Re: 42,000 feet

Date:         20 Mar 97 02:34:55 
From: (andrew m. boardman)
Organization: Quiche Eaters Anonymous
References:   1 2
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Andrew Goldfinger ( wrote:
>     I recently flew at 41,000 feet in a commercial airliner (actually FL
>410, I assume).  According to a friend, at around 42,000 feet free
>flowing oxygen is no longer adequate for maintenance of life, and
>pressure breathing is required.  Therefore, the emergency oxygen system
>(as an SWA flight attendent put it: "these are not party hats") on
>commercial aircraft would not be sufficient.

The short answer: they're quite sufficient for maintenance of life, and
even reasonably productive consciousness, in most people.

The longer answer: motor skills and judgement start going to pot when the
partial pressure of oxygen in the air we breathe gets below about 120mB.
"Standard" sea level pressure is 1014mB, and the normal (21%) portion of
oxygen in that gives 213mB of oxygen; no problem.  At FL180, though, with
ambient pressure around 500mB, the partial pressure of oxygen is only
around 105.  This definitely leads to the sillies, and trying to pilot a
plane would be a Bad Idea; a greater portion of oxygen than 21% is called
for.  25%, yielding 125mB O2, would do.  (25%, by the way, is about the
best obtainable with a cannula, which is why full oxygen masks are
required for crew at cabin altitudes above FL180.)

Now, if the masks are providing 100% oxygen to breathe, that means that,
for full functionality, the ambient pressure should be 120mB or more,
which, altitude-wise, is somewhere in the high 40s.  Even above that,
though, consciousness is maintainable, if not much fun. :-)  Note again
that the *pressure* of what you're breathing doesn't need to be above the
ambient, so long as most or all of it is oxygen, which is why you can use
oxygen masks instead of pressure suits.

You might have noticed a fair amount of equivocating in the numbers
above; the effects of oxygen vary a *lot* from person to person, and
there are no hard answers.  The numbers are from memory, too, although
they should be as accurate as any.