“We actually saw significant progress in 2014 and 2015,” said Lauri Myllyvirta, a senior coal campaigner at Greenpeace East Asia in Beijing. “But there’s been no improvement since March or April, which really coincides with a shift in economic policy.”

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Beijing had made a huge effort to clean up its air: The last of the coal-fired power plants in the capital is due to close in March 2017, coal-burning boilers have been replaced by gas or electric boilers in the center of the city, and investment poured into expanding a subway system to curb road traffic.

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Greenpeace says that it is showing results: The levels of arsenic, cadmium and lead — dangerous carcinogens linked to coal burning — have fallen sharply in the past three years.

But the capital still has a big problem, largely out of its control: It is surrounded by what Greenpeace calls “the biggest man-made hotspot of air pollution in the world” — a huge cluster of steel producers and other heavy industries situated in the province of Hebei.

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Hebei’s government depends on heavy industries for tax revenues, jobs and ultimately for social stability, and although central government funds have been set aside to shift it away from polluting heavy industries, no one wants a sudden jump in unemployment.

This year’s economic stimulus has pushed up steel prices, and some of Hebei’s most polluting steel factories have expanded production. When Beijing’s pollution is at its worst, it is usually because slow winds from the southwest have pushed a mass of polluted air hundreds of miles from Hebei and neighboring Shanxi provinces into the capital, Myllyvirta says.

This chart shows a 30-day moving average of PM2.5 concentrations in Beijing for the last four years. It measures most dangerous particles — smaller than 2.5 micrometers — that travel through the lungs and into the bloodstream and cause a range of health problems, from heart disease to severe asthma.

You can see the improvement Myllyvirta refers to. The annual average PM2.5 level in Beijing stood at 103.9 micrograms per cubic meter in 2013, fell slightly to 97.8 in 2014 and more significantly to 79.2 last year. But it’s still not falling fast enough.

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Millions of people die from the effects of air pollution globally every year, and the numbers of deaths are expected to continue rising.

In January 2014, the mayor of Beijing, Wang Anshun, told a meeting of municipal party leaders that he had signed a “life and death contract” with the central government to reduce PM 2.5 levels in the capital to a target of 60 micrograms per cubic meter.

“If we fail to deliver on air pollution in 2017, the boss said ‘bring me your head,’” he was quoted as saying. “It’s a joke as well as an order with weight.”

Nearly three years later, air pollution is set to miss the 2017 target and Wang has been replaced. Soon after taking office, Beijing’s new acting mayor, Cai Qi, implicitly acknowledged this by issuing a revised target of 56 micrograms per cubic meter by 2020 — giving him another three years.

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Cao Junji at the Institute of Earth Environment at the Chinese Academy of Science said that was more realistic and suitable target.

“Personally I think it gets harder and harder as the efforts to curb air pollution continue,” he said. “It’s no good setting high targets and failing to deliver.”

Here’s another Greenpeace chart that shows the link between air pollution and industrial production growth for the country as a whole.

While London's smog problem in past decades was largely caused by coal and Los Angeles's mainly by cars, bad air in Beijing has a number of causes, including cars, trucks, construction site dust and burning of corn stalks in the countryside. But the biggest regional source of polluted air is coal and heavy industry.

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In trying to curb industrial emissions, a big issue is the enforcement of existing rules.

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Laws that came into force last year and this year now require major factories to install air pollution monitoring devices and cities to disclose that information to the public.

So far compliance is patchy, says Ma Jun, at the Institute of Public and Environmental Affairs in Beijing — much better in Shandong province than Hebei for example — but progress is slowly being made toward greater transparency.

Officials with the Ministry of Environment Protection trying to enforce pollution controls still face “interference” from local officials in league with heavy industry, experts say. In one widely publicized case, three officials in the city of Xi’an were arrested last month for having stuffed cotton gauze into air pollution monitoring devices, in order to make the air seem cleaner.

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The ministry has also begun naming and shaming factories that fail to install pollution devices, report data or meet standards. But it does not always have the power to enforce compliance.

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“That is the reason we need deeper transparency, because we need public support to overcome that local interference,” Ma Jun said.

Attention now needs to focus on the major industrial sources of pollution, he said, “fixed discharge points where the standards are quite clear.” Beijing should also focus its attention on the relatively small number of heavy diesel trucks entering the capital, rather than trying to deal with millions of cars.

Luna Lin contributed to this report.