An old man wearing facial mask walks at the Temple of Heaven Park in Beijing, capital of China, Feb 22, 2014. Thick smog shrouding Beijing and its surrounding areas becomes heavier as the capital has raised its four-tiered alert system to "orange". (Xinhua/Gong Lei)

Beijing on Monday maintained its orange pollution alert, the second-highest alert level, as severe smog is forecast to linger in the Chinese capital for another three days.

The density of PM2.5, particles smaller than 2.5 microns in diameter, which have been a major contributor to the smog, climbed again Sunday night and remained at high levels, the city's office for severe air pollution emergency response said in a statement.

The agency advised children and the elderly to stay indoors and to wear masks while going out. It also urged residents to take public transport and reduce driving. The agency urged middle schools, primary schools and kindergartens to reduce outdoor activities.

City authorities on Friday ordered 36 companies to halt production and another 75 to reduce production as part of a response mechanism when the pollution alert was raised to orange from yellow.

Beijing issued a yellow pollution alert on Thursday, the first since last October, when the emergency response system was put into place.

Beijing has a four-tier alert system, with blue, yellow, orange and red indicating the air pollution level in order of increasing severity.

A red alert indicates the most serious air pollution (AQI above 300) for three consecutive days. An orange alert indicates heavy to serious air pollution (AQI between 200 and 300) alternately for three consecutive days. A yellow alert indicates severe pollution for one day or heavy pollution for three consecutive days.

A red alert requires traffic to be cut with alternate driving days for even- and odd-numbered license plates, and schools to be suspended. Industrial plants are closed or told to reduce production when an orange alert is issued.

Despite a spate of control measures, Beijing and neighboring cities, mainly in Hebei Province, are prone to frequent air pollution.

The current smog, which started on Thursday, is also forecast to persist in neighboring Tianjin Municipality, Hebei province and parts of Shandong province for the next three days, according to the China Meteorological Administration.