They act as a vehicle to carry other contaminants such as acids and oils. They also work their way into porous surfaces such as fabrics and paper or become embedded in soft surfaces such as paint and varnish. Unfortunately, the process of cleaning an object once it is soiled always causes some damage. The first line of defense against dirt is to remove as much as possible from the museum environment. Most of the dirt in a museum environment enters as airborne particles or is carried in on people and objects.
Cleanliness therefore begins outside the museum.
A well-maintained and swept walk will markedly reduce the amount of mud and dirt visitors carry in the door on their feet. At the door there should be a rubber-backed rug or mat that is cleaned frequently. If your museum is not air conditioned, all open windows need screens. Screens cut out a surprising amount of airborne dirt and are a barrier to insects that carry dirt and leave tiny droppings that are highly acidic. To maintain a proper environment, museums have to remove the dirt they can't keep out.