Von Hevesy first applied radioactive tracers to a biological problem in 1923 when he traced lead absorbed by plants. In 1935, von Hevesy began using artificial radioisotopes as tracers. He won the Nobel Prize in 1943 for his work with radioactive tracers. Radioactive tracers have been used for many purposes. For instance, doctors use minute amounts of radioactive substances to diagnose the presence of tumors, ulcers, or non-functioning organs.
Biologists use tracers to follow the path of nutrients through the food chain.
Earth scientists use tracers to follow the path of rainwater as it moves through the groundwater to lakes, rivers, and reservoirs.