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I am not a chemist so I'm not going to give specific amounts of radon; different countries use different units. My question: why do some areas have more radon? For instance, Australia has a spreadsheet of all locations and their radon levels. I've looked it up and where I used to live, nearby, has half the listed Radon levels of where I live now. Wikipedia was a bit general to me for American examples:

Iowa has the highest average radon concentrations in the United States due to significant glaciation that ground the granitic rocks from the Canadian Shield and deposited it as soils making up the rich Iowa farmland.

So it is rock or soil types or weather or something else that causes the different amounts?

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    $\begingroup$ Well, there are parts of the earth that have significantly more water than others. $\endgroup$
    – Martin - マーチン ♦
    Commented Jun 27, 2019 at 12:46
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    $\begingroup$ While radon and groundwater may have some association (I truly don't know), Radon is part of the U238 decay chain, so what you are looking for is types of rocks, particularly those that contain uranium. Granite happens to be one. $\endgroup$
    – Jon Custer
    Commented Jun 27, 2019 at 12:55
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    $\begingroup$ My point was actually why would you expect uniform radon levels around the earth? Yes, the comment was snarky, and I apologise for that, but the point remains: the earth is not uniform, so there will be differences in abundance in certain elements. So the why part of your question can't really be answered (at least not on chemistry.se), but the part of where it comes from might be answerable. $\endgroup$
    – Martin - マーチン ♦
    Commented Jun 27, 2019 at 13:03
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    $\begingroup$ @athornton To me this seems more like a question of "What are the different natural or artificial events that lead to radon production?"; obviously radon is more likely to be found where such events are abundant. Their distribution is uneven, hence, so is the radon distribution. Could you please clarify if this is not your question? $\endgroup$
    – Gaurang Tandon
    Commented Jun 27, 2019 at 13:43
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    $\begingroup$ So, you need uranium-bearing rocks (granite is good). In the Canadian shield, that means huge pieces of solid, continuous granite. So, any radon generated inside a huge chunk of granite isn't getting out anytime soon, and will decay, ultimately making lead at the end of the decay chain. Now, pulverize the rock and all of a sudden much of the radon being produced gets out of the time bit of rock and into the soil, diffusing up to the surface where humans are and can breath it in and potentially get cancer from the radon decay occurring in their lungs. $\endgroup$
    – Jon Custer
    Commented Jun 27, 2019 at 14:32

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Radon nuclides are short-lived members of the natural radioactive decay chains of thorium and uranium. On a time scale that would be relevant for geology, you should better ask why some locations have more uranium or thorium. The distribution of radon concentrations in the ground air is a result of the distribution of uranium or thorium in the ground.

The main reason for the inhomogeneous distribution is igneous differentiation. The original liquid magma might have been rather homogenous. But when the magma cooled down, fractional crystallization started. This removal of mineral precipitates also changed the composition of the remaining melt. Uranium and thorium ions are incompatible, i.e. they do not fit well in the crystal lattices of the silicate minerals that precipitate first. Therefore, they were accumulated in the remaining melt. Primary deposits of uranium and thorium minerals (such as pitchblende) were formed where the last parts of the cooled magma solidified.

Furthermore, secondary deposits were formed when uranium or thorium minerals were dissolved, transported and precipitated in ground waters. (This includes the nicely coloured and fluorescent uranyl ($\ce{UO2^2+}$) minerals.

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