[1] Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about (7–10) × 10−13 to 1 with respect to the stable chlorine isotopes.

In the top meter of the lithosphere, 36Cl is generated primarily by thermal neutron activation of 35Cl and spallation of 39K and 40Ca.

[2] The half-life of this isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years.

[4] Its properties make it useful as a proxy data source to characterize cosmic particle bombardment and solar activity of the past.

[5] Additionally, large amounts of 36Cl were produced by irradiation of seawater during atmospheric and underwater test detonations of nuclear weapons between 1952 and 1958.