Atmosphere

There are several different types of atmospheres to consider.

Lunar atmosphere[1] Earth's atmosphere[2] Atmospheric pressure on Earth[3]

The most important for Lunar development is the pressure inside your space suit or habitat. Here is a listing of the relative pressures of other environments and the percentage oxygen in each.

Earth Sea Level Oxygen (21%) - 101.3 kPa Mercury Program (100%) - 34.5 kPa Apollo Program (100%) - 34.5 kPa Skylab (70%) - 34.5kPa Space Shuttle (28.5%) - 70.0 kPa

Cabin Pressurization[4]

Generally if you don't get enough oxygen pressure you will get altitude sickness[5]

In the extreme case of a severe leak in space, you have to consider what the Death Zone does to climbers on Everest[6]

Most desirable for long term habitats in space is somewhere around 40kPa total with about 60% oxygen. In the event of an sudden depressurization this ratio means you do not have a hard transition to switch to your backup breathing systems. The rest is Nitrogen, water vapor and trace amounts of CO2 which can be isolated quite easily.