Patent Publication Number: US-2023142434-A1

Title: Face mask apparatus with secondary filter

Description:
TECHNICAL FIELD 
     The disclosure relates to face masks, and in particular to face masks including a primary filter and for use with a removable secondary filter. 
     BACKGROUND 
     Face masks are used in a wide variety of applications to protect a respiratory system from particles or gases in the air. Such masks include filter masks which are commonly worn by persons who are in polluted environments in an effort to protect themselves from inhaling airborne contaminants. Filter masks typically have a fibrous or sorbent filter that is capable of removing particulate and/or gaseous contaminants from the air. 
     An example of a filter mask is disclosed in Patent Application Publication WO 2018/045456 filed by the present Applicant. The filter mask includes a face seal for providing an airtight flexible seal around the nose and mouth of a user, a support sealably attached to the face seal, wherein the support has an open area that allows for passage of incoming air and outlet valves for expelling exhaled air, a front shell for removably attaching to the support, wherein the front shell has inlet holes for allowing the incoming air to pass through the open area of the support, and a filter for filtering particulate elements from air. The filter is configured to be housed between the front shell and the support. The face seal provides a direct connection between the filter and the user. 
     Some users of filter masks may need to replace a filter and/or filter mask while remaining in a potentially hazardous environment. For example, medical personnel may need to replace a mask and/or filter when exiting a first room and prior to entering a second room to avoid carrying contaminants from the first room to the second room on the mask and/or filter. Medical personnel may need to replace a mask while remaining in a potentially hazardous environment. For example, medical personnel may need to replace a mask in a hallway between a first room and a second room, even when there is a risk that an airborne contaminant may be in the hallway. Medical personnel may be exposed to an airborne contaminant while replacing a mask. 
     There is accordingly a need for an improved face mask that addresses at least one of the above concerns. 
     SUMMARY 
     According to a first aspect, there is provided a face mask apparatus for filtering air, the face mask apparatus comprising a face seal for providing an airtight flexible seal around the nose and mouth of the user; a support sealably attached to the face seal, wherein the support has an open area that allows for passage of incoming air and outlet valves for expelling exhaled air; a front shell for removably attaching to the support, wherein the front shell has at least inlet holes for allowing the incoming air to pass through the open area of the support; a filter for filtering particulate elements from air, wherein the filter is configured to be housed between the front shell and the support, and the face seal provides a direct connection between the filter and the user; and a receiver projecting from an exterior surface of the front shell for receiving a secondary mask. 
     In some embodiments, the receiver includes a mounting feature for mounting the secondary mask to the front shell. 
     In some embodiments, the receiver includes a standoff for maintaining an airflow gap between the exterior surface of the front shell and the secondary mask. 
     In some embodiments, the receiver includes a receiver seal for providing an airtight flexible seal between the secondary mask and the exterior surface of the front shell around the inlet holes. 
     In some embodiments, the receiver seal is a continuous flexible raised periphery. 
     According to a second aspect, there is provided a face mask apparatus for filtering air, the face mask apparatus comprising a face seal for providing an airtight flexible seal around the nose and mouth of the user; a support sealably attached to the face seal, wherein the support has an open area that allows for passage of incoming air; a front shell for removably attaching to the support, wherein the front shell has at least one inlet hole for allowing the incoming air to pass through the open area of the support; a filter for filtering particulate elements from air, wherein the filter is configured to be housed between the front shell and the support, and the face seal provides a direct connection between the filter and the user; and a receiver projecting from an exterior surface of the front shell for receiving a secondary mask. 
     According to a third aspect, there is provided a face mask apparatus for filtering air, the face mask apparatus comprising a face seal for providing an airtight flexible seal around the nose and mouth of the user; a support sealably attached to the face seal, wherein the support has an open area that allows for passage of incoming air; a front shell for removably attaching to the support, wherein the front shell has at least one inlet hole for allowing the incoming air to pass through the open area of the support; a filter for filtering particulate elements from air, wherein the filter is configured to be housed between the front shell and the support, and the face seal provides a direct connection between the filter and the user; and a secondary filter releasably secured to an exterior surface of the front shell and disposed upstream of the at least one inlet hole to pre-filter an air flow entering the inlet hole. 
     In some embodiments, the secondary filter is a compression molded filter mask. 
     According to a fourth aspect, there is provided a filtration method, comprising applying a face mask apparatus to a face of a user, the face mask apparatus including a primary face mask to cover a nose and a mouth of the user and having a face seal for providing an airtight flexible seal around the nose and mouth of the user, the face mask apparatus including an inhalation passage through the face mask apparatus, a primary filter received in the inhalation passage to filter air passing through the inhalation passage; applying, while maintaining the face mask apparatus on the face of the user and the primary filter in the inhalation passage to filter air passing through the inhalation passage, a first secondary filter to the face mask apparatus to filter air prior to the air passing through the inhalation passage; removing, while maintaining the face mask apparatus on the face of the user and the primary filter in the inhalation passage to filter air passing through the inhalation passage, the first secondary filter and disposing of the first secondary filter; and applying, while maintaining the face mask apparatus on the face of the user and the primary filter in the inhalation passage to filter air passing through the inhalation passage, a second secondary filter to the face mask apparatus to filter air prior to the air passing through the inhalation passage. 
     In some embodiments, applying the first secondary filter to the face mask apparatus includes covering a forward exterior surface of the face mask apparatus with the first secondary filter. 
     In some embodiments, applying the second secondary filter to the face mask includes covering the forward exterior surface of the face mask apparatus with the second secondary filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings included herewith are for illustrating various examples of systems, methods, and apparatus of the present specification. In the drawings: 
         FIG.  1    is an exploded view of a first face mask and first secondary filter; 
         FIG.  2    is a schematic diagram of a user wearing the first face mask and first secondary filter of  FIG.  1   ; 
         FIG.  3 A  is cross sectional view of a second face mask and second secondary filter; 
         FIG.  3 B  is a perspective view of the second face mask and second secondary filter of  FIG.  3 A ; 
         FIG.  4 A  is cross sectional view of a third face mask and third secondary filter; 
         FIG.  4 B  is a perspective view of the third face mask and third secondary filter of  FIG.  4 A ; 
         FIG.  5 A  is cross sectional view of a fourth face mask and fourth secondary filter; 
         FIG.  5 B  is a perspective view of the fourth face mask and fourth secondary filter of  FIG.  5 A ; 
         FIG.  6 A  is cross sectional view of a fifth face mask and fifth secondary filter; 
         FIG.  6 B  is a perspective view of the fifth face mask and fifth secondary filter of  FIG.  6 A ; 
         FIG.  7 A  is cross sectional view of a sixth face mask and sixth secondary filter; 
         FIG.  7 B  is a perspective view of the sixth face mask and sixth secondary filter of  FIG.  7 A ; 
         FIG.  8 A  is cross sectional view of a seventh face mask and seventh secondary filter; 
         FIG.  8 B  is a perspective view of the seventh face mask and seventh secondary filter of  FIG.  8 A ; and 
         FIG.  9    is perspective exploded view of a prior art face mask with a first front shell; 
         FIG.  10    is a perspective view of a second front shell; 
         FIG.  11    is a side elevation view of the second front shell of  FIG.  10   ; 
         FIG.  12    is a front elevation view of the second front shell of  FIG.  10   ; 
         FIG.  13    is a perspective view of a third front shell; 
         FIG.  14    is a side elevation view of the third front shell of  FIG.  13   ; 
         FIG.  15    is a front elevation view of the third front shell of  FIG.  13   ; 
         FIG.  16    is a perspective view of an eighth face mask apparatus having a fourth front shell; 
         FIG.  17    is a side elevation view of the eighth face mask apparatus of  FIG.  16   ; 
         FIG.  18    is a front elevation view of the eighth face mask apparatus of  FIG.  16   ; and 
         FIG.  19    is a rear perspective view of an offset member of the eighth face mask apparatus of  FIG.  16   . 
     
    
    
     DETAILED DESCRIPTION 
     Various apparatus or processes will be described below to provide an example of each claimed embodiment. No embodiment described below limits any claimed embodiment and any claimed embodiment may cover processes or apparatuses that differ from those described below. The claimed embodiments are not limited to apparatuses or processes having all of the features of any one apparatus or process described below or to features common to multiple or all of the apparatuses described below. 
     Referring now to  FIG.  1   , depicted is a face mask apparatus  100 . Face mask apparatus  100  includes a primary face mask  104  to cover the nose and the mouth of a user. Primary face mask  104  includes a face seal  110  for providing an airtight flexible seal around the nose and mouth of the user. Face seal  110  may be formed of, for example, a soft silicone and/or a harder coating over a soft base such as a hard nylon over a silicon base. 
     Primary face mask  104  has a interior side  114  and a exterior side  118  opposite the interior side  114 . At least one respiration passage  120  is provided through the face mask  104  between the exterior side  118  and the interior side  114 . The at least one respiration passage  120  extends between at least one exterior respiration port  124  and at least one interior respiration port. In the illustrated example, the at least one exterior respiration port  124  is a single exterior respiration port  124  and the at least one interior respiration port is a plurality of interior respiration ports  130   a ,  130   b.    
     The at least one respiration passage  120  includes at least one inhalation passage. In some examples, the at least one respiration passage  120  also includes at least one exhalation passage separate from the inhalation passage or partially separate from the inhalation passage. In the illustrated example, the at least one respiration passage  120  includes at least one inhalation passage extending between a single exterior respiration port  124  and a plurality of interior respiration ports  130   a . The illustrated at least one respiration passage also includes an exhalation passage extending between the single exterior respiration port  124  and a single interior exhalation respiration port  130   b.    
     In some examples, the optional at least one exhalation passage is closed by a filter and/or a vent, such as a one-way vent inhibiting flow through the exhalation passage from the exterior side  118  to the interior side  114 . In some examples, a filter is received in the inhalation passage. In the illustrated example, face mask apparatus  100  includes a filter  108 . 
     In some examples, filter  108  is a primary filter and face mask apparatus  100  is to be used also with a secondary filter. Primary filter  108  is provided for filtering air and is received in the at least one inhalation passage to filter air passing through the at least one inhalation passage. Primary filter  108  optionally filters substantially all air passing through the at least one inhalation passage. In some examples, primary filter  108  is a particulate filter. In some examples, face mask apparatus  100  is designed to filter at least 95% of 0.3 micron particles. In some examples, face mask apparatus  100  is a designated N95 mask. 
     In some examples, the primary filter  108  is sealingly received in the at least one inhalation passage to filter all air passing through the inhalation passage. In some examples, a perimeter of the primary filter  108  is sealed against a wall of the inhalation passage around an entire periphery  134  of the filter. In some examples, the primary face mask  104  includes a flexible primary filter seal in the inhalation passage to receive the primary filter  108  with a periphery of the primary filter  108  seated against the flexible primary filter seal. In some examples, the primary filter  108  includes a flexible primary filter seal around a periphery  134  of the primary filter  108  to be positioned against the wall of an inhalation passage. 
     A primary filter  108  may be a sheet filter, as in the illustrated example, or a shaped filter such as a pleated filter. A primary filter  108  may optionally be provided as a replaceable cartridge, including a supporting primary filter frame. A replaceable cartridge may include a cartridge retaining feature, such as a snap fit member, to cooperate with a primary face mask retaining feature, such as a snap fit member, to removably retain the cartridge in the at least one inhalation passage. 
     In the illustrated example, the primary filter  108  is a sheet filter and does not include a supporting primary filter frame. Illustrated face mask  104  includes a support  138  bearing the face seal  110 . Face mask  104  also includes a front shell  142 . One or both of the support  138  and the front shell  142  may be formed of a firm material such as a rigid or semi-rigid plastic. Illustrated primary filter  108  is received between the support  138  and the front shell  142 . The illustrated front shell  142  can be releasably secured to the support  138 . At least a portion of the primary filter perimeter  134  can be held between the support  138  and the front shell  142  to retain the primary filter  108  in the at least one inhalation passage. One or both of the primary filter  108  and the primary face mask  104  may include a primary filter seal, such as a bead of soft silicone, to further inhibit air from bypassing the primary filter  108  when moving through the at least one inhalation passage. 
     In some examples, the primary face mask  104  may be used with a head strap to hold the mask to the head of a user. In the illustrated example, a head strap  146  is secured to primary face mask  104 . Head strap  146  may be passed around a head of a user to hold the face mask apparatus  100  to the head of the user. In the illustrated example, the head strap  146  is secured to the face mask apparatus  100  by passing through clasps  148  of support  138  of face mask  104 . 
     In some examples, a primary face mask may be used with a secondary face mask having a secondary filter. In some examples, the secondary filter is a replaceable filter that can be removed and replaced without removing the primary filter, such as to reduce the user&#39;s exposure to airborne contaminants. For example, a user may need to remove the secondary filter to avoid carrying contaminants on the secondary filter from a first area to a second area. The user may move from the first area such as a first room to an interim area such as a hallway, and then remove a first secondary filter and apply a second secondary filter before moving from the interim area into a second area such as a second room. In some examples, the act of removing a first secondary filter may itself release contaminants from the first secondary filter into the air. 
     In some examples, the primary filter is also provided to bolster the effect of the secondary filter. For example, a primary filter may be a high-quality filter to protect the user while the secondary filter may be a surgical mask to capture droplets, such as cough droplets, to prevent the droplets from being deposited on the face mask. In some examples, a primary face mask with a primary filter may be a certified N95 mask, and the secondary mask with a secondary filter may be applied to the face mask to protect the face mask from droplets and/or other airborne contaminants. In some examples, a secondary filter and/or secondary mask may prevent droplets which escape the face mask, such as via an exhalation port, since droplets would be caught in the secondary filter and/or secondary mask as well. 
     In some examples, a secondary mask may be applied over the primary face mask and/or in the inhalation passage. In the illustrated example of  FIG.  1   , secondary filter  152  is a surgical mask which may be worn over the face mask apparatus  100  and includes a filter sheet  154  and a pair of ear loops  156 . 
     In some examples, the at least one exterior inhalation port  124  is spread across a substantial portion of the forward surface  176  of the face mask apparatus  100 , as in the illustrated example. In some examples, the at least one exterior inhalation port  124  is limited to a small portion of the forward surface  176  of the face mask, such as limited to a leading portion  177  of the forward surface  176 . In some examples, the at least one inhalation port  124  is limited to a planar front surface extending generally perpendicular to the airflow direction. Limiting the at least one exterior inhalation port  124  to a leading portion or a planar front surface may facilitate drawing the secondary filter against the forward surface  176  over the at least one inhalation port  124  to filter air entering the at least one inhalation port. 
     Referring now to  FIG.  2   , in some examples the secondary face mask is a disposable face mask. In the illustrated example the secondary filter  152  is worn by a user  158  over the primary face mask. The user  158  also carries a pouch  160  of extra secondary face masks. 
     In some examples, a secondary receiver is provided on an exterior surface of a front shell of a primary face mask to receive a secondary mask and/or secondary filter. In some examples, a secondary filter receiver is a protruding and/or projecting receiver. In some examples, a receiver includes at least one of a mounting feature, a standoff, and a receiver seal. 
     In some examples, a receiver includes a mounting feature for mounting the secondary mask to the front shell. Examples of mounting features include snap-fit members, bump interfaces, hook and loop members, and magnetic members. A mounting feature may also be provided to inhibit movement of a secondary filter and/or secondary mask relative to the face mask. Particularly where the face mask includes a hard forward surface the secondary filter may easily slide against the hard forward surface, which may result in an undesirable positioning of the secondary filter. 
     In some examples, a receiver includes a standoff to maintain an air gap between the exterior surface of the front shell and the secondary mask. Examples of standoffs include projecting ribs and wire frames. In some examples, a standoff keeps the secondary mask and/or secondary filter a small distance from the exterior surface of the front shell to allow air to more freely move across the exterior surface of the front shell under the secondary mask and/or secondary filter. 
     In some examples, a receiver includes a receiver seal for providing an airtight flexible seal between the secondary mask and the exterior surface of the front shell around the inlet holes. Examples of receiver seals include silicone ribs and continuous silicone beads. In some examples, a receiver seal helps prevent air from bypassing a secondary filter when passing into the face mask. Air bypassing the secondary filter may reduce the effectiveness of the secondary filter in filtering air. 
     Referring now to  FIGS.  3 A and  3 B , illustrated is a face mask apparatus  200  with a secondary filter receiver  270  on the primary face mask exterior side. Face mask apparatus  200  and secondary filter  252  are similar in many respects to face mask apparatus  100  and secondary filter  152 , and like features are identified by like reference numbers incremented by 100. 
     In some examples, a secondary filter receiver  270  includes at least one mounting member for use in holding the secondary filter to the face mask. Secondary filter receiver  270  of face mask apparatus  200  includes a pair of lateral side mating components  272  as mounting members. A compression molded secondary filter  252  may be designed to accommodate mating components on the mask. The compression molded secondary filter  252  may be made of a stiff, deformable material to be shaped by a user to fit over the face mask apparatus  200 . In some examples, a compression molded secondary filter  252  includes a material which may be manually shaped by a user. In some examples, a compression molded secondary filter  252  is stamped to shape using a stamping machine. In some examples, the natural tension of the secondary filter  252  may allow it to stretch and snap over the pair of lateral side mating components  272 , causing the secondary filter  252  to be held in place on the face mask apparatus  200 . In some examples, a compression molded filter forms a compression molded secondary filter mask. In some examples, a secondary filter mask formed of a compression molded filter does not include any secondary coupling features. For example, a compression molded filter mask may not include an elastic or a metal clip to be used to secure the compression molded filter mask to a face mask apparatus. In some examples, a compression molded filter mask is formed of a single sheet of compression molded material, and may be a stamped compression molded filter mask. In some examples, a compression molded filter mask formed of a single sheet of compression molded material allows for more rapid manufacturing and/or less waste during manufacturing. 
     The face mask apparatus  200  and/or secondary filter receiver  270  may include a forward exterior surface  276  which matches an interior surface  278  of the secondary filter  252  or which the secondary filter  252  may be compression molded to match. The secondary filter  252  may then seal to the face mask apparatus  200  by neatly overlaying the forward exterior surface  276 . Portions of the secondary filter  252  are stretched over the lateral side mating components  272  and held behind the lateral side mating components  272  to hold the secondary filter  252  against the forward exterior surface  276 . 
     In some examples, the secondary filter receiver  270  may also or alternatively include a raised periphery surrounding the at least one exterior inhalation port  224 . The raised periphery may be a continuous flexible rib or rim of a flexible material such as silicone or rigid plastic. The raised secondary filter sealing member may also or alternatively be a bead of a flexible material forming a raised surface that the secondary filter  252  may be pressed into to inhibit air passing into the at least one exterior inhalation port  224  without first passing through the secondary filter  252 . 
     Illustrated secondary filter  252  includes at least one pull tab  282  to provide a gripping area for use in pulling the secondary filter  252  on or off of the face mask apparatus  200 . In some examples, at least one pull tab  282  may assist in minimizing contact between the fingers a user and the secondary filter  252  during removal of the secondary filter. 
     Referring now to  FIGS.  4 A and  4 B , illustrated is a face mask apparatus  300  with a secondary filter receiver  370  on the primary face mask exterior side. Face mask apparatus  300  and secondary filter  352  are similar in many respects to face mask apparatus  200  and secondary filter  252 , and like features are identified by like reference numbers incremented by 100. 
     In the illustrated example, secondary filter receiver  370  is a flexible rib  380 , such as a silicone rib, and the secondary filter  352  is held to a head of a user by ear loops  356  which draw the secondary filter  352  against the face mask apparatus  300  and the secondary filter receiver  370 . Secondary filter  352  may be a standard surgical mask or may be shaped for the face mask apparatus  300  to avoid air gaps between the secondary filter  352  and the flexible rib. For example, the secondary filter  352  may be shaped to fit more easily over clasps  348  of mask  300 . In some examples, secondary filter  352  is a compression molded filter that may be stamped to shape. In some examples, secondary filter  352  is cut to form over the base mask under tension. In some examples, secondary filter  352  is pleated or darted with sonic welded joints to be tailored to fit the base mask. 
     Referring now to  FIGS.  5 A and  5 B , illustrated is a face mask apparatus  400  with a secondary filter receiver  470  on the exterior side. Face mask apparatus  400  and secondary filter  452  are similar in many respects to face mask apparatus  300  and secondary filter  352 , and like features are identified by like reference numbers incremented by 100. 
     Illustrated secondary filter  452  is a compression molded filter formed of a stiff moldable material to allow a user to shape the filter over the face mask apparatus  400 . The face mask apparatus  400  includes a series of flexible silicone ribs  480  which create a multi-layered seal with the compression molded filter  452 . The series of flexible silicone ribs  480  are also mounting members and assist in holding the secondary filter  452  on in a friction fit. In some examples, a secondary filter receiver assists in preventing fogging of eyewear. For examples, ribs  480  inhibit movement of air through a gap between the face mask  400  and the secondary filter  452  and thereby help prevent fogging of eyewear adjacent the face mask  400 . 
     Referring now to  FIGS.  6 A and  6 B , illustrated is a face mask apparatus  500  with a secondary filter receiver  570  on the primary face mask exterior side. Face mask apparatus  500  and secondary filter  552  are similar in many respects to face mask apparatus  400  and secondary filter  452 , and like features are identified by like reference numbers incremented by 100. 
     Secondary filter receiver  570  of face mask apparatus  500  includes a raised periphery  580 . Secondary filter  552  is a flexible material sheet  584  with a perimeter elastic  586 . Similar to the way in which a perimeter elastic of a hair net may be stretched over a retaining feature, the perimeter elastic  586  may be stretched over raised periphery  580 . Raised periphery  580  is therefore both a mounting member and provides a filter seal between the primary face mask and the secondary filter  552 . In some examples, the primary face mask of face mask apparatus  500  includes a foam or silicone track in which the elastic  586  may rest to allow the tension of the elastic  586  to create a seal between the secondary filter  552  and the mask  500 . 
     Referring now to  FIGS.  7 A and  7 B , illustrated is a face mask apparatus  600  with a secondary filter receiver  670  on the primary face mask exterior side. Face mask apparatus  600  and secondary filter  652  are similar in many respects to face mask apparatus  500  and secondary filter  552 , and like features are identified by like reference numbers incremented by 100. 
     In the illustrated example, the secondary filter receiver  670  includes a set of continuous raised beads of silicone  688  surrounding the at least one exterior inhalation port  624  for providing a filter seal between the primary face mask and the secondary filter  652 . Secondary filter  652  is a mask having ear loops to hold the mask against a head of a user and against the beads of silicone  688 . 
     Referring now to  FIGS.  8 A and  8 B , illustrated is a face mask apparatus  700  with a secondary filter receiver  770  on the primary face mask exterior side. Face mask apparatus  700  and secondary filter  752  are similar in many respects to face mask apparatus  600  and secondary filter  652 , and like features are identified by like reference numbers incremented by 100. 
     Secondary filter receiver  770  of face mask apparatus  700  includes a raised periphery  780 . Raised periphery  780  includes an outwardly directed flange  790 . Secondary filter  752  includes a lip  792  which may fit under outwardly directed flange  790  to assist in retaining secondary filter  752  on face mask apparatus  700 . 
     Referring now to  FIG.  9   , illustrated is a prior art face mask apparatus  800  disclosed in Patent Application Publication WO 2018/045456 filed by the present Applicant. Face mask apparatus  800  is similar in many respects to face mask apparatus  100 , and like features are identified by like reference numbers incremented by 100. 
     Face mask apparatus  800  is a face mask apparatus for filtering air. Face mask apparatus  800  includes a face seal  810  for providing an airtight flexible seal around the nose and mouth of the user. 
     Face mask apparatus  800  also includes a support sealably attached to the face seal wherein the support  838  has an open area  839  that allows for passage of incoming air and outlet valves  841  for expelling exhaled air. Face mask apparats  800  also includes a front shell  842  for removably attaching to the support  838 . The front shell  842  has inlet holes  824  in the exterior surface  876  for allowing the incoming air to pass through the open area  839  of the support  838 . 
     Face mask apparatus  800  also includes a filter  808  for filtering particulate elements from air. The filter  808  is configured to be housed between the front shell  842  and the support  838 . The face seal  810  provides a direct connection between the filter  808  and the user. 
     Referring now to  FIGS.  10  to  12   , illustrated front shell  942  is similar in many respects to front shell  842 , and like features are identified by like reference numbers incremented by 100. Illustrated front shell  942  includes a filter receiver  980  projecting from an exterior surface  976  of the front shell  942  for receiving a secondary mask. 
     Illustrated filter receiver  980  includes a receiver seal  981  for providing an airtight flexible seal between the secondary mask and the exterior surface  976  of the front shell  942  around the inlet holes  924 . Illustrated receiver seal  981  includes a pair of continuous flexible ribs  983 . 
     Referring now to  FIGS.  13  to  15   , illustrated front shell  1042  is similar in many respects to front shell  942 , and like features are identified by like reference numbers incremented by 100. Illustrated front shell  1042  includes a receiver  1080  projecting from an exterior surface  1076  of the front shell  1042  for receiving a secondary mask. 
     Illustrated receiver  1080  includes a receiver seal  1081  for providing an airtight flexible seal between the secondary mask and the exterior surface  1076  of the front shell  1042  around the inlet holes  1024 . Illustrated receiver seal  1081  includes a pair of continuous flexible ribs  1083 . 
     Illustrated receiver  1080  also includes a standoff  1085  for maintaining an airflow gap between the exterior surface of the front shell and the secondary mask. Illustrated standoff  1085  includes a pair of raised ribs  1087  to hold the secondary mask and/or secondary filter away from the exterior surface  1076 . 
     Referring now to  FIGS.  16  to  18   , illustrated front shell  1142  is similar in many respects to front shell  1042 , and like features are identified by like reference numbers incremented by 100. Illustrated front shell  1142  includes a receiver  1180  projecting from an exterior surface  1176  of the front shell  1142  for receiving a secondary mask. 
     Illustrated receiver  1180  includes a receiver seal  1181  for providing an airtight flexible seal between the secondary mask and the exterior surface  1176  of the front shell  1142  around the inlet holes  1124 . Illustrated receiver seal  1181  includes a continuous flexible rib  1183 . 
     Illustrated receiver  1180  also includes a standoff  1185  for maintaining an airflow gap between the exterior surface of the front shell and the secondary mask. Illustrated standoff  1185  includes a detachable offset member  1189  to hold the secondary mask and/or secondary filter away from the exterior surface  1076 . Offset member  1189 , shown in greater detail in  FIG.  19   , may come in a variety of shapes and be detectably mountable to the front shell  1142 . In the illustrated example, offset member  1189  includes snap-fit legs to fit into inlet holes  1124  and has a circular forward portion with a flat leading surface. In some examples, the offset member may have a square forward portion and/or have an arched leading surface. 
     In some examples, a standoff and/or offset member may include a non-smooth surface to prevent the secondary filter and/or secondary mask from sliding across the front shell  1142 . In some examples, a standoff and/or offset member may include a silicone leading surface. 
     The present invention has been described here by way of example only. Various modification and variations may be made to these exemplary embodiments without departing from the scope of the invention, which is limited only by the appended claims.