Tri-atomic oxygen that exists in the Earth's atmosphere as a gas. Ozone is highest in concentration in the stratosphere (10-50 kilometres above the Earth's surface) where it absorbs the Sun's ultraviolet radiation. Stratospheric ozone is produced naturally and helps to protect life from the harmful effects of solar ultraviolet radiation. Over the last few decades levels of stratospheric ozone have been declining globally, especially in Antarctica. Scientists have determined that chlorine molecules released from the decomposition of chlorofluorocarbons are primarily responsible for ozone destruction in the stratosphere.It is also abundant near the the Earth's surface in highly polluted urban centers. In these areas, it forms as a by product of photochemical smog, and is hazardous to human health.