Ozone is a gas that is made up of three oxygen atoms (O₃).  Due to Rayleigh scattering of light, ozone has a pale blue tint that is visible only in high concentrations.  The word ozone comes from the Greek word ozein meaing "smell", but the strong odor associated with ozone (what you'd smell after a lightning strike) is not from the ozone, but rather from the ionization (freeing of electrons) that takes place during ozone formation and destruction. Near the surface it is considered a pollutant and is detrimental to the human respiratory system. However, in the stratosphere, ozone is beneficial as an absorber of ultraviolet radiation coming in from space. Ozone is produced both naturally (via lightning) and anthropogenically (auto exhaust), and a molecule will last about a half hour without disturbance. Other human produced gases such as Chloroflorocarbons (CFCs) act to destroy ozone by breaking away one of the oxygen molecules. When this occurs in the stratosphere, the ability of the atmosphere to block the ultraviolet radiation is diminished. These ozone destroying gases may be responsible for the "holes" (reductions) in the stratospheric ozone (ozone layer).