Thallium: A metallic element having the symbol Tl and the atomic number 81 and an atomic weight of about 204.
Approximately 70% of thallium production is used in electronics with the remaining used in pharmaceuticals, optics, infrared detectors, and nuclear medicine. Thallium was historically used as rat poison and insecticides, however due to its nonselective toxicity, it has been reduced or eliminated in many countries.
Thallium enters the environment primarily from coal-burning and smelting, in which it is a trace contaminant of the raw materials. It stays in the air, water, and soil for a long time and is not broken down. It builds up in fish and shellfish.
The major source of exposure to thallium for most people is eating food contaminated with thallium. Other sources of exposure include breathing workplace air in industries that use thallium, smoking cigarettes, living near hazardous waste sites containing thallium, and touching (or, for children, eating) soil contaminated with thallium.
Exposure to high levels of thallium can result in death. Studies in people who ingested large amounts of thallium over a short time have reported vomiting, diarrhea, temporary hair loss, and effects on the nervous system, lungs, heart, liver, and kidneys. A study on workers exposed on the job over several years reported nervous system effects, such as numbness of fingers and toes, from breathing thallium. It is not known what the effects are from ingesting low levels of thallium over a long time.
The word "thallium" is a Latinized version of the Greek "thallos" meaning "green stalk" because of a characteristic bright green line in the spectrum of the element.