The hole in the South Pole ozone layer has reached a record size.
It now stretches over an area of 10 million square miles - about twice the size of Antarctica.
Most of the region it covers is uninhabited. But New Zealanders and people living at the southern tip of South America could be at risk.
Normally, the ozone layer shields us from the cancer-causing effects of the sun's ultraviolet rays.
The hole grew rapidly last month and is now larger than it has been at any time in early September.
But British Antarctic Survey scientists monitoring its progress are not unduly alarmed - in fact, they see the observations as good news.
They believe the hole is nearing its peak size, and will soon start to shrink as the level of the ozone-depleting CFC pollutants in the atmosphere diminishes.
Dr Alan Rodger, from the BAS, told Britain's biggest scientific meeting at Salford University today: "The exciting thing this year is that the Antarctic ozone hole appears to be bigger than it's been at any other stage since recent records began.
"We're just at a point where we'd expect the ozone hole to be reaching its maximum size. Over the last 20 years it has been getting bigger, but the rate of change is now slowing. We're probably getting close to turning the corner."
Speaking at the British Association Festival of Science, he said it could be another 10 years before scientists knew for sure that the ozone layer was recovering.
PA