Uranium

Thorium

Atomic Symbol

U

Th

Source

Occurs naturally in trace amounts

Occurs naturally in trace amounts

Common Forms

Uranium is both radioactive and a chemical toxin. Uranium is comprised of three naturally occurring isotopes. U²³⁸ comprises 99.284% of natural uranium by weight, U²³⁵ 0.711%, and U²³⁴ 0.005%. U²³⁵ is the form used in nuclear weapons and power plants. However, U²³⁸ is often converted to Pu²³⁹ for such purposes.

Thorium is both radioactive and a chemical toxin. Three main isotopes of throrium occur in nature: Th²³² is a primordial radionuclide. Th²³⁴ and Th²³⁰ exist naturally as part of the uranium decay chain.

Decay²

Alpha, low-energy gamma

Alpha, low-energy gamma

Exposure

Uranium is found in nature and therefore minute concentrations are present in food, water, and air. Increased exposure is common in occupations where uranium dust is prevalent, like in nuclear materials processing and mining.

Thorium is found in nature and therefore may be present in air, food, and water. The highest chance of serious exposure occurs when thorium dust is present, as for example in the workplace.

Excretion

Particles in lungs may be coughed or breathed out, or may enter the blood, pass through the kidneys and be excreted as urine. Ingested uranium particles can be excreted in feces. Some particles remain in the body where they can build up lung, or enter the blood stream where it can accumulate in bone tissue.

Excretion pathways are similar to those of uranium.

Health Effects

Because alpha particles and gamma radiation emitted by uranium are relatively weak, uranium poses little health hazard outside the body. However, kidney disease has been observed in uranium miners and animals that ingest large quantities of uranium, attributable to the element's toxic chemical properties. Due to its radioactive properties, exposure to uranium increases the risk of lung, bone, leukemia, and soft tissue cancers, particularly when inhaled or ingested. Animal studies show that uranium may affect reproduction and the developing fetus. Uranium-238 also decays into dangerous radionuclides such as radium-226 and radon-222 decay products.

Thorium dioxide is classified by the U.S. Agency for Toxic Substances and Disease Registry as a "known carcinogen." Animal studies suggest that thorium may be absorbed through skin, but thorium poses little health hazard outside of the body. Workers who are exposed to thorium have been shown to have an increased chance of lung disease, lung cancer, and pancreatic cancer. Thorium has also been shown to cause liver disease, blood disorders, and changes to genetic material. Large acute (one time) doses have been shown to lead to metal poisoning in animals. Birth defects have been observed in animals exposed to thorium.