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close this book Prevention and treatment of mold in library collections with an emphasis on tropical climates: A RAMP study
View the document Preface
View the document Acknowledgements
close this folder 1. Introduction
View the document 1.1 Climate
View the document 1.2 Materials
View the document 1.3 Modifying the environment
View the document Literature cited
close this folder 2. Mold
View the document 2.1 Structure of mold
close this folder 2.2 Environmental and nutritional factors in growth and survival
View the document 2.2.1 Temperature
View the document 2.2.2 Moisture
View the document 2.2.3 Nutrients
View the document Literature cited
close this folder 3. Implications for library materials
close this folder 3.1 Vulnerability of materials
View the document 3.1.1 Paper - cellulose, sizes, coatings
View the document 3.1.2 Bookcloth
View the document 3.1.3 Leather
View the document 3.1.4 Adhesives
View the document 3.1.5 Film and related materials
close this folder 3.2 Environmental factors
View the document 3.2.1 Circulation
View the document 3.2.2 Relative humidity
View the document 3.2.3 Temperature
View the document Literature cited
close this folder 4. Prevention
close this folder 4.1 Building design and modification
View the document 4.1.1 Location
View the document 4.1.2 Structural considerations in environmental modification
close this folder 4.2 Interior modifications in existing facilities
View the document 4.2.1 Location of stack and storage areas
View the document 4.2.2 Stack arrangement
View the document 4.2.3 Localized environmental modification
View the document 4.2.4 Creating microclimates in cabinets and cases
View the document 4.3 Stack maintenance
View the document Literature cited
close this folder 5. Fungicides and fumigation
View the document 5.1 Fungicides
View the document 5.2 Fumigation
View the document 5.3 Toxicity of fumigants
View the document Literature cited
close this folder 6. Treatment
close this folder 6.1 Small outbreaks - localized high relative humidity
View the document 6.1.1 Books
View the document 6.1.2 Unbound materials (documents, maps, works of art on paper)
View the document 6.1.3 Photographs, negatives and microfilm
View the document 6.1.4 General area
close this folder 6.2 Moderate outbreaks - Major and prolonged periods of high humidity or minor flooding
View the document 6.2.1 Books
View the document 6.2.2 Unbound materials
View the document 6.2.3 Photographs, negatives and microfilm
View the document 6.2.4 General area
close this folder 6.3 Major outbreaks - Major flooding and prolonged exposure
View the document 6.3.1 Priorities and planning
View the document 6.3.2 Prevention of mold growth on site
View the document 6.3.3 Freezing
View the document 6.3.4 Drying
View the document Literature cited
close this folder 7. Equipment and supplies
View the document 7.1 Monitoring equipment
View the document 7.2 Prevention
View the document 7.3 Treatment
View the document 7.4 Emergency treatment
View the document 8. Selected bibliography

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 70 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.