Carbon-14 is a radioactive isotope used to date organic material. Its consistent rate of decay allows the age of an object to be determined by the proportion of carbon-14 to other carbon isotopes. This process is called radiocarbon dating. Carbon-14 is also used as a radioactive tracer for medical tests.
Carbon dating works by comparing the amount of carbon-14 in a sample to the amount of carbon-12. Because organisms stop taking in carbon-14 at death, the age of the material can be precisely determined by this ratio of carbon isotopes.
Human remains, fossils and organic materials from archaeological sites are all dated using carbon-14. Books, clothing and food remains are all archaeological artifacts that can be carbon dated. Carbon-based rocks, such as bitumen and tephra, can also be dated in this manner.
Carbon dating is most effective on material that predates the 1940s; this is due to above-ground nuclear tests increasing the amount of carbon-14 in the environment. This inconsistent amount of carbon-14 renders the test less accurate but opens up testing possibilities not available for older samples. For example, it is possible to determine the age of a person born after the 1940s using the carbon-14 content of teeth.