The most serious level of air pollution is now a red alert rather than orange, according to a new guideline released by Beijing Meteorological Bureau on Thursday, the Beijing News reported on Thursday.

The bureau has also included PM 2.5, or airborne particles measuring 2.5 microns or less in diameter, as an element in the appraisal criteria for air quality.

According to the revised guideline, when the PM 2.5 reading exceeds 500 and visibility is reduced to less than 5,000 meters, a red alert will be announced.

Previously, the alerts were based only on visibility and humidity.

The Beijing municipal government aims to decrease the average density of main air pollutants by 2 percent in 2013, according to the municipal government's annual work report released earlier this year.