We hear a lot about ozone and its relation to our atmosphere and the air we breathe, but what exactly is it, and why can it be both safe and dangerous? Well, ozone is a highly reactive gas that forms in the atmosphere when three atoms of oxygen are combined through a chemical reaction. In contrast, the oxygen gas we breathe is chemically formed from two atoms of oxygen. Ozone is not emitted directly into the air but, at ground level, is created by a chemical reaction between oxides of nitrogen and volatile organic compounds in the presence of sunlight. Ozone has the same chemical structure whether it occurs high above the earth or at ground level. High above the earth is the ozone that protects us from some of the most dangerous radiation from the sun. Ozone on the ground can be very unhealthy and dangerous in sufficient quantities.
Ground-level ozone is primarily formed as a result of human activities. The troposphere extends to a level about 10 miles up, where it meets the second layer of the atmosphere, the stratosphere. In this second layer of the atmospheric, the stratospheric or "good" ozone layer extends upward from about 10 to 30 miles. The stratospheric ozone layer naturally occurs and protects life on Earth from the sun's harmful ultraviolet rays.
High ozone levels can damage leaves on trees and crops, reducing growth rates and crop yields. In 1995, ground-level ozone caused $2.7 billion in crop damage nationwide, according to the U.S. Environmental Protection Agency. Due to its reactive nature, ozone also can prematurely degrade and wear out rubber, paints and other materials.
Ozone is primarily a problem during the summer months, when heat and sunlight are more intense. Ozone levels also vary on a daily basis. Ozone levels peak in the afternoon, when temperatures are higher, and then drop at night in most areas. Because of unique atmospheric conditions, ozone can stay high on mountain peaks during the night.