Jim Scott wrote:
Only if you've been sitting on boulders in Cornwall...

I'll get me coat...

Since Radon gas is emitted from granite, it collects under floors in
Cornwall, and you can get a grant to fit an extractor fan.

I believe the highest concentration found was in the toilets of a health
centre.

This could well be of interest for those wondering about whether
they be exposed to a radon hazard:

http://www.hpa.org.uk/radiation/publ.../1996/ern8.pdf