in conditions where high percentages (wet) and low percentages (dry) are present. High relative humidity can result in mold growth, salt efflorescence, rapid corrosion, and swelling of wood. Low relative humidity can result in cracking of wooden objects and embrittlement of paper and organic textiles. Rapid fluctuations in relative humidity can also be damaging, as “Rapidly fluctuating temperature and relative humidity compound all of these effects.” Institutions should aim to keep the RH constant in exhibitions and storage areas because many organic objects expand and contract as both temperature and RH change. Some changes undergone by objects are reversible by adjusting the RH, but damage like cracks may be irreversible. Keeping the RH within an appropriate range for the type of material and as consistent as possible will prevent most RH based damage. Limiting storage and display spaces to between 40-60% RH will avoid most damaging effects, but maintaining a stable RH is considered more important than adhering to absolute ranges. Light Light, as it relates to collections maintenance, is primarily concerned with the visual and ultraviolet light (UV) ranges of the electromagnetic spectrum. Both types of light can cause damage as “light radiation falling on a surface provides energy to induce chemical changes in the molecules of the material.” Damage from light, including loss of color and strength, is cumulative and irreversible. It is therefore crucial that light levels are monitored. Visible light Visual light, measured in lux or foot-candles (fc), cannot be eliminated, as it is required both to view collections in detail and to move safely in the presence of collections. Unfortunately, this means that harmful oxidation effects which visual light makes possible also cannot be eliminated, but merely reduced to the amount necessary for the task at hand. The effects of visual light began to be studied by artists and color manufacturers as early as the 18th century, but it was not until the mid-20th century that the chemical damage caused by different lighting situations was researched in depth. In recent decades the cumulative nature of light degradation has become better understood by conservation science. Comprehensive studies began to emphasize long-term effects and allow for short-term variation in light levels depending on the specific situation: standard viewing, viewing by the aged, complex study or treatment, and observation of low contrast details all may have different requirements. Practical compromise between protection of cultural heritage and allowing the