This graph shows a correlation between the levels of CFC₁₂ and ozone over Antarctica. As CFC₁₂ increased, ozone declined (a negative correlation). When CFC₁₂ amounts levelled off, around the year 2000, the ozone hole over Antarctica started to recover. (Ozone concentration is expressed in Dobson units, DU, a measure of how thick the air sample would be if it were compressed at 1 atmosphere of pressure at 0°C; 1 DU = 1 mm thick.)