Ozone was a triatomic oxygen molecule that was created by excess energy in the air, often caused by the firing of laser and blaster weapons, as well as the use of lightsabers. The gas had an odor which was pungent and noticeable. Ozone odors were also known to precede ion storms.
Many planets had a layer of ozone in their upper atmosphere; Roon was one such example. This ozone restricted ultraviolet light's ability to pass freely to the surface of a planet. Some planets, such as Af'El, lacked a layer of ozone in their atmosphere. On Af'El, the native Defel adapted to this by developing the ability to see in the ultraviolet spectrum of light.