A library collection’s life expectancy is most dependent on its environmental conditions. When conditions are sub-par, damage generally occurs slowly, making it easy to go unnoticed. One of the most common forms of damage is mold growth, which first becomes apparent when a collection gains a musty smell. Because cellulose-based materials are vulnerable to several types of mold, humidity control for libraries is an essential step to maintaining an ideal environment. By knowing which environmental factors are crucial to preserving collections, you can take steps to ensure they are within acceptable ranges.