Mold can take hold if any book is left in a warm, humid environment for too long. In many cases, especially if the book is placed in a cool, dry environment, the mold will become inactive and little permanent damage will result. In some cases, however, the mold can cause serious damage. Active mold can secrete an enzyme that breaks down the cellulose fibers of paper, as seen in the top photo. The paper in affected areas has powdered away or become thin and extremely weak. The pages of this book were reinforced with thin, transparent Japanese paper to prevent further damage. Storage in cool, dry conditions will prevent reactivation of the mold.