to harmful environmental conditions, the more likely that it will experience deterioration. Many museums and libraries have permanent exhibitions, and installed exhibitions have the potential to be on the view without any changes for years. Damage from a long exhibition is usually caused by light. The degree of deterioration is different for each respective object. For paper-based items, the suggested maximum length of time that they should be on display is three months per year, or 42 kilolux hours of light per year – whichever comes first. An exhibition log report, including records of the length of the exhibition time and the light level of the display, may prevent objects from being exhibited too frequently. Displayed items need to be inspected regularly for evidence of damage or change. It is recommended that high-quality facsimiles of especially delicate or fragile materials be displayed in lieu of originals for longer exhibitions. Individual cases Library or archival materials are usually displayed in display cases or frames. Cases provide a physically and chemically secure environment. Vertical cases are acceptable for small or single-sheet items, and horizontal cases can be used for a variety of objects, including three-dimensional items such as opened or closed books, and flat paper items. All these objects can be arranged simultaneously in one horizontal case under a unified theme. Materials used for case construction should be chosen carefully because component materials can easily become a significant source of pollutants or harmful fumes for displayed objects. Outgassing from materials used in the construction of the exhibition case and/or fabrics used for lining the case can be destructive. Pollutants may cause visible deterioration, including discoloration of surfaces and corrosion. Examples of evaluative criteria to be used in deeming materials suitable for use in exhibit display could be the potential of contact-transfer of harmful substances, water solubility or dry-transfer of dyes, the dry-texture of paints, pH, and abrasiveness. New cases may be preferred, constructed of safe materials such as metal, plexiglass, or some sealed woods. Separating certain materials from the display section of an exhibition case by lining relevant surfaces with an impermeable barrier film will help protect items from damage. Any fabrics that line or decorate the case (for example, polyester blend fabric), and any adhesives used in the process, should also be tested to determine any risk. Using internal buffers and pollutant absorbers, such as silica gel, activated carbon, or zeolite,