Editing Background radiation (section)

===Terrestrial sources===
{{Main|Environmental radioactivity}}
'''Terrestrial background radiation''', for the purpose of the table above, only includes sources that remain external to the body. The major [[radionuclide]]s of concern are [[potassium]], [[uranium]] and [[thorium]] and their decay products, some of which, like [[radium]] and [[radon]] are intensely radioactive but occur in low concentrations. Most of these sources have been decreasing, due to [[radioactive decay]] since the formation of the Earth, because there is no significant amount currently transported to the Earth. Thus, the present activity on Earth from [[uranium-238]] is only half as much as it originally was because of its 4.5&nbsp;[[1000000000 (number)|billion]] year half-life, and [[potassium-40]] (half-life 1.25 billion years) is only at about 8% of original activity. But during the time that humans have existed the amount of radiation has decreased very little.

Many shorter half-life (and thus more intensely radioactive) isotopes have not decayed out of the terrestrial environment because of their on-going natural production. Examples of these are [[radium]]-226 (decay product of thorium-230 in decay chain of uranium-238) and [[radon-222]] (a decay product of [[radium]]-226 in said chain).

Thorium and uranium (and their daughters) primarily undergo [[alpha decay|alpha]] and [[beta decay]], and are not easily detectable. However, many of their [[daughter product]]s are strong gamma emitters. [[Thorium-232]] is detectable via a 239&nbsp;keV peak from [[lead-212]], 511, 583 and 2614&nbsp;keV from [[thallium-208]], and 911 and 969&nbsp;keV from [[actinium-228]]. Uranium-238 manifests as 609, 1120, and 1764&nbsp;keV peaks of bismuth-214 (''cf.'' the same peak for atmospheric radon). Potassium-40 is detectable directly via its 1461&nbsp;keV gamma peak.<ref name="DetNuc"/>

The level over the sea and other large bodies of water tends to be about a tenth of the terrestrial background. Conversely, coastal areas (and areas by the side of fresh water) may have an additional contribution from dispersed sediment.<ref name="DetNuc"/>