China's notoriously polluted capital of 17 million reached the clean-air day target on Sunday, 31 days ahead of schedule, Beijing's Municipal Environmental Protection Bureau said.

'The quality of our city's air has shown constant improvement over the last 10 years,' Mr Du Shaozhong, deputy director of the bureau, said in a statement.

Beijing had only 100 blue-sky days in 1998, when it introduced a clean-air campaign and began investing more than US$15 billion (S$22.9 billion) to improve air quality, the official Xinhua News Agency said.

The long-term measures as well as more drastic efforts taken ahead of the Olympic Games in August helped reach the goal, the bureau said.

Beijing pulled half the city's 3.3 million vehicles off the roads, halted most construction and closed some factories in the capital and surrounding provinces ahead of the games.

The Olympics proved that controlling emissions is the main way to reduce pollution, the bureau said. Car emissions, Beijing's main source of pollution, were reduced by 60 per cent from a year earlier because of the measures, it said.

So far this year levels of inhalable particulate matter - tiny dust particles that are among the worst pollutants - were reduced by 16 per cent from a year earlier, and other pollutants such as carbon monoxide and sulfur dioxide showed reductions of more than 20 per cent, the bureau said.

China's daily air pollution index, which ranges from 1 to 500, uses a standard calculation derived from levels of major pollutants.

A reading below 50 is considered good, and 51 to 100 is moderate.

Below 100 is considered a 'blue-sky day.' Only 56 days have measured 'good' so far this year, the bureau said. But environmentalists say a blue-sky day is still more polluted than what is considered healthy by the World Health Organisation.

Mr Steven Andrews, an independent environmental consultant based in Washington, said Beijing's claims of improved air quality are not reliable because the city has moved monitoring stations to less-polluted areas and has varied the way it has measured pollutants since 1998.

'They've measured different things during that time period and it has a huge impact on the number of days that meet the national standard,' Mr Andrews said in a telephone interview.

Such inconsistencies mean that the increase in the number of blue-sky days may be due to the change of monitoring locations, rather than a reduction in overall pollution levels, he said. -- AP