The ozone layer protects Earth from harmful ultraviolet solar radiation. Ozone is a gas made of three oxygen atoms, and just like any other gas it circulates in the atmosphere. During the fall months, chemical reactions combine with circulation patterns high in the atmosphere to reduce the concentration of ozone over Antarctica. Areas with ozone concentrations less than 220 Dobson Units are called "holes" in the layer. NOAA's polar orbiting satellites are used to monitor the ozone hole and the data taken from the POES satellites over the past year is show in this animation. In 2009, the ozone hole reached its 10th largest measured size since careful measurements began in 1979. It appears, though, that the ozone hole is repairing itself after passage of the Montreal Protocol in 1989 that banned the use of ozone-depleting compounds such as chlorofluorocarbons(CFC's) and hydrofluorocarbons (HFCs).