Paper is an incredibly strong material, despite its apparent fragility, provided it is treated properly. There are as many reasons for deterioration as there are kinds of paper. Works of art on paper are composed essentially of two layers: the paper itself and the pigment(s) lying on the surface.
Humidity – fluctuations of humidity are as damaging as extremes of humidity. An extremely dry atmosphere can lead to brittleness. A damp atmosphere can trigger moulds, whether microscopic or in broad patches. Iron gall ink, the brown ink used by so many old masters, composers and others, deteriorates at a greater pace when the humidity fluctuates regularly.
Light – too much light falling on a drawing will cause watercolour and pastel pigments to fade, and will start breaking down the cellulose of which paper is made, resulting in a sheet so discoloured and brittle that it cannot be handled safely.
Insects – different insects are drawn to different components of a work on paper. The types of damage caused are varied too; some leave deposits on the surface, some eat all the way through the paper, some graze the surface, and some will eat only specific media resulting in just one colour being damaged.
Composition – it is also important to understand the incompatibility of certain materials, and this is often a problem for a work of art on paper. Iron gall ink, as it breaks down, will degrade the support and "eat" through it, leading to the complete deterioration of the work.
Handling – because paper readily picks up the natural oils from fingers (which then attracts dirt) it should not be handled without wearing gloves.
Storage – as paper absorbs pollutants readily, the mounting, framing and boxing materials must be acid-free. Cheap mount-board is often acidic, causing discolouration and embrittlement. The primary purpose of a mount is to keep the artwork away from the glass of the frame; it also acts as a filter for atmospheric pollutants and a buffer against sudden changes in humidity.
Tapes and adhesive – probably the worst enemy of paper is the adhesive on pressure-sensitive tapes, which is oily and stains paper, sometimes irreversibly, and can only be removed by using toxic solvents. Even re-positional sticky notes leave sufficient adhesive behind to pick up dirt for a long time afterwards.
Inherent instability – this may include fragments of metal from machinery dislodging and being formed in the sheet. With time this metallic fragment will deteriorate, particularly if exposed to moisture, resulting in deterioration and discolouration such as rust.