The chemical formula for ozone is O3. Ozone is a triatomic molecule made up of three oxygen atoms and is a pale blue gas with a distinct, sharp smell. It occurs naturally in the Earth's stratosphere, where it forms a protective layer that absorbs the majority of the sun's harmful ultraviolet (UV) radiation, thus protecting living organisms from the damaging effects of UV light. Ozone is also present at ground level where it can form as a secondary pollutant from industrial emissions and vehicle exhaust, contributing to smog and posing health risks in urban areas.