NOAA and NASA observations found with the ozone hole shrinking to a minimum of 29 years. The hole in the ozone layer over Antarctica peaked on September 11–an area equivalent to the United States–7.6 million square miles, 8.9 million square miles, 1.3 million square miles from the 2016 maximum, and 3.3 million square miles smaller than the 2015 peak. Since 1991, the average maximum size of the hole in the ozone layer is about 10 million square kilometres. Although stratospheric higher than average temperature conditions in the past two years have reduced the depletion of the ozone layer, the current area of the ozone layer remains large, as substances that lead to ozone depletion, such as chlorine and bromine, are still high enough to produce significant ozone depletion. Scientists believe that the contraction of the ozone hole over the past two years is a natural change, and does not represent a fast-healing signal.