Thallium is number 81 in the periodic table with the symbol Tl. This soft gray or bluish-white malleable metallic element is a lead-like metal which discolors when exposed to air. It is a by-product recovered in some areas from flue dusts and residues from copper, zinc and lead ore smelting. Approximately 60-70% of thallium production is used in the electronics industry, with the rest being used in the pharmaceutical industry and in glass manufacturing. It is highly toxic and is used in rat poisons and insecticides. The radioactive isotope 201 is used in cardio vascular imaging.