Extreme variations in either can cause mold growth, cracks in pictures, premature aging and other damage. Although 68 F and 50 percent relative humidity are frequently cited as being close to ideal, stability plays a major role in long-term preservation, so it is better to have a stable temperature of perhaps 78 F and a relative humidity of 30 percent than extreme variations in either. Another environmental factor to consider is light. All light damages paper and photographs to some degree, so they should be stored in a dark or dimly lit area if possible. Direct sunlight on valued materials must be avoided, because it causes severe damage very quickly, as witnessed by a newspaper that is left outside for only one day.
Air quality is a third factor to consider.
Air pollution, especially in large cities, contains many gases that combine with water to form acids that eat everything from paper to bronze statues. By properly maintaining air conditioning filters and regularly cleaning materials with non-oily dust cloths, much of this type of damage can be avoided. Creating and maintaining a good climate for an entire house can be difficult and expensive. Interior closets, those located away from the outside walls of the building, tend to provide an environment which is more stable. If they are kept shut, fluctuations in temperature and relative humidity will be reduced, and the interior will be dark and less dusty.