The U.S. agency NOAA said that the ozone hole spanned about 9.7 million square miles, making it the fifth largest on record.

The largest Antarctic ozone hole ever recorded developed in 2006, when it covered 10.6 million square miles.

While ozone is a pollutant on the surface, it forms a protective layer high in the atmosphere that filters out much of the sun’s dangerous ultraviolet radiation.

The use of manmade chemicals like chlorofluorocarbons was found to cause a chemical reaction in the stratosphere that destroys the ozone.

The annual hole grew steadily during the 1980s and '90s, but appeared to level off after 2000 due to a 1989 international treaty to restrict the use of ozone-destroying chemicals.

But the chemicals linger in the atmosphere for a long time, meaning it could take several decades for the ozone hole to heal.

Earlier this year, the first significant Arctic ozone hole on record formed around the North Pole, mainly due to unusually frigid air that gathered high in the atmosphere.

Image: NASA Ozone Hole Watch