Re: How far up in the atmosphere can you go and still breathe?

Thanks for writing to MadSci Network!  

  As you know, our bodies require oxygen to function.  Red blood cells 
carry oxygen from the lungs to all the cells in the body, exchange the 
oxygen for carbon dioxide,  and carry this waste product back to the lungs 
where it is breathed out.   For more than you ever wanted to know about 
breathing, though, check out B*R*E*A*T*H*E (http://pages.nyu.edu/~ttk7953).  

The atmosphere consists of other gases besides oxygen, so not every 
molecule we breathe in is oxygen.  Close to the earth's surface, the air is 
concentrated enough that there are plenty of oxygen molecules in each 
breath we take.  However, as we move farther from the earth's surface, for 
instance when climbing a mountain, the air gets less and less dense.  This 
is because the air pressure decreases as we move away from the sea level.  
Since the oxygen molecules are more spread out,  each breath takes in fewer 
oxygen molecules.  Your body accommodates by speeding up your breathing 
rate, called hyperventilation.  As altitude increases, the body has a 
harder and harder time adjusting.
	
The High Altitude Medicine Guide (http://www.gorge.net/hamg/AMS.html) does 
a super job of explaining how the body reacts to high altitudes.  Though 
I'll offer a summary here, be sure to read their page.

First,  what is high altitude?  For medical purposes, 5000-11500 feet is 
considered high altitude, 11500-18000 ft is very high altitude, and 
anything above 18000 ft is extremely high altitude.  

Hyperventilation skews the body's delicate balance of oxygen and carbon 
dioxide.  Since concentrations of these gases in the blood signal the brain 
to inhale and exhale at the appropriate rate, those sleeping at very high 
altitudes often have very irregular breathing patterns.  Altitude sickness 
occurs primarily above 8000 ft when the body fails to acclimate.  Thus it 
is important for climbers to go slowly enough so that their bodies can 
adjust and monitor themselves for symptoms, including confusion, headache, 
and nausea.  Altitude sickness therapy includes descent and occasional drug 
therapy.  Severe forms of altitude sickness are dangerous, and include 
hemorrhaging in the brain and fluid accumulation in the lungs.

In answer to your specific question, the body has enormous difficulty 
adjusting above ~21000 ft.  For reference, Mount Everest is 29000 ft tall, 
meaning that climbers must take oxygen tanks for the last part of the 
ascent.  The reason for this is not a lack of oxygen in the atmosphere as a 
whole, but a lack of oxygen in each breath that results from the molecules 
being very spread out at high altitudes.

Good luck and keep the questions coming!
-Tinsley