Thallium (Tl) is widely distributed in the natural environment although at very low concentrations. It mainly occurs in the oxidation state Tl (I), whilst Tl (III) increases under acid and oxidizing conditions. Geochemical behaviour of Tl is analogous to that of potassium. Thallium does not occur in a free state in nature although several minerals contain it as a major constituent. The most common Tl-containing minerals are Lorandite and Crooksite. This element is mobilised by the combustion of fuels and other industrial processes and tends to persist in soils, depending on the soil type. It is considered a non-essential element and highly toxic to living organism. It is relatively easily taken up by plants and enters the food chain and it has been shown to accumulate in fish and other animals, with toxic effects. Peer Reviewed