Silica is a substance naturally found in certain types of stone, rock, sand and clay. Working with these materials can create a very fine dust that can be easily inhaled.
Once inside the lungs, the dust particles are attacked by the immune system. This causes inflammation (swelling) and gradually leads to areas of hardened and scarred lung tissue (fibrosis). Lung tissue that’s scarred in this way does not function properly.
Read more at:
http://www.nhs.uk/conditions/Silicosis/Pages/Introduction.aspx