An atmosphere was a collection of gases that surrounded a celestial body with sufficient mass, and therefore enough to cause gravitational attraction. Examples included planets, moons, and large asteroids. Almost all species in the galaxy required a form of atmosphere to breathe. Humans, as well as most other species, were known to breathe oxygen.
Type I atmospheres
The majority of sentient life in the galaxy had a biochemistry based on carbon compounds, with water as a solvent. They typically evolved in atmospheres that contained a mixture of oxygen, nitrogen, and hydrogen. When the gases were present in appropriate levels to be breathable by a wide range of similar species, they were known as Type I atmospheres. Planets that maintained a Type I atmosphere had almost always developed some forms of indigenous life.
Planets with a Type I atmosphere relied on the air being naturally refreshed by plant life or some other chemical mechanism that freed oxygen and recycled it in a breathable form. Without it, all the oxygen in the atmosphere would be bound up to other chemicals and locked away in compounds such as carbon dioxide. Planets like Jakku maintained a Type I atmosphere.
Type II atmospheres
Breath mask recommended
A Type II atmosphere typically maintained adequate levels of atmospheric oxygen and nitrogen to support indigenous life. Some non-native species were capable of Type II atmospheres without difficulty, but it depended on their physiology. As a result of a lack of sufficient pressure, or the presence of contaminants and trace gases, breathing such gaseous mixes caused discomfort or harm to humans and many other species over a short period of time. The effects were akin to the altitude sickness that humans experienced when climbing high peaks, or the choking effects that came from breathing air pollution or smoke.
Type III atmospheres
Breath mask essential
Planets with a Type III atmosphere were sometimes capable of supporting native life, and some species could breathe certain types of such atmospheres, but it depended on physiology and biochemistry. For humans and most other species, breathing a Type III atmosphere caused immediate or gradual discomfort, which was then followed by incapacitation, and usually a degree of long-term harm. It may be due to either the lack of appropriate levels of oxygen or nitrogen in the atmosphere, insufficient atmospheric pressure, or the presence of contaminants and trace gases.
For oxygen-breathing species, the possible harmful effects meant that a breath mask was required. However, as there were no elements in a Type III atmosphere that would affect exposed skin or other tissue, an environment suit was not necessary.
Type IV atmospheres
Environment suit required
The classification of Type IV was used for atmospheres that were either combustible or toxic. It was also used to denote worlds where the atmosphere was so thin as to be effectively non-existent. All Type IV atmospheres were able to kill or incapacitate most species in the galaxy, unless visitors were protected by a suit that possessed its own hermetically sealed environment. Planets that held a Type IV atmosphere rarely supported life, although a few species could tolerate or even thrive in such atmospheres.