You can also view previously answered questions in the HSW Question Archive . During the summer I am always hearing about ozone warnings in my city. The thing that makes ozone both good and bad is its location. See this page for a good description of the chemical reaction that allows ozone to absorb UV light.
Ozone is a very reactive gas that is hard on lung tissue.
It also damages plants and buildings. Any ozone at ground level is a problem. Unfortunately, chemicals in car exhaust and chemicals produced by some industries react with light to produce lots of ozone at ground level. In cities the ozone level can rise to a point where it becomes hazardous to human health. That's when you hear about it as an ozone warning on the news.