Measurements taken at SMHI's station in Norrkoeping, just south of Stockholm, showed the ozone layer was at its thickest in February since recordings there began in 1988, with a measurement of 426 Dobson units (DU).

At the Vindeln station in northern Sweden, where measurements started in 1991, a record high of 437 DU was recorded.

"We have to go as far back to the measurements taken in Uppsala between 1951 and 1966" to find levels that high, SMHI said in a statement.

There, the highest level for February was in 1957, when a value of 439 DU was recorded.

The circumpolar whirl over the Arctic -- a polar high pressure system formed of a distinct column of cold air that develops during the long polar night -- disappeared very quickly in mid-January, and the stratosphere warmed up quickly in the space of a few days, SMHI explained.

As a result, "the low temperatures that usually cause rapid depletion of the ozone layer did not take place," it said.

The institute, which only a year ago recorded the second-thinnest levels of ozone ever, said it was too early to tell whether the ozone layer was improving in general.

"We would need to see more high values before we can say with certainty that the ozone layer is growing thicker. However we are now in a period where the decrease appears to have halted and we expect to see a thickening," it said.

The ozone layer over Sweden usually reaches its thickest level during the spring, before thinning during the summer and reaching a minimum during the winter, according to SMHI.

Ozone provides a natural protective filter against harmful ultra-violet rays from the sun, which can cause sunburn, cataracts and skin cancer and damage vegetation.

Its depletion is caused by extreme cold temperatures at high altitude and a particular type of pollution, from chemicals often used in refrigeration, some plastic foams, or aerosol sprays, which have accumulated in the atmosphere.

Most of these chemicals, chlorofluorocarbons (CFCs), are being phased out under the 1987 Montreal Protocol, but they linger in the atmosphere for many years.

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