Thallium is a naturally occurring metal in the earth’s crust. It is mainly found in the environment combined with other elements such as oxygen, sulfur, and chlorine. The primary sources of thallium release into the environment are coal burning and smelting and the production of cement. Thallium is used in superconductors, low-melting glass, photoelectric cells, and radiation detection equipment. In the past, thallium pesticides were used to control rodents and insects; this use was banned in 1972. U.S. production of thallium ended years ago.