SHANGHAI • China has publicly named more than 20 enterprises that it said broke environmental rules during this week's outbreak of hazardous smog in the country's north.

In its latest attempt to bring lawbreaking firms to account by shaming them, the Ministry of Environmental Protection (MEP) gave the names of steel and paper mills, cement plants, power generators and chemical producers accused of committing a range of offences.

Its investigation, published on Thursday, revealed that six steelmakers failed to comply with a Dec 18 order to suspend sintering operations, which are highly polluting.

The ministry also accused three more provincial steel firms of "maliciously" restarting sintering facilities as soon as its inspectors had left the site.

Most of the firms were from smog-hit Hebei province.

But Mr Alex Wang, an expert in Chinese environmental law at the University of California, Los Angeles, said such shaming tactics have a short-term impact and polluting firms need to be put under constant and systematic real-time scrutiny.

Greenpeace estimated that 460 million people - the equivalent of the populations of the United States, Canada and Mexico combined - were affected by the worst episode of smog to hit northern China in the past week.

"These campaign-style inspections still only catch violators intermittently (and) China is going to need to make more dramatic reforms," he said, adding that citizens should be given "bounties" for reporting violations.

A new law enacted early last year laid out heavier punishments and ruled environmental violations must be disclosed to the public. The MEP was also given new powers this year to conduct spot inspections anywhere in China and summon officials for their actions.

Greenpeace estimated that 460 million people - the equivalent of the populations of the United States, Canada and Mexico combined - were affected by the worst episode of smog to hit northern China in the past week.

More than 20 cities, including Beijing, issued red alerts from Dec 16 to Dec 21, triggering emergency measures to close schools, restrict traffic and shut thousands of factories and steel plants. Many plants reopened on Thursday as winds cleared the polluted air.

Pollution alerts are frequent during the winter when demand for energy, which is largely coal-fired, soars. But closures on the scale seen in the past week are rare.

In neighbouring Mongolia, the PM2.5 concentration peaked at 1,985 micrograms a cubic metre on Dec 16 in the capital's Bayankhoshuu district, according to government data.

Bloomberg reported that the PM2.5 level in Mongolia was five times worse than that recorded in Beijing during the past week.

Public anger over the Mongolian government's handling of pollution has been growing on social media, where residents share pictures of the smog, promote methods of protection and call on the authorities to do more to protect citizens.

After first cutting the night-time electricity tariff by 50 per cent to encourage residents to heat their homes with electric heaters - instead of raw coal, or other flammable material that is often toxic - Prime Minister Erdenebat Jargaltulga announced yesterday that the tariff would be eliminated entirely as of Jan 1.

In the long term, he proposed using a loan from China to build apartments and replace makeshift housing, doing more to encourage electric heating, and reducing poverty to slow migration to the capital, according to a government statement.

REUTERS