Thanks. (Though we're really getting off-topic now:)

The radiation in question was from Radon gas. We were looking at a property a few years ago and it was disclosed in the documentation that a survey has been done and over a 3 month period the level had never exceeded 114 Bq/M³. This being well below the UK government 'action limit' for households of 200 Bq/M³, it was, we were assured, "nothing to worry about".

A little research discovered that the 200 level was correct for households, and that for businesses it was 400. But looking around, the recommendations in different countries vary quite widely from as high as 1200, to as low as 150, and some research pointed to increased cancer risk at levels lower than 100 or even 80 of them Bq (whatever they are) /M³.

The variation, and the breadth of the variation got me to thinking about how the limits were determined, by whom, when, where and why. And without trying to retrace all the paths I took, I was left with the conclusion that they were quite literally "plucked from thin air". There seemed to be no one value, nor piece of definitive research, nor authoritative body that determined such values. That I could find anyway.

Since then I've read several articles and a seen a TV program about the aftermath of Chernobyl, and the most surprising and significant thing about them is that so far the affects of the radiation are not living up to their original cataclysmic predictions.

Of course there is some question as to whether there is full disclosure of the human costs by the local authorities. But other indicators, like birds nesting on the reactor building without any apparent harm coming to them, and the dramatic increases in wildlife, especially large mammalian wildlife in the exclusion zone. All these things make you wonder: a) how were those original cataclysmic predictions made; b) how do they decide upon the 'safe values' for short and long term exposure?