I was interested in this a while ago and the interwebz is your friend. There is an article about Terras levels somewhere.
Old Gunnislake has a radioactive X-Course and is quite hot.
The units are confusing.
Bequerels, Grays and Sieverts and Working Levels.
I like this analogy:-
Bequerel = How much it's raining.
Gray = How much of it hits you.
Sievert = How wet you get.
Working Levels = Some HSE bull$hit in the same order as crystal healing, calculated by someone with brown shoes and a 2CV.
We can all run around like headless chickens, wringing our hands and shouting "think of the children" but the bottom line is that your meter records decay events entering the window at the end of the tube. This is Bq and not a very good measure of how much there is going on in a given volume. To get this, you suck a given amount of air though a filter and then measure how radioactive it is, this gives you a Bq/volume reading, which is a good start of how you transfer a reading concerning volume to something like your lungs or exposure.
Radon is a funny one and a Geiger tube doesn't really discriminate against different sorts of the same radiation, for instance, Polonium has a very high energy particle which does a lot of damage to your lungs, yet may only be recorded as 1 event in the tube and something else may barely produce a particle. So, your Bq reading has loads of variables in it anyway, it's at best an indicator. PLUS, you have a whole load of different species present in mine gas and the concentrations of these vary according to variations in the mine atmosphere, rather like trace gases.
To actually transfer this into a Sievert reading (how much it has messed you up) involves a lot of equations, assumptions and the term Cb (which stands for crystal ball). Basically, if your geiger tube goes utterly mental or stops working, it might be a good idea to think about how much is going in your eyes or lungs.
Before you've got that far, it's a good idea to have a think about how the radon daughters (the real nasties) are behaving. Dry and dust is bad, as are smokes (you know who your are) moisture is less bad.
Essentially, radon decays and the remaining particle is charged and adheres to the nearest thing, if that is a certain sized piece of dust, you can breathe it in and it will get stuck long enough to decay and nuke a bit of your lung.
I'd avoid smoking, I'd avoid kicking up a dust and I'd avoid breathing through my mouth too much and spending too much time in radioactive hotspots.
We looked at the 18mSv/hr "temperature" of S Terras and went in there with Breathing Apparatus. We got about 100 yards in where the stoping had been dynamited by a famous mineral collector (who expired from cancer).
Personally, I'd say you are more likely to get gassed, fall through a false floor, have a rope accident, or get buried by a bad roof. Well, perhaps that's just me!
Some of these uni spods will write anything to pass the time.