The natural substance, Silica, is found in many products such as bricks and concrete. When such materials are cut, sanded, carved etc., they create dust, some of which is fine enough to be invisible in normal lighting and can be breathed deeply into your lungs, causing harm to your health. The fine dust is called respirable crystalline silica (RCS).
The WEL (Work Exposure Limit) for respirable dust is 0.1 mg/m3, averaged over 8 hours. Similar to a small pinch of salt and generated within minutes of cutting concrete.