[ben_w]

Wikipedia says radon gets into it:

"""Radon is found in some petroleum. Because radon has a similar pressure and temperature curve to propane, and oil refineries separate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can become radioactive because of decaying radon and its products.[84]

Residues from the petroleum and natural gas industry often contain radium and its daughters. The sulfate scale from an oil well can be radium rich, while the water, oil, and gas from a well often contains radon. Radon decays to form solid radioisotopes that form coatings on the inside of pipework.[84]""" - https://en.m.wikipedia.org/wiki/Radon

Unfortunately citation 84 is a dead link: "Potential for Elevated Radiation Levels In Propane" (PDF). National Energy Board. April 1994. Retrieved 2009-07-07" - http://www.neb-one.gc.ca/clf-nsi/rsftyndthnvrnmnt/sfty/sftyd...

[jhgb]

Except natural gas does not get distilled propane added into it (or at least I'm not aware of anyone doing that; the varying composition of natural gas is due to its varying geological origins).