According to the Library of Congress, two of the main factors that degrade paper are acid and moisture. Acid and moisture are produced from a variety of sources, including climatic conditions, like humidity. When these two factors combine, it produces a reaction called acid hydrolysis, which degrades paper.
Cotton papers are less vulnerable to degradation than many modern papers, partially because cotton papers have a natural alkalinity, which battles against acids that might damage them. Natural alkaline reserves, such as chalk, are often added to modern papers.
Once acid hydrolysis begins, it produces a domino-effect of degradation, as the process itself naturally produces additional acid.
Warmth and humidity degrade paper. Paper of any kind is more likely to survive in dry, cool environments.
As paper products are constructed of fibers, and fibers, in turn, are constructed of cellulose chains, the fibers with the shortest chains are the most vulnerable to damage. Cotton fibers have some of the longest chains of any paper, while newsprint, which is constructed of non-treated groundwood pulp, has some of the shortest chains.
Cellulose itself produces acids that naturally degrade paper, such as oxalic, lactic, acetic and formic. These acids build-up over time as they molecular bond to the paper.
Light naturally degrades paper in a process called photolytic degradation, to which--once again--newsprint is the most vulnerable. Oxidation also produces limited damage to paper, though it is most degrading when combined with nitrogen oxide pollutants.
Papers absorb pollutants such as sulfur and nitrogen oxides. Newsprints are the most vulnerable to pollutants.
According to the Library of Congress's website in 2010, emerging studies are suggesting that paper bound together (such as in books) degrades faster than loose sheets of paper with ventilation between them. Paper at the center of books may also be weaker than paper at the edges, though studies are ongoing.