In doing a little research into natural background radiation, I came upon a table from the National Council on Radiation Protection and Measurement (NCRP). It shows that inhaled radon gas is by far the largest contributor to average annual dose equivalent from background radiation, for people living in the United States. [1] That table lists "other internally deposited radionuclides" as contributing only a small fraction of what inhaled radon gas contributes. Among those "other internally deposited radionuclides" is potassium-40.

According to another source, the typical activity of potassium-40 in the human body is about 0.1uCi. [2] Typical activity of radon in outside air is said to be 0.4pCi/L, while for indoor air the average is 1.3pCi/L. [3] Given that the typical volume of air the average adult inhales is 0.5L, at any given time an adult has about 0.4pCi of radon in their lungs. [5] Note that the stated activity levels for radon are about a million times lower than those for potassium-40 (0.4 pCi versus 0.1 uCi).

I also learned that radon typically decays via alpha decay. [4] Potassium-40, on the other hand, mostly decays via beta decay. [2] I understand that alpha radiation is given more weight than beta radiation, generally by a factor of 20 or so, when estimating the biological effects of radiation. [6] However, a factor of 20 falls far short of the 10 million or so times higher effect stated for radon versus all other internally deposited radionuclides in the NCRP table. The table implies that the one million times greater activity of potassium-40 results in only about a tenth (or less) the equivalent dose. That seems way beyond what could be accounted for with just a weighting factor.

Why doesn't potassium-40 contribute much more to the equivalent dose? What am I missing?

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    $\begingroup$ This may be a biology question. Is the answer something to do with where the exposure happens - radon in the lungs, vs potassium in the digestive system? $\endgroup$ – EnergyNumbers Sep 12 '13 at 16:28

In addition to the deposition location when EnergyNumbers mentions in the comment the types of radiation and the length of the decay chain are an issue.

Radon sits at the top of a long sequence of decays many of which are alpha emitting (quality factor $\approx 10$--$20$) including Po-210 (5.34 MeV alpha, yikes!). Also the radon has a non-zero fission branch (quality factor $\approx 20$+). Basically per deposited MeV the radon is doing a lot more damage.

Then the alphas do their damage in a highly localized place and the K-40 (being a beta emitter) spread the damage around.

  • $\begingroup$ But the OP is asking about a factor of $10^6$ discrepancy. $\endgroup$ – Ben Crowell Sep 12 '13 at 19:30
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    $\begingroup$ @Ben, You are right, of course, but I can get a couple of orders of magnitude from the high energy of many of the alpha decays, and the large number of them in the chain. Then one order from the quality factor. So that only leaves three orders of magnitude to explain, and I think you may have the right track for that. $\endgroup$ – dmckee Sep 12 '13 at 19:54

We need to explain a discrepancy amounting to a factor of $10^6$, so clearly the explanation can't lie in things like quality factors for alphas versus betas.

Your calculation assumes that exposure due to radon is only due to the air that's in the person's lungs at any given time. But this article says:

[...] charged radon particles can easily bind to available surfaces, including walls, floors, clothing, and aerosolized particles such as dust and other particulates. Once bound to aerosolized particles, the charged [decay products] can easily be transported throughout the environment via wind action, and more importantly, can be inhaled by respiring animals and humans. The [decay products] can be inhaled either as free particles or particles that are attached to dust. Because they are ionized, the [decay products] preferentially attach to the respiratory epithelium, particularly the bronchi, the site of most lung cancers. Most of the radon gas inhaled will be exhaled (due to the relatively long half-life of radon gas) before it can decay and deposit a significant radiation dose to the lung tissue.

So I think the exposure comes from matrial that is incorporated into your body for long periods of time, not from the neutral radon atoms that are temporarily resident in your lungs during a breath.

  • $\begingroup$ This may well be product along the decay chain. Radon is a noble gas and doesn't bind to much of anything, but the various nth generation daughters bind to lots of things. $\endgroup$ – dmckee Sep 12 '13 at 19:52
  • $\begingroup$ @dmckee: Yeah, all my "[decay products]" edits in square brackets were "progeny" in the original article. I assume that's what they mean by that word. The first sentence seems to be talking about radon itself that has been ionized so that it sticks to things. $\endgroup$ – Ben Crowell Sep 12 '13 at 20:23

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