Some people are a lot more qualified to answer Ediger's questions than
I am, but seeing as none of them has jumped in....
CO2 of volcanic origin in pretty rare in caves. Main sources are decay
of organic matter and oxidation of material in the bedrock, the former
more often than the latter. (The decay of organic matter might not be
occurring in the cave. Soil CO2 can be carried in drips or stream
water.) But I understand that there is a relatively large amount of
petroleum in the limestone in which the caves around San Saba have
formed, so that might be a reason "bad air" is so often present there
compared to a lot of other places. I doubt if the limestone around
there contains much uranium, the source of radon, but there might be
some in deeper rocks.
Anyway, CO2 accumulations are a sign of poor ventilation, and that is
the same circumstance that would allow radon to accumulate, if there
were any of it seeping from the rock. However, the radon hazard in
caves has for twenty years or so been generally overhyped every time
it is "rediscovered." I doubt if there are any circumstances under
which recreational cavers, at least non-smokers, need to worry about
it in any natural caves. Stay out of old uranium mines. The radon risk
was first observed in uranium miners who smoked. The management agency
for Horsethief Cave, Wyoming, used to keep track of the radon exposure
of cavers to whom they'd given permits; there was a reasonable amount
of uranium in some of the fill in the cave. Don't know whether they're
still doing that or not. But the government worries about anything
that will increase your lifetime risk of getting lung cancer by a few
percent relatively, when it's only about 1% anyway if you haven't
smoked a lot. That is, from, say, 1% up to 1.1%. Big deal. Some show-
cave guides might have enough exposure to radon to worry about. Radon
is thought to be the greatest cause of lung cancer in non-smokers, but
it's still a very rare disease in non-smokers. (Contrary to what one
might think from all the propaganda, the chance of even a smoker
getting lung cancer during his lifetime is not more than 10 percent.)
-- Mixon
Incidentally, the accumulation of CO2 in the lower part of a cave has
nothing to do with the fact that it's heavier than air. (I don't
remember the exact numbers, but to get a doubling--nowhere near what
we call bad air--of CO2 just because of its molecular weight, you'd
need a pit full of completely still air thousands of meters deep.)
When it accumulates deep in a cave, it's just being created or
released there faster than it is removed by diffusion or air
movements. -- Mixon
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