The air quality in Beijing Municipality fell to dangerous levels again on Saturday after only a few days of blue skies.

Fog started lingering in the city on Friday night, taking the PM2.5 concentration to between 300 and 400 micrograms per cubic meter of air, or Level VI, which is at a dangerous level, according to Beijing Municipal Environmental Monitoring Center statistics.

PM2.5 refers to fine particulate matter that are 2.5 microns or less in diameter.

The weather forecast bureau has issued yellow alerts for fog and haze, the third highest level following red and orange.

Visibility in the city's southern region will be less than 500 meters during daytime Saturday. A haze with a visibility less than 3,000 meters is expected to cover most of the city.

The city's environmental protection departments have advised residents, especially the elderly and children to stay indoors, and urged relevant departments to tell companies to take measures to curb emissions.

According to the weather forecast, it will snow on Saturday evening through to Sunday morning, which is likely to improve the air quality.

Beijingers suffered from heavy smog for seven days through until Wednesday, when a cold front with wind dispersed it.

The air quality indices were off the charts during the seven days, exceeding the "maximum" level of 500 in the city, as well as many other cities in central and north China.