My idea is to build a universe with various alien species. There are dozens of different species and every species evolved differently on a different planet. The problem arises when they want to meet each other on space stations. It is very uncomfortable to wear an environmental space suit, which regulates your own atmosphere, at a bar or club during your visit. So it was decided to set the atmospheric conditions of the space stations to 'fit' most of the species. The big question is: Does this work (on a long visit/for the staff)?
Let me give you some further constraints:
- The temperature does not matter. There are cooler and hotter places, but the differences of those temperature is fairly low, like walking into an AC-cooled building. The aliens can deal with it.
- The aliens have a human-like respiratory sytem. They all breathe in oxygen and exhale carbon dioxide, but are used to different oxygen levels in the air at different atmospherical pressures.
- The rest of the atmosphere is non-toxic (and not very reactive). It consists mainly of nitrogen and some trace gases, like our air.
- The atmospheric pressure is fixed, so there is no decompression needed between visiting different section of a space station. There might be decompression required when visiting or leaving the space station.
The easiest solution is that every alien is required to wear a mask covering mouth and nose (like Bane in Batman, but more stylish), which regulates the amount of breathable oxygen to stay in the safe zone. If an alien species requires more oxygen at the given atmospheric pressure, ventilators in the mask accumulate more air volume per breath, extract the additional oxygen, expel the rest and thus enriche the oxygen level. For lower levels of oxygen the mask just recycles exhaled air.
The thing is how to deal with the pressure. Too low pressure causes blood to boil or cells to swell (like the accident of Joe Kittinger). Too high pressure can cause nitrogen narcosis, but might be prefered due to lower overall oxygen requirements. It would seem that mild symptoms of nitrogen narcosis start to appear above 2 bar, so a safe pressure would be around 1.5 - 2 bar, assumed that the internal structure of the space station can support that pressure against vacuum.
But what about long term effects (like weeks/months on humans) of high (around 1.5 - 2 bars) or low pressures? And what about the effects of increased oxygen levels on the human skin? It is the biggest organ and houses a very complex microbiome, which is (intentionally) not protected from higher oxygen levels.
Can aliens (or humans) live on such a space station or under such atmospheric conditions just wearing a mask instead of a full environmental space suite?