Ozone (O₃) is a gas that occurs both in the upper atmosphere and near the Earth's surface, and is of particular concern to human health and for the environment. Ground-level O₃ is a widespread, highly reactive air pollutant that is a main ingredient of urban smog and poses a serious health risk. It can also impact crops, trees, and other commercially important vegetation by damaging leaf tissues, reducing plant growth, and making them more susceptible to other stresses such as drought. The stratospheric O₃ layer extends upward from about 6 to 30 miles and protects life on Earth from the sun's harmful ultraviolet (UV) rays. This natural shield has been gradually depleted by man-made chemicals like chlorofluorocarbons (CFCs). A depleted O₃ shield allows more UV radiation to reach the ground and can affect human health.