The ozone hole over Antarctica this year could be one of the smallest seen in three decades, say scientists.
Observations of the gas's depletion high in the atmosphere demonstrate that it hasn't opened up in 2019 in the way it normally does.
The EU's Copernicus Atmosphere Monitoring Service (CAMS) says it's currently well under half the area usually seen in mid-September.
The hole is also off-centre and far from the pole, the EU agency adds.
CAMS' experts, who are based in Reading, UK, are projecting a decline in the area of thinning in coming days.
Ozone is a molecule that is composed of three oxygen atoms. It is responsible for filtering out harmful ultraviolet radiation from the Sun.
The gas is constantly being made and destroyed in the stratosphere, about 20-30km above the Earth.
In an unpolluted atmosphere, this cycle of production and decomposition is in equilibrium. But chlorine and bromine-containing chemicals released by human activity have unbalanced the process, resulting in a loss of ozone that is at its greatest in the Antarctic spring in September/October.
The Montreal Protocol signed by governments in 1987 has sought to recover the situation by banning the production and use of the most damaging chemicals.
This past week has seen the area of deep thinning cover just over five million square km. This time last year it was beyond 20 million square km, although in 2017 it was just above 10 million sq km.
Vincent-Henri Peuch, who leads CAMS, says the small size seen so far this year is encouraging but warns against complacency.
"The recovery of the ozone layer is dependent on climate change as long-term cooling in the stratosphere can exacerbate ozone loss and delay the process. Also, the possibility of unauthorised emissions of ozone depleting substances cannot be ruled out – in fact, emissions of the second most abundant chlorofluorocarbon (CFC-11) were detected even last year," he said.
"It is very important to maintain international efforts for monitoring the recovery of the ozone layer over time and the annual ozone hole events."
CAMS is an EU service run by the European Centre for Medium-range Weather Forecasts.
It has access to a range of space-borne and ground observations. Principal data sources include the European Metop weather satellites and the EU's own Sentinel-5P spacecraft. The four platforms all carry ozone sensors and routinely cross the pole.
The conditions for thinning occur annually just as the Antarctic emerges from Winter. The reactions that work to destroy the gas in the cold stratosphere are initiated by the return of sunshine at high latitudes.
Scientists say that while losses started earlier than normal this year, they were truncated by a sudden warming event in the high atmosphere.
The WMO-sponsored 2018 Scientific Assessment of Ozone Depletion said a recovery of the ozone layer to pre-1970 levels could be expected around 2060./BBC news