In 2013, the air quality was deemed unhealthy or hazardous for over half the year.

Peaking in Beijing at 35 times the World Health Organisation's recommended limit.

It was so bad that Premier Li Keqiang declared a "war on pollution" at China's annual high profile National People's Congress.

Five years later, in March 2019, as Premier Li again opened the NPC meetings, the smog outside was still ten times worse than what the WHO defines as healthy.

Even as China cracks down on pollution like never before, the country remains one of the world's worst polluters.

This is your Bloomberg QuickTake on China's smog.

China overtook the U.S. as the world's biggest source of greenhouse gases in 2006, helping put the globe on a path to miss United Nations' targets aimed at stemming the rise in the earth's temperature.

This cheap power from coal and cheap factory production that's powered by coal has helped China turn into this economic giant.

That has helped produce cheap goods for the rest of the world and helped drive the world's economy.

And so in a sense, Chinese people are paying a tax in breathing this bad air for the benefit of consumers all over the world.

The WHO estimates more than 1 million Chinese died from dirty air in 2016.

Another study puts the tally even higher at 4,000 deaths a day.

Pollution is said to have been the main cause of social unrest in recent years.

With social media helping to amplify complaints.

On Weibo, China's Twitter-like online platform, people blamed factories for polluting the air and the government for not doing enough.

In February 2015, a Chinese investigative journalist published a self-funded documentary about the country's air pollution problem.

More than 100 million people watched "Under the Dome" before it was banned from Chinese video websites six days after its release.

Shortly after, President Xi Jinping pledged to unleash an iron hand to punish environmental polluters.

In the last few years, the government has spent billions of yuan tightening environmental regulations.

Scrapping coal-fired power plants and switching millions of homes and businesses from coal to natural gas.

The regulations and policies have worked.

The US State Department monitors particulate matter in the air in its Beijing embassy.

And these recordings showed that 2018 was the lowest level in a decade.

And the winter of 2017 and 2018 was one of the best as far as air quality goes.

It's not perfect, but it's much, much better than the heyday of the 2013 pollution problems.

China is now the world's biggest investor in green energy.

It spent over $100 billion in 2018, which was 56% more than the U.S.

Initiatives have included supporting the electric vehicle industry by providing subsidies for EV buyers.

And helping build out infrastructure that allows electric cars to drive and charge around its cities.

EV sales are huge in China. It's the biggest market in the world for electric vehicles.

It's not just cars as well. Electric buses are a huge deal in China.

China is betting big on solar energy as well.

In 2019, over a third of the world's solar panels are estimated to be installed in China.

But the war on pollution promises to be a long one.

Four decades of breakneck economic growth have turned China into the world's biggest carbon emitter.

And it's still going to depend on coal for years to come.