By Michael Bristow
BBC News, Beijing
Despite the claimed improvements, Beijing still has smoggy days
Beijing is enjoying the best air quality this decade because of measures taken during last year's Olympic Games, officials have said.
They say there were 171 days of low pollution in the first seven months of this year, 22 more than in 2008.
One year after the games, independent experts agree that air quality in the Chinese capital has improved.
But they say the city still has some way to go before it can be compared with the world's cleanest cities.
And the clean-up came at a cost. China will find it hard to spend similar amounts of money improving its other polluted cities.
"All the major measures taken by the city were expensive and not easily replicated elsewhere," said Yang Ailun, of Greenpeace in China.
'Blue sky days'
Beijing's notorious air pollution was a major concern for athletes, spectators and Olympic officials before the games.
Chinese officials did not want the biggest international event ever staged in communist China being overshadowed by pollution.
So the government initiated a series of clean-up programmes for the Olympics. According the United Nations Environment Programme (UNEP), China spent a total of $17bn on these schemes.
Anti-pollution measures produced clear skies for some days of the Olympics
Polluting factories were moved, private cars were banned from the roads and coal boilers were converted so they could use cleaner natural gas.
This massive spending spree worked. Pollution was down by 36% in Beijing during the games last August compared to previous years, according to a UNEP report.
And the benefits have continued this year with an increased number of "blue sky days".
This uniquely Chinese standard measures the quantity of three air pollutants - which must fall below certain levels - rather than the colour of the sky or whether it is even visible.
Independent air quality expert Ivo Allegrini, who monitored the city's air quality during the Olympics, said Beijing's air will only get better.
"The government has a good working plan to reduce emissions over the next few years," said the Italian expert, who has kept a close eye on the city's air quality since the Olympics.
But he said that does not mean the Chinese capital's air is as clean as in other cities across the world at the moment.
Some critics add that the current pollution figures issued by the Chinese government under-estimate the real problem.
Beijing's government said the city's pollution level on Friday was "good", despite the fact that it was hazy for most of the day.
Improved air quality does not always mean blue skies. The WHO air quality guideline level is 50 micrograms/cubic metre
The US Embassy in Beijing has also been monitoring the air around its compound using a stricter measure.
At midday on the same day, it described the pollution level as "unhealthy".
It is also clear that some of the gains made during the Olympic Games were not sustained when the athletes left.
That is because some of the clean-up measures were only short-term schemes that have now been reversed.
China banned half the city's private cars - there are more than three million - from the roads during the games. But most of those were soon back on the streets.
Other aspects of city life, such as vegetable deliveries, were also restricted during the sporting event, an unsustainable solution to a city's pollution problems.
Ms Yang, the climate and energy campaign manager at Greenpeace, said a tax on fuel was a better way to discourage people from driving their cars.
"That kind of policy would be much more efficient than simple administrative measures," she said.