What is the real meaning of a pressure between 0 and 1 bar?

A.T. said:

Also: The capitol of Bolivia is at 3600-3800m. And there are many smaller settlements with permanent inhabitants even higher than that. People have climbed the Mount Everest (8,848.86 m / 29,031.7 ft) without oxygen support.

It all goes to show how adaptable humans can be. Spending time and training at high altitude is a technique used by competitive runners. The temporary high concentration of red blood cells tends to give them an advantage in races near sea level.
People who have climbed Mt Everest without oxygen must be serious outliers. You can easily die of ‘Mountain Sickness’ if you are not extremely fit. High altitude flying is much more common than mountaineering and even passengers, sitting in their seats and taking it easy, need pressurised cabins to be sure of remaining fit and well on their journey.

At the other extremes of pressure, divers have to ‘decompress’ after spending time at lower than 10m (+1Bar). Compression sickness and Caisson disease present serious problems after extended times (working hard) at only moderate depths. Special gas mixes are needed for working under ‘saturation diving’ conditions.

I read that even operating theatre staff can suffer from the effects of long term exposure to positive pressure ventilation. I looked for a link but SARS and Covid 19 seem to dominate all the google hits I could find just now.

Time is a big factor at depth. Submarine crews have escaped from depths up to 200m with escape suits which allow them to surface at a safe rate and have no serious ill effects after the relatively short time at depth. (You just have to keep breathing out as the air is expanding all the time and never try to hold your breath.)

It all goes to show how adaptable humans can be. Spending time and training at high altitude is a technique used by competitive runners. The temporary high concentration of red blood cells tends to give them an advantage in races near sea level.People who have climbed Mt Everest without oxygen must be serious outliers. You can easily die of ‘Mountain Sickness’ if you are not extremely fit. High altitude flying is much more common than mountaineering and even passengers, sitting in their seats and taking it easy, need pressurised cabins to be sure of remaining fit and well on their journey.At the other extremes of pressure, divers have to ‘decompress’ after spending time at lower than 10m (+1Bar). Compression sickness and Caisson disease present serious problems after extended times (working hard) at only moderate depths. Special gas mixes are needed for working under ‘saturation diving’ conditions.I read that even operating theatre staff can suffer from the effects of long term exposure to positive pressure ventilation. I looked for a link but SARS and Covid 19 seem to dominate all the google hits I could find just now.Time is a big factor at depth. Submarine crews have escaped from depths up to 200m with escape suits which allow them to surface at a safe rate and have no serious ill effects after the relatively short time at depth. (You just have to keep breathing out as the air is expanding all the time and never try to hold your breath.)