3.2.2 Relative humidity
Paper, cloth and leather are all hygroscopic, that is, they absorb moisture from the air and retain it. Thus, in humid climates, most materials in the library contain a relatively high percentage of water. In these conditions, even a slight increase in ambient relative humidity is enough for the item to sustain mold growth, if the other requirements are present.
There are several different ways to measure moisture. Absolute Humidity is the weight of water in a given volume of air (g/m3 ). Moisture content is the weight of water in any given material (kg/kg). Both of these measurments are variable, i.e., warm air can hold more moisture than cold air, and the moisture content of materials varies with the absolute humidity of the surrounding air. Neither absolute humidity nor moisture content can be effectively determined in a library environment. Therefore, the only useful measure from the point of view of collections maintenance is that of Relative Humidity (RH). Relative humidity is the amount of water in a given volume of air relative to the maximum amount of water air can hold at that temperature, and is expressed as a percentage.
When warm air is cooled it can hold less moisture. This moisture condenses on the surface of items or is absorbed by them if they are hygroscopic, If, for example, at 70º F, the RH is 50%, it requires only a ten degree drop in temperature to raise the RH to 70%. Plenderleith and Werner8 include a chart which shows the curves relating a reduction in temperature to the corresponding rise in relative humidity. In humid tropical climates lowering the temperature without reducing the relative humidity can result: in rampant mold growth, as many institutions have discovered to their dismay after installing a series of window air conditioners in an attempt to improve their environment. While air conditioning does remove some moisture from the air, and is generally adequate in a more temperate environment with a naturally lower ambient RH, in tropical climates with year round RH of 80 to 90%, a window airconditioning unit cannot remove enough of the moisture to prevent the cooled air from reaching the dew point.
The literature contains a variety of recommendations for RH levels that will prevent the growth of mold. They range from a high of 60% to a low of 45%, and seem to have declined steadily over the years. In 1940, Beckwith found that of the molds in his experiment, none would grow at a relative humidity below 75%, even when additional nutrients were added to the culture.9 While not definitive, this would help to explain why tropical libraries and museums (whose RH is seldom as low as 60%, let alone 45%) are not constantly blanketed in mold. Certainly lower relative humidities are safer, but it is apparent that the incidence of growth can be minimized at significantly higher levels of humidity.
Because relative humidity is so dependent on temperature, all figures are relative, and subject to a number of variables. As seen above, a change in one results in a change in the other and achieving the correct balance is the critical factor.