Ballast systems for, particularly, inflatable decontamination shelters are described. The systems may be integrated into the overall structures of the shelters so as to avoid need for sand bags or other discrete weight-providing objects. They additionally may use liquids (rather than or in addition to solids) for weight-providing purposes, with the liquids including water supplied by the same plumbing system that supplies a shower of a shelter.

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 61/955,889, filed Mar. 20, 2014, entitled “Decontamination Shelter With Fully Integrated Ballast System,” the entire contents of which are hereby incorporated herein by this reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to ballast systems and more particularly, but not necessarily exclusively, to ballast systems for use in connection with, and integrated into, decontamination shelters. 
     BACKGROUND OF THE INVENTION 
     Decontamination shelters are increasingly in demand. Portable shelters are especially useful for “first responders” such as mobile medical crew and law enforcement officers as well as for military personnel and others working in the field (i.e. not in permanent institutional settings). Recent outbreaks of the Ebola virus in west Africa, for example, have highlighted need for equipment and structures useful to decontaminate medical workers treating infected populations. 
     U.S. Pat. No. 4,800,597 to Healey details a relatively simple decontamination shelter. As shown in the Healey patent, the shelter may include multiple adjacent compartments. One compartment may be designated a shower area and include a shower head. According to the Healey patent, “[t]he shelter desirably is comprised of flexible waterproof material and scaffolding in the form of hollow tubes with connecting fittings which can be erected to support the shelter and easily dismantled when the shelter has served its purpose.” See Healey, col. 1, 11. 52-56. No inflation of any portion of the shelter occurs, however. Moreover, presumably because it employs substantial structure in the form of “tubular metal posts,” see id., col. 3, 11. 17-19, the Healey patent fails to contemplate utilizing any ballast for stabilizing the shelter when constructed. 
     U.S. Pat. No. 7,624,543 to Sample, et al., discloses another decontamination shelter intended to be portable, “lightweight, and rapidly deployable.” See Sample, col. 1, 11. 15-16. Preassembled, the shelter “comprises a frame movable between a stowed configuration and a deployed configuration and a canopy associated with the frame.” See id., col. 1, 1. 66 to col. 2, 1. 1. As with the shelter of the Healey patent, those of the Sample patent are not inflated and have self-supporting frames including substantial structure in the form of multiple aluminum struts. See id., col. 4, 11. 4-8. Fabric straps or other “support elements” may be used “in cases where additional structural support is desired, such as . . . in windy conditions.” See id., col. 4, 11. 24-61. 
     Finally, U.S. Pat. No. 8,365,804 to Genovese, et al., identifies a portion of yet another decontamination shelter. Designed “to form a gas-impermeable barrier in a structural location such as a hallway,” see Genovese, Abstract, 11. 1-3, the device “is composed of an inflatable support section which contains two doorways separated by an inner compartment, and an outer, expandable bladder.” See id., col. 1, 11. 48-50. Because intended for use within a structural location (i.e. indoors), the device is not subject to windy conditions or other destabilizing hazards and thus too lacks any ballast. 
     SUMMARY OF THE INVENTION 
     By contrast, shelters of the present invention are both inflatable and useful outdoors (as well as indoors). They further may be lightweight and capable of being stowed compactly. Shelters of the present invention nevertheless may function well in windy conditions and other potentially-destabilizing environments. 
     Versions of the invention may employ ballast to enhance their operating effectiveness in destabilizing conditions. Preferably, though, the ballast need not be in the form of sand bags or similar discrete weight-providing objects placed against or atop portions of a shelter. Instead, a ballast system may be integrated into the overall structure of the shelter. Moreover, the ballast may be liquid rather than a traditional solid (e.g. sand). Yet additionally, the liquid ballast may be or comprise water—including water supplied by the same source (and possibly by the same plumbing system) that supplies a shower of the shelter—thus avoiding need for any ballast material separate from that already available at the shelter. 
     Furthermore, by integrating the ballast system into the shelter structure, the volume of the ballast system may be incorporated within the footprint of the inflatable tube assembly of the shelter. This arrangement allows the ballast system to avoid consuming additional floor space either within or outside of the shelter, producing superior mobility for both operating crew and users of the shelter. Combined with the likely absence of any sand bags or other discrete objects, this arrangement also reduces risk of tripping hazards as well as damage to the ballast structures from foot traffic or related activities. 
     It thus is an optional, non-exclusive object of the present invention to provide ballast systems integrated into other structures. 
     It is also an option, non-exclusive object of the present invention to provide ballast systems for use in connection with decontamination shelters. 
     It is another optional, non-exclusive object of the present invention to provide systems utilizing ballast of the same type as employed for other purposes within the systems. 
     It is a further optional, non-exclusive object of the present invention to provide systems in which the ballast is or comprises water. 
     It is an additional optional, non-exclusive object of the present invention to provide ballast systems whose volume is incorporated within inflatable tube assemblies of decontamination shelters. 
     Other objects, features, and advantages of the present invention will be apparent to those skilled in the relevant art with reference to the remaining text and the drawings of this application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a portion of a decontamination shelter including a ballast system useful in connection with the present invention. 
         FIG. 2  is a schematized, plan view of, among other things, the footprint of the decontamination shelter of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Depicted in  FIG. 1  is a portion of an exemplary decontamination shelter  10  consistent with the present invention. Shelter  10  may include three-dimensional structure  14  configured, when deployed, to bound volume sufficient for occupancy by at least one person. Preferably, shelter  10  includes a shower  12  and has height adequate for a person to stand underneath a nozzle or head of the shower  12 . Structure  14  also preferably is covered, in part or whole, by material suitable to provide privacy for the user of shelter  10  and isolate the user from the environment surrounding shelter  10 . 
     At least portions of structure  14  may be inflatable for use.  FIG. 1  illustrates various support tubes  18  of structure  14  configured to receive inflation air or other gas. Tubes  18  may have any desired size and shape and may be formed of any suitable gas-impervious (or substantially so) material. Advantageously, tubes  18  are formed of light weight, pliable material that may be folded or otherwise collapsed into a smaller volume when uninflated. 
     Structure  14  additionally may include other components and equipment, some or all of which are not typically configured for inflation. Examples of such components and equipment shown in  FIG. 1  comprise shower plumbing  22  and cross-support  26 . Further examples may be attachment assemblies  30  by which plumbing  22  may be connected to tubes  18  for use as well as floor  34 . 
     Further illustrated in  FIGS. 1-2  is that structure  14  may comprise one or more ballast tubes  38 . As with tubes  18 , ballast tubes  38  may have any desired size and shape and beneficially may be formed of light weight, pliable material. Ballast tubes  38  may be integral with tubes  18  or other parts of structure  14 ; alternatively, ballast tubes  38  may be fastened or otherwise connected to one or more other components of the structure  14 . Advantageously, though, ballast tubes  38  are constructed and placed so that, when not in use, they may fold or collapse into a smaller volume together with other parts of structure  14  for integrated storage. 
     Although ballast tubes  38  may, if desired, be constructed of gas-impervious material, they preferably are formed of material that is liquid-impervious (or substantially so). This is because ballast tubes  38  are configured to receive quantities of liquid in use, with the weight of the liquid serving as ballast for structure  10 . A preferred ballast liquid is water, which if desired may derive from the same source  40  as supplies the shower  12  of shelter  10 . In this case ballast plumbing  42  may provide liquid communication between source  40  and ballast tubes  38 . Ballast plumbing  42  may either be wholly distinct from plumbing  22  or share some piping or other components. 
     As shown in  FIG. 1 , control valve  46  optionally may be interposed between source  40  and each of plumbing  22  and ballast plumbing  42 . Interposing control valve  46  in this manner allows operating personnel or a user to direct flow of water from source  40  only to the shower  12 , only to ballast tubes  38 , or to both the shower  12  and ballast tubes  38 . To reduce pressure of water flow from source  40  and consequent risk of damaging or overfilling ballast tubes  38 , pressure regulator  50  optionally may be interposed between control valve  46  and ballast plumbing  42 . 
     Additionally illustrated in  FIGS. 1-2  is that ballast tubes  38  effectively form a base of structure  14  atop the ground or other surface on which shelter  10  is positioned. When deployed, structure  14  thus defines a footprint (see  FIG. 2 ) with respect to such surface. Ballast tubes  38  need not extend this footprint of structure  14  beyond that which would otherwise be provided by tubes  18 , nor do ballast tubes  38  subtract from the area of floor  34  available to operating personnel and users of shelter  10 . These characteristics offer superior mobility of operating personnel and users both within and outside shelter  10  as reflected in  FIG. 2 . They also reduce risk of tripping hazards to humans and damage to the ballast itself as compared with using conventional sand bags or similar discrete objects. 
     By using common water source  40  to fill ballast tubes  38 , no separate ballast fluid or solid is needed for the ballast tubes  38 . Similarly, routing both plumbing  22  and ballast plumbing  42  to the same source  40  avoids need for separate reservoirs for the shower  12  and ballast fluid. Shelter  10  hence may be more quickly and easily constructed than are conventional decontamination shelters. 
     Shelter  10  may be deployed in any appropriate way for use. One exemplary deployment method includes transporting the shelter  10  to a suitable site and constructing structure  14  at least by inflating support tubes  18  and attaching plumbing  22  thereto. Also as part of the construction, plumbing  22  and ballast plumbing  42  may be connected directly or indirectly to source  40  and liquid from source  40  added to ballast tubes  38 . Those skilled in the art will, of course, recognize that other actions may be required to construct structure  14  and render shelter  10  fully functional. 
     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of the present invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of the invention. Incorporated herein by this reference are the entire contents of the Healey, Sample, and Genovese patents.