Some materials in use in storage rooms in museums emit gases which tend to attack the objects. Metal objects, the main subject discussed here, are particularly vulnerable in this respect. All kinds of wood emit acidic vapors, e.g., acetic and formic acid. Among them oak is notorious in releasing considerable amounts of acid capable of causing acidity in the environment of about pH 3-4. The emission of acid is long-lasting as is illustrated by 17th-century oak which continues to exude as temperature and relative humidity exceed 25°C and 50% respectively. There is as yet no known way of sealing wood with a gas-tight coating, except by means of metal foil. Even acid concentrations of less than 0.05 ppm corrode metals, although at such a rate that it takes years before the discoloration becomes visible. Nevertheless, this is alarming, as the acetic acid vapor is corrosive to practically all metals. Many types of fresh wood also emit volatile oils which may weaken protective resinous coatings on metal objects. Acetic and formic acid are, moreover, the final product of the vapors issuing from plywood and the urea formaldehyde adhesive in it. Vulcanized rubber emits sulfur and this stains silver, copper, and lead. The same holds true for materials such as chamois leather, suede leather, felt, and fish glue. Synthetic materials such as polyester, polyformaldehyde, polyurethane, polyvinyl compounds, and polysulfides emit vapors that attack metals.