For most people, cosmic radiation is the major source of adsorbed radiation. A person traveling by commercial jet aircraft on a long flight, such as Los Angeles to London, can receive as much as 10 mrem during the flight. When cosmic radiation interacts with gases in the atmosphere, it causes nuclear transformations that release neutrons and protons. Carbon-14 is responsible for less than 1 mrem per year of absorbed radiation in humans, and tritium about 1 microrem. Long-lived radioisotopes in the earth's crust are also a source of radiation.
Potassium is one of the most abundant elements, and an essential component of food.
Until the beginning of the 20th Century, geologists had no method by which to determine the absolute age of a material. The age of the earth was believed to be at most tens of millions of years. By 1907, the discovery that lead was the final product of uranium decay provided evidence that geologic time should be measured in billions of years. Naturally occurring uranium contains 14 isotopes, all of which are radioactive. In contrast to chemical reactions, where the isotopes of an element behave similarly, in nuclear reactions, isotopes behave quite differently.