A mold outbreak is a big threat to any library or archival collection. Mold growth can alter and stain paper documents, and mold can pose dangers for people with allergies, respiratory problems, and weakened immune systems.

Because many institutions have never experienced a major mold outbreak, they are often unprepared when an outbreak does occur. That’s why it helps to know a bit about how mold grows and reproduces, and what can be done to prevent an outbreak.

Mold is a type of fungus, and like all fungi, it obtains its nourishment from organic matter. There is a great variety of known species of mold, and although the individual species exhibit unique behaviors, there are common patterns that are helpful to delineate.

Mold spreads by producing great numbers of spores, which travel through the air, and germinate when met with the right conditions. Upon germinating, the spores grow hyphae—fuzzy-like, branching structures—which then produce more spore sacs that burst and release new spores. The enzymes mold releases during reproduction cause the deterioration of many materials, like paper.

The conditions needed for mold activity are a humid environment, high temperatures, stagnant air, and a food source. (When these conditions are not present, spores are inactive and do little damage.) Moisture is the key to mold reproduction—either as humidity in the air (relative humidity or RH) or the wetness of the object itself (a book damaged by water, for example).

High RH levels promote the rapid growth of mold. An RH level of more than 75 percent lasting for a month can lead to mold growth. If the RH level jumps to 80 percent, mold can start to grow in just two weeks. And if the RH is at 90 percent, mold growth can appear in mere days. You can imagine that a water-damaged book in a humid environment is doubly threatened by mold growth.

To prevent mold growth, it’s important to store collections in environments with humidity control. As mentioned, high temperatures and stagnant air also promote mold growth and must be avoided. Damp basements, for example, are dangerous storage spaces for collections, as the combination of humidity and stagnant air can lead to a mold outbreak. 

Here are some general tips to keep in mind: maintain the humidity below 60 percent RH and the temperature below 70°F; promote good air circulation to prevent mold spores from settling easily; keep storage areas clean and protect collections in special cases whenever possible to avoid dust (dust contains mold spores); check all newly acquired books and items for mold; keep windows closed (to prevent more mold spores from entering); replace heating, ventilating, and air conditioning filters promptly or use high efficiency particulate air (HEPA) filters.

Taking steps to prevent a mold outbreak can save institutions time and money in the long run. It will also help ensure the longevity of your collection.