The association's calculations show that the consumption of legal foaming agents in China from 2011-2017 is consistent with PU foam output, meaning that "it's impossible to have marked consumption of CFC-11 in the country", it said.

"We welcome international scientists' long-term monitoring of CFC-11 and other banned substances included in the Montreal Protocol, which helps alert parties to problems and take measures accordingly," the statement said.

"We found in the study, however, great uncertainty, over quantifying the discharge amount and locating discharging sources, which makes us realize the importance of atmosphere monitoring," it added.

It said the ministry has initiated a research project on the construction of an ODS monitoring network as it makes efforts to enhance its capability monitoring and assess progress phasing out ODS.

China in 2021 may start building stations to monitor the background density of ODS in the atmosphere, it said.

It also said China will continue to support research on implementing the protocol, and a new special fund of 10 million yuan on scientific research has been approved.

According to the center, China unearthed 24 cases of illegal ODS manufacturing, 44 cases of illegal use and five cases of illegal selling from 2010 to the first half of 2018.

It also said 14 of the cases were related to CFC-11, in which 84 metric tons of the substance was seized and destroyed, with all facilities producing the substance demolished.

It also said the Ministry of Ecology and Environment combed 1,172 companies across the country in the second half of 2018 and found CFC-11 in some raw materials in 10 companies.

The ministry also destroyed two sites that illegally produced CFC-11, seizing 29.9 tons of the substance and 177.6 tons of various kinds of raw materials.