The ozone hole over Antarctica shrank to its smallest peak since 1988, NASA said yesterday.
The US space agency said that this year, the huge hole in Earth's protective ozone layer reached its maximum in September, and was 19.6 million square kilometres wide.
The hole size shrunk again after mid-September.
This year's maximum hole was more than twice as big as the United States, but was 3.36 square kilometres less than last year and 8.55 square kilometres smaller than 2015.
NASA said stormy conditions in the upper atmosphere warmed the air and kept the chemicals chlorine and bromine from eating ozone.
This year's drop was mostly natural, but came on top of a trend of smaller steady improvements likely from the banning of ozone-eating chemicals in a 1987 international treaty.
The ozone hole hit its highest in 2000 at 29.86 million square kilometres.
Ozone is a colourless combination of three oxygen atoms.
High in the atmosphere, about 11 to 40 kilometres above the Earth, ozone shields Earth from ultraviolet rays that cause skin cancer, crop damage and other problems.
Scientists at the United Nations a few years ago determined that without the 1987 treaty, there would have been an extra two million skin cancer cases by 2030.
They said overall the ozone layer is beginning to recover because of the phase-out of chemicals used in refrigerants and aerosol cans.
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