Atmospheric temperature varies non-uniformly due to the non-uniform heating of the air by sunlight or by the ground. The upper stratosphere and thermosphere are warm because they absorb various short wavelengths of sunlight; the rest passes through the air to the ground, heats up the ground, then heats up the lower troposphere. The upper troposphere is not heated by the sun or the ground, so it will be cooler than the lower troposphere.

Given that there are no space colonies above the ground, and no topographic features higher than about 10 km, humans are limited to breathing the air in the troposphere. Thus, most air pollution problems occur in the troposphere and not elsewhere, since contaminating the other parts will not cause harm to things at the ground. The exceptions would be the depletion of the stratospheric ozone layer and global climate change due to accumulation of certain gases in the global atmosphere, since the harm to humans would not come from breathing the pollutants; rather, the harm comes from changes in the environment (more UV radiation, higher temperatures, etc.).