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A recurrent problem in libraries, particularly in subtropical and tropical climates, is the presence of mold on library materials (Figure 20). Mold results when spores land on a substrate that has the correct temperature and surrounding humidity to initiate their germination. When spores germinate, they put out fine strands of mycelia, which invade the substrate and use it as a food source. The mold mycelia exude liquid enzymes that dissolve the substrate, and this food is then used in the production of more mycelia and eventually millions of spores.
Each cubic foot of air contains thousands of mold spores which land on surfaces and objects in the library every day. Attempts at mold control in library materials by using various chemicals therefore is usually ineffective. Chemicals such as thymol, ortho phenylphenol (OPP), alcohol, and dilute bleach solutions kill some of the mold spores on the surface as well as some of the mycelia. As soon as these chemicals have volatilized from the surface, the object is again vulnerable to new mold spores that land on its surface. If conditions are correct, the spores will germinate and more mold will be produced. These types of chemicals do not impart residual control for mold or mold spores.
Similarly, fumigation with poisonous gases in a chamber does not impart any residual mold-control effects. Much of the fumigation that is done in libraries is not warranted. Changing the environment that produces the conditions suitable for mold growth in the first place is the only effective means of retarding and eliminating mold growth. If a spore lands on a substrate which is not suitable for growth, and the humidity and temperature conditions surrounding the spore are inadequate to trigger spore germination, in time the spore will desiccate and die. As long as the conditions of the substrate and surrounding microenvironment are not suitable for spore germination, the spores will not germinate and mold will never appear. If mold is present on materials, moving them to a drier, less humid area will stop the mold growth and the spores will eventually desiccate and die.