PARIS • The ozone layer that protects life on earth from deadly ultraviolet radiation is unexpectedly declining above the planet's most populated regions, a new study has found.

Industrial aerosols that chemically dissolved ozone in the high atmosphere, especially above Antarctica, were banned under a 1987 treaty called the Montreal Protocol.

Today, after nearly three decades, the ozone hole over the South Pole and the upper reaches of the stratosphere are showing clear signs of recovery.

At the same time, however, ozone in the lower stratosphere, 10km to 24km overhead, is slowly disintegrating, said an international team of two dozen researchers. The stratosphere starts about 10km above sea level and is about 40km thick.

"In tropical and middle latitudes, the ozone layer has not started to recover, yet," said the study's lead author, Dr William Ball of the Swiss Federal Institute of Technology in Zurich.

"It is, in fact, slightly worse today than 20 years ago," he said.

At its most depleted, around the turn of the 21st century, the ozone layer had declined by about 5 per cent, earlier research has shown.

The new study, published in the Atmospheric Chemistry and Physics journal, estimates that it has now diminished by an additional 0.5 per cent.

If confirmed, it would mean that the level of ozone depletion is now at its highest level ever, Dr Ball said.

Two possible suspects stood out, the study said. One is a group of chemicals collectively known as "very short-lived substances". The other possible culprit is global warming.

AGENCE FRANCE-PRESSE