Prevention of Mold in Library Collections (edited)                Home Up

     by Mary Wood Lee -- June 1988

See Also:
Mold & Mildew Summary
Understanding Mold
Treatment of Mold
Storage Materials

There is no question that the prevention of mold growth is much easier when the library environment is controlled, but the technology for such control is expensive to install and maintain. Modification of the environment though less expensive, is not without cost. Care of collections is as important as acquisition and organization, and should be budgeted by every library and archive. Although there are no panaceas, there are a number of possibilities for environmental modification that will enable institutions to reduce likelihood of mold damage to collections.

Location of stack and storage areas

Because of the high water table common in tropical climates, buildings are usually constructed without the basements and subbasements common in temperate climates. In the event that the building does have one or more levels below ground, every effort should be made to avoid using these areas for either stacks or storage of unused collections. Sub-surface areas are difficult if not impossible to adequately seal, and moisture from the ground will wick through the walls. Even if walls are coated with moisture barrier sealants there is a tendency for moisture and salts to build up below the surface of the coating until the surface of the wall and the coating begin to flake, (a condition known as spelling) exposing the interior of the wall and allowing the moisture to come through into the interior of the building. Adequate ventilation is also difficult to maintain. These factors result in warm, damp, still air and virtually assure the growth of mold.

Even if underground areas are not used for storage, both walls and floors should be sealed as thoroughly as possible to prevent the elevation of relative humidity throughout the building. Frequent inspections should be made to monitor conditions in these areas, and staff should be aware of potential trouble spots in the building.

Sealed interior rooms should also be avoided, unless they can be environmentally controlled by mechanical systems to control both temperature and relative humidity. Such areas should be monitored regularly. In buildings designed with such areas, ventilation may be improved by replacing solid interior walls with a half wall of louvered windows which can provide cross ventilation either naturally or with the use of fans.

Stack arrangement

Stacks should not be placed directly against exterior walls as heat and moisture transfer is greatest there, and circulation will be severely limited. Even a foot of air space between the wall and the stack will improve circulation and prevent condensation of moisture on the wall from creating a micro-climate.

Stacks should be arranged parallel to the air flow, so that the prevailing air movement is across the spines of the books as they stand on the shelves. Stacks should never block air flow from existing windows or ventilation created by fans.

Stacks should be open backed, particularly free standing stacks which are joined at the back. This will improve the ventilation on all sides of the volumes. If strength or stability of the stacks is a concern, cross braces should be used rather than the solid panel supports provided in many commercially available stacks. Compact shelving, however desirable from the stand point of space saving, should be avoided in the tropics, primarily because a micro-environment may be created when the stacks are closed. In addition, the mechanisms for moving the stacks tend to become inoperable in high humidities.

Closed cabinets should be avoided whenever possible. If they are necessary for the storage of microfilm or locked case books both the back and front of the cabinet should be ventilated, or a favorable microclimate should be created in the closed cabinet to counteract the high relative humidity.

Localized environmental modification

In addition to taking advantage of natural conditions in modifying the overall building environment, there are various technological methods for modifying the environment in localized areas within the library. In most collections there are materials which warrant special protection. Rare and valuable works, and items of particular historic importance are often included in this category. There is therefore a tendency to want to create a special area within the library where these materials can be kept secure, and receive the benefits of a more nearly ideal environment. Localized environmental modification may be used in addition to the measures cited above, but should not be considered substitutes for the modification of environment in the building as a whole.

Monitoring Existing Conditions

Before attempting to alter the environment in a specific location, it is imperative that existing conditions be understood. This requires comprehensive monitoring of the existing environment. Information should be available for conditions at given locations at all hours of the day and for all seasons of the year. If air conditioning units are to be installed, it is important to be sure that the lower temperature will not cause an unacceptable increase in the relative humidity, either immediately or during certain seasons.

Paper humidity indicator strips are of relatively little use in truly tropical climates. They almost always register in the pink (humid) range, and most indicate only very broad changes in relative humidity. Their best use may be in closed cases where a drier microclimate has been created, but they must be monitored regularly in order to be of use.

Air Conditioning

The term air conditioning, as used in this study, refers to the utilization of individual mechanical units to cool and filter air within a localized area of a building. Central plants which provide environmental control for the entire building are beyond the scope of this work. Air conditioning units fall into two basic categories: evaporative cooling and chilled water cooling.

Evaporative cooling is the simplest and least expensive system, however it is generally not suitable for areas with year round high temperatures and relative humidities.

Chilled water cooling units include a refrigeration device which lowers the air temperature, and a heating unit which warms the air slightly before it enters the room. In humid tropical climates, this procedure is critical, since the air introduced must be above the dew point to prevent an unacceptable increase in relative humidity. A change in temperature of only 1 F will result in a change in relative humidity of 3%.  The cost of a refrigeration unit may be double the cost of an evaporative unit, and is considerably more costly to operate in terms of energy costs. There is often a tendency to reduce operating costs by shutting down the heating unit. This inevitably results in severe environmental problems. If the unit is to be purchased and installed, it must be operated properly.

The kind of filtration chosen and the degree of recirculation desirable depend very much on local conditions. Filters should be cleaned or replaced on a regular basis. Not only will this improve the air filtration, it will result in more economical operation of the system. Electrostatic systems should be avoided because they produce ozone which can damage organic materials.

One other consideration regarding the use of air conditioning units should be mentioned. Air introduced into a particular locale will, like water, find its own level. Cool air entering at or slightly above floor level will remain there while the warm air floats above it. Air conditioning units should be installed as high up in the wall or window as possible to achieve maximum circulation in the area. Stacks and cabinets should be positioned in such a way that they do not block the air flow.


In humid tropical climates, dehumidification may be the most important factor in preventing mold growth. Its strongest challenger is good air circulation, not air conditioning. As noted above, air conditioning may make dehumidification even more necessary. Portable dehumidification units should be available in every library, and for some the installation of a permanent system may be necessary.

The most effective and economical systems for warm climates are the refrigeration units. Moisture is removed from the air as it condenses on refrigerated coils. Portable units operate on the same principle, and require very little maintenance and energy. Most have very basic internal monitoring devices, and can be set to maintain a given level of relative humidity.

One of the major advantages of dehumidification systems is that they do not require the major duct work that air conditioning systems entail. According to Gates, water vapor will migrate to the point of lowest moisture content. Thus, even portable machines can be left in place and adequately dehumidify a room. In large areas, several machines will be necessary.

Creating microclimates in cabinets and cases

A microclimate is any variation from the prevailing temperature and relative humidity of the surrounding environment. It may be either negative or positive in its effects and may occur unsolicited or be artificially induced and maintained.

It may, at times, be necessary to create a microclimate within the larger building environment. This could be occasioned by the nature of the materials, the necessity of protecting valuable items, or by the desire to remove them from a controlled environment and exhibit them in one that is uncontrolled. Microfilm, maps and documents stored in file cabinets are obvious candidates for microclimates in high humidity environments. While the incidence of mold growth can be reduced in the collection as a whole through improved circulation, the closed metal cabinets designed for the storage of microfilm, maps and documents tend to retain moisture, especially if they are not frequently used. By artificially lowering their interior relative humidity a beneficial microclimate can be maintained.

A reduced humidity microclimate can be established in a closed storage cabinet through the use of desiccants which absorb moisture from the air. There are a number of products which can be used as desiccants. Two of the most readily available are silica gel (which is available in various grades) and is widely used in the U.S. and Europe, and Nikka pellets (also called Kaken Gel) which is used in Japan and the Far East. Nikka pellets have been found to be more effective than silica gel at humidities above 60%.12 Silica gel often includes a color indicator which turns from blue to pink as moisture is absorbed and indicates when the material has reached its maximum absorption and must be reconditioned.

Before use, the desiccant must be conditioned to 0% relative humidity. This is done by heating the material in an oven to drive off moisture. The pellets or crystals can be reconditioned and used many times without losing their absorption capacity. After conditioning, the desiccant may be placed in the cabinets, either in trays in the base, or in small cloth bags in individual drawers. If the desiccant used is not a color indicator, a hygrometer or indicator strips must be placed in the cabinet to indicate when reconditioning is necessary. Once the cabinet reaches the desired humidity, and an equilibrium has been reached, the desiccant will require reconditioning less often. If the cabinets are used frequently, reconditioning may continue to be required at frequent intervals. The larger the quantity of desiccant used, the longer the microclimate can be maintained before reconditioning is necessary.

Stack maintenance

Routine cleaning and maintenance lack glamour in every library, however it is particularly important where environmental control is lacking. When natural ventilation is used to maintain adequate circulation, dirt and dust is a constant problem. Since these particles are hygroscopic, attracting and holding moisture from the air, and since they often contain nutrients required by molds, the constant cleaning of stack areas is essential in tropical climates. Vacuuming will also reduce, even if only temporarily, the number of spores on materials. In savanna climates, a thorough cleaning just before the rainy season may eliminate outbreaks of mold altogether.

A routine schedule for vacuuming of all books in the stacks on an annual basis (or as often as possible) should be established and maintained. Frequent inspection of the stack areas is also important, particularly little used sections and storage areas.

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