Library materials published since about 1800, present the biggest preservation problem with which American libraries must contend. These are the materials which are not rare enough or intrinsically valuable enough to justify restoration, but are of sufficient value to justify restoration in the original format, but are of sufficient value to justify preservation of the intellectual content. The major cause of deterioration of modern books was the high acid content of the paper being used. The combination of acid with environmental factors of heat, humidity and ultra-violet light further deteriorated books.