First we have to understand what causes specimens to deteriorate.
Light waves are units of energy. This energy can break chemical bonds and/or create new chemical bonds. This can cause fading, loss of color, or changes in color. It can also cause drying out and brittleness.
Temperatures that are too high can dry out specimens and make them brittle. If temperatures are too high AND combined with high relative humidity, mold and mildew can form on the specimens. Temperatures that are too low can dry a specimen out as well (think about the effects of food you've left in the freezer too long.)
If the relative humidity is too low, it dries out specimens. This causes brittleness and shrinking, and we see splits, cracks, and holes in specimens. If its too high, mold and mildew can form. Moisture vapor can also cause swelling, and permanent, irreversible physical changes.
We include not only major forces, such as earthquakes, but forces as simple as mishandling of fragile specimens, vibration, and abrasion. These cause breakage or deformation, which often can not be repaired.
If a mammal specimen gets wet and then is dried, it goes through an expansion/contraction cycle. This weakens the skin and during the drying part of cycle, we might get tears and breaks in the specimen. Water can also cause mold and mildew if the drying is not fast enough or temperatures are high. Water can cause the total loss of specimens. Imagine pressed plants or pinned insects soaked with water; they become so fragile that the slightest movement destroys them. See herbarium specimen.
The worst case is total loss of the specimen. But there can also be smoke and soot damage (which rarely can be cleaned up entirely), heat damage, and water and chemical damages due to fire suppression systems.
Pests (e.g. dermestids, moths, silverfish, rodents) can cause as little damage as small holes or as much damage as a complete loss of specimen. Frass or insect excrement can stain specimens and cause chemical change in the specimen over time. Insects and rodents can be attracted by other insects.
Dirt and particulates are abrasive and can cause loss of specimen details, change of colors and loss of parts of specimens. They provide an attractive habitat for pests. Dirt and particulate matter can hold moisture against the surface of the specimen, creating a microclimate. This microclimate can initiate deterioration such as mold and the other characteristics already listed for temperature, relative humidity and water. Chemically based pollutants, when reacting with moisture in air, form acids which in turn cause breakdown of the specimen materials and color changes.
Theft and vandalism can result in complete loss of specimens or damage that is expensive to repair.
Neglect refers to inappropriate handling or environmental conditions that are not noticed or ignored. These unobserved or ignored problems lead to the loss of mounts and specimens, and damage to specimens and their enclosures.