The ozone hole over Antarctica that lets dangerous ultraviolet light reach the Earth is bigger than ever this year, scientists say.

Preliminary satellite data from the U.S. National Aeronautic and Space Administration found that the hole encompasses 10.4 million square miles, 5 percent larger than the previous record set in 1996, the government agency Antarctica New Zealand said this week.The hole is actually an area of thinned ozone, where the gas is depleted by chemical reactions in air trapped over the South Pole during the Antarctic winter. The area covers most of Antarctica.

Ozone helps block damaging ultraviolet rays from the sun, and scientists have been concerned that, if it becomes depleted, there could be an increase in skin cancer and cataracts. Certain chemicals believed to damage the ozone have been banned, including chlorofluorocarbons that are widely used in aerosol propellants, refrigerators and air conditioners.

The area of reduced ozone expands at this time of year - spring in the Antarctic - as sunlight returns to that region.

Like the 1996 hole, this year it developed more rapidly in late August and early September than in previous years.

Measured in "Dobson Units," NASA satellites found the ozone cover fell below 100 units, close to the all-time low of 88 units measured in 1994, Antarctica New Zealand said.

"The ozone hole has probably reached its greatest area extent for this year, but the lowest amounts of ozone are likely to be seen in the next week," the agency said.

Ozone levels usually range from 250 units to more than 500 units over different parts of the world. The ozone hole was first observed in the 1980s.