But I thought ozone was a bad thing?
Not at all...in fact it is just the opposite!!
When referring to air quality they often report ozone levels. What they are really concerned with is the smog level. 'Smog' is a complex ever changing mixture of toxic gasses and particles, which is very difficult to measure accurately. Early on it was noted that when smog levels are high, it is accompanied by a high level of ozone in the air. When smog levels are low, there is a low level of ozone in the air. The Ozone, is actually there to destroy (oxidize) the toxic chemicals in smog and return the air to a breathable status.
Since ozone is so easy to measure, scientists gave up trying to measure smog and instead rely on ozone levels to indicate how much smog is in the air. Since they are measuring ozone levels, the word "Ozone" became synonymous with 'smog' and everyone gets the idea that ozone is bad (which of course, it is not)..