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".