A hazardous, ozone-depleting compound is still being used in China, even though it is banned worldwide by the Montreal Protocol, a new study finds.

Eastern China has emitted significant amounts of this substance — known as carbon tetrachloride (CCl4) — which is known to eat away at the ozone, a protective layer in Earth's atmosphere that shields the world from dangerous ultraviolet radiation.

The new research is yet another piece of evidence pointing toward China as the source of the ozone-destroying emissions. This past summer, an investigation by The New York Times also found that factories in the country were releasing banned substances that destroyed the ozone layer. [Earth from Above: 101 Stunning Images from Orbit]

The ozone sits high in Earth's stratosphere, about 6.2 miles (10 kilometers) above the ground, where it absorbs much of the sun's ultraviolet (UV) radiation. This radiation increases the risk of cancer and eye damage in humans. A human-made hole already exists in the ozone layer over Antarctica. So, to protect the ozone, all the countries in the world collectively agreed to ban the substances that destroy that layer, including CCl4, which was banned worldwide in an update to the Montreal Protocol in 2010. Despite this agreement, about 44,000 tons (40,000 metric tons) were mysteriously being emitted each year, recent studies have shown.

To investigate, an international team of scientists from Australia, South Korea, Switzerland, the United Kingdom and the United States worked together to pinpoint the origin of these puzzling emissions. The team used data from ground-based and airborne atmospheric-concentration sensors from near the Korean Peninsula, as well as two models that simulated how gases move throughout the atmosphere.

These techniques paid off; the researchers found that about half of these mystifying emissions came from eastern China between 2009 and 2016.

"Our results show that emissions of carbon tetrachloride from the eastern Asia region account for a large proportion of global emissions," study lead author Mark Lunt, a visiting research associate of chemistry at the University of Bristol in England, said in a statement. "And [these emissions] are significantly larger than some previous studies have suggested."

The CCl4 emissions are so vast that "despite the phaseout of carbon tetrachloride production for emissive use in 2010, we found no evidence for a subsequent decrease in emissions," Lunt said. Some regions, including the Shandong Province of China after 2012, have even pumped out more emissions than they did previously, he said.

However, the scientists noted that they're unsure where the other the CCl4 emissions are coming from. It's possible that large amounts of this gas are inadvertently being created when other chemicals, such as chlorine, are produced, the researchers said.

"Our work shows the location of carbon tetrachloride emissions," study co-author Matt Rigby, a reader in atmospheric chemistry at the University of Bristol, said in the statement. "However, we don't yet know the processes or industries that are responsible. This is important because we don't know if it is being produced intentionally or inadvertently."

More atmospheric research could uncover other culprits. "There are areas of the world — such as India, South America and other parts of Asia — where emissions of ozone-depleting gases may be ongoing but detailed atmospheric measurements are lacking," Rigby said.

Going forward, these findings may help scientists and regulators identify exactly where and why these emissions in China are happening. After all, the sooner these emissions are stopped, the faster the ozone will recover, the researchers said.

"There is a temptation to see ozone depletion as a problem that has been solved," Lunt said. "But the monitoring of man-made ozone-depleting gases in the atmosphere is essential to ensure the continued success of the phaseout of these compounds."

The study was published online Sept. 28 in the journal Geophysical Research Letters.

Originally published on Live Science.