From: roberts@cmr.ncsl.nist.gov (John Roberts)
Subject: Re: Space suit research?
X-Added: Forwarded by Space Digest
Organization: National Institute of Standards and Technology formerly National Bureau of Standards
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU
Distribution: sci
Date: Sun, 13 Dec 1992 02:31:08 GMT


-From: fcrary@ucsu.Colorado.EDU (Frank Crary)
-Subject: Re: Space suit research?
-Date: 25 Nov 92 04:45:43 GMT
-Organization: University of Colorado, Boulder

-In article <17722@mindlink.bc.ca> Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) writes:
-[JR]:
->> Apparently anything significantly below 3 psi partial pressure of oxygen
->> is a potential health risk, unless you have many generations of ancestors
->> who lived in the mountains.

-I'm not sure what you mean by "significantly", but this certainly doesn't
-apply to ~2 to 2.5 psi oxygen: The residents of Aspen, many (most?) of
-whom have zero generations of ancestors who lived in mountains, live
-and work at such oxygen pressures, with no visible problems. Further,
-people with no hereditary _or_ personal adjustment to low pressures
-routinely go they to ski (which, if anything, requires slightly above
-normal consumption of oxygen.) 

I finally found the article. It's in the October, 1992 issue of Scientific
American, pp. 58-66, and is "Mountain Sickness", by Charles S. Houston, a 
mountain-climbing physician who specializes in high-altitude research. I'll
try to post a summary of the article:

The only problems dealt with in the article are those associated with
hypoxia - breathing air at low pressure, or breathing oxygen-depleted air
at sea level pressure. Prolonged hypoxia leads to a set of physiological
changes called acclimatization. These physiological changes can lead to
serious health problems. (Note that full partial pressure of oxygen but
lower total pressure is a separate issue.)

Mountain sickness encompasses a wide range of symptoms, which vary widely
from individual to individual. Among the observed phenomena:
 - The reduced supply of oxygen leads to "overbreathing", which in turn
   seriously depletes levels of carbon dioxide, causing the pH of the 
   body fluids to change. The kidneys are then forced to excrete biocarbonate
   to try to restore the balance. The rate of breathing is controlled by
   centers in the midbrain that are responsive to CO2 levels and blood pH,
   and also by the carotid bodies (small collections of cells in the neck),
   which are responsive to oxygen levels. The condition of hypoxia often
   causes dominance to shift between these two control mechanisms, resulting
   in periodic, or Cheyne-Stokes breathing, in which a period of rapid deep
   breathing is followed by a shallow breathing, and a complete cessation of
   breathing for a period of eight to ten seconds before the cycle repeats.
   This effect is more pronounced during sleep, and results in a net reduction
   in oxygenation of the blood. It is common above 9000 feet, and universal
   at higher altitudes.
 - Hypoxia tends to lead to increased blood flow to the brain, which causes
   an accumulation of fluid in the brain. This in turn causes general and
   localized increases in pressure, which results in headaches, staggering,
   difficulty in performing fine-motor skills, mental confusion, hallucinations,
   and sometimes death. These effects can occur at altitudes as low as 9000
   feet. Countering cerebral edema is hypocapnia (decreased carbon dioxide),
   which can lead to decreased blood flow to the brain (and oxygen deprivation).
   Whether the net blood flow to the brain is increased or decreased depends
   on the individual.
 - There are many hormonal changes. An increase in the hormone erethropoetin
   stimulates the production of red blood cells. This in turn, can actually
   impede oxygen flow to tissue, increases the viscosity of the blood, and
   can cause extensive clotting, and sometimes congestive heart failure (the 
   volume output of the heart is greatly increased during acclimatization).
 - Normally, between 20 and 30 percent of the body's capillaries are inactive.
   With acclimatization, the number of capillaries in active use can be
   increased. Enzyme changes in the cells can enhance anaerobic metabolism.
 - Above 10000 feet, the supply of supply of oxygen to the eyes is decreased
   to the extent that vision in dim light is decreased by 50%. Above
   14000-15000 feet, the veins and arteries in the retina can double in size,
   often causing small hemorrhages. These are usually unnoticed, and it is
   not known whether they represent a health hazard. There is some concern that
   bleeding takes place elsewhere in the body, for instance in the brain, 
   which can lead to permanent damage after repeated or prolonged exposure.
 - Hypoxia tends to increase the contractility of the small arteries in the
   lungs, increasing pressure. This in turn stimulates the production of
   biologically active substances, which can have a whole range of side
   effects, and also causes the leakage of plasma and red blood cells.
   Most people going to even such moderate elevations as 8000 feet experience
   some buildup of fluid in the lungs. This fluid can enter the alveoli, and
   sometimes causes drowning.
 - The "sodium pump" mechanism of the cells, which normally uses as much as
   20% of the body's intake of oxygen, can undergo partial failure under
   hypoxia. This causes the cells to build up excess sodium and leak excess
   potassium, upsetting the water balance, and leading to edema.

In many cases, acclimatization can partially offset the symptoms of mountain
sickness. But even so, every 1000 foot increase in elevation reduces maximum
work capacity by 3 percent. Populations that have lived for many generations
at high altitudes show adaptations such as larger lungs, greater amounts
of hemoglobin, and more or larger or differently placed mitochondria. But
no humans have adapted permanently to elevations above 17000 feet - sea-level
dwellers who move to 17000 feet can stay there for only a few months before
deterioration outstrips acclimatization.

The author recommends that climbers ascend no more that 2000 feet a day 
above 7000 feet. There are several drugs that can be used to relieve
some of the symptoms of mountain sickness - taking more salt than usual
is *not* recommended. It is emphasized that if serious symptoms develop,
the most important thing is to get to a lower elevation (especially at night).

It appears to me that while mountain sickness affects some people much more
than others, a low partial O2 pressure causes some harmful effects to just about
everybody. In light of the many physiological changes that take place, I think
that it would be unsafe to introduce a low-oxygen atmosphere to manned 
spacecraft without considerable further investigation.

John Roberts
roberts@cmr.ncsl.nist.gov

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