Ozone is a gas comprised of three oxygen (O3) atoms. It is unstable and reacts easily to other gases. In its natural state, it is a component in several levels of the atmosphere. At high altitudes, it provides protection by absorbing large amounts of ultraviolet radiation. At ground level, however, high concentrations pose a health and environmental threat. It is also one of the principal components of smog. Ozone levels are generally higher in summer, during hot sunny weather. Concentrations are usually highest in late afternoon. Ozone is a pollutant that can travel large distances. Periods of high ozone levels can persist anywhere from a few hours to a few days, depending on weather conditions.