Atmosphere (Earth-2042)

An atmosphere is a collection of gases that surround a celestial body that possess sufficient mass, and therefore sufficient gravity to hold it. Examples include many planets, moons, and large asteroids. Almost all species in the galaxy need some form of atmosphere to breathe. During the height of the Galactic Empire's colonial period, a system of classification for atmospheres was established. This system, which was afterward adopted by nearly the New Republic, separates atmospheres into four distinct categories.

Type I atmospheres
Type I atmospheres have appropriate levels of oxygen, nitrogen, and hydrogen to be breathable by the majority of species in the galaxy. Some contaminants or other trace gases can have had long-term detrimental effects. Planets with these types of atmospheres almost always have some form of indigenous life. Plant life or some analog that frees oxygen is 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 atmospheres
Type II atmospheres usually have 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 cause detrimental effects to Humans and most other species over a shorter period of time. Many species can breathe Type II atmospheres without any trouble depending on physiology. Planets with a Type II atmosphere usually support some form of indigenous life.

Type III atmospheres
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 have to be worn. For Humans and most other species, these atmospheres cause immediate or eventual impediments or even incapacitation. Some species can breathe these types of atmospheres depending on physiology and biology. Planets with a Type III atmosphere sometimes support native life.

Type IV atmospheres
Type IV atmospheres are either toxic, flammable, extremely high pressure, or nonexistent; these types of atmospheres cause immediate incapacitation or death in most species of the galaxy. Few species can breathe in this type of atmosphere; planets with a Type IV atmosphere rarely support life.

Source

 * https://starwars.fandom.com/wiki/Atmosphere/Legends