Major pollutants in Beijing are down and air quality in the city has improved for a ninth consecutive year, an environment official said at a news conference on Wednesday.

Between 1998 and 2007, the amount of sulphur dioxide in the capital's air was down 60.8 percent, while carbon monoxide was cut 39.4 percent. Nitrogen dioxide was reduced 10.8 percent and particles were down 17.8 percent, said Du Shaozhong, the Beijing Environment Protection Bureau deputy head, quoting the latest statistics.

The number of days that met the "blue sky" standard had increased from 100 to 246 during the period and the figure is expected to rise to 256 this year, he told reporters.

Apart from the particles, Du said the other indices had met national standards. "Particles and dust at construction sites, which cover about 100 million square meters in Beijing, are a major source of pollution," he said.

"We have taken action in the past few phases, but we will still stiffen requirements for sites that produce dust. We also need businesses to discipline themselves to follow environmental rules, and the public to help supervise."

China adopted the air quality standards, which were introduced by the World Health Organization to developing countries, in 2005, under which the daily density of inhalable particles should not exceed 150 micrograms per cubic meter.

Hitting back at reports that Beijing environment staff sampled areas where pollution was not serious to make the statistics look good, Du defended said "the improvement of Beijing's air quality is an indisputable fact". He denounced the reports as "untrue and distorting the true picture".

He said all environment statistics are available at the bureau's website for reference.

In further attempts to remove dirty air, Beijing would implement more stringent environmental standards to reduce pollutants in the coming months ahead of the August Olympics.

Du outlined a new set of measures this year to contain air pollution. This included removing 2,300 cars, 1,500 buses and 2,000 cabs that failed to meet emission standards.

The bureau will continue to target industrial emissions by polluting businesses such as chemical, metallurgy and cement manufacturers, and require residents to contribute to emission-cutting by switching from coal to electricity.