A FACTORY fire in eastern China went unnoticed for three hours because of the thick smog that blanketed large stretches of the country this week, state media have said.

After firefighters in Zhejiang province were finally alerted to the blaze in the early hours of Monday they took 10 hours to extinguish the flames, which destroyed a large amount of furniture, the Xinhua news agency said.

"Because of the thick fog pervading the air at the time, the initial smoke and flames produced by the fire took an unexpected almost three hours to be discovered by nearby residents," Xinhua said.

The unusually dense air pollution across much of China this week reached levels dangerous to human health and prompted public criticism of the nation's breakneck development.

At the height of the smog authorities in the capital said readings for PM2.5 - particles small enough to penetrate the lungs - hit 993 micrograms per cubic metre, almost 40 times the World Health Organisation's safe limit.

The reduced visibility contributed to a 20-car pileup elsewhere in Zhejiang on Tuesday that left two people dead and eight injured, Xinhua said in a separate report, and forced flights in Beijing to be cancelled.

- "Drastic change needed"

China has cleaned up its air before but experts say if it wants to avoid the kind of smog that choked the country this week, it must overhaul an economy that is fuelled by heavily polluting coal and car use.

Several days of hazardous air quality across large swathes of northern China triggered an emergency response which saw schoolchildren kept indoors, factories closed and government cars parked.

Experts are urging authorities to take firmer action to confront the consequences of China's rapid industrialisation - and the spending habits of the middle classes created by its economic boom.

The calls for cleaner air come after new Communist Party leader Xi Jinping said the government would strive to meet demands for "more comfortable living conditions, and a more beautiful environment".

China began releasing pollution data last year, which observers say triggered a huge backlash against the smog, both from a newly aware public who took to Internet message boards in droves, and by normally partisan state-run media.

As the noxious haze blanketed Beijing this week, driving sales of face masks and air purifiers through the roof, state news agency Xinhua said the party's promise to build a beautiful China was "in jeopardy".

"A country with a brown sky and hazardous air is obviously not beautiful," it said in a commentary.

When the eyes of the world were on Beijing for the 2008 Olympics, the Chinese capital implemented strict measures for two months or more that reduced toxic emissions by 60 per cent.

Car users could only drive every other day, construction work ceased and factories were shut down.

Such steps were "extreme resolutions which are not sustainable", said professor John Cai, director of the Centre for Healthcare Management and Policy at the China Europe International Business School in Shanghai.

"What needs to happen is for the government to take long-term action by centralising the heating systems, so that individual families in rural areas do not burn coal, and also cutting down on car use in the city," he told AFP.

China would face "short-term economic losses" from sweeping changes, Mr Cai said, but added the government should look to the long term.

Growing car use has been both an important driver of the economy and an overt symbol of China's mounting wealth.

But with five million vehicles already in Beijing and 20,000 more each month, Mr Cai said addressing it was essential, suggesting owners be forced to pay high licence fees on each vehicle to reduce emissions.

Tackling coal use is even more problematic, as while families might be encouraged onto a central heating system, the vast majority of China's electricity is generated from the fuel, powering China's breakneck development.

A total of 79 percent of China's electricity is generated by coal, the World Coal Association says, and experts say much of that is the low-grade "dirty" kind rather than the "clean" coal now favoured in many Western markets.

"Beijing has taken emergency measures to tackle the current pollution, but in the long term we need a region-wide policy to cut coal use to make the air quality better," Zhou Rong, climate and energy campaigner at Greenpeace, said.

A former senior official in the London mayor's office, John Ross, who is now a visiting professor at Jiaotong University in Shanghai, drew comparisons with the "Great Smog" of 1952 in the British capital.

The pollution was eventually cleared after legislation forced power stations out of the city and banned many smoke-emitting fuels.

"The lesson from London (is) industrial pollution can be eliminated but it takes strict laws, heavy investment and over 10 years," Ross said on his account on Sina Weibo, a version of Twitter.

Beijing said in May that it would get rid of 1200 high-polluting enterprises by 2015 to improve air quality, and has set up bicycle rental kiosks in a bid to ease traffic congestion.

But implementing strict controls that can last requires firm political will from China's new leaders, professor Cai said.

Given Xi's professed stand in favour of an environmental clean-up, he anticipated "substantial, radical changes" within two years.

But many ordinary Chinese do not share such optimism.

"Air pollution is an emergency as severe as an earthquake, flood or chemical spill," said one weibo poster.

"The government needs to mobilise all of society to solve this problem - not just its environmental protection department."