Ozone loss over the Arctic this year has been so severe that for the first time it could be called an "ozone hole" like that the Antarctic one, according to scientists.
Writing in the journal Nature, they say 80% of the ozone about 20km above the ground was lost.
The cause was an unusually long spell of cold weather at altitude. In cold conditions, the chlorine chemicals that destroy ozone are at their most active, the BBC reports.
Scientists say it is currently impossible to predict if such losses will occur again.
Ozone-destroying chemicals originate in substances such as chlorofluorocarbons that came into use late in the 20th century in appliances including refrigerators and fire extinguishers.
The destructive effects of CFCs were first documented in the Antarctic, which now sees severe ozone depletion in each of its winters.
Their use was progressively restricted and then eliminated by the 1987 Montreal Protocol and its successors.
The ozone layer blocks ultraviolet-B rays from the Sun, which can cause skin cancer and other medical conditions.
The hole above the Antarctic remains relatively stable.