Here's the Answer From the EPA...
That depends on who you are and how much ozone is in the air. Most people only have to worry about ozone exposure when ground-level concentrations reach high levels. This can happen more frequently during the summer months in many U.S. communities. In general, as the ground-level ozone concentrations increase, more and more people experience health effects, the effects become more serious, and more people are admitted to the hospital for a variety of respiratory problems.
When ozone levels are very high, everyone should be concerned about ozone exposure. Scientists have found that about one out of every three people in the United States is at a higher risk of experiencing ozone-related health effects. If you are a member of a "sensitive group" you should pay special attention to ozone levels in your area.
