The days of Beijing being the world's most polluted capital are long gone (thanks Delhi!), but that doesn't mean the city of 22 million people is in the clear.

Key points: Beijing and other Chinese cities routinely suffer from heavy pollution

Beijing and other Chinese cities routinely suffer from heavy pollution Sydney's current pollution levels are dangerous but still lower than Beijing's

Sydney's current pollution levels are dangerous but still lower than Beijing's Severe smog events in the Chinese capital shut schools and heavy industry

While blue-sky days have increased substantially in recent years, the capital still has semi-regular bouts of thick smog.

As Sydney suffers through extraordinarily high levels of bushfire haze, it's worth noting Beijing's air is still substantially worse for the most dangerous type of pollutant.

Sydney's air quality index (AQI) is reported at government monitoring stations as a 24-hour average, but the visibility is updated every hour, explaining why an air quality monitor at Macquarie Park could report a staggeringly hazardous 1,647 on Tuesday lunchtime, while the most dangerous component — PM2.5 — is still lower than Beijing.

A picture taken from the Manly ferry shows Sydney blanketed in bushfire smoke. ( ABC News: Michael Troy )

Usually, Sydney's AQI — including the dangerous microparticles known as PM2.5 — would be around 20 or less.

That specific reading for the tiny particles that can be breathed into the lung and enter the bloodstream was hovering below 200 for most of Sydney on Tuesday but is likely to rise as the next 24-hour average is released.

Signs across Beijing warned motorists to "slow down" in smog events that severely limited visibility. ( Reuters: Jason Lee )

In Beijing, the PM2.5 level for Tuesday is close to 300.

But that's still not bad enough to trigger any of the major measures that authorities introduced seven years ago to cope with the smog.

2014 was a significant year in China's battle with pollution.

Air quality had been deteriorating for years in line with the country's increasing economic growth, but a series of sustained smog periods triggered the first-ever "red alert".

And although there is a four-colour system ranging from a moderate blue warning to the ominous red, it is only when things get really bad that disruptive measures swing into action.

In Beijing, the PM2.5 level for Tuesday was close to 300. ( ABC News: Bill Birtles )

Beijing students play under purified domes

A red alert for PM2.5 pollution above 200 for four straight days, 300 for two days or 500 for a single day means the government pulls its fleet of cars off the roads.

Classes are cancelled in a bid to prevent children having to go outside to travel to school and outdoor barbeques are banned, affecting the many small street vendors who grill spiced lamb sticks over charcoals.

In high-priced international private schools, air purification systems are a major selling point to attract parents.

Many such schools in Beijing have erected massive inflatable domes over their playing fields containing only purified air.

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The vast majority of children in Beijing at local schools have to still contend with outdoor playgrounds, but outdoor activities are cancelled at the second-worst, orange, alert level.

During red alerts, heavy industry around the capital is forcibly shut down or scaled back by authorities, although environmental groups point out that coal-fired power plants in nearby provinces are much bigger contributors to the smog that blows in and settles within the hills of the city.

Offices across the city are equipped with air purifiers and several years ago, when the smog was much worse, the release of new models could generate online buzz in a similar way to smartphones.

But now that's changed. China's efforts to cut pollution in the capital have been quite successful.

There hasn't been a red alert declared since early 2017 and that was for fog rather than pollution.

Smog cloaks Beijing's China Central Television building in 2013. ( Reuters: Jason Lee )

"Air quality has been significantly improving year-on-year since 2013, and Beijing will likely drop out of the world's 200 most polluted cities this year", said Yann Boquillod, the director of air quality monitoring for IQAir Visual, a monitoring company.

The regular social icebreakers about air quality apps and debates about the best air purifiers are noticeably down now on a few years ago, even though the PM2.5 reading still from time to time shoots up to hazardous levels.

And the number of people on the street wearing pollution masks is noticeably down as well, although the occasional multi-filter mask can still be spotted.

Beijingers are breathing easier, but that doesn't mean the city's battle with pollution has been won just yet.