The average concentration of PM 2.5 in Beijing's air over the past 24 hours has exceeded the limits for "air with acceptable quality", as defined by the Ministry of Environmental Protection's air quality standards.

PM 2.5 refers to particles in the air with less than 2.5 micrometers in diameter, which are believed to pose health risks.

Beijing's environmental monitoring center showed that all the 27 outdoor monitoring stations detected an average concentration of PM 2.5 higher than 100 micrograms per cubic meter from 6 pm yesterday to 5 pm today.

The highest average concentration was 254 mcg/cu, detected by an air monitoring station in Beijing's southwest Fangshan district.

Air quality that is "moderate", or better than that, should contain a 24-hour average concentration of PM 2.5 of no more than 75 mcg/cu, according to air quality standards. Otherwise, the air is deemed "polluted".

An average concentration of more than 250 mcg/cu falls in the category of "severely polluted" air, with people advised against outdoor activities, according to the ministry's standards, which will take effect in 2016.

Air quality is likely to improve tomorrow. The city's meteorological bureau has predicted that a cold air mass will reach the city and bring wind, which will improve conditions.

However, Beijing's air quality is expected to remain lackluster for some time.

"Based on the air quality of Beijing, excessive PM 2.5 concentration will be detected often for a long time to come," Zhao Yue, deputy director of the environmental monitoring center, told Xinhua News Agency on Oct 4.