Humidity Control
Relative humidity (RH) is a ratio of water vapor in the air to the amount it could hold if fully saturated, and is expressed as a percentage. Low levels of relative humidity mean dry conditions since the air is then capable of taking up moisture. High values of RH mean that moisture can be transferred to paper materials. Temperature is measured with a thermometer; RH with a hygrometer; a thermohygrometer measures both.
Humidity control is very important to your health for a couple of reasons. Mold, which thrives in high humidity, is dormant or dies when the humidity is low. Mold occurs around 68% humidity. Dust mites also die when the humidity stays below 50%. On the other hand, below 30% relative humidity, people can be uncomfortable and can suffer from dry mucus membranes which can lead to nosebleeds and infections. Low humidity can aggravate the symptoms of asthma and high humidity at high temperatures can sometimes lead to heat exhaustion or heat stroke. The range of RH for human comfort is 45 to 55%. Control humidity with proper ventilation and air conditioners, humidifiers or dehumidifiers.
Temperature and humidity are key agents of deterioration of library materials. Extreme, or rapidly fluctuating, relative humidity poses a major threat, especially to organic materials. Their hygroscopic nature (they can rapidly absorb and release water) means that they quickly expand and contract; generating stresses which cause damage such as cracking. The goal for most types of library collection is avoiding rapid RH change (no greater than +/- 3% in one hour, +/- 5% in 24 hours) while staying within the range of 40-55% RH.
If relative humidity is controlled, temperature control is generally less crucial. Ironically, however, since the human body is far more sensitive to heat and cold than to humidity, it is temperature control that is more often seen as the priority in public buildings. Public areas are usually kept between 17-19oC (63-66oF). For collections care, a range of 15-25oC is acceptable for most collections, while stores can be kept at lower temperatures (there is no lower limit, provided humidity is still controlled) to both save energy and reduce decay rates.
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