updated 28 May 2021
The recent interest in the radon health concern has resulted in some questions about radon’s origins. As we know, there is a concern about radioactive radon gas in dwellings, especially those in the Canadian Shield, such as those at Chandos. Where does this radon come from? This note attempts to be a bit of a primer on the subject.
The decay of uranium-238 into the radioactive gas radon-222.
Uranium-238 is a naturally occurring radioactive element with a half life of 4.5 billion years, meaning that it decays very slowly. It is very common in the earth’s crust. The nucleus of Uranium-238 is unstable, and so it naturally decays towards a more stable state, eventually becoming the stable element lead-206. Uranium has an atomic number of 92, meaning that this is the number of protons in its nucleus. 238 is its atomic weight, which includes, along with the protons, the neutrons plus other particles. Uranium-238 contains 92 protons plus 146 neutrons. Uranium has several isotopes, meaning that there are various forms of uranium with the same atomic number, but with different atomic weights. Actually, there are 16 such isotopes, with 3 naturally occurring ones: U-238, U-235, and U-234.
The decay of uranium-238 terminating at lead-206, is called the “uranium series”. There are other pathways from the isotopes of uranium to the isotopes of radon, but the uranium series is the main one of interest to us.
In simple terms, here are the main intermediate states in the uranium series as the uranium atom morphs on its way to radon:
- 238U decays to 234Th (thorium has an atomic number of 90)
- 234Th decays to 234U (uranium has an atomic number of 92)
- 234U decays to 230Th
- 230Th decays to226Ra (radium has an atomic number of 88)
- 226Ra decays to 222Rn (radon has an atomic number of 86)
Radon-222 is an unstable element and thus is busy transforming itself into polonium-218 on the road to becoming lead, as all the while it is a gas seeping out of the ground into our dwellings. Radon-222 has a relatively short half-life of 3.8 days and so any given atom isn’t around very long, but of course it continues to be replenished from the ongoing decay of radium. (who remembers having a watch dial painted with radium, to glow in the dark?) In a closed building it does not build up indefinitely but to a level where the natural decay is in equilibrium with the amount new coming into the building.
The amount of radon is measured in becquerels per cubic metre (Bq/m3), The current Canadian Guideline is a maximum of 200 Bq/m3. USA units are picocuries per liter (pCi/L), with 4 pCi/L being the maximum advisable. The conversion factor between units is 37. See the EPA document here.
Radon is a gaseous substance, in fact the heaviest noble gas known. (noble meaning it is inert, and doesn’t react with other elements.) This means it lays low in buildings, especially basements.