The gaping hole in the ozone layer around Earth, over the planet’s polar regions (mainly Antarctica), has been consistently growing smaller since the 1990s, following a worldwide ban on the use of chemicals that caused its depletion. But now, scientists have found the recovery of the ozone layer has been mostly limited to the poles, while it continues to become thinner in the lower latitudes.

A large team of researchers from Europe and North America found that ozone continues to recover in the upper stratosphere but has been unexpectedly declining in the lower layers of the stratosphere, between 60 degrees north and south of the equator.

William Ball from ETH Zurich, who led the analysis, said in a statement Tuesday: “The finding of declining low-latitude ozone is surprising, since our current best atmospheric circulation models do not predict this effect. Very short-lived substances could be the missing factor in these models.”

Ozone is a form of oxygen, wherein each molecule contains three oxygen atoms instead of the usual two. The ozone layer — part of Earth’s atmosphere which absorbs most of the sun’s harmful ultraviolet radiation — is not made of ozone entirely. It is a part of the planet’s stratosphere which has a relatively high concentration of ozone particles, about 10 per million (compared to 0.3 parts per million on average throughout Earth’s atmosphere).

The stratosphere itself starts at about 8-10 kilometers over Earth’s surface and goes up to about 50 kilometers. The ozone layer is mainly in the lower stratosphere, and without it, the UV radiation would damage the DNA in all life-forms the sunlight would touch.

The Montreal Protocol, signed in 1987 and effective from 1989, banned the use of ozone-depleting substances, and scientists reported seeing a recovery of the layer since the 1990s, mainly in the upper layers of the stratosphere. In 2016, even the hole over Antarctica was declared to be healing. Which is why the continued depletion of ozone in lower latitudes caught scientists somewhat by surprise.

Researchers reasoned the sudden discovery, made using analysis of satellite measurements over three decades, was because of human-made ozone in the troposphere — the lowest layer of the atmosphere, where we live. This would be created by the use of certain chemicals in solvents, paint strippers and degreasing agents. Or, it could be a result of climate change, which has been exacerbated by human activity, and the changes it is causing to atmospheric circulation.

“This anthropogenic ozone, which causes summer smog, partially masks the stratospheric decline in the satellite measurements,” Ball explained in the statement.

The study found that while the amount of ozone over the poles has increased, the overall amount of ozone in the atmosphere has not gone up, indicating a thinning at non-polar latitudes. And that gives the problem a different twist.

“The potential for harm in lower latitudes may actually be worse than at the poles. The decreases in ozone are less than we saw at the poles before the Montreal Protocol was enacted, but UV radiation is more intense in these regions and more people live there,” Joanna Haigh from Imperial College London, a coauthor on the study, said in another statement.

The study was published online Tuesday in the journal Atmospheric Chemistry and Physics, under the titled “Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery.”