A planet that is to a large part urbanized. The exhaust gasses of millions of vehicles have poluted the atmosphere to a point that it's toxic for most living species, causing many of the original lifeforms to die out.
To grant adequate living conditions, the cities are supplied with artificially recycled air. All buildings and complexes are connected via air locks. To enter a vehicle the lower level population needs gas masks and isolated suits, while the upper class inhabitants have specially isolated garages to enter their vehicles safely.
The high grade atmospheric polution also caused the Khetarians, along with a few insects, to be the only life forms inhabiting the planet.