Heat also encourages biological agents, such as insects and mold, and directly affects relative humidity. Organic materials do not have a high degree of thermal expansivity, or swelling due to heat, but variations in heat effect relative humidity, and fluctuations in relative humidity can cause structural damage to paper-based materials. Over time, fluctuations in temperature and relative humidity actually cause pages to separate from their bindings. Collection materials with components of animal origin such as vellum or gelatin are particularly sensitive to fluctuations in relative humidity. Fluctuations due to seasonal variations in temperature should be managed so that such changes are gradual.
Rapid fluctuations do the most damage to collections.
Therefore, it is recommended that repositories maintain the temperatures outlined below. If this is not possible, repositories can contribute to the longevity of their materials by maintaining constancy of temperature within the specified ranges. In general, warmer air holds more moisture. High humidity, like high temperature, accelerates the rate of chemical reactions and increases the rate of deterioration of library and archival materials. A certain level of relative humidity is recommended for paper to retain its flexibility.