Casts made of plaster are particularly vulnerable to damp conditions. Larger casts can become unstable if the iron armatures inside them start to corrode (rust), staining and cracking the plaster from within. Surfaces that were originally painted with oils, waxes or even milk can grow mould.
Plaster is a soft material which is easily scratched and chipped by fingernails or hard dusting brushes, and when they are moved, accidental damage is a high risk.
Fine dirt particles can make their way into the porous surface and their removal is very difficult and potentially damaging.
Light will not harm plaster but can age and discolour surface coatings.