Thallium comes primarily from the Earth’s crust, where it exists at an estimated concentration of about 0.7 parts per million. Despite being reasonably abundant at that level, it rarely forms its own mineable deposits. Instead, nearly all commercially recovered thallium is extracted as a byproduct from sulfide ores of copper, lead, and zinc. The rest enters the environment through coal combustion, industrial processes, and natural weathering of rock.
Thallium in the Earth’s Crust
Thallium is a soft, bluish-gray metal scattered thinly throughout rocks and soil worldwide. It tends to associate with potassium-containing minerals in clays, granites, and soils, essentially substituting for potassium atoms because the two elements share similar chemical behavior. This wide, dilute distribution means thallium is not commercially recoverable from ordinary rock or soil. There are a handful of rare minerals that contain meaningful concentrations of thallium, but they occur in quantities far too small to support mining on their own.
The practical consequence is that virtually all thallium on the global market is a secondary product. When miners process copper, lead, or zinc sulfide ores, thallium tags along in trace amounts. It accumulates in the flue dusts and residues generated during smelting, and refiners separate it out from there. No country mines thallium as a primary commodity.
Coal Combustion and Industrial Release
Burning coal is one of the largest ways thallium moves from underground into the environment. Most coals contain between 0.5 and 3 micrograms of thallium per gram. Roughly half of that thallium escapes into the atmosphere during combustion. Concentrations in airborne fly ash from coal-burning power plants range from 29 to 76 micrograms per gram, a significant enrichment compared to the original coal. Flue gas emissions from coal-fired plants can reach 700 micrograms per cubic meter.
Cement manufacturing is another notable source. Cement kilns process limestone and clay at extremely high temperatures, and thallium present in those raw materials can volatilize and settle as dust in the surrounding area. Steel manufacturing and oil and gas operations also release low levels of thallium into the air and soil near their facilities. These industrial sources create localized hotspots where thallium concentrations in soil and dust exceed normal background levels.
How It Ends Up in Food
Because thallium sits in soil, plants absorb it through their roots. Not all plants are equally efficient at this. A meta-analysis covering 35 vegetable species found that leafy vegetables accumulate the most thallium, followed by root and stalk vegetables, with fruit-bearing vegetables (like tomatoes and peppers) absorbing the least. Kale and taro stood out as the strongest accumulators among all species studied, with kale showing a bioconcentration factor more than double that of most other crops.
Soil chemistry matters enormously. The amount of thallium a plant takes up correlates with both the total thallium in the soil and the soil’s pH. Higher pH soils tend to make thallium more available to plant roots. In contaminated areas near smelters, mines, or industrial sites, vegetables can accumulate enough thallium to pose a health concern. Kale, beet, sweet potato, potato, taro, pepper, turnip, Chinese cabbage, eggplant, and carrot have all been flagged as potential risks when grown in thallium-enriched soil. For kale specifically, researchers calculated that soil thallium levels as low as 0.24 milligrams per kilogram could push the crop above safe thresholds, depending on pH.
Historical and Current Uses
Thallium compounds were once widely used as rat poison, a role they filled effectively because thallium sulfate is odorless, tasteless, and lethal in small doses. The United States banned thallium-based rodenticides in 1972 after too many accidental poisonings, particularly in children. Old stockpiles of thallium rat poison still cause occasional poisoning incidents to this day. In many developing countries, thallium-based rodenticides remain in use.
Modern applications are more specialized. Thallium is used in electronic devices, semiconductor manufacturing, and the production of specialized glass and optical lenses. Certain thallium compounds improve the refractive properties of glass, making them valuable in high-performance optics. Thallium-based materials also play a role in superconductivity research.
Thallium in Medical Imaging
One of the most familiar uses of thallium today is in cardiac stress tests. A radioactive form called thallium-201 is injected into a patient’s bloodstream during exercise or a chemically simulated stress test, then tracked with a special camera to reveal how well blood flows through the heart muscle. Healthy tissue absorbs the tracer readily, while areas with poor blood supply show up as dark spots on the scan.
Thallium-201 is produced in particle accelerators called cyclotrons. Technicians bombard a target made of enriched, stable thallium with high-energy protons. This triggers a nuclear reaction that first creates a short-lived form of lead, which then decays into thallium-201. The process requires precisely controlled proton energies and careful timing to maximize the yield of the desired isotope while minimizing unwanted radioactive byproducts.
Why It Spreads So Easily
Thallium’s environmental persistence comes down to its chemistry. It dissolves readily in water, doesn’t bind tightly to most soils, and mimics potassium closely enough that living organisms absorb it through the same cellular channels they use for potassium. This means it moves through ecosystems with relative ease, passing from soil to water to plants to animals. Unlike some heavy metals that stay locked in sediment, thallium remains mobile and biologically available for long periods. Even at the low concentrations found in uncontaminated environments, this mobility ensures a continuous, low-level presence in food and water supplies worldwide.