Odorless and colorless, ground level ozone is the main pollutant in photochemical smog. Ozone is a gas naturally found in the Earth's upper atmosphere, but at ground level it is largely a human caused pollutant. Where it occurs determines if it is beneficial or harmful. Good ozone is found in the stratosphere 30 miles above earth where it removes excess ultraviolet radiation. This layer is being slowly thinned and destroyed by manmade chemicals, creating the ozone hole.

Harmful ozone is found at ground level where it is a manmade pollutant and the main gas in photochemical smog. Ozone is a powerful oxidant, capable of making rubber and synthetic materials brittle.

Ozone smog sends tens of thousands of Americans into hospital rooms annually and affects many areas. While emissions may originate in urban areas, wind may transport the chemicals downwind into rural, even remote areas, where ozone forms often at higher levels than that found in cities.

In Iowa, ozone can reach unhealthy levels during warm weather months. It has been found in both cities and rural locations. Often when levels are high, it can affect many counties or large areas of the state.

• How Photochemical Smog Forms
• Health Effects of Ozone
• Environmental Effects of Ozone