Ozone (O3) is a constituent of the troposphere (it is also an important constituent of some regions of the stratosphere commonly known as the ozone layer). The troposphere extends from the Earth's surface to between 12 and 20 kilometers above sea level and consists of many layers. Ozone is more concentrated above the mixing layer, or ground layer. Ground-level ozone, though less concentrated than ozone aloft, is more of a problem because of its health effects.