Ozone is made up of three oxygen atoms. It has benefits and dangers depending on where in our atmosphere it occurs: near ground level, it is a pollutant that affects respiration; 10-50km up, in the stratosphere, the ozone layer philtres out potentially harmful ultraviolet rays (which cause skin cancer) from reaching the earth. Ozone also functions as a greenhouse gas, though it is considered a less potent one than CO2.