The hole in the ozone layer over the Antarctic has begun to shrink, signalling good news for the environment several decades after an international accord to phase out certain pollutants, researchers in the US have said.

The study found that the September ozone hole has shrunk by four million square kilometers (1.5 million square miles since 2000) - an area about the size of India.

"It's a big surprise," said lead author Professor Susan Solomon, an atmospheric chemist at the Massachusetts Institute of Technology, in an interview with Science magazine.

"I didn't think it would be this early."

The ozone hole was first discovered in the 1950s.

It reached record size in October 2015, but Prof Solomon and colleagues determined that this was due to the eruption of the Chilean volcano Calbuco.

The overall trend toward recovery became apparent when scientists studied measurements from satellites, ground-based instruments and weather balloons in the month of September, not October when the ozone hole typically peaks in size.

"I think people, myself included, had been too focused on October, because that's when the ozone hole is enormous," said Prof Solomon.

"But October is also subject to the slings and arrows of other things that vary, like slight changes in meteorology."

The study attributed the ozone's recovery to the "continuing decline of atmospheric chlorine originating from chlorofluorocarbons (CFCs)," or chemicals that were once emitted by dry cleaning, refrigerators, hairspray and other aerosols.

Most of the world signed on to the Montreal Protocol in 1987, which banned the use of CFCs.

"We can now be confident that the things we've done have put the planet on a path to heal," said Prof Solomon.