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[Phys-L] Re: inhaling helium



On 04/15/05 17:37, Anthony Lapinski wrote:
Regarding the speed of sound in different gases, is it safe to inhale
helium to show this? (I've never tried it.) I've never heard it wasn't
safe, but a student today told me a friend of his "passed out" while
attempting this (details are vague). The helium is less dense than air,
and should easily clear out of your lungs. So it should be relatively
safe. Right?

I would say the experiment can easily be done
safely, indeed with a wide margin of safety.

OTOH yes, if you breathe nothing but helium for
several breaths in a row, you might pass out.
So don't do that.

To perform the demonstration, it suffices
to take one breath of helium, or maybe some
small number, and if you space them out with
breaths of real air in between, you should
be fine.

It is a fun fact that the symptoms of hypoxia
are rather subtle, vastly more subtle than most
people would have guessed. The horrible I-need-to-
breathe-right-now feeling you get when you hold your
breath not due to a deficit of oxygen, but rather
an excess of CO2. You will *not* get that feeling
from breathing helium, even up to the point where
you become significantly hypoxic. The exact
symptoms differ from person to person, but hypoxia
(due to altitude, or CO poisoning, or prolonged
goofing around with helium) may be manifested
as
-- feeling giddy or euphoric, or
-- feeling worn-out or sleepy, or
-- loss of visual acuity, or
-- impaired reasoning and judgement, or
-- some combination of the above.

The good news is that it is very hard to do
yourself lasting harm by breathing helium, unless
you pass out and hit your head on something.
The Air Force has altitude chambers wherein they
routinely subject pilots to hypoxia, so pilots
can become familiar with the symptoms. Restoring
the normal everyday partial pressure of oxygen
promptly and completely reverses the effects of
hypoxia.

So to maximize your margin of safety:
-- Don't try it unless you are in ordinary good
health to begin with.
-- In any given minute, you don't want more
than 1/3rd of your breaths to be helium; the
rest should be normal air. This corresponds
to suddenly going up to an altitude 10,000
feet above what you're used to ... which most
people can handle easily.
-- You might want to practice in private, to
make sure your personal reaction to mild
hypoxia isn't too far out of the mainstream;
remember idiosyncratic reactions are common.
If you are a smoker and/or are anemic, your
tolerance will be reduced.
-- If you feel you need to "push the envelope"
make sure you are well seated *and* arrange
for a spotter. You need somebody to tell you
when to end the experiment, because by the time
your judgement is impaired, you may well be too
impaired to realize how impaired you are.
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