Since the 1970s, NASA and NOAA have used a variety of instruments, including satellites and balloons, to measure the ozone layer. Ozone concentrations are measured in Dobson Units (DU); the average thickness of the ozone layer is about 300 DU, or some 3 millimeters (the height of two pennies stacked together). The ozone hole is the region where concentrations drop below 220 DU. The ozone hole, depicted in red, begins forming every year in early September, when the spring sunlight ends Antarctica’s long, dark winters. Mid-1980s: Ozone layer sees a noticeable decline from its average level of about 300 DU. 1992: Ozone layer is 100 DU. 2006: Ozone layer dips below 93 DU. 2012: Total ozone reached 124 DU on Oct. 1 and 136 DU on Oct. 5 with the hole covering 8.2 million square miles (equivalent to the area of North America).