encompasses all actions taken to prolong the life of an object." and is an important element of museum policy. Members of the museum profession are entrusted to create and maintain a protective environment for the collections in their care. A good preventive conservation program minimizes the need for conservation treatment by blocking, avoiding, or minimizing the agents of deterioration. Emergency planning, environmental safeguards and monitoring are all types of preventive conservation. Scientific research continues to discover new ways of safeguarding collections. Today various monitoring devices assist in the observation of changes in the Agents of Deterioration and other changes that may assist in diagnosing destructive activity before it is a disaster. In the image to the right a device is being attached to the Liberty Bell to monitor any changes in the crack. Metallic heritage objects are sensitive to environmental conditions such as exposure to light and ultraviolet light, temperature, relative humidity, water and moisture, and various pollutants especially chloride salts. Safeguards of protection against threats of natural disasters such as flood or fire need to be planned for and maintaining an environment that keep all Agents of Deterioration within safe limits and controlling their fluctuation will assist in the preservation of metals. Whether in storage, on display, or in transit metals are best preserved in a "moderate climate that avoids extreme temperature and RH fluctuations and that excludes daylight and/or filters out ultraviolet light and infrared radiation and air pollution provides the appropriate environment for collection preservation." A controlled environment can protect metals from polluted air, dust, ultraviolet radiation, and excessive relative humidity - ideal values are temperature of 16-20 °C and up to 40% (35-55% according to recent Canadian Conservation Institute recommendations) relative humidity, noting that if metal is combined with organic materials, relative humidity should not be below 45%. Archaeological objects are best stored in rooms (or plastic boxes) with very low relative humidity, except if they come from a bog or high water environment, in which case the right equilibrium with the environment needs to be found. Particularly valuable items can be placed in sealed micro-climate containers with an inert gas such as nitrogen or argon. Metals with active corrosion fare better with lower relative humidity: copper or copper alloy objects up to 35% RH and iron objects 12-15% RH. Clean and well organized storage areas are important but materials in the environment are also considered.