China has ordered thousands of factories to shut as it grapples with swathes of choking smog that were nearly 24 times safe levels on Tuesday, casting a shadow over the country's participation in Paris climate talks.
A thick grey haze shrouded Beijing, with the concentration of PM 2.5, harmful microscopic particles that penetrate deep into the lungs, climbing as high as 598 micrograms per cubic metre.
The reading, given by the US embassy, dwarfs the maximum recommended by the World Health Organisation, which is just 25 micrograms per cubic metre
Levels in Jinan, a provincial capital hundreds of kilometres away, reached over 400.
Authorities in Beijing ordered the closure of 2,100 highly polluting businesses, the state-run China Daily said, and advised citizens to stay indoors.
Airlines cancelled over 30 flights from Beijing and Shanghai, many to highly polluted Shaanxi province, a key coal producer.
The environmental woes came after Chinese President Xi Jinping took the stage at crucial international talks aiming to limit dangerous climate change.
He vowed "action" on greenhouse emissions, repeating existing pledges and telling the summit that poor nations should not have to sacrifice economic growth.
Most emissions come from coal burning which spikes in winter along with demand for heating, which also causes smog.
China is estimated to have released between nine and 10 billion tonnes of carbon dioxide in 2013, nearly twice as much as the United States and around two and a half times the European Union.
Beijing pledged last year that carbon dioxide output would peak by "around 2030" -- suggesting at least another decade of growing emissions.
Social media users in China were sceptical about the chances of a clean up, with many circulating a picture of a Beijing newspaper front page from 1999.
It cited officials as proclaiming: "We absolutely will not let big pollution enter the new century."