Primary school students in Shenyang, Liaoning province, wear masks to protect themselves from pollution on Friday. The city has seen severe smog for several days. Provided to China Daily

With the coal-burning winter heating season right around the corner, Beijing issued a yellow alert for thick smog on Friday, after the city registered unhealthy air pollution for three straight days.

The city's Air Quality Index, which is calculated to gauge PM2.5 and other toxic pollutants, reached 260 on Friday morning. PM2.5 is airborne particulate matter with a diameter of less than 2.5 microns that can penetrate the lungs and harm health.

Based on China's national standard, an AQI reading of between 201 and 300 is considered "heavy pollution".

The pollution reduced visibility to less than 4 kilometers on Friday, and visibility is expected to fall to less than 500 meters on Saturday, the Beijing Meteorological Service reported on its website.

Conditions will not ease until Sunday, when smog-dispersing cold air is forecast.

Since October, Beijing has issued air pollution alerts four times, twice for blue alerts and twice for yellow alerts. The country has a four-level, color-coded weather warning system: Red represents the most serious conditions, followed by orange, yellow and blue.

In response to a yellow alert, Beijing requires the cessation of building demolition and construction work that involves digging or filling with dirt and rock.

Cities in the capital's neighboring Hebei province also have issued yellow alerts since Tuesday.

Cities in northern China typically suffer heavy air pollution when winter begins and household heating starts, mainly by burning coal, which contributes significantly to air pollution.

The National Meteorological Center said smog was also expected to hit parts of Henan and Shandong provinces on Friday and Saturday.

Wang Xiaoli, director of the weather emergency and warning center at the Hebei Environmental Protection Bureau, said that compared with previous years, the overall air quality has been improving year by year as the provinces take steps to cut pollution.