effects on nearly all cultural heritage materials, with ceramics and glass being exceptions to these effects in most cases. Metals face the risk of corrosion as RH increases, a risk which is enhanced by surface contaminants and emphasizes the need for proper housing. Additionally, mold growth is far more likely as humidity increases, which not only could cause allergic reactions for viewers but it also weakens the collections afflicted and attracts other pests. In contrast to this requisite for dry conditions, if the atmosphere is not humid enough wooden objects could crack or warp, and many organic materials face embrittlement below 40% RH. Relative Humidity (RH) is the amount of water held in the air as a percentage of how much could be held in fully saturated air at a given temperature. The possible amount of moisture held at a given time is directly related to temperature. Warm air can hold more water than cold air. It is important to measure the RH of spaces regularly by using a number of tools including humidity indicator cards, thermo-hygrographs, hygrometers, psychrometers and data loggers. Once these data are being monitored, there are several ways to adjust relative humidity by using humidifiers, dehumidifiers, improving heating and air conditioning systems, and adjusting the temperature of the space. In recent decades, it has become understood that even delicate organic materials have some elasticity in their response to relative humidity (RH) fluctuations, allowing the materials to swell or contract as necessary. This phenomenon is naturally reversible within a range of 50% ± 15% RH. Destructive chemical and mechanical processes, such as hydrolysis at high RH and embrittlement and cross-linking at low RH, can be held to a minimum within a similar range for a general museum collection. Although minor and gentle fluctuations in RH can reasonably be withstood by most collections, quick or drastic shifts can be harmful. Anisotropic materials such as wood and ivory are especially responsive to humidity changes, and RH issues are compounded when they are attached to inorganic materials such as a metal. The metal acts as a restraint, hindering the organic materials’ ability to expand and contract as needed. Thus, cultural heritage objects composed of highly responsive materials or a combination of organic and inorganic materials should ideally be in carefully controlled climates and buffered against the atmosphere by their display or storage housing. Damage due to incorrect relative humidity occurs