Washington, Oct 26 (IANS): The hole in Earth's protective ozone layer in Antarctica this year was slightly smaller than average in recent decades, US space agency NASA said.

The average size of the hole was 21 million sq km in September-October 2013, while the average size measured since the mid-1990s was 22.5 million sq km, reported Xinhua Friday citing a NASA statement.

However, it is too early to determine whether a filling of the hole has begun, the space agency said.

The single-day maximum area this year reached 24 million sq km Sep 16, which was equal to the size of North America. The largest single-day ozone hole since the mid-1990s was 29.9 million sq km Sep 9, 2000, it said.

The ozone hole in the stratosphere is a seasonal phenomenon which starts forming in August and September.

However, the 1987 Montreal Protocol -- an international treaty to protect the ozone layer by phasing out production of ozone-depleting chemicals has decreased the level of such chemicals in the atmosphere. Thus the size of the hole has stabilized, with variation from year to year driven by changing meteorological conditions.