Official statistics show that Beijing's air has seen a marked decrease in the density of PM2.5, particulate matter with a diameter of 2.5 microns or less, over the past decade and the downward trend is expected to continue.

The average annual PM2.5 density in Beijing has decreased from 100-110 micrograms per cubic meter of air in 2000 to 70-80 micrograms per cubic meter in 2010, said the Beijing Municipal Environmental Protection Bureau.

Although Beijing did not monitor air-quality using the PM2.5 gauge 10 years ago, PM2.5 data were available for laboratory use, according to the bureau.

China currently uses PM10, or particulate matter under 10 microns in size, to measure air quality. But finer PM2.5 particulates are considered more hazardous to people's health as it can go deeper into the lungs.

The Ministry of Environmental Protection said it has been considering revising present standards and a new index standard that includes measurements of PM2.5 and ozone density is scheduled to be fully implemented nationwide in 2016 with pilot projects conducted in certain regions ahead of the national deadline.