Much like cholesterol, there is "good" ozone and "bad" ozone. The good ozone (atmospheric) lies in the upper reaches of the atmosphere, helping protect us from ultraviolet radiation from the sun. However, when ground level ozone (tropospheric) is in the air we breathe, it's not healthy. What we commonly call "smog" is really ozone build-up in the air. The use of ethanol-blended gasoline has been shown to help reduce tropospheric ozone levels.