Mounted head of a roan antelope, Hippotragus equinus, severely damaged by low relative humidity in mammal storeroom.
Conservators measure relative humidity (RH) to determine whether environmental conditions are appropriate for specimens. RH is the amount of water in a given volume of air relative to the maximum amount of water that air can hold at a given temperature, expressed as a percentage. For example, when the RH is 100% the air is saturated, holding all the water it can; at 50% RH, air holds half the water it can.
Controlling RH levels is crucial to the long-term preservation of specimens. Organic materials such as wood, leather, skin, hair, feathers, ivory, fibers and paper are most susceptible to RH changes. Because these materials give off moisture when the RH is low and absorb moisture when the RH is high, they expand and contract when RH fluctuates. Repeated cycles of these changes eventually lead to warping, cracking and possible disintegration. Consistently low RH over a long time also causes damage because organic materials give off moisture, including water that is part of their chemical make-up, and can become dry, brittle, shrunken and warped. A combination of fluctuating and low RH levels over time have caused the skin on the neck of the antelope above to shrink and crack.