Guess that makes sense, since radon and sunlight are the two main sources of naturally-occurring ionizing radiation.

I heard this several years ago, but it is interesting,
A customer of mine used to participate if International scientific ocean surveys, during which he measured radon in seawater to determine water density.

Doesn't fair too well on a cost/benefit analysis at this point! Hopefully, this is the beginning of something new...

Gordon, I, for one, am not surprised that AARST was not contacted to provide a counter-argument, but I am disappointed.
I wonder how long it will be until a disgruntled Homeseller quotes the article to me.

I have a W. Johnson sniffer, which I understand is very highly thought of. They are no longer made, but sometimes become available on ebay or from retiring radon pros. I bought mine from used form another inspector, and have heard that the manufacturer maintains a wiring list. My sniffer CAN be used to perform a professional radon test, Karen, as it has capability of measuring 48-hr intervals, as well as very short sniffs of few seconds.
The RadonAway version is simpler, and also functions as a manometer.
There are also the RadElec RAD7 (more of a lab instrument), and the CT007R and VS472 from radon sniffer.com, which I know nothing about.

Firstly, if there are pipes in the crawlspace, it needs to be MADE accessible. Sooner or later there will be a plumbing issue. (I once hand dug a basement for that very reason).
Secondly, the living area above the garage should definitely be tested: the slab is a totally different dynamic than the crawlspace.
Don't discount the possibility that the slab is the entire problem, and that nothing needs to be done with the crawlspace. This is a great time to use a sniffer to check the radon concentrations in all the different areas.
An HRV may be a good choice depending on the layout and test results.

Heck, yeah, I send a mitigation quote along with the test results! Strike while the iron is hot!
Radon is my only business; I have been told that some states/municipalities forbid Home Inspectors from giving repair quotes on properties they inspect.