Mould, corrosion and oxidation can damage art works, documents and other objects of value in museums and art galleries, requiring high expenditures on restoration. The organic materials in these items already contain the micro-organisms which can cause or exacerbate problems, so a key preventative measure is control of environmental factors such as humidity and temperature.

While maintaining consistent temperatures helps, humidity control is generally a more important – and more expensive – concern. There is no consensus on the optimum humidity for museum environments. In the general space, human comfort as well as the stability of materials need to be taken into account. Often, the goal for buildings is to maintain a relative humidity of between 45 and 55% throughout the whole year, but lower humidity may well be better. Sudden weather changes can be highly problematic, especially if several occur over a short period, causing warping or cracking. Older artefacts are more fragile, and the accumulating effects of the small defects caused by improper maintenance of the surroundings will eventually become apparent. Humidity control systems with proven reliability are therefore an essential part of any gallery’s inventory.

Often, passive buffering for humidity control can be achieved using desiccant-containing enclosures. These create microclimates in which humidity is kept low and changes very slowly. However, these systems can be expensive and will not function as expected if there are any leaks.

Alternatively, self-contained dehumidifier units or a central unit equipped with ducting may be used. Desiccant dehumidifiers are used in colder climates, refrigerative dehumidifiers in hotter ones. Usually, humidifiers in these environments operate on the evaporative principle.