I was thinking more on the lines that there are many areas of the world with much higher natural levels of radiation than would be permitted in the US or most countries if from man-made sources. There is no evidence that people living in these area are affected adversely in any way (such as higher cancer rates). Some studies even show beneficial effects.
See for example http://heinonline.org/HOL/Page?handle=hein.journals/risk13&div=6&g_sent=1&collection=journals
