Scientists say they have pinpointed the source of a globally banned chemical that damages the Earth’s protective ozone layer: China.

In a report published Wednesday in the journal Nature, the scientists who monitor the planet’s atmosphere say the recent rise in the emission of the ozone-depleting chemical CFC-11 has been traced to two provinces in eastern China.

Any production and use of CFC-11 is a violation of the Montreal Protocol, a 1987 agreement that phased out chlorofluorocarbons that cause damage to the ozone layer.

Ozone is critical to life, limiting the amount of harmful ultraviolet solar radiation that reaches Earth’s surface. The recovery of the ozone had been touted as an environmental success story. But since 2012, air samples had shown a troubling amount of CFC-11 present in them. Because the chemical doesn’t occur in nature, it indicated an illegal use of the chemical.

Over the last two years, scientists have used air monitoring stations in Japan and Korea along with water studies to pinpoint the offenders as foam factories in the provinces of Shandong and Hebei in eastern China.

Scientists say the report will help Chinese authorities find the exact sources and stop the emissions before they deal a major setback to ozone hole recovery.

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