Which gas makes up the ozone layer that filters ultraviolet light?

Ozone is oxygen in an unusual form, three atoms bonded together rather than the two in the oxygen we breathe. High in the stratosphere, ultraviolet light splits ordinary oxygen molecules apart, and the loose atoms attach to intact molecules to make ozone. The same ultraviolet light then breaks ozone back down, and this continuous cycle of creation and destruction absorbs a large share of the Sun's most damaging ultraviolet before it reaches the ground, which matters because that radiation damages DNA and drives skin cancer and cataracts. The layer thinned badly in the late twentieth century because chlorofluorocarbons used in refrigeration and aerosols drifted upward and released chlorine, and a single chlorine atom can destroy many ozone molecules in a chain reaction. An international agreement phased those chemicals out, and the layer has been slowly recovering since. Near ground level, by contrast, ozone is a harmful pollutant that irritates the lungs.

Ozone has a sharp smell that people sometimes notice near sparking electrical equipment or after a thunderstorm.