How to Colonize an Asteroid
When the asteroid is first hollowed out and the shaft plugged, there will probably be a residual atmosphere left over from the mining operations and natural outgassing from the rocks. Although this mixture will be at extremely low pressure, it could contain some toxins and should be tested thoroughly or pumped out into space before we start to establish a permanent atmosphere.
Much of the initial nitrogen and oxygen will have been processed and stored before the cavern was completed, and can be released directly. Other processing operations might have to be set up on other asteroids to provide a steady supply of nitrogen and carbon dioxide.
Depending on the source of water for filling the ponds in the next step, there could be a great deal of gas released when cometary cores are melted. We must watch for gases in the cometary cores which could be harmful to our asteroid environment.