Toward the Galactic Nucleus
Type I Atmosphere
Type I atmospheres had appropriate levels of oxygen, nitrogen, and hydrogen to be breathable by the majority of species in the galaxy. Some contaminants or other trace gases could have had long-term detrimental effects. Planets with these types of atmospheres almost always had some form of indigenous life. Plant life or some analog that frees oxygen was present on all planets with a Type I atmosphere—all oxygen in the atmosphere would have been bound to other elements within a matter of months without such life to free it up.
Type II Atmosphere
Breath mask Suggested
Type II atmospheres usually had appropriate levels of atmospheric oxygen and nitrogen. Due to lack of sufficient pressure or the presence of contaminants and/or of other trace gases, however, these atmospheres usually caused detrimental effects to Humans and most other species over a shorter period of time. Many species could breathe Type II atmospheres without any trouble depending on physiology. Planets with a Type II atmosphere usually supported some form of indigenous life.
Type III Atmosphere
Breath mask Required
Within Type III atmospheres, due to either the lack of appropriate levels of atmospheric oxygen, nitrogen, sufficient pressure, the presence of contaminants and/or of trace gases, a breath mask had to be worn. For Humans and most other species, these atmospheres caused immediate or eventual impediments or even incapacitation. Some species could breathe these types of atmospheres depending on physiology and biology. Planets with a Type III atmosphere sometimes supported native life.
Type IV Atmosphere
Environment Suit Required
Type IV atmospheres were either toxic, flammable, or nonexistent; these types of atmospheres caused immediate incapacitation or death in most species of the galaxy. Few species could breathe in this type of atmosphere; planets with a Type IV atmosphere rarely support life.