The hole in the protective ozone layer over Antarctica has grown bigger this year than at any time since scientists began measuring it fifteen years ago.
Ozone gas in the upper atmosphere blocks harmful ultraviolet radiation from the sun.
Ozone helps protect people from the sun's harmful ultraviolet rays.
Too much of this radiation can cause skin cancer or damage the eyes.
U-V radiation can also threaten agriculture and the environment.
Scientists say they are concerned that low ozone levels in the upper atmosphere could threaten heavily populated areas around the world.
In the Nineteen-Seventies,scientists expressed concern about man-made chemicals called chlorofluorocarbons.
They said these C-F-Cs were destroying the protective ozone layer.
C-F-Cs destroy ozone in the presence of sunlight.
They were once widely used in aerosol sprays and in cooling equipment.
But most countries agreed to stop producing C-F-Cs after a worldwide ban in the late Nineteen-Eighties.
Over the years,scientists have observed a huge loss in ozone over Antarctica.
Antarctica is the ice-covered continent that covers and surrounds the South Pole.
The loss of ozone takes place during its extremely cold winter season from June until September.
Scientists believe that sunlight causes chemical reactions in the cold air over Antarcitica that can destroy ozone.
This creates a huge ozone hole.
The hole gets smaller as temperatures begin to rise in November.
The temperature over Antarctica affects the size of the hole each year.
The National Oceanic and Atmospheric Administration said that the ozone hole increased to more than twenty-seven-million square kilometers for a few days last month.
Scientists say this is the largest size ever recorded.
It is an area about the size of North Amercia.
Record low temperatures in the upper atmosphere are believed to the cause.
For several years,the hole has extended over populated areas at the southern end of South America.
This year,cities in Chile and Argentina were hit with dangerous levels of ultraviolet radiation.
And experts say it is possible that the situation could affect areas in South Africa,Australia or New Zealand.