Fluctuating relative humidity (RH) causes stress on materials. Rapid humidity fluctuation damages a wider range of museum objects than does temperature change. A change in RH causes dimensional alteration in hygroscopic materials (for example, wood, ivory, skin, and other organic materials), resulting in warping, splitting, and delamination of sensitive materials. Seasonal slow drifts are less harmful to structures and objects than abrupt changes. High RH (above 65%) can cause mold growth and metal corrosion. Low RH (below 25%) can cause embrittlement of hygroscopic materials such as leather and paper.
However, if you are in a wet or dry climate, it may not be possible to maintain the ideal RH level. Try to set your relative humidity level so that it is stable somewhere between 25% and 65%. Above 65% mold will grow, more rapidly as the RH rises. Below 25%, the materials may lose structurally important water. If you cannot achieve even these levels, achieve a reasonable level that does not fluctuate. If this level is above 65%, make sure you have good air circulation and regular inspections for mould growth.
Temperature is the major factor in the speed at which ”natural aging” occurs. Materials last longer at cooler temperatures. However, lower temperatures are hard on visitors to the museum. A rapid change in temperature, if the relative humidity is constant, may have damaging effects on stressed metals, stone, films, plastics or wax, materials from which many modern collections are made. High temperatures increase deterioration reaction rates and melt heat-susceptible materials. Store inherently unstable materials, like plastics and rubber, in cool temperatures and lower RH levels to decrease the rate at which they naturally deteriorate. Waxes and plastics are damaged by freezing, so should be kept cool, but not frozen.