[hinckley buzzard]

Ozone is considered a pollutant at ground level because it doesn't naturally occur there and when it is generated it combines with nitrogen oxides to form smog, while high up in the atmosphere it is formed naturally and serves to shield some of the solar uv rays.

(Approximate non-chemist answer)

[Eva]

So we should all be walking around laughing our heads off?

[editor-surveyor]

"Ozone is considered a pollutant at ground level because it doesn't naturally occur there and when it is generated it combines with nitrogen oxides to form smog"

In reality, ozone and oxides of nitrogen are both quite beneficial at ground level. Oxides of nitrogen enable trees to produce more cellulose, which is the structural element of plants. Ozone is beneficial to all creatures that have lungs, because it aids in regulating the plentiful bacteria that attack them constantly. The reason that they both were declared to be "smog" is because they gang up to give the lower atmosphere a yellow color which people have been conned into believing is harmful.

What is really harmful, and extremely so, is the H2S and other sulphur compounds that are produced by the catalytic converters on gasoline powered vehicles. They are the cause of the burning in your throat and eyes when driving up a long steep grade.