In the year's worst air pollution episode, more than 200 million citizens across six provinces were experiencing hazardous levels of smog, with a further 260 million experiencing "heavy" pollution, Greenpeace said.

In Shijiazhuang, the capital of northern Hebei province, air quality readings of PM2.5 fine particulate matter soared beyond 1000 micrograms per cubic metre on Monday night, more than 100 times levels the World Health Organisation's recommended annual average.

The Ministry of Environmental Protection said as of Monday evening eight cities had shown Air Quality Index readings that were "beyond index" or above official maximum reading levels of 500, including steel-making cities Handan, in Hebei, and Anyang, in Henan province.

Pollution red alerts are issued in Chinese cities when AQI readings are forecast to exceed 200 for more than four days in succession, 300 for more than two days or 500 for at least 24 hours. The first red alert was announced in Beijing in December last year, in a four-tier system first introduced by Chinese authorities in 2013 amid widening popular anger at the country's severe air pollution.

Worsening air pollution in Chengdu, the capital city of Sichuan, triggered calls for a mass protest on social media earlier this month, though they were shut down by police before they began. Guerrilla-style protests had also seen activists place masks on statues in parks and public squares.

There has also been controversy over mooted moves by authorities in Beijing and Shanghai to classify smog as a meteorological phenomenon, prompting concerns it was an effort from authorities to shirk accountability.

The cities experiencing the most severe pollution are all among China's largest steel or coal industry clusters, which have recently experienced a major rise in production driven by "retrograde stimulus policies", Greenpeace said.

"The scale of the red alert measures show that the Chinese government is taking air pollution seriously," said Greenpeace climate and energy campaigner Dong Liansai.

"However, the ongoing 'airpocalpyse' is further evidence that China must implement far stricter limitations on coal consumption and accelerate the restructuring of the economy away from the heavily polluting sectors."

In Beijing, where a pollution red alert has been in place since last Friday, air quality index readings hovered neared 400 since Monday despite strict measures closing schools, factories and polluting industries, while ordering half the city's traffic off the roads. At least 181 flights were cancelled at the capital's major airports, with sections of a major ring road closed due to poor visibility.