Abstract:
A biological safety cabinet is provided that includes a frame. The frame defines a protected work area and encloses the work area on all but one side. A sash is coupled to the frame that at least partially encloses the side that is not enclosed by the frame. A blower is coupled to the frame generally above the work area. The blower is adapted to circulate air through the work area to make the work area a negative pressure area so that harmful materials are confined. A sash grill is coupled to the frame generally below the sash that has a curved top surface. The curved sash grill provides a superior and less turbulent air-flow into the work area, thereby better containing any harmful materials. The curved sash grill is perforated, and the curvature and perforations of the sash grill compensate for partial blockage by such things as the user&#39;s arms and other objects. The curvature of the sash grill also avoids a sharp angle at the same height as the work surface which reduces the chance of contact and possible breakage of labware as it is moved into the cabinet.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to biological safety cabinets. 
     Biological safety cabinets are laboratory containment devices equipped with High Energy Particulate Air (HEPA) filters. These cabinets are used in microbiological laboratories and provide a work area with safe environment in which a variety of experiments and studies can be performed. Rather than providing only a hood above a working surface, these cabinets provide a more protective working environment. The safety cabinet has a frame that surrounds the work area on all but one side. The remaining open side is enclosed by a moveable sash. The sash may be moved upwardly to provide access to the work area, so that work can be performed. The sash may be moved downwardly to partially or completely close the work area. A blower unit is provided in the cabinet above the work area. The blower is used to circulate air downwardly through the safety cabinet. A portion of this downward air flow forms an air curtain at the front of the cabinet work area and passes beneath the floor of the work area and a portion is directed to the back of the cabinet where it is drawn upwardly through a plenum chamber. This air may be contaminated by materials being used within the working environment. Therefore, prior to being exhausted into the room or a fume system, the air is first passed through a HEPA exhaust filter. 
     The blower is operated so there is sufficient air flow through the work area to insure that any harmful materials are contained and eventually passed to a filter area rather than escaping into the room or exhausted into the atmosphere. To this end some air is drawn into the safety cabinet about the open perimeter formed when the sash is in an open or partially open position. 
     The prior art safety cabinets are typically provided with a sash grill located below the bottom of the sash. This sash grill forms the lower-most surface of the opening into the work area. Typically, the sash grill is provided with a number of perforations, through which air can flow. Air flows downwardly from the blower along the back of the sash and into these perforations. Air is also drawn inwardly from the exterior of the cabinet along the surface of the sash grill and into the perforations. The air flowing through the sash grill flows under the work surface and upwardly through the plenum at the back of the cabinet to be recirculated or exhausted. 
     Safety cabinets have heretofore utilized a sash grill having a generally flat surface which gives rise to a number of disadvantages. The flat surface may be used by those operating the safety cabinet as a surface on which to place a variety of labware. This is undesirable because objects located on the sash grill present a source of possible contamination of the room, and may be inadvertently broken if bumped or knocked onto the floor. Moreover, by placing an object on the sash grill, a portion of the perforations therein may be blocked, which can adversely affect the air flow of the safety cabinet. The flat surface of the sash grill also results in a large portion of the perforations therein becoming blocked by a user&#39;s arm as the user performs work within the safety cabinet. As the user&#39;s arm blocks the perforations in this fashion, it is difficult to properly maintain the negative pressure environment about the user&#39;s arm, thereby risking possible contamination. The flat sash grills of the prior art also present a right angle with the work surface which projects far enough above the work surface that labware is sometimes broken when it bumps against the projecting vertical face. It is thus desirable to provide a sash grill which does not provide a flat surface and does not present a right angle corner at the entrance to the work area opening. 
     Another drawback of prior art sash grills is attributable to the fact that the grills are formed with a front face that is at a right angle to the flat top of the grill. This orthogonal relationship results in an air flow that is less than desirable. When air is drawn inwardly and through the perforations in the sash foil, it may cause a turbulence in the air flowing downwardly along the back of the sash and through the working environment. This turbulence is increased by the right angle relationship, as the air encountering the front face of the grill will be partially directed upwardly over the front face before being drawn through the perforations in the flat top of the grill. Therefore, a biological safety cabinet is needed with a sash grill that improves the air flow and safety of the cabinet. 
     Similarly, air may be drawn into the opening of the safety cabinet along the sides of the cabinet adjacent the opening when the sash is in an open or partially open position. In prior art safety cabinets, the front sides of the cabinet are oriented at right angles relative to the interior side walls. When air is drawn into the cabinet along these sides, it will initially be directed away from the interior surface of the interior walls. However, it is much more desirable to cleanly “sweep” the interior walls of the cabinet, to ensure the best possible containment of any harmful materials. A biological safety cabinet having a construction that draws air inwardly to cleanly sweep the interior side walls is needed. 
     After the safety cabinets have been used for a certain period of time, they must be decontaminated. One method for performing this decontamination involves sealing the front of the safety cabinet with a plastic sheet. When the prior art safety cabinets are being decontaminated, it is often necessary to first remove the sash to insure proper decontamination. This is attributable to the location of the sash within a U-shaped channel where contaminants may accumulate. This procedure is time consuming and risks damage to the sash. If the sash is dropped it may shatter, and contaminate an entire room. Thus, a biological safety cabinet which can be decontaminated without removal of the sash is needed. 
     Another drawback of prior art safety cabinets involves the lower edge or handle of the moveable sash. When the sash is in an open or partially open position, two bodies of air are coming together adjacent the handle of the sash. One body of air is flowing from the exterior of the cabinet into the interior thereof. The second body of air is flowing downwardly from the blower unit of the safety cabinet along the back of the sash. In prior art cabinets, the sash handle has transitioned from the front face to the bottom face at a right angle. This results in the inwardly flowing air meeting the downwardly flowing air at a right angle, causing turbulence. As noted above, turbulent air flow adjacent the opening of the cabinet is undesirable. A sash handle that reduces turbulence would represent an improvement over the prior art. 
     As stated above, the biological safety cabinet is operated with the benefit of a blower which provides an air flow so that harmful materials are contained within the cabinet. The cabinets are constructed with the blower above the working environment, and the working environment is subject to a continual flow of air to contain contaminants and then move them to a filter area. Above the working environment and beneath the blower, is a supply filter and a positive pressure plenum. The pressure plenum receives air from the blower and directs it through the supply filter. 
     To monitor the pressure within the cabinet, prior art safety units have used a pressure gauge mounted on the exterior of the cabinet, with the pressure being monitored in the positive pressure environment of the pressure plenum immediately below the blower. Monitoring the positive pressure allows a more meaningful pressure reading to be obtained and used by the laboratory personnel. However, the air within the pressure plenum immediately below the blower has not yet been filtered. As such, the air may contain harmful materials from the working environment below. If the gauge on the exterior of the cabinet were to leak, contaminated air would be allowed into the room. In some instances this concern has been addressed by placing a HEPA filter in the pressure line to the readout gauge. This of course results in additional expense both initially and for ongoing maintenance. Another method of addressing the potential problem of contamination through the pressure gauge has been to monitor the air pressure in a negative pressure environment (relative to the atmosphere surrounding the cabinet) thus eliminating the possibility of contamination as a result of leakage through the gauge into the room. Monitoring and displaying a negative pressure, however, is more difficult to translate into meaningful and usable numbers by laboratory personnel. A monitoring apparatus is therefore needed which does not require any additional filters and allows the monitoring and display of a positive pressure, while eliminating the risk of possible contamination of the room environment. 
     It has been found that it is desirable to equip the safety cabinet with a “towel catch” to catch or filter out large objects from the returning air flow prior to being recirculated through the blower. This towel catch removes such things as paper towels and small laboratory items from the returning air stream. Prior art safety cabinets have located this towel catch in the plenum formed by the rear wall of the work area and the rear wall of the safety cabinet. While this location is effective in removal of the desired items, it is impossible to visually inspect without taking the cabinet apart. One method typically utilized for inspecting these prior art towel catchers is to reach up within the plenum and feel the towel catch to determine if any paper towels or other objects are lodged within or against the towel catcher. This method can be uncomfortable and dangerous to the extent that pieces of broken laboratory glass and other sharp objects may be lodged within the towel catch. The towel catch itself is normally formed from metal with sharp edges which presents a safety hazard in and of itself if it is placed in a traditional location where it is not visible to a worker cleaning it. Therefore, a towel catch that is readily accessible and can be visually inspected is needed. 
     Another drawback of prior art safety cabinets involves the construction of the sash. The sash of the safety cabinet is moveable upwardly and downwardly, to allow better access to the working environment when needed and to more fully enclose the working environment when access is no longer needed. In prior art safety cabinets, the rear of the sash is provided with a seal to prevent any contaminated air from escaping the working environment. The seal wipes the back of the sash as the sash is raised. This arrangement is disadvantageous in that the wiping action may create an aerosol containing contaminants from the rear of the sash. While in other prior art constructions holes communicating with the exhaust system have been utilized in place of seals, such constructions have not been particularly effective, largely because there has been no means for insuring a uniform negative pressure across the exhaust holes. Thus, an arrangement is needed for a biological safety cabinet that eliminates the need for a wiping seal at the rear of the sash and instead provides for a uniform negative pressure which will insure removal of any contaminated air from the back side of the sash. 
     Yet another drawback of existing prior art safety cabinets involves the design of the positive pressure plenum box. This box is located in the area below the blower and above the work area. More specifically, in prior art cabinets, air leaving the blower is directed to a perforated plate and then through a supply filter prior to be recirculated downwardly through the work area. The perforated plate is used to more evenly distribute the air flow over and through the supply filter. The perforated plate creates an undesirable increased load on the blower and can interfere with the function of the supply filter. Moreover, this prior art construction does not distribute air across the supply filter as evenly as desired. Therefore, a structure is needed that both evenly distributes the flow over and across the supply filter while not overly increasing the load on the blower or interfering with the function of the supply filter. 
     Prior art safety cabinets are typically equipped with exhaust control systems. As contaminated air passes through the blower of the safety cabinet, some of the air is recirculated through the supply filter as described above and some of the air is routed through an exhaust filter. This exhaust air is either discharged into the room, or it passed to an exhaust system associated with the safety cabinet which moves the air out of the building. In cabinets routing the exhaust air directly back into the room, the prior art cabinets have merely routed the air directly upwardly. Prior art units routing the air into a building exhaust system direct typically employ duct work coupling the safety cabinet exhaust to the building exhaust system. Both prior art embodiments require a certain amount of additional space above the ceiling of the safety cabinet to allow for the exhaust control systems. This need for space can place limitations on the rooms in which the safety cabinets can be used. 
     In addition to routing the exhaust air, the exhaust control systems of the safety cabinets are used to balance the air flow through the safety cabinet. Prior art exhaust control systems use a guillotine damper to allow more or less air to be exhausted, as needed to balance the air flow through the safety cabinet and achieve the proper pressure within the cabinet. This damper places some additional load on the blower by restricting air flow to the filter. Furthermore, a damper is not aerodynamically efficient and interferes with the uniform flow of air. Such dampers are normally not readily accessible for making adjustments. The use of such a damper also tends to cause air to flow unevenly through the filter thus not effectively using the entire filter surface area. Therefore, a more efficient exhaust control system is needed for a biological safety cabinet that reduces undesired blower loading, makes better utilization of available filter surface area and is readily accessible. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a biological safety cabinet having a novel sash grill that more effectively prevents contaminated air from leaving the cabinet, and more effectively draws air into the cabinet. 
     It is another object of this invention to provide a sash grill for a biological safety cabinet that prevents objects from being placed thereon. 
     It is a further object of the invention to provide a biological safety cabinet having exterior front side panels that allow incoming air to more effectively sweep the sides of the cabinet and that allow the cabinet to more easily be decontaminated. 
     It is yet another object of the invention to provide a handle for the sash of a biological safety cabinet that allows air to more effectively flow thereover. 
     It is still another object of the present invention to provide a biological safety cabinet in which the pressure gauge measures a positive pressure environment while being contained within the safety cabinet so that any risk of contamination through the gauge is reduced while also eliminating the need for a separate HEPA filter for the gauge. 
     Another object of the present invention is to provide a towel catch for a biological safety cabinet that is visible to the user thereof and that can be easily removed without disassembling the safety cabinet. 
     Yet another object of the present invention is to provide a biological safety cabinet that eliminates the need to wipe the back of the sash with a seal so that still another risk of contamination is reduced. 
     It is another object of the present invention to provide a biological safety cabinet with a plenum box that evenly distributes the air flow across a supply filter without increasing the load on the blower of the cabinet. 
     A still further object of the present invention is to provide a biological safety cabinet with a low profile, externally adjustable exhaust control that does not require decontamination before adjusting and provides for more uniform distribution of air across the exhaust filter. 
     It is yet another object of the present invention to provide a plenum chamber seal and tensioning device for the exhaust filter of a biological safety cabinet that allows the supply filter and exhaust filter to be simultaneously sealed. 
     According to the present invention, the foregoing and other objects are attained by a biological safety cabinet that includes a frame that defines a protected working environment and encloses the working environment on all but one side. A sash is coupled to the frame that at least partially encloses the side that is not enclosed by the frame. A blower is coupled to the frame generally above the working environment. The blower is adapted to circulate air through the working area so that harmful materials are confined. A sash grill is coupled to the frame generally below the sash and has a curved top surface. The curved sash grill provides a superior and less turbulent air-flow into the working environment, thereby better containing any harmful materials. The curved sash grill is perforated, and the curvature and perforations of the sash grill compensate for partial blockage by the user&#39;s arms and other objects. The curvature of the sash grill also presents a surface on which objects cannot be easily placed, thereby avoiding a safety hazard. The curved grill also eliminates a protruding right angle corner at the cabinet opening which has been known to cause breakage of labware being placed inside the cabinet. 
     Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art upon examination of the following, or may be learned from practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings which form a part of this specification and which are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views: 
         FIG. 1  is a perspective view of the biological safety cabinet of the present invention, with parts being broken away to show particular details of construction; 
         FIG. 2  is a front elevation view of the safety cabinet of  FIG. 1 , with parts being broken away to show particular details of construction; 
         FIG. 3  is a side cross sectional view taken along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a partial cross sectional view taken along line  4 - 4  of  FIG. 3 ; 
         FIG. 5  is an enlarged view of the encircling line  5  of  FIG. 2 , showing the sealing arrangement between the supply filter and the exhaust filter; 
         FIG. 6  is an enlarged view of the encircling line  6  of  FIG. 1 ; 
         FIG. 7  is a partial sectional view taken along line  7 - 7  of  FIG. 3  showing a partial top plan view of the sash grill used in the safety cabinet of  FIG. 1 ; 
         FIG. 8  is a partial sectional view taken along line  8 - 8  of  FIG. 3 , showing an elevation view of the towel catch used in the safety cabinet of  FIG. 1 ; 
         FIG. 9  is perspective view of an alternate embodiment of the exhaust body used in the safety cabinet of  FIG. 1 ; and 
         FIG. 10  is an enlarged view of the encircling line  10  of FIG.  3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIG. 1 , a biological safety cabinet according to the present invention is broadly designated in the drawings by the reference numeral  10 . A broad overview of the construction of cabinet  10  is set forth below, followed by a more detailed description of certain features of the cabinet. Broadly, cabinet  10  has a bottom panel  14  and a pair of upwardly extending opposing side panels  16  which are rigidly coupled to bottom panel  14 , such as by welding. Extending upwardly from the bottom panel  14  and rigidly coupled between side panels  16  is a rear panel  18 , as best seen in FIG.  3 . Rear panel  18  extends upwardly from bottom panel  14  as do side panels  16 . Bottom panel  14 , side panels  16  and rear panel  18  form apartial frame in which the other components of cabinet  10  are held. A baffle  20  is coupled between side panels  16  and is spaced outwardly away from real panel  18 . The bottom of baffle  20  is spaced upwardly away from bottom panel  14 . Panels  14 ,  16  and  18 , as well as baffle  20  are preferably made from metal, such as stainless steel. 
     As best seen in  FIG. 3 , a work surface  22  is suspended above bottom panel  14 . Work surface  22  is used to hold the objects necessary to perform experiments within cabinet  10 , such as beakers, flasks and other conventional labware. Extending generally along the front of cabinet  10  between side panels  16 , and extending from work surface  22  to bottom panel  14 , is a sash grill  24 , the importance of which is further described below. 
     As best seen in  FIGS. 2-4 , a blower assembly  26  is located in the upper part of cabinet  10 . Assembly  26  includes a blower  28 , an exhaust filter  30 , a supply filter  32  and a plenum box  33  which is in communication with the blower outlet. A top panel  34  presents the enclosed top of the cabinet. Panel  34  extends from rear panel  18  to the front of the cabinet and between side panels  16 . An exhaust control cap  36  is coupled to top panel  34  directly above exhaust filter  30 . Top panel  34  also has coupled thereto an electronics housing  38 . Housing  38  houses and protects the electronics necessary to operate cabinet  10 . As best seen in  FIGS. 1 and 3 , a cover panel  40  that is coupled to top panel  34  and extends between side panels  16 . Panel  40  extends only partially down cabinet  10  from top panel  34 . A movable sash  42  is mounted between side panels  16  in a manner allowing it to be moved upwardly and downwardly. Work surface  22 , baffle  20 , side panels  16  and an air diffuser plate  43  below supply filter  32  form a protective work area  44  within which work can be performed. 
     In use, blower  28  of cabinet  10  is operated to provide an air-flow through the cabinet, and particularly through work area  44 . Prior to the air entering the work area  44 , it is first passed through supply filter  32  to remove any contaminants. Cabinet  10  may be operated with sash  42  located a specified distance away from sash grill  24 , as is shown in FIG.  3 . To ensure that contaminants do not escape through the opening between sash  42  and grill  24 , blower  28  will direct air downwardly along the rear of sash  42  and into the perforations of grill  24  from above the work area to provide a protective curtain of air that facilitates containment within work area  44 . A portion of the air from blower  28  also moves toward the rear of the surface  22  as will be explained hereinafter. The action of blower  28  provides a certain amount of suction, causing an air flow inwardly along the opening defined by the bottom of sash  42 , side panels  16  and sash grill  24 . Air which is drawn through this opening also passes through the perforations in sash grill  24 . The air, once drawn through sash grill  24 , will travel beneath work surface  22  and through the plenum defined by baffle  20  and real panel  18  as it is drawn upwardly by blower  28 . The air moving from the blower to the rear of surface  22  will also be drawn into this same plenum. 
     Air that has passed through working environment  44  is likely to contain contaminants and thus, before being recirculated or exhausted to the room, is first passed through a HEPA filter. Prior to being recirculated into working environment  44  the air passes through supply filter  32 . Similarly, prior to being exhausted to the room, the air is passed through exhaust filter  30 . Filters  30  and  32  are both High Efficiency Particulate Air (HEPA) filters of a type well known to those skilled in the art. Thus, cabinet  10  is used to perform experiments within work area  44  and to contain any contaminated air within the cabinet. Particular and novel details of construction are more fully set out below. 
     As best seen in  FIG. 3 , work surface  22  is positioned above bottom panel  14  by a number of supports  46  that are preferably screwed directly into bottom panel  14  (additional support is provided by a rear lip to be described hereinafter). Supports  46  are thus easily removable and can be decontaminated and cleaned after removal from bottom panel  14  as needed. Work surface  22  rests directly upon supports  46  and is thus spaced from bottom panel  14 . The spacing between bottom panel  14  and work surface  22  allows air to circulate beneath work surface  22 . Surface  22  can be made from a material such as stainless steel and is placed on supports  46  so that the rear edge thereof rests on a lip at the bottom of baffle  20 . Work surface  22  may be held in place through the use of removable fasteners which require no tools. Work surface  22  is thus mounted within safety cabinet  10  in a manner allowing the easy removal thereof, such as may be needed for decontamination and cleaning of the safety cabinet. 
     Sash grill  24  extends between the front of work surface  22  and bottom panel  14  from one side panel  16  to the other. As best seen in  FIGS. 6 and 7 , grill  24  has a plurality of main perforations in  48  therein. Perforations  48  allow air to flow through sash grill  24  as air passes downwardly along the rear of sash  42  and inwardly as air enters the safety cabinet adjacent the surface of sash grill  24 . Preferably, perforations  48  extend generally from one side of sash grill  24  to the other. However, as best seen in  FIGS. 6 and 7 , a series of enlarged side holes  50  are provided along each side of grill  24 . Enlarged holes  50  provide additional air flow adjacent side panels  16  and operate to better contain the air within working environment  44 . Further, grill  24  is provided with a front row of scavenger holes  52 . Scavenger holes  52  operate to provide an additional source of protection should the main perforations  48  become blocked along the length of sash grill  24 . 
     As best seen in  FIGS. 3 and 6 , sash grill  24  has a curved surface. This curved surface provides a number of advantages. First, it prevents objects from being placed on the sash grill and blocking any of the perforations within sash grill  24 . This not only prevents blockage of the perforations, but also eliminates any possibility of objects being placed on the grill and then knocked off and broken. The curved shape of the grill also eliminates a sharp edge at the same level as that of the work surface which greatly reduces the possibility of accidental contact when labware is being moved in and out of the work area. Contact at this point has been a source of breakage of glass labware in the past. Further, the curvature provided also prevents all of the main perforations  48  in a particular area from being blocked by a relatively linear object, such as a person&#39;s arm. Safety standards require a certain minimal opening for the sash while a user is performing a task in the work area with the sash raised. This means that there must be a certain minimal distance between the bottom of the sash and the top of the sash grill. With the curved grill of the present invention, since the height of the grill relative to the floor is lower than it would be if the grill was flat, the minimal distance between the bottom of the sash and the grill can be met with the sash lower relative to the floor than with prior flat grills. This results in the sash handle, which interferes with the view of the worker, being in a lower position and improves the worker&#39;s available viewing area. It also improves work safety by increasing the distance between the opening and the worker&#39;s face. The curved surface of grill  24  also operates to allow the air flowing downwardly along the back of sash  42 , and the air flowing inwardly from the opening in cabinet  10 , to more effectively sweep across the grill surface and enter the work area. In prior art systems, the air flowing inwardly is confronted with a front face that is located at a right angle to the flat horizontal surface of the sash foil. This air is then forced in an upward arc away from the surface of the sash grill prior to entering any perforations therein. With the novel curved sash grill of the present invention, the downwardly moving air is not confronted with a surface at a sharp (right) angle to the direction of air flow, which allows it to more effectively enter through the perforations within the sash grill with less turbulence. The curved surface of grill  24  also promotes smooth flow of air across the grill into the work area from outside the cabinet. Less turbulence is experienced then with prior art designs where the grill presents a right angle relative to the work surface. 
     Turning to the rear of cabinet  10 , baffle  20  is mounted between side panels  16  and can be secured in place such as by bolting or welding. The lower-most edge of baffle  20  may be provided with a support lip  58  as best seen in FIG.  3 . Lip  58  is used to support work surface  22  and may be provided with a number of threaded holes to secure work surface  22  to baffle  20 . Located above the lower most surface of baffle  20  and extending from one side of baffle  20  to the other, are a number of slots  60 , as best seen in FIG.  8 . Slots  60  are provided to allow air flowing downwardly from blower  28  to pass there through and into the plenum formed by baffle  20  and rear panel  18 . 
     As best seen in  FIG. 3 , a pressure gauge  62  is mounted within baffle  20  above slots  60 . Gauge  62  can be viewed by the user of safety cabinet  10  through sash  42 , which is made from a clear material such as tempered glass. Gauge  62  is used to measure a positive pressure within a plenum box  64  that is located immediately below blower  28 . Measuring the positive pressure within plenum box  64  allows the user of cabinet  10  to obtain a more accurate indication of the load on filters  30  and  32 . To measure the pressure within plenum box  64 , a hose barb  66  is placed through the rear plate of plenum box  64 . A piece of tubing  68  is mounted to hose barb  66  and extends downwardly through the rear plenum and is connected to a plastic Y-hose barb  70 . Another piece of tubing  72  extends from the lower end of barb  70  downwardly and into the space between bottom panel  14  and work surface  22 . Finally, the remaining end of hose barb  70  is connected to a third piece of tubing  74  which is coupled to the high pressure port of gauge  62 . Gauge  62  thus is mounted entirely within safety cabinet  10  and is adapted to measure the positive pressure within plenum box  64 . Should any leakage occur within gauge  62 , any contaminants within tubing  68 ,  72  or  74  would be contained within cabinet  10  and would be filtered prior to being exhausted into the room. 
     As best seen in  FIGS. 3 and 8 , cabinet,  10  is also provided with a perforated towel catch  78 . More specifically, a towel catch  78  extends from lip  58  at the bottom of baffle  20  downwardly to bottom panel  14 . Preferably, catch  78  is angled rearwardly as shown in  FIG. 3 , and is mounted to baffle  20  with the same screws that are used to attach work surface  22  to baffle  20 . This mounting allows towel catch  78  to easily be removed, such as may be necessary to clean towel catch  78  or bottom panel  14  in the event of a spill. As best seen in  FIG. 8 , catch  78  has a number of rectangular slots  80  which allow air to pass through catch  78  and upwardly behind baffle  20 . Moreover, the lower tubing  72  associated with pressure gauge  62  may be passed through one of the slots  80 . Catch  78  is used to prevent objects such as broken pieces of glass and paper towels from traveling upwardly through the rear plenum and into blower  28 . In use, work surface  22  may be pulled away from baffle  20  which allows towel catch  78  to be visually inspected for any blockage. If an object is lodged against towel catch  78 , it may be easily removed by the user of safety cabinet  10 . Moreover, the visual inspection allows the user of safety cabinet  10  to avoid contact with the catch which might result in injury and to be forewarned if a sharp of dangerous object is lodged against the catch. Prior art safety cabinets have located the towel catch associated therewith upwardly from the bottom of the safety cabinet. Generally, such a prior art towel catch would be located somewhere above the rear intake of the exhaust plenum  20 . In such a location the towel catch becomes a safety hazard in and of itself and can also result in injury if sharp objects are restrained by it. Location of towel catch  78  as described for the present invention allows the towel catch  78  to be visually inspected and cleaned. Further, the towel catch may be much more easily removed from safety cabinet  10  if needed, such as when surface  22  is to be removed for cleaning beneath it. 
     Turning to details of the plenum box  33  and associated filters, and as best seen in  FIGS. 3 and 4 , the supply filter  32  is located above work area  44  at the upper end of the baffle  20 . Air diffuser  43  is located immediately below supply filter  32 . Diffuser  43  operates to properly direct the air as it exits supply filter  32  to obtain the desired air flow through work area  44 . Immediately above supply filter  32  is the plenum box  33 . Box  33  directly abuts supply filter  32  and is held against it as described below. As best seen in  FIG. 4 , plenum box  33  extends from the exit of blower  28  and provides a structure for evenly distributing the air flow to both the supply and exhaust filters. More specifically, box  64  includes a distribution baffle  88  that tapers upwardly from the exit of blower  28  as it extends across the side of safety cabinet  10 . Preferably, baffle  88  extends from the front of plenum box  64  to the back thereof. A portion of the output from blower  28  will pass upwardly to exhaust filter  30  while a portion will be directed into a narrow channel  90 . The air leaving channel  90  is directed to a first curved deflector  92 , as shown on the left-hand side of FIG.  4 . Deflector  92  operates to redirect the air downwardly and to the right as viewed in FIG.  4 . Deflector  92  is preferably made from a rigid material such as steel and is rigidly mounted within plenum box  33 , such as by welding. As the air travels back to the right as viewed in  FIG. 4 , distribution baffle  88  forces the air downwardly and into a second narrow channel  94 . The angle of baffle  88  is selected to insure that the volume of air passing across supply filter  32  is relatively constant across the entire width of the filter. The angle will vary depending upon the output of the blower and the size of filter  32 . The air at the far right hand portion of plenum box  64 , as viewed in  FIG. 4 , is directed downwardly by a second deflector  96 . Thus, construction of plenum box  64 , with baffle  88  and deflectors  92  and  96 , operates to evenly distribute the air flow across and through supply filter  32 . This is done without restricting the air flow, such as with the use of a prior art perforated plate. Therefore, the above construction of plenum box  64  achieves a more uniform distribution of air across supply filter  32  without placing an increased load on blower  28 . 
     As best seen in  FIG. 4 , the upper end of plenum box  64  has an exhaust channel  98  therein that communicates directly with exhaust filter  30 . Baffle  88  directs some of the air leaving blower  28  upwardly through exhaust channel  98  and exhaust filter  30  ultimately exiting cabinet  10  through exhaust control cap  36 . As best seen in  FIG. 5 , exhaust filter  30  is held in position with an exhaust frame  100 . Frame  100  includes a recessed portion  102  which is shaped to conform to the outer perimeter of exhaust filter  30 . Portion  102  thus operates as a placement guide when filter  30  is to be replaced. Frame  100  also includes an upper bracket  104  and a lower leg  106 , which extends downwardly into a labyrinth seal  108 . As shown in  FIG. 5 , seal  108  includes a pair of upwardly extending plates  110  which are bolted to the top of plenum box  64 . Leg  106  extends between the plates  110  and is movable there between. 
     To adjust the position of filter  30 , the upper bracket  104  includes a pair of threaded holes  112 , through which are placed a plurality of bolts  114 . A retaining nut  116  is rigid with bracket  104  and in alignment with each bolt  114 . Each bolt  114  has a head  114 a, a threaded portion  114 b and a length such that it extends to the upper surface of plenum box  64 , and as shown in  FIG. 5 , may extend to the upper surface of a horizontal portion of plates  110  of the labyrinth seal  108 . Exhaust frame  100  cooperates with bolts  114 , the top of plenum box  64  and labyrinth seal  108  to simultaneously position and seal exhaust filter  30  upwardly and supply filter  32  downwardly. More specifically, in use, both head  114 a is turned with a wrench to move portion  102  upwardly or downwardly along threaded portion  114 b. When portion  102  is lowered, lower leg  106  will move lower within labyrinth seal  108 . Thereafter, the exhaust filter  30  may be replaced by placing a new or clean exhaust filter  30  within recessed portion  102 . Exhaust filter  30  is then raised into place by turning bolt  114  in the opposite direction. Bolt  114  may be rotated sufficiently to place a downward force on plenum box  64 . This downward force on plenum box  64  forces exhaust filter  30  into a sealing engagement with top panel  34 . Thus, bolt  114  in cooperation with portion  102  and nut  116  serves as a jack screw to raise and lower the filter housing and apply pressure in opposite vertical directions to hold the filter firmly in place. 
     Any air that is not recirculated through supply filter  32  and work area  44  must be filtered and exhausted from the cabinets. If air is to be exhausted into the room, exhaust control cap  36  is used. As best seen in  FIGS. 1 through 3 , exhaust control cap  36  is mounted on top of top panel  34  and directly above exhaust filter  30 . Control cap  36  is generally rectangularly shaped and has a pair of mounting flanges  122  extending from each side thereof. Flanges  122  are used to mount control cap  36  to top panel  34 . Control cap  36  has a solid top  124  and sides  126  which have a plurality of exhaust apertures  128  extending there through. Apertures  128  are preferably varied in diameter and operate to accommodate outward flow of exhaust air in a lateral as opposed to a vertical direction. As can be seen, control cap  36  thus provides a low profile mechanism for directing the exhaust air from safety cabinet  10  in a horizontal direction. As seen in  FIG. 2 , removable plugs  130  may be used to block the apertures  128 . The number and size of the blocked apertures, in combination with the blower output, determines the volume of air that is exhausted through the control cap. The control cap  36  can therefore be used to regulate the flow of air being exhausted from safety cabinet  10 . This regulation is done while evenly distributing the flow of exhaust air over the entire surface exhaust filter  30  and without placing an increased load on blower  28  by significantly restricting the passage of air. 
     The above described embodiment of control cap  36  is utilized when the exhaust air from safety cabinet  10  is exhausted directly into the room. In an alternative embodiment, the air is not exhausted directly into the room, but rather is directed into an exhaust system that removes the air from the building. In this embodiment, a different exhaust control cap  131  used, and is shown in FIG.  9 . As shown, control cap  131  has mounting flanges  132  that secured to top panel  34 . In this embodiment, rather than the side surfaces  133  being provided with apertures  128 , the side surfaces  133  are solid. In this embodiment however, a top surface  134  is provided with an exhaust duct  135 . Preferably, duct  135  is cylindrical. Duct  135  may be provided with a damper  136  as is known to those of skill in the art. An apertured plate  138  mounted below duct  135  and above the exhaust filter  30  provides a mechanism for controlling the flow of air through the exhaust filter in much the same manner as control cap  36  described above. As shown in  FIG. 9 , the apertures  140  within plate  138  can be varied in size. Further, selected apertures  140  may be plugged to regulate the volume of air passing through plate  138 . Plate  138  is preferably attached to control cap  131  with screws  142 . Control cap  131  preferably includes an access port  144  along one side thereof, which is covered with a plate  146  in normal use. Plate  146  may be bolted or screwed to control cap  131 . Port  144  is used to visually inspect plate  138  and obtain access thereto without removing plate  138 . In use, the desired number of apertures  140  are plugged within plate  138  to regulate the amount of air flowing through cap  131 . Plate  138  is then secured within control cap  131 . Thereafter, the exhaust system associated with safety cabinet  10  is coupled to duct  135  so that air passing through exhaust filter  30  would be directed through control cap  131  and into the exhaust system. In the case of both cap  36  and plate  138  the fact that the mechanical device for controlling air flow is located on the “clean” side (i.e the downstream side) of the exhaust filter means that it can be accessed for adjustment or service without danger of contamination to either the worker or the room environment. 
     The front of cabinet  10  also has a novel construction. As best seen in  FIG. 3 , front panel  40  is coupled to top panel  34  and extends between side panels  16  to enclose the area above supply filter  32 . Front panel may be held in place with any suitable attachment mechanism, such as by bolting. Sash  42  is held within cabinet  10  and travels along a pair of sash tracks  150 , as best seen in FIG.  7 . Tracks  150  are defined by a pair of front trim panels  152 . As best seen in  FIGS. 2 and 7 , trim panels  152  have a wide and angled front face  154 . Face  154  thus forms an acute angle with its associated side panel  16 . The angle of face  154  directs air downwardly toward the sash opening and then inwardly to the interior side surfaces of work area  44 . The angle of face  154  thus allows the air entering work area  44  to sweep the interior side surfaces of the work area as it passes over grill  24 . 
     As best seen in  FIG. 3 , the lower-most edge of sash  42  is provided with a handle  156 . Handle  156  is used to raise and lower sash  42  as may be needed to gain access to work area  44 . As seen in  FIG. 3 , handle  156  is equipped with a curved or angled lower surface  158 . While surface  158  is shown as being flat, but angled, it should also be understood that surface  158  could be curved in a concave shape. In use, surface  158  provides for a smooth interface of two bodies of air. The first body of air is that which is entering the cabinet from the outside through the sash opening. This air will travel along surface  158  as it approaches the sash opening. The second body of air is that which is moving downwardly along the back side of the sash inside the cabinet as a result of blower  28 . By providing an angled or curved surface  158 , the two bodies of air will not be meeting at a right angle, resulting in less turbulence and better containment of the air within work area  44 . A third body of air is that which flows from the blower toward the rear of the work area. 
     Referring to  FIGS. 1 ,  3  and  10 , as sash  42  is moved upwardly within tracks  150 , it will slide behind an upper sash pocket  160 . As best seen in  FIGS. 1 and 3 , sash pocket  160  is preferably bolted to front panel  40  and trim panels  152 . Pocket  160  is shaped to extend from one side of sash  42  to the other, and is enclosed along the top thereof. Pocket  160  thus cooperates with front panel  40  to enclose the top and sides of sash  42  as it is moved upwardly along tracks  150 . Pocket  160  acts to prevent the operator of cabinet  10  from accessing the upper portion of sash  42  as it slides away from work area  44 . As best seen in  FIG. 10 , there is no physical contact between the rear of sash  42  and any type of seal. In the prior art, a wiping seal would exist in the area of a screw  133  shown in FIG.  10 . This wiping seal resulted in certain disadvantages as explained above. Such a seal is not needed with the present invention. A front cover  165  is secured over the front of cabinet  10 . More specifically, cover  165  is placed over sash pocket  160  and front panel  40  to present a more appealing front face for cabinet  10 . The design of face  154  also facilitates decontamination of the cabinet as is required from time to time by safety regulations. Decontamination may occur by leaving pocket  160  in place and lowering the sash. The entire front of the cabinet is then sealed with plastic which is secured by tape to the angled surfaces  154 . Alternatively, sash pocket  160  may be removed and the sash completely lowered followed by sealing off the front of the cabinet with plastic. Another alternative is to remove pocket  160  and place the sash in the fully raised position before the front face is sealed with plastic. In the latter two cases the pocket  160  may be placed inside the cabinet so that it will be decontaminated. In all three cases effective decontamination is accomplished without the need to actually remove the sash. 
     As can be seen in  FIG. 10 , there is no physical contact with the back of sash  42  and the prior art wiping seal has been eliminated. In order to insure that contaminated air from the work area  44  does not escape into the room a plurality of upper scavenger holes  168  are provided immediately above work area  44  along the front of cabinet  10 . Any air leaving environment  44  will be drawn back through holes  168  and will not be leaked into the room. While the use of scavenger holes in this location has been taught by prior art constructions, it has been discovered that the effectiveness of these holes  168  is greatly enhanced if structure is provided to insure that the area in front of these holes will be a uniform negative pressure area relative to the work area  44 . To this end a restrictor plate  172  is coupled between air diffuser plate  43  and a filter shelf  170  used to hold supply filter  32  in place. Restrictor plate  172  is preferably held in place with a series of screws  174 . The location of plate  172  may be altered by loosening screws  174  and sliding the plate inwardly or outwardly. By adjusting the location of plate  172  the balance between air flow down into the work area and air flow passing through the exhaust is maintained in favor of exhaust air. Plate  172  serves to even out any pressure differences in the area of holes  168  resulting from the competing air flows and the fact that the holes are interrupted with solid areas. This insures that air will flow into the holes and out the exhaust rather than out into the room in the area behind the sash. It is to be understood that holes  168  extend across the entire front of the cabinet to insure that the entire back side of the sash is effectively “sealed” against contaminate air entering the room. 
     As can be seen from the above, the invention provides a biological safety cabinet with a number of improved features and achieves a better air-flow into and through the cabinet. From the foregoing, it will be seen that this invention is one well adapted to attain all of the ends and objects herein above set forth, together with other advantages which are inherent to the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.