A mysterious surge in emissions of an illegal ozone-destroying chemical has been tracked down to plastic foam manufacturers in China, according to an on-the-ground investigation published on Monday.

The chemical, trichlorofluoromethane or CFC-11, has been banned around the world since 2010 and is a potent destroyer of ozone, which protects life on Earth from UV radiation, and strong greenhouse gas. A shock rise in the gas in recent years was revealed by atmospheric scientists in May, but they could only narrow the source to somewhere in East Asia.

The Environmental Investigation Agency, a non-governmental organisation, has now identified widespread use of CFC-11 factories in China that make insulating foams. The EIA’s investigators identified factories that sold the chemicals needed for foam-making, then contacted and visited them.

“We were dumbfounded when out of 21 companies, 18 of them across China confirmed use of CFC-11, while acknowledging the illegality and being very blase about its use,” said Avipsa Mahapatra at the EIA. Furthermore, the companies said the use of CFC-11 was rife in the sector. “It was very clear. These companies, again and again, told us everybody else does this,” she said.

China is a major producer of the rigid polyurethane foams involved and the EIA calculates that if the illegal use of CFC-11 is pervasive in the 3,500 small- and medium-sized companies that make up the sector, then this would explain the surge. Without action, the CFC-11 emissions would delay the recovery of the planet’s ozone hole by a decade, scientists estimate.

“We didn’t know what on Earth someone would be using CFC-11 for – well, here’s one answer and that’s a surprise,” said Steve Montzka at the US National Oceanic and Atmospheric Administration in Colorado, whose team revealed the surge. “Despite efforts to get rid of this activity, it continues.”

He praised the EIA work, but cautioned that CFC-11 might be also produced by other activities and that this should not be ruled out: “If this one issue is targeted within China, we want to be sure that will take care of the problem.” New atmospheric measurements in east Asia should narrow down the total amount of CFC-11 being leaked and any hotspot locations in the next nine months, he said.

The EIA’s evidence has been passed to the Chinese government, which has already inspected and taken samples from some sites, and to officials at the Montreal Protocol (MP), the treaty that phased out ozone-killing chemicals. An MP working group is meeting in Vienna on 11 July and will consider the next steps.

“This week will be a critical moment for dialogue, resolve and action to ensure any illegal activities are fully investigated and urgently halted,” said Erik Solheim, the head of UN Environment, which hosts the MP. He said the EIA evidence was part of a wider body of scientific investigation taking place.

PU foams are used mainly as insulation in buildings, either sprayed into cavities or applied as solid panels, and are in high demand due to China’s construction boom. CFC-11 is easy to produce and $150 (£113) a tonne cheaper than the ozone-friendly alternative, according to the companies to which the EIA investigators spoke. The penalty in China for its use is a fine.

“The profit margins were very high, the demand was high and the risks were very low,” said Mahapatra. “That enabled these companies to use it so blatantly and is why we think this is so pervasive.”

A representative of one company, Aoyang Chemical Co, in Dacheng, Hebei province, told the EIA that 99% of its foams used CFC-11, bought from “shady and hidden” factories in Inner Mongolia. Another, from the nearby Wan Fu Chemical Co, said it was easy to avoid inspections: “When the municipal environmental bureau runs a check, our local officers would call me and tell me to shut down my factory. Our workers just gather and hide together.”

The EIA’s findings are supported by official Chinese government documents, with a 2016 report from environmental officials in Shandong province, a key region for foam production, stating: “There is still a large volume of illegally produced CFC-11 being used in the foam industry” and that its production is “highly concealed”. Other documents from Shandong showed one factory alone to have been producing 1,100 tonnes of CFC-11 in a year.

The EIA report acknowledges significant uncertainties in its calculation but believes it has been conservative in estimating 10,000-12,000 tonnes a year of CFC-11 leaking into the atmosphere from foam-making in China from 2012-17. The scientific study that revealed the surge estimated emissions between 8,000 and 18,000 tonnes over the same period.