-pollution face mask (100) comprising:

i) a filter segment (50) comprising the filtration material (51 ) as defined above adapted to cover the mouth and nostrils of the user,

ii) a support segment (10) adapted for dismountable placement around the head of the user, and adapted to align the filter segment (50) over the mouth and nostrils of the user, wherein the filter segment (50) and support segment (10) are joined by a seam in abutting alignment, and

iii) one or more exhalation valves located in the filter segment (50).

Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein the support segment (10) is formed essentially from the elastic foam preferably neoprene rubber, poly vinylchloride, polyethyl, polyurethane or polyester.

Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein the elastic foam is overlaid by a textile element on one or both sides. Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein the support segment (10) comprises a central portion (13) provided with a cut-out (1 1 ) for the filter segment (50) encompassing at least partially the nose and mouth, which central portion extends horizontally in opposing directions to form the tails of a strap (12, 12'). Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein the ends of the tails bear reciprocal parts of a fastener arrangement, preferably a Velcro® fastener (14, 16). Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein the filter segment is disposed with an exhalation valve (20).

Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein the exhalation valve (20) is configured to direct exhaled air in a dorsal direction.

Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein the support segment (10) is provided with a plastically deformable nose clip (40).

Another embodiment of the invention is an air-pollution face mask (100) as described above, wherein filter segment (50) is raised, preferably domed.

FIGURE LEGENDS FIGs. 1 A, B and C depict an embodiment of a face mask of the invention. FIG. 1 A shows the front (atmosphere facing) view, FIG. 1 B a side view, and FIG. 1 C a rear (skin contacting) view.

FIG. 2 shows a transverse cross-section through the filtration material of a filtration segment of the invention to reveal the individual layers

FIG. 3 shows a rear (skin contacting) view of a schematic illustration of a support segment devoid of the filter segment.

FIG. 4 shows an exploded view of an embodiment of an exhalation valve of the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to a first aspect of this invention, and with reference to FIGs. 1 a to c there is provided an air-permeable face mask 100 of a shape suitable to be placed over a user's mouth and nose and to sealingly contact the user's face, provided with a support segment 10 to hold the mask in place on the user's face, and comprising a filter segment 50 positioned such that inhaled and/or exhaled air of the user passes through the filter segment 50. The filter segment 50 is preferably raised in a sub-region (e.g. central region), in a direction away from the skin facing side of the mask e.g. domed so as to contact the skin only at the periphery. One or more exhalation valves 20 may be disposed in the filter segment 50. An adjustable nose clip 40 may be provided to adapt the shape of the mask over the nose. The support segment 10 is provided with a strap portion 12 which terminates in a pair of fasteners (e.g. Velcro) 14, 16 that fix the mask 100 around the back of the head. The filter 50 and support segments 10 may be in abutting alignment, joined at least partially at their edges by way of a seam 30. The support segment 10 positions the filter segment 50 to intercept a stream of inhaled breathing air. The user is preferably typically human, though it is not necessarily limited thereto, for example, it may be applied to a pet animal such as a dog, cat etc.

Thus, one embodiment of the invention concerns an air-pollution face mask 100 for wearing over the oral and nasal orifices of a subject user, comprising:

i) a filter segment 50 comprising the filtration material 51 as defined below, adapted to cover the mouth and nostrils of the user,

ii) a support segment 10 adapted for dismountable placement around the head of the user, and adapted to align the filter segment 50 over the mouth and nostrils of the user, wherein the filter segment 50 and support segment 10 are joined by a seam in abutting alignment, and

iii) one or more exhalation valves located in the filter segment 50.

According to a second aspect of the invention, and with reference to FIG. 2, there is provided a filtration material 51 for filtering contaminant-bearing air. The filter segment 50 of the face mask 100 comprises the filtration material 51 that is layered and comprises:

- a core layer 52 that is a non-woven material impregnated with sodium chloride,

- an atmosphere-facing outer layer 58 formed from a silver nano-particle-coated non- woven material,

- a skin-facing outer layer 60 formed from a silver nano-particle-coated non-woven material,

- a non-woven filtration layer 54 disposed between one side of the core layer 52 and the atmosphere-facing outer layer 58, and

- a non-woven activated carbon layer 56 disposed between the other side of the core layer 52 and the skin-facing outer layer 60.

Contaminant-bearing air (e.g. carrying bacteria, viruses, fungal spores, dust, aerosols) first comes into contact with the atmosphere-facing outer layer 58 which rapidly neutralizes airborne pathogens such as bacterial and fungal particles. Contaminants that pass through are trapped by the filtration layer 54. The subsequent core layer 52 processes the inhaled air by invigorating it with sodium chloride. The activated carbon layer 56 removes remaining stale odours. The skin-facing outer layer 60 neutralizes bacterial and fungal particles exhaled by the wearer, and eliminates breeding of pathogens in the moist and warm environment of the mask. Thus, as the air passes through the filtration material 51 , the contaminants are neutralised, securely trapped and adsorbed, the air deodourised. An advantage of the filtration material 51 employed in the filter segment 50 is that an air- permeable mask 100 can be made in a light-weight form. The support segment 10 in abutting alignment with the filter segment 50 preferably has an elastic property. Pressure can be applied along the peripheral edge of the non-expandable filter segment 50, holding the filter segment 50 in intimate sealing contact with the wearers face. Moreover, it can support a raised filter segment 50 (e.g. domed shaped) that creates a space over the nose and prevents contact of most of the filter segment 50 with the skin i.e. sealing contact is effected only at the periphery of the filter segment 50.

The core layer 52 of the filtration material 51 is formed from a non-woven material impregnated or coated with sodium chloride. The non-woven is implicitly porous i.e. air- permeable. Examples of non-woven materials include terylene, polypropylene, polyethylene, polyester, nylon, PET and PLA; preferably the filtration material 51 is formed from terylene or polyester. The density of the non-woven impregnated or coated with sodium chloride may be 10 g/m ² , 20 g/m ² , 30 g/m ² , 40 g/m ² , 50 g/m ² , 60 g/m ² , or value in the range between any two of the aforementioned values, preferably between 40 and 60 g/m ² , most preferably 50 g/m ² . The core layer 52 has a sheet or pad structure having two opposing sides; one side being adjacent to the skin and an opposing side being adjacent to the atmosphere.

In a preferred manufacturing process to make the core layer 52 of the invention, the sodium chloride may be incorporated in a liquid vehicle, the non-woven material may then be wetted with the resulting liquid composition, and the liquid vehicle allowed or caused to evaporate to thereby leaving the sodium chloride deposited on the non-woven.

Such a resulting liquid composition is herein termed a "loading solution". The liquid vehicle may be aqueous, e.g. water or a mixture of water and an alcohol (e.g. methanol, ethanol, propanol). The sodium chloride may be dissolved or suspended in the liquid vehicle. The loading solution may incorporate any additional substances. This loading solution may also be adjusted to a suitable pH if necessary, for example pH 2 - 3, typically -2.5. For example, an alkali such as sodium hydroxide, or a buffer such as a citrate e.g. sodium citrate, may be included into the loading solution to achieve such a pH. Wetting of the non-woven may be achieved by simply coating the non-woven with the so- formed dispersion, e.g. dipping the substrate into the loading solution. Alternatively the non-woven may be sprayed with the loading solution. On an industrial scale spraying is preferred for convenience. The wetted non-woven may then be dried, e.g. by evaporation in the ambient air or in a drying tunnel. A suitable drying temperature in such a tunnel is less than 100 deg C.

A loading solution suitable for use in the process for making the filter material is a further aspect of this invention. For example such a loading solution may be made in the liquid vehicle comprising 0.5 - 10 wt%, typically 1 - 5 wt% of sodium chloride; 0.5 - 10 wt%; and 0 - 4.0 wt% surfactant, e.g. 1 - 2 wt% surfactant, e.g. Polysorbate or Tween 20. A suitable stabilizer is EDTA disodium salt at 0.01 %. Accordingly, the core layer 52 may be a product obtainable or obtained by the process of wetting the air-permeable non-woven with the loading solution, and causing or allowing the liquid vehicle to evaporate therefrom so as to deposit substances in the loading solution onto the substrate. In use, the sodium chloride in core layer 52 permeates the inhaled air with salt which has the effect of dissolving phlegm in the bronchial tubes and killing micro-organisms that cause infections. This makes breathing easier, alleviates sneezing, coughing, and shortness of breath. The filtration material 51 , therefore, is especially suited to suffers of respiratory disorders associated with asthma, allergy, bronchitis, emphysema, COPS, cystic fibrosis etc. For the first time, suffers of respiratory disorder can wear an air- purification mask that does not exacerbate the condition, and which ameliorate the symptoms. Studies have found that allergic disease are an increasing problem in Western Society and reports show, for example, that approximately 20% of the UK population (and about 75% of those with asthma) suffer with some form of rhinitis. The use of sodium chloride within the mask utilises the benefits of salt therapy to maintain an optimum respiratory function and to help the wearer breath more easily. By using the mask, the wearer not only benefits from an effective barrier for preventing infection from

microorganisms which the other layers provide, but be assisted in the recovery of respiratory disorders. A non-active ingredient may also be present that enhances the effectiveness of the sodium chloride.

The separate atmosphere-facing outer layer 58 and skin-facing outer layer 60 are each formed from a non-woven material impregnated or coated with silver nanoparticles. The non-woven is implicitly porous i.e. air-permeable. Examples of non-woven materials include polypropylene, polyethylene, polyester, nylon, PET and PLA; preferably the non- woven material is formed from polypropylene. Such a material may be in the form of a non-woven sheet or pad having two opposing sides. The density of the non-woven impregnated or coated with silver nanoparticles may be 10 g/m ² , 20 g/m ² , 30 g/m ² , 40 g/m ² , 50 g/m ² , 60 g/m ² , or value in the range between any two of the aforementioned values, preferably between 40 and 60 g/m ² , most preferably 50 g/m ² .

Nanosized silver particles are known as such to disinfect the air. They are highly sensitive to oxygen, resulting in formation of partially oxidized silver with chemisorbed silver ion, which has an additional contribution to the bactericidal properties of silver. Nano-sized silver can be used to safely and effectively inhibit the growth of bacteria, mould and fungi. Typically, the silver nanoparticles having a diameter of 100 nm or less e.g. 90 nm, 80 nm, 70 nm, or less are most effective; they are applied as a colloid to the non-woven substrate. The preparation of non-woven material coated with silver nano-particles is well known in the art and is described, for example, in WO 2009/070123, which is incorporated herein by reference. Atmosphere-facing outer layer 58 and/or skin facing outer layer 60 may be dyed to blend in or match with the colour of the support segment 10.

The atmosphere facing outer layer 58 provides the first level of defence, inactivating bacteria present in the air that is drawn through the mask. Bacteria that manage to pass through the atmosphere facing outer layer 58 are trapped by the filtration layer 54, thus making it possible to prevent infection by bacteria and other pathogens. The atmosphere facing outer layer 58 also acts as a barrier against self-contamination from the wearers own body, for example, from the hands. The polypropylene, when dyed, further reduces the water / moisture absorbs ion rates; typically they are < 0.01 % over a 24 hour period. The skin facing outer layer 60 destroys bacteria released as droplets from the wearer's mouth, preventing their dispersal and reducing the spread of infection. Because the skin facing outer layer 60 comes into direct contact with the skin, the non-woven material adopted provides both a comfortable, non-irritant feeling, and does not readily generate fuzz. While the skin facing outer layer 60 of the mask utilizes the same basic material as the atmosphere facing outer layer 58, it has a further advantage over conventional masks. Being close to the wearers face and with the fabric being dyed, the absorption of perspiration from the wearer is minimised, hence providing more comfort for the wearer facilitating prolonged use. It also reduces exposure of the wearer to contamination due to the bacteria static nano-finish which prevents bacterial and fungi from culturing in the warm and moist environment of the mask.

A non-woven filtration layer 54 is formed from a non-woven material. The non-woven is implicitly porous i.e. air-permeable. Examples of non-woven materials include

polypropylene, polyethylene, polyester, nylon, PET and PLA; preferably it is formed from polypropylene, preferably melt-blown polypropylene. Such a material may be in the form of a non-woven sheet or pad having two opposing sides. The density of the non-woven may be 20 g/m ² , 30 g/m ² , 40 g/m ² , 50 g/m ² , 60 g/m ² , 70 g/m ² or a value in the range between any two of the aforementioned values, preferably between 50 and 70 g/m ² , most preferably 60 g/m ² . Advantageously, the filtration layer 54 is formed from P2 filter material. It is non-toxic, has no smell, exhibits high filtration efficiency and has low breathing resistance.

Within the mask, the filtration layer 54 provides the second level of defence against specifically microorganisms, and due to the density of its structure, against certain types of dust. The atmosphere-facing outer layer 58 provides additional protection and

performance of the filtration material 51.

A non-woven activated carbon layer 56 is formed from a non-woven material containing activated charcoal, preferably as a particulate. The non-woven is implicitly porous i.e. air- permeable. Examples of non-woven materials include polypropylene, polyethylene, polyester, nylon, PET and PLA; preferably it is formed from polypropylene. The density of the non-woven impregnated or coated with activated charcoal may be 30 g/m ² , 40 g/m ² , 50 g/m ² , 60 g/m ² , 70 g/m ² , 80 g/m ² or value in the range between any two of the aforementioned values, preferably between 60 g/m ² and 80 g/m ² , most preferably 70 g/m ² . Such a material may be in the form of a non-woven sheet or pad having two opposing sides. The activated charcoal is efficient at preventing passage of micro-organisms and absorbing pollution e.g. from volatile organic compounds. The respective layers 52, 54, 56, 58, 60 may be bonded together, for example, using heat treatment, adhesive or the like. Alternatively, the layers may be joined together along a seam around their respective edges. The seam may be stitched, glued, ultrasonically welded, or joined using any known technique.

Typically the filtration material 51 may be in sheet or pad form, suitable for use in the above-mentioned face mask. Such sheet or pad form materials can be made into a suitable shape for a face mask of generally known shape in a known manner. Face masks can be made from such materials using known mask-making processes, e.g. moulding, folding, joining one or more parts using a seam.

As shown in FIG.s 1a to 1 c, the filter segment 50 is formed from two pieces of filtration material 51 joined along their mutually-facing edges with a sewn seam so as to form a raised or domed-shaped filter segment. The filter segment 50 is preferably raised in a sub- region (e.g. central region), in a direction away from the skin facing side of the mask to create a space between the inside of the mask and the nose and mouth. This allows the segment to contact the skin only at the periphery, contrary to systems of the art which are in complete contact with the wearers face and hence skin. Direct contact with the skin generates a moisture build up from perspiration, and this in itself can create conditions for bacterial growth and aid the receptiveness of air bourn microorganisms. Masks of the art do not lend themselves towards usage beyond a working shift (e.g. 8 hours), whereas the present mask can be used for extended periods and repeatedly without discomfort, and without risk from bacterial and/or fungal growth in otherwise ideal incubation conditions.

In a further aspect of this invention a process for making a face mask 100 is provided comprising providing a filtration material 51 as described herein and forming the filtration material into a face mask 100 or a filter segment 50 therefor. The support segment 10 is attached to the filter segment 50, and is provided to affix the mask 100 by a strap to the head of the user and position the filter segment 50 over the nose and mouth. Preferably, the support segment 10 comprises a central portion 13 as shown in FIG. 3 configured for placement at least over the bridge of the nose. The central portion 13 preferably extends downwards towards the mouth, and is provided with a cut- out 11 for the filter segment encompassing at least partially the nose and mouth. The edge of the cutout 11 is joined by way of a seam to the filter segment 50. The profile of the cutout 11 compliments that of the filter segment 50. The support segment 10 may extend horizontally in opposing directions from the central portion 13 to form a pair of wing-like appendages 10', 10" that are the tails 12, 12' of a strap. The each end of the strap tails 12, 12' may bear reciprocal parts of a fastener arrangement 14, 16, such as Velcro® hook and loop material.

The support segment 10 is preferably in abutting alignment with the filter segment 50 meaning the respective segments do not overlap beyond the joining seam 30. The arrangement limits the quantity of filter material required to cover the mouth and face, and allows the filter segment to be raised or domed. The seam may be stitched, ultrasonically welded, or joined using any known technique.

The support segment 10 may be made from any suitable material that is non-irritating to the skin and is breathable i.e. moisture permeable to reduce accumulation of perspiration. It is preferably has elastic properties, particularly in the case where support segment 10 extends to form a Velcro®-ended strap. According to a preferred embodiment, the support segment 10 is formed from an elastic foam, such as neoprene rubber, poly vinylchloride, polyethyl, polyurethane or polyester. Foams of all types of cell structure can be used, for example open cell, compressed open cell, dosed cell, com- pressed closed cell or skeleton foams. Preferably the elastic foam is overlaid by a textile element on one or both sides. The textile element may be a woven, knitted or non-woven material. The textile element may be colour-dyed at least partially on the atmosphere facing side.

A plastically deformable nose clip 40 may be incorporated in the support segment 10, in a position corresponding to the bridge of the nose. It may be made from any suitable material such as annealed aluminum.

According to one aspect of the invention, the mask is provided with one or more (e.g. 1 , 2, 3, 4, 5, 6) exhalation valves 20. An exhalation valve 20 is configured for the passage of exhaled air. It may be a one-way (non-return) valve that limits the passage of inhaled air. It may be provided with a directional vent adapted to direct exhaled air, for example, in the dorsal direction (towards the back) of the wearer i.e. towards the tails 12, 12' of the strap. The exhalation valves enables easier exhalation and helps to reduce exhaust gases in the mouth; again it provides the wearer with greater comfort for prolonged use. An exhalation valve is preferably disposed on the filter segment. The filter segment may be provided with one or more apertures (e.g. 2, 3, 4, 5, 6) in which an exhalation valve 20 is fixed. The apertures are preferably located in a position corresponding to the mouth. According to one aspect of the invention, the exhalation valve is a non-return valve. The non-return valve may be any of the art, for instance, and with reference to FIG. 4, it may comprise a valve cover 21 with openings 22, a freely movable valve disc 23, an annular valve seat 24 which projects into the filter segment 50 through the aperture, and an annular retaining member 25 which clamps the periphery of the aperture to the valve seat 24 and cover 21. The openings 22 are limited to segment of the cover 17, allowing exhaled air to be directed, for example, away from the front and towards the tails 12, 12' of the strap or dorsal region of the subject.