The hole in the ozone layer over Antarctica this year is the fifth biggest on record, according to the latest data from NASA's atmospheric scientists.

NASA said on Sept. 12, 2008, the ozone hole reached its annual maximum area of 27.2 million square kilometres, up from last year's 25.1 million square km. By comparison, the total land area of North America is about 24.7 million square km.

The hole is considered "moderately large," said NASA atmospheric scientist Paul Newman. The largest ozone hole ever recorded occurred in 2006, at a size of about 27.4 million square kilometres.

Ozone, a bluish gas composed of three oxygen atoms, is harmful to breathe, but absorbs ultraviolet radiation that harms living organisms. Its presence in our stratosphere helps keep harmful UV rays originating from the sun from reaching the Earth.

The hole typically reaches its maximum extent in September or early October, and then dissipates into smaller parts throughout southern latitudes, affecting the UV exposure in places such as New Zealand, Australia and South America, NASA said.

NASA has been monitoring the depletion of ozone above Antarctica through satellite observations since the 1970s. The increasing size of the hole in the 1980s was the motivation behind restrictions on the use of chlorofluorocarbons, or CFCs.

Newman said that while the ozone hole is large, the amount of human-produced substances that contribute to its depletion has actually decreased about 3.8 per cent from peak levels set in 2000.

In addition to CFCs, the potential harmful substances include hydrochlorofluorocarbons, which have been used as a refrigerant in air conditioners and fridges, and halons, which act as a propellant for fire extinguishers.