Arsenic is classified chemically as a metalloid, having both properties of a metal and a nonmetal. It is a natural component of the Earth’s crust, generally found in trace quantities in all rock, soil, water and air. However, arsenic is usually found in the environment combined with other elements such as oxygen, chlorine, and sulfur. Arsenic combined with these elements is called inorganic arsenic. Arsenic combined with carbon and hydrogen is referred to as organic arsenic. However, concentrations may be higher in certain areas due to either natural conditions or human activities.