Atmospheric Pressure

[Lmoy]

Assuming an unending supply of oxygen, what is the lowest survivable atmospheric pressure in which one would not need to wear a pressurised suit? I mean for long term exposure, in a range of several days to months and years. I've heard values ranging between 400 millibars down to 180 millibars, but I'm not entirely sure.

 

[Quick_Nick]

I think this is useful information for long-term: http://en.wikipedia.org/wiki/Space_suit#Operating_pressure

 

[Lmoy]

That page references the Armstrong limit, which I knew would probably be the answer but I couldn't remember what it was called thanks.
And that's 62 millibars apparently. Rather lower than I thought.

 

[N_Molson]

You'll find very precise data in this document, the HIDH :

http://ston.jsc.nasa.gov/collections/TRS/_techrep/SP-2010-3407.pdf

 

[Thunder Chicken]

The Armstrong limit of 62 millibar only describes the total pressure where water will boil at body temperature (37 C).

Another limit is the partial pressure of oxygen in air at sea level (approximately 212 millibars). This would be the approximately the minimum pressure required to survive and function on 100% oxygen through an unpressurized mask. Some (many?) could survive much lower partial pressures, but your cognition and functioning would take a serious hit.

An extreme example would be those who have climbed Mt. Everest without supplemental oxygen. Total pressure is about 300 millibars at the summit, for a partial pressure of oxygen of about 60 millibars. Very few have done this and survived. Here is a summary of the Messner/Habeler ascent of Everest in 1978:

http://www.pbs.org/wgbh/nova/everest/history/firstwoo2.html