BEIJING – China’s proposal to halt some metals production to fight air pollution over the winter would create shortages of alumina but have a more limited impact on aluminium supply, according to China’s top industry body, which has been consulted on the plan.

The proposal involves an alumina suspension in three provinces that would affect about a fifth of the nation’s operating capacity producing the raw material for aluminium. The halts to aluminium, which is used in everything from cans to window frames, would be less severe – about a tenth of the country’s operating capacity would be targeted, across four provinces, according to the plan.

A draft was circulated by the Ministry of Environmental Protection earlier this month and is subject to change pending industry feedback, according to a person with knowledge of plan, who asked not to be identified because it’s confidential. The period targeted runs from November to March, when pollution peaks due to coal-fired heating. While the intention is to implement the plan next winter, it hasn’t been decided whether it would come into force over the remainder of this season, the person said.

The impact on aluminium production would likely be limited at about one-million metric tons, the deputy chairman of the China Nonferrous Metals Industry Association, Wen Xianjun, said by phone on Wednesday. For alumina, the impact would be bigger and create an imbalance in supply and demand, he said, without giving figures.

If the plan materializes, it would lead to a 12% production loss for alumina and a 4% loss for aluminium, Citigroup analysts including Jack Shang and Ada Gao said in a note on Wednesday.

China churns out more than half the world’s aluminium and produced a record volume last year of almost 32-million tons, according to the statistics bureau. It had been expected to boost output further to put the global market into surplus in 2017. Alumina production in 2015, the latest for which figures are available, was 56-million tons, according to state-backed researcher Antaike Information Development.

News of the proposal pushed aluminium prices in London to their highest level in 20 months on Tuesday, while on Wednesday the nation’s biggest smelter China Hongqiao Group surged as much as 8.4% in Hong Kong; the No. 2, Aluminium Corp of China, or Chalco, rose as much as 5.1%.

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There’s limited downside for aluminium prices with alumina supply remaining tight in 2017, even without the production cut plan, according to Citigroup. The bank said the proposal would affect China Hongqiao more in terms of volume than Chalco.

Researcher SMM Information & Technology said in a note Tuesday it doubts the proposal will be implemented, as the halt would cost aluminium smelters about 2.25-billion yuan ($327-million) to stop and resume production, and risks other capacity coming online to fill the supply gap.

Under the proposal, 30% of running capacity at some aluminium smelters in Hebei, Shandong, Henan and Shanxi provinces would be ordered to halt over the period, according to the person. The operations targeted account for more than 11-million tons, or about 30% of the nation’s total. For alumina, 50% of running capacity in Shandong, Henan and Shanxi provinces would be affected, operations which account for about 28-million tons, or 40% of the nation’s total.

Nobody responded to a fax requesting comment from the Ministry of Environmental Protection’s news department. An official at China Hongqiao, which also producers alumina, declined to comment. An e-mail to Chalco didn’t get a response.