The control of temperature and relative humidity is a vital factor in the preservation of a collection. Inappropriate of fluctuating environmental conditions can cause irreparable damage to works of art. High temperatures and high levels of relative humidity can initiate chemical reactions that may lead to deterioration, cause physical changes, promote corrosion, and permit mold to grow. Low levels of relative humidity may cause desiccation, embrittlement, and irreversible dimensional changes. Rapidly fluctuating temperature and relative humidity compound all of these effects. The contents and condition of a collection must be reviewed to determine what range of temperature and relative humidity can be tolerated. A collection may need to be organized by materials with similar needs. Pieces with exceptional problems may require special rooms or cases. Following the assessment of a collection, a building's environment must be monitored to establish which areas are most in need of adjustment. An understanding of the interrelationship of temperature and relative humidity and their effects upon the materials of works of art will make it possible for staff to establish safe parameters for a collection.