Humidity

The term relative humidity (RH) refers to the percentage of water vapor in the air at a given temperature. When the temperature is high, air can contain a great deal of water in the form of vapor, but when the temperature drops, vapor condenses into water. In other words, temperature is closely linked to relative humidity. Because cooler air cannot hold as much vapor as warmer air, a rapid lowering of temperature can result in condensation, causing absorbent materials such as paper to become damp.



For example, if a daytime temperature of 20°C (70° F) with an RH of 50 percent drops to 15°C (60° F) at night, the RH will increase drastically to 70 percent. An example of this effect may be noted on the accompanying chart, where point A shows a temperature of 20°C with an RH of 50 percent, and point B a temperature of 15°C with an RH of 70 percent. Most paper expands and contracts with fluctuations in RH, which causes stress that can shorten the life of paper and binding materials. In general, low RH is less damaging than high RH. High RH can cause mold, especially when air circulation is poor. High RH also provides a hospitable environment for insects, which need moisture to survive and flourish. A range of 35 to 40 percent RH is acceptable and achievable in tropical climates with the right equipment. Failing this, libraries should try to lower RH as much as possible, perhaps by increasing the rate of air flow in the building.

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