Potassium-40 (^40K) is a radioactive isotope of potassium which has a very long half-life of 1.251×10^9 years. It makes up 0.0117% of the total amount of potassium in the earth’s crust. Potassium-40 is created from the decay of argon-40 in the earth’s atmosphere.
The half-life of ^40K is so long that it can be used to date extremely old fossils, as well as rocks. When ^40K decays, it produces Argon-40 and Calcium-40. Potassium-40 is also used to date sea water samples, as well as determine the age of meteorites.
Potassium-40 is not dangerous to humans because it is not absorbed by the body, and thus poses no radiation risk. However, it can be dangerous if inhaled or ingested in large quantities, since it can then be absorbed by the body and lead to high levels of radiation exposure.